Torgeir Bjørge Andersen

• Grund, Saskia; John, Timm; Vrijmoed, Johannes Christiaan; Austrheim, Håkon Olaf & Andersen, Torgeir Bjørge (2025). A Mechanistic Look at the Amphibolitization of Mafic Crust: Insights From the Kråkeneset Gabbro Body, Western Gneis Region, Norway. Journal of Metamorphic Geology. ISSN 0263-4929. 43(4), p. 385–405. doi: 10.1111/jmg.12809. Full text in Research Archive Show summary

  ABSTRACT Fluid–rock interactions play a key role in the formation, evolution and recycling of the Earth's crust. For fluids to infiltrate rocks and enable and sustain fluid-mediated mineral transformations, fluid pathways are required. In this study, we examined the potential mechanisms of formation of such pathways via detailed mineralogical, petrophysical and thermodynamic analysis of a dry, essentially ‘non-porous’ gabbro that was hydrated and transformed into an amphibolite under amphibolite-facies conditions. During a previous regional HP eclogite-facies metamorphism, the gabbro did not equilibrate and preserved almost entirely its igneous textures and magmatic minerals. Rock transformation during amphibolitization was triggered by fluid infiltration through a newly opened N–S striking fracture network. An equally spaced fracture network formed by mode I opening related to the formation of an E–W striking shear zone at the northern and southern borders of the gabbro body. The amphibolitization process allowed the fluid to pervasively infiltrate the rock from the fracture into the pristine gabbro. The essentially fully amphibolitized sample exhibits some unaffected gabbroic mineral relicts. Even though the amphibolitization process led to the formation of ~70 vol.% hydrous phases, it was accompanied by densification and related porosity formation. The modes and compositions of minerals within partly amphibolitized rocks indicate that besides the uptake of H2O, no significant mass exchanges were necessary for this transformation, at least on the thin section scale. Thermodynamic modelling and petrological data show that the transition from gabbro to amphibolite favours porosity formation. In the model, the reaction front proceeded as soon as the gabbro at the reactive interfaces of the affected minerals was sufficiently transformed. At this point, fluid was not consumed further but remained as a free fluid phase, which progressed through the newly formed pore space and advanced amphibolitization. Once the gabbro was almost entirely amphibolitized, its mineral content and mineral chemistry no longer changed, so the progress of amphibolitization progress was controlled by fluid availability. This case study shows that fluid–rock interaction leading to hydration of a rock can be efficiently maintained in almost non-permeable, dry and mafic crust and, therefore, strongly affects the petrophysical properties of the Earth's crust.

• Austrheim, Håkon Olaf & Andersen, Torgeir Bjørge (2023). Extreme enrichment of arsenic and antimony during alteration of serpentinized peridotites to form listvenite-like dolomite–quartz rocks and Ni–Cr-rich jasper and quartzites in the Highland Border Complex of Scotland. Journal of the Geological Society. ISSN 0016-7649. 180(5). doi: 10.1144/jgs2023-054. Full text in Research Archive Show summary

  Alteration of serpentinised peridotitesof Highland Border Complex in Scotland took place in two steps. Listvenite-like dolomite-quartz rocks formed by addition of CaO, Sr and CO2 at constant MgO and SiO2 involving a mass increase of ca. 140 %. Stage-two involved the dissolution of dolomite evinced by the abundant pores and rhombohedral-shaped grains of quartz to form Cr- and Ni-rich jasper and quartzites. Formation of the jasper-quartzites involve a mass reduction of ca. 80%. The listvenite-like and jasper-quartzite rocks have enrichment in ACCEPTED MANUSCRIPT the fluid mobile elements Ba, Sr, Cs, As and Sb. The As is present in the Aluminium- Phosphate-Sulphate group of minerals formed during alteration of Cr-spinel. Cr-spinel also alters to porous hematite and ferrihydrite with patches containing up to 5.5 wt% As2O3. Enrichment of As, related to alteration of chromite, is previously unknown from natural rocks, but strongly resembles efficient methods used for remediation of this toxic element. Formation of quartzite and jasper from peridotite and their common presence as pebbles both in the Devonian Old Red conglomerates, in the Highland Border Complex and in Devonian Basins in the Scandinavian Caledonides, highlights their importance and potential for provenance- and tectono-stratigraphic correlations.

• Osmundsen, Per Terje; Svendby, Anne Kathrine; Braathen, Alvar; Bakke, Beate & Andersen, Torgeir Bjørge (2023). Fault growth and orthogonal shortening in transtensional supradetachment basins: Insights from the `Old Red´ of western Norway. Basin Research. ISSN 0950-091X. 35(4), p. 1407–1432. doi: 10.1111/bre.12759. Full text in Research Archive Show summary

  For basins that evolve adjacent to large-magnitude normal faults, tectonic controls on sedimentation involve isostatic back rotation of an exhuming footwall and, commonly, the evolution of kilometre-scale extension-parallel folds. Based on observations from classic localities in western Norway, we propose a three-stage evolution scenario for transtensional supradetachment basins where the basins become progressively re-arranged because of core complex exhumation and subsequent orthogonal shortening. Extension-parallel transverse synclines initially form due to a normal displacement gradient, but when displacements accumulate beyond a certain magnitude, the hanging wall increasingly responds to core complex exhumation and the original depocentre, formed close to the original area of maximum displacement, will become inverted and dismembered above the core complex. Two new synclinal depocentres will develop along the flanks. Because these synclines form by extensional fault growth rather than by shortening, they will be associated with widening of the basin and onlap onto basement at high angles to the maximum elongation trend with overall grain-size decrease and retrogradational stacking patterns. Further, because these synclines grow away from the evolving core complex, sedimentary units will become asymmetrically distributed inside each syncline in such a way that the oldest deposits in the syncline will be preserved on the flank most proximal to the core complex. In transtensional environments, a third evolutionary stage may involve constrictional strain where extension-parallel folds and reverse faults produced by orthogonal shortening enhance or interact with other structures. Ultimately, initial extensional sub-basins may become warped across extension-parallel folds. Hanging wall deformation will be manifested in shifting accommodation patterns, with depocentres that generally migrate in the direction of the detachment fault. Accommodation patterns initially related to megafault growth may conceptually evolve into depocentres controlled by orthogonal shortening.

• Jakob, Johannes; Andersen, Torgeir Bjørge; Mohn, Geoffroy; Kjøll, Hans Jørgen & Beyssac, Olivier (2022). Revised tectono-stratigraphic scheme for the Scandinavian Caledonides and its implications for our understanding of the Scandian orogeny. In Kuiper, Yvette D. (Eds.), New Developments in the Appalachian-Caledonian-Variscan Orogen. The Geological Society of America. ISSN 9780813725543. p. 335–374. doi: 10.1130/2022.2554(14. Full text in Research Archive Show summary

  The Scandinavian Caledonides formed during the continental collision between Baltica and Laurentia. During the collision, a complex nappe stack was thrust over the Baltican continental margin. The orogen can be subdivided into segments based on architectural differences within the Scandian nappes. The southern and central segments of the orogen link up in the Gudbrandsdalen area in south-central Norway. Alpine-type metaperidotite-bearing metasedimentary complexes occur in the southern and central segments and can be traced continuously along the strike of the orogen from one into the other segment. Traditionally, these units have been assigned to different tectono-stratigraphic levels, one below the Middle Allochthon and one above the Middle Allochthon. Here, we trace the Alpine-type metaperidotite-bearing units from Bergen to Esandsjøen and show that these units exhibit a common geologic and metamorphic history, consistent with the metaperidotite-bearing units representing a single tectonic unit. We suggest that the metaperidotite-bearing units can be used as a “marker level” to revise the tectono-stratigraphy of the Gudbrandsdalen and adjacent areas. The tectono-stratigraphic revisions imply that the Scandian nappe stack consists of seven tectono-stratigraphic levels that can be traced throughout the southern and central segments of the Scandinavian Caledonides. Moreover, the revision of the tectono-stratigraphy and new U-Pb geochronology data also suggest a revision of the timing of the succession of tectonic events leading up to the Scandian continental collision. The available evidence indicates that Baltica-derived tectonic units collided with the Iapetan/Laurentian subduction complexes as early as ca. 450 Ma. The initial collision was followed by in-sequence nappe formation of Baltican-derived units, which occurred contemporaneously with the opening of a marginal basin in the upper plate. After the arrival of thick, buoyant, unthinned Baltican crust at the trench, the main zone of convergence stepped outboard, the marginal basins closed, and those basins were thrust out-of-sequence over the previously assembled nappe stack.

• Andersen, Torgeir Bjørge; Jakob, Johannes; Kjøll, Hans Jørgen & Tegner, Christian (2022). Vestiges in the Pre-Caledonian Passive Margin of Baltica in the Scandinavian Caledonides: Overview, Revisions and Control on the Structure of the Mountain Belt. Geosciences. 12(57). doi: 10.3390/geosciences12020057. Full text in Research Archive Show summary

  Abstract: The Pre-Caledonian margin of Baltica has been outlined as a tapering wedge with increas- ing magmatism towards the ocean–continent transition. It is, however, well known that margins are complex, with different and diachronous evolution along and across strike. Baltica’s vestiges in the Scandes have complexities akin to modern margins. It included a microcontinent and magma-poor hyperextended and magma-rich segments. It was probably up to 1500 km wide before distal parts were affected by plate convergence. Characteristic features are exhumed mantle peridotites and their detrital equivalents, some exposed to the seafloor by the pre-orogenic hyperextension. A major change in the architecture of the mountain belt occurred across the NW–SE trending Sveconorwe- gian front in the Baltican basement. This coincided with the NE termination of the Jotun-Lindås- Dalsfjord basement nappes, the remains of the Jotun Microcontinent (JMC) formed by hyperexten- sion prior to the orogeny. Mantle with ophicalcite breccias exhumed by hyperextension are covered by deep-marine sediments and local conglomerates. Baltican basement slivers are common in the transitional crust basins. Outboard the JMC, the margin was magma-rich. The main break-up mag- matism at 605 ± 10 Ma was part of the vast Central Iapetus Magmatic Province. The along-strike heterogeneity of the margin controlled diachronous and contrasting tectonic evolution during the later Caledonian plate convergence and collision. Keywords: Scandinavian Caledonides; Baltica; passive margin; hyperextension; mantle exhuma- tion; microcontinent; magma-rich; magma-poor

• Jakob, Johannes; Kjøll, Hans Jørgen & Andersen, Torgeir Bjørge (2022). A Fossil Magma-rich Rifted Margin in the Scandinavian Caledonides, Continental Rifted Margins 2: Case examples. ISTE. ISSN 9781789450620. p. 185–202. Full text in Research Archive Show summary

  Rifted margins mark the transition between continents and oceans, which are the two first-order types of land masses on Earth. Rifted margins contribute to our understanding of lithospheric extensional processes and are studied by various disciplines of Earth Science (geology, geophysics, geochemistry). Thanks to better and wider public access to high-quality data, our understanding in these areas has improved significantly over these last two decades. This book summarizes this knowledge evolution and details where we stand today, with a series of case examples included. It is structured in a practical way, with concise text descriptions and comprehensive diagrams. Continental Rifted Margins 2 is a useful resource for students and newcomers to the rifted margin community – a “cookbook” of sorts to facilitate the reading of scientific publications and provide basic definitions and explanations.

• Corfu, Fernando & Andersen, Torgeir Bjørge (2022). A hyperextension assemblage, imbricated in Archean– Paleoproterozoic crust, at the base of the Kalak Nappe Complex in the northern Scandinavian Caledonides. Journal of the Geological Society. ISSN 0016-7649. 179(4). doi: 10.1144/jgs2021-140. Full text in Research Archive Show summary

  Abstract The breakup of continents generates dyke swarms, basins, and hyperextension assemblages. Once incorporated in new orogens the latter can provide crucial information on origins and tectonic processes. The Nålfjell Complex in the Caledonian Skillefjord Nappe of northern Norway has many of the characteristics of an hyperextension assemblage, notably the presence of solitary serpentinite bodies exposed by exhumation of serpentinized mantle, and now embedded in schists, amphibolites, mylonites, marbles, and felsic gneisses The Skillefjord Nappe is a lithologically diverse, imbricated and discontinuous allochthton. It comprises felsic gneisses and dykes dated by zircon U-Pb to ca. 3100, 2940, 2830, 2510 and 1800-1750 Ma. These rocks yield titanite ages of 2810-2700, 1750, 1660-1590 Ma and 420-430. Metagabbro intruded at 1995 Ma. The ages and evolution of the Skillefjord Nappe are very distinct from those of the structurally higher Svaertholt Terrane (>1030 Ma sediments deformed and intruded by granite at 980-950 Ma), and the Sørøy Terrane (characterized by a multistage 900 to 500 Ma evolution). The assembly of these disparate elements was completed at 420 Ma. The timing of hyperextension is uncertain. The serpentinites have a primitive Pb isotopic composition indicating old depleted mantle lithosphere. Extensional processes from the Palaeoproterozoic to Ediacaran are considered.

• Kaatz, Lisa; Zertani, Sascha; Moulas, Evangelos; John, Timm; Labrousse, Loic & Schmalholz, Stefan Markus [Show all 7 contributors for this article] (2021). Widening of hydrous shear zones during incipient eclogitizationof metastable dry and rigid lower crust, Holsnøy, Western Norway. Tectonics. ISSN 0278-7407. 40(3). doi: 10.1029/2020TC006572. Full text in Research Archive Show summary

  The partially eclogitized crustal rocks on Holsnøy in the Bergen Arcs, Norway, indicate that eclogitization is caused by the interplay of brittle and ductile deformation promoted by fluid infiltration and fluid-rock interaction. Eclogitization generated an interconnected network of millimeter- to kilometer-wide hydrous eclogite-facies shear zones, which presumably caused transient weakening of the mechanically strong lower crust. To decipher the development of those networks, we combine detailed lithological and structural mapping of two key outcrops with numerical modelling. Both outcrops are largely composed of preserved granulite with minor eclogite-facies shear zones, thus representing the beginning phases of eclogitization and ductile deformation. We suggest that deformation promoted fluid-rock interaction and eclogitization, which gradually consumed the granulite until fluid- induced reactions were no longer significant. The shear zones widen during progressive deformation. To identify the key parameters that impact shear zone widening, we generated scale-independent numerical models, which focus on different processes affecting the shear zone evolution: (i) rotation of the shear zones caused by finite deformation, (ii) mechanical weakening due to a limited amount of available fluid, and (iii) weakening and further hydration of the shear zones as a result of continuous and unlimited fluid supply. A continuous diffusion-type fluid infiltration, with an effective diffusion coefficient around 𝐷 = 10−16 𝑚2, coupled with deformation is prone to develop structures similar to the ones 𝑠 mapped in field. Our results suggest that the shear zones formed under a continuous fluid supply, causing shear zone widening, rather than localization, during progressive deformation.

• Hosseinzadehsabeti, Elham; Ferre, Eric C.; Andersen, Torgeir Bjørge; Geissman, John W; Bilardello, Dario & Toro, Giulio Di (2021). Focal mechanisms of intraslab earthquakes: insights from pseudotachylytes in mantle units. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 126(4). doi: 10.1029/2020JB021479. Full text in Research Archive Show summary

  Devastating seismic events occur mainly in subduction zones, and a significant percentage of them are intraslab earthquakes. The geologic record of these events holds valuable information that needs to be investigated for a comprehensive seismic risk assessment. Here we investigate pseudotachylytes formed in oceanic peridotites and that are interpreted to result from intraslab seismic rupture. Each vein has recorded the seismic slip direction and slip sense of a single coseismic shear-heating event. The well-preserved exposures, showing individual veins up to 7 m in length and about 3 cm in width, of Cima di Gratera, in the Schistes Lustrés ophiolitic units of Corsica, offer unparalleled opportunities to investigate intraslab rupture kinematics in mantle rocks. The principal magnetic phases in these rocks is a Ti-poor magnetite. We use the anisotropy of magnetic susceptibility (AMS) recorded in pseudotachylyte generation veins (bulk susceptibilities range from 600 to 20,000 × 10-6 [SI] volume, with P’ ranging from 1.05 to 2.5) toreconstruct the co-seismic deformation parameters, i.e., fault plane attitude, direction and sense of slip. These new results, internally consistent at the vein level, span across oblate and prolate symmetries and reveal that seismic deformation recorded in these veins was kinematically diverse and included strike slip, normal and reverse mechanisms acting along the same subduction zone. In addition, our investigations show that the magnetic fabric of peridotite- hosted pseudotachylytes provides key information bearing on the complex dynamics of frictional melts at a unprecedently high spatial resolution.

• Zertani, Sascha; Vrijmoed, Johannes Christiaan; Tilmann, Frederik; John, Timm; Andersen, Torgeir Bjørge & Labrousse, Loic (2020). P wawe anisotropy caused by partial eclogitization of descending crust demonstrated by modelling effective petrophysical properties. Geochemistry Geophysics Geosystems. 21(6). doi: 10.1029/2019GC008906. Full text in Research Archive Show summary

  eismological studies of large-scale processes at convergent plate boundaries typically probe lower crustal structures with wavelengths of several kilometers, whereas field-based studies typically sample the resulting structures at a much smaller scale. To bridge this gap between scales, we derive effective petrophysical properties on the 20-m, 100-m, and kilometer scales based on numerical modelling with the Finite Element Method. Geometries representative of eclogitization of crustal material are extracted from the partially eclogitized exposures on Holsnøy (Norway). We find that the P wave velocity is controlled by the properties of the lithologies rather than their geometric arrangement. P wave anisotropy, however, is dependent on the fabric orientation of the associated rocks, as fabric variations cause changes in the orientation of the initial anisotropy. As a result, different structural associations can result in effective anisotropies ranging from ~0-4% for eclogites not associated with ductile deformation to up to 8% for those formed during ductile deformation. For the kilometer-scale structures, a scale that in principle can be resolved by seismological studies, we obtained P wave velocities between 7.7 and 8.0 km-1. The effective P wave anisotropy on the is ~3-4% and thus may explain the backazimuthal dependence of seismological images of, for example, the Indian lower crust currently underthrusting beneath the Himalaya. These results imply that seismic anisotropy could be the key to visualize structures in active subduction and collision zones that are currently invisible to geophysical methods and thus can be used to unravel the underlying processes active at depth.

• Kjøll, Hans Jørgen; Andersen, Torgeir Bjørge; Corfu, Fernando; Labrousse, Loic; Tegner, Christian & Abdelmalak, Mohamed Mansour [Show all 7 contributors for this article] (2019). Timing of break-up and thermal evolution of a pre-Caledonian Neoproterozoic exhumed magma-rich rifted margin. Tectonics. ISSN 0278-7407. 38(6), p. 1843–1862. doi: 10.1029/2018TC005375. Full text in Research Archive Show summary

  During the terminal stages of Wilson cycles, remnants of magma-poor margins may be incorporated into the orogens, whereas the magma-rich margins often are lost in subduction due to low buoyancy. The understanding of magma-rich margins is therefore mostly based on drill holes and geophysical observations. In this contribution, we explore the temporal evolution and the ambient conditions of a magma-rich rifted margin preserved within the Scandinavian Caledonides. The Scandinavian Dike Complex was emplaced into a sedimentary basin during the initial break-up and opening of the Iapetus Ocean 615 to 590 million years ago. The dike complex constitutes 70-90% of the magma-rich, syn-rift basins and is locally well-preserved despite the complex Caledonian history. This contribution provides new observations about the geometry, relative timing, and development of the margin. Jadeite-in-clinopyroxene geothermobarometry, titanium-in-biotite geothermometry, and garnet isopleth modeling show that the ambient pressure and temperature conditions were similar for the entire dike complex at 0.25 to 0.45 GPa, with contact metamorphic temperatures up to c. 700°C. In the northernmost part of the study area U-Pb dating of magmatic zircon shows that partial melting of the sedimentary host-rock, at relatively shallow levels, occurred at 612 Ma. This shows that the crust was molten already 6 million years before the northernmost dike swarm was emplaced at 605.7 ± 1.8 Ma. We propose that the locally pervasive partial melting occurred due to high geothermal gradients and introduction of mafic melt in the lower crust. These processes significantly reduced the strength of the crust, eventually facilitating continental break-up.

• Incel, Sarah; Labrousse, Loic; Hilairet, Nadège; John, Timm; Gasc, Julien & Shi, Feng [Show all 11 contributors for this article] (2019). Reaction -induced embrittlementog the lower continental crust. Geology. ISSN 0091-7613. 47(3), p. 235–238. doi: 10.1130/G45527.1. Full text in Research Archive Show summary

  Field observations and geophysical data reveal a causal link between brittle seismic failure and eclogitization of the lower continental crust. We present results from experimental deformation of plagioclase-rich samples at eclogite-facies conditions and quantify the link between rock rheology and the kinetics of the eclogitization reactions. The deformation was ductile both in the absence of reaction and when the progress of eclogitization was fast compared to the imposed strain rate. However, when the reaction rate was relatively slow, the breakdown of plagioclase into nanocrystalline reaction products induced a weakening that triggered seismic failure. Fluid-induced plagioclase breakdown under eclogite- facies conditions is an exothermic reaction accompanied by a negative change in solid volume. This is similar to other mineral transformations that are known to trigger transformational faulting. We demonstrate that mineral reactions lead to brittle deformation in situations where reaction rates are slow compared to the deformation rate. This reaction-induced instability may provide a generic mechanism for embrittlement at depths beyond the normal seismogenic zone.

• Kjøll, Hans Jørgen; Galland, Olivier; Labrousse, Loic & Andersen, Torgeir Bjørge (2019). Emplacement mechanisms of a dyke swarm across the brittle-ductile transition and the geodynamic implications for magma-rich margins. Earth and Planetary Science Letters. ISSN 0012-821X. 518, p. 223–235. doi: 10.1016/j.epsl.2019.04.016. Full text in Research Archive Show summary

  gneous dykes are the main magma transport pathways through the Earth’s crust, and they are considered to mainly accommodate for tectonic extension in volcanic rifts. Dykes are typically considered to result from brittle fracturing, even in the ductile crust. A common assumption is that dyke orientation is controlled by tectonic stresses, such that dykes in rifts are expected to be vertical and perpendicular to extension. Here we report on detailed field observations of a spectacularly well-exposed dyke swarm to show that dykes were not systematically emplaced by purely brittle processes and that dyke orientation may differ from the dominant tectonic stress orientations. The dyke complex formed near the brittle-ductile transition during opening of the Iapetus Ocean and is now exposed in the Scandinavian Caledonides. Distinct dyke morphologies related to different emplacement mechanisms has been recognized: 1) Brittle dykes that exhibit straight contacts with the host rock, sharp tips, en-echelon segments with bridges exhibiting angular fragments; 2) Brittle-ductile dykes that exhibit undulating contacts, rounded tips, ductile folding in the host rock and contemporaneous brittle and ductile features;3) Ductile “dykes” that exhibit rounded shapes and mingling between the soft ductile host rock and the intruding mafic magma. The brittle dykes exhibit two distinct orientations separated by c. 30◦ that are mutually cross-cutting, suggesting that the dyke swam did not consist of only vertical sheets perpendicular to regional extension, as expected in rifts. We were able to use the well-exposed host rock layers as markers to perform a kinematic restoration to quantify the average strain accommodating the emplacement of the dyke complex: it accommodated for >100% extension, but counter-intuitively it also accommodated for 27% crustal thickening. We infer that the magma influx rate was higher than the tectonic stretching rate, implying that magma was emplaced forcefully, as supported by field observations. Finally, our observations suggest that the fast emplacement of the dyke swarm triggered a rapid shallowing of the brittle-ductile transition, and lead to a considerable weakening of the crust. Our study can potentially have large implications significant for surface topography and seismicity in active rifts and volcanic areas around the world.

• Jakob, Johannes; Andersen, Torgeir Bjørge & Kjøll, Hans Jørgen (2019). A review and reinterpretation of the architecture of the South and South-Central Scandinavian Caledonides—A magma-poor to magma-rich transition and the significance of the reactivation of rift inherited structures. Earth-Science Reviews. ISSN 0012-8252. 192, p. 513–528. doi: 10.1016/j.earscirev.2019.01.004. Full text in Research Archive Show summary

  nterpretations of the pre-Caledonian rifted margin of Baltica commonly reconstruct it as a simple, tapering, wedge-shaped continental margin dissected by half graben, with progressively more rift-related magmas towards the ocean-continent transition zone. It is also interpreted to have had that simple architecture along-strike the whole length of the margin. However, present-day rifted margins show a more complex architecture, dominated by different and partly diachronous segments both along and across strike. Here, we show that the composition and the architecture of the Baltican-derived nappes of the South and South Central Scandinavian Caledonides are to a large extend rift-inherited. Compositional variations of nappes in similar tectonostratigraphic positions can be ascribed to variations along-strike the rifted margin, including a magma-rich, a magma-rich to magma-poor transition zone, and a magma-poor segment of the margin. The architecture of the nappe stack that includes the Baltican-derived nappes was formed as a result of the reactivation of rift-inherited structures and the stacking of rift domains during the Caledonian Orogeny.

• Zertani, Sascha; John, Timm; Tilmann, Frederik; Motra, Hem B.; Keppler, Ruth & Andersen, Torgeir Bjørge [Show all 7 contributors for this article] (2019). Modification of the seismic properties of subducting continental crust by eclogitization and deformation processes. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 124(9), p. 9731–9754. doi: 10.1029/2019JB017741. Full text in Research Archive Show summary

  Subduction zone processes and the resulting geometries at depth are widely studied by large- scale geophysical imaging techniques. The subsequent interpretations are dependent on information from surface exposures of fossil subduction and collision zones, which help to discern probable lithologies and their structural relationships at depth. For this purpose, we collected samples from Holsnøy in the Bergen Arcs of western Norway, which constitutes a well-preserved slice of continental crust, deeply buried and partially eclogitized during Caledonian collision. We derived seismic properties of both the lower crustal granulite-facies protolith, and the eclogite-facies shear zones by performing laboratory measurements on cube-shaped samples. P- and S-wave velocities were measured in three perpendicular directions, along the principal fabric directions of the rock. Resulting velocities agree with seismic velocities calculated using thermodynamic modeling and confirm that eclogitization causes a significant increase of the seismic velocity. Further, eclogitization results in decreased VP/VS-ratios and, when associated with deformation, an increase of the seismic anisotropy due to the crystallographic preferred orientation of omphacite that were obtained from neutron diffraction measurements. The structural framework of this exposed complex combined with the characteristic variations of seismic properties from the lower-crustal protolith to the high-pressure assemblage provides the possibility to detect comparable structures at depth in currently active settings using seismological methods such as the receiver function method.

• Zertani, Sascha; Labrousse, Loic; John, Timm; Andersen, Torgeir Bjørge & Tilmann, Frederik (2019). The interplay of eclogitization and deformation during deep burial of the lower ontinental crust – a case study from the Bergen Arcs (western Norway). Tectonics. ISSN 0278-7407. 38(3), p. 898–915. doi: 10.1029/2018TC005297. Full text in Research Archive Show summary

  The island of Holsnøy in the Bergen Arcs, which belong to the Caledonides of western Norway, represents an excellent example of how fluid‐induced eclogitization modifies material deeply buried by subduction and continental collision. We produced a new detailed map of the northwestern part of Holsnøy, differentiating not only the magnitude of eclogitization but also the strain intensity at different spatial scales: from the outcrop to the entire massif. Using structural data from eclogite‐facies shear zones and eclogitized low‐strain domains, we show that fluid‐mediated eclogitization not only progresses via the development of shear zones (dynamic eclogitization) but occurs over large areas of the island without associated deformation, creating a characteristic static eclogite‐facies overprint. Static eclogitization preserves the structural features of the granulitic protolith while the rock body is transformed from a granulite‐ to an eclogite‐facies mineral assemblage. The extent of static eclogitization was underestimated strongly in the past, a finding also relevant for the interpretation of seismological images in currently active orogens, where presumably similar processes are currently occurring. In addition, we find that the general structure of the eclogite‐facies shear zones is scale‐independent over several orders of magnitude. Although crustal‐scale eclogite‐facies complexes are rarely preserved without significant modification during exhumation, this implies that similar geometrical configurations are likely produced at the scale of the whole lower crust during subduction or continental collision and therefore shape the crustal geophysical signature.

• Tegner, Christian; Andersen, Torgeir Bjørge; Kjøll, Hans Jørgen; Brown, Eric L.; Hagen-Peter, Graham & Corfu, Fernando [Show all 8 contributors for this article] (2019). A mantle plume origin for the Scandinavian Dyke Complex: a "piercing point" for 615 Ma plate reconstruction of Baltica? Geochemistry Geophysics Geosystems. 20(2), p. 1075–1094. doi: 10.1029/2018GC007941. Full text in Research Archive Show summary

  Abstract The origin of Large Igneous Provinces (LIPs) associated with continental breakup and the reconstruction of continents older than c. 320 million years (pre-Pangea) are contentious research problems. Here we study the petrology of a 615 - 590 Myr dolerite dyke complex that intruded rift-basins of the magma-rich margin of Baltica and now is exposed in the Scandinavian Caledonides. These dykes are part of the Central Iapetus Magmatic Province (CIMP), a LIP emplaced in Baltica and Laurentia during opening of the Caledonian Wilson Cycle. The >1000 km long dyke complex displays lateral geochemical zonation from enriched to depleted basaltic compositions from south to north. Geochemical modelling of major and trace elements shows these compositions are best explained by melting hot mantle 75-250°C above ambient mantle. Although the trace element modelling solutions are non- unique, the best explanation involves melting a laterally zoned mantle plume with enriched and depleted peridotite lithologies, similar to present-day Iceland and to the North Atlantic Igneous Province. The origin of CIMP appears to have involved several mantle plumes. This is best explained if rifting and breakup magmatism coincided with plume generation zones at the margins of a Large Low Shear-wave Velocity Province (LLSVP) at the core mantle boundary. If the LLSVPs are quasi-stationary back in time as suggested in recent geodynamic models, the CIMP provides a guide for reconstructing the paleogeography of Baltica and Laurentia 615 million years ago to the LLSVP now positioned under the Pacific Ocean. Our results provide a stimulus for using LIPs as piercing points for plate reconstructions.

• Jamtveit, Bjørn; Moulas, Evangelos; Andersen, Torgeir Bjørge; Austrheim, Håkon Olaf; Corfu, Fernando & Petley-Ragan, Arianne Juliette [Show all 7 contributors for this article] (2018). High Pressure Metamorphism Caused by Fluid Induced Weakening of Deep ontinental Crust. Scientific Reports. 8. doi: 10.1038/s41598-018-35200-1. Full text in Research Archive Show summary

  Studies of mineral equilibria in metamorphic rocks have given valuable insights into the tectonic processes operating at convergent plate margins during an orogeny. Geodynamic models simulating orogenesis and crustal thickening have been constrained by temperature and pressure estimates inferred from the mineral assemblages of the various lithologies involved along with age constrains from increasingly precise geochronological techniques. During such studies it is assumed that the pressure experienced by a given rock is uniquely related to its depth of burial. This assumption has been challenged by recent studies of high pressure (HP) and ultrahigh pressure (UHP) rocks. Here, we describe an example of Caledonian HP metamorphism from the Bergen Arcs in western Norway, and show that the associated formation of Caledonian eclogites at the expense of Proterozoic granulites was related to local pressure perturbations rather than burial, and that the HP metamorphism resulted from fluid-induced weakening of an initially dry and highly stressed lower crust when thrust upon the hyperextended margin of the Baltic shield.

• Jakob, Johannes; Aisaif, Manar; Corfu, Fernando & Andersen, Torgeir Bjørge (2017). Age and origin of thin discontinuous gneiss sheets in the distal domain of the magma-poor hyperextended pre-Caledonian margin of Baltica, southern Norway. Journal of the Geological Society. ISSN 0016-7649. 174(3), p. 557–571. doi: 10.1144/jgs2016-049. Full text in Research Archive Show summary

  Abstract: The vestiges of the pre-Caledonian magma-poor hyperextended margin of Baltica are preserved in a mélange zone in southern Norway. The rock assemblage in the distal domains of magma-poor hyperextended margins includes extensional allochthons and basement slivers, which share a pre-rift tectonometamorphic history with the non-rifted basement. After incorporation into an orogenic belt, these distinct fingerprints may be used to test whether such heterogeneous mélange units were derived from the basement units below or have been transported to the orogen from elsewhere. Later magmatic additions and tectonometamorphic events may provide additional information on the geological evolution of the margin. We present U–Pb results from 12 gneisses and meta-igneous rocks from the mélange zone. We find Proterozoic gneisses of Telemarkian affinity over the length of the mélange zone that support the formation of the mélange along the ancient Baltican margin. We also find latest Cambrian–early Ordovician meta-igneous rocks that may be linked either to shortening of transitional crust formed in the Ediacaran or, alternatively, to an episode of extension in the Ordovician. Scandian thrusting initiated between 438 and 427 Ma and was followed by the emplacement of syntectonic ‘Scandian’ granitoids at 421 Ma; that emplacement coincides with peak metamorphism.

• Ferre, Eric C.; Meado, Andrea L; Geissman, John W; Toro, Giulio Di; Spagnuolo, Elena & Ueda, Tadamasa [Show all 11 contributors for this article] (2017). Earthquakes in the Mantle? Insights From Rock Magnetism of Pseudotachylytes. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 122(11), p. 8769–8785. doi: 10.1002/2017JB014618. Full text in Research Archive Show summary

  Ultramafic pseudotachylytes have been regarded as earthquake fossils formed at mantle depths (i.e., >30 km). Here we show that pseudotachylytes hosted by ultramafic rocks from three localities have distinct magnetic properties. Fresh host-peridotites contain only small amounts of coarse-grained magnetite. In contrast, the ultramafic pseudotachylytes contain variable amounts of significantly finer magnetite that formed coseismically through melting. Among each locality, magnetite abundance in the pseudotachylytes ranges over several orders of magnitude (4~2000 ppm), and magnetic grain size varies considerably (from single domain to multidomain). Because the host-peridotites are compositionally similar, the pseudotachylyte magnetic properties are interpreted to primarily reflect the physical and cooling conditions prevailing during seismic slip. Further, the examination of laboratory- produced ultramafic pseudotachylytes shows that quenching does not produce superfine magnetite. We hypothesize that the magnetic properties of ultramafic pseudotachylytes are controlled by fO2 and in consequence vary systematically with depth of formation. Therefore these properties can be used to assess if the ruptures producing the earthquakes that these pseudotachylytes represent nucleated at actual mantle depths or at shallow depths during exhumation of mantle rocks.

• Jakob, Johannes; Boulvais, Philippe & Andersen, Torgeir Bjørge (2017). Oxygen and carbon isotope compositions of carbonates in a prominent lithologically mixed unit in the central South Norwegian Caledonides. International journal of earth sciences. ISSN 1437-3254. p. 1–19. doi: 10.1007/s00531-017-1551-0. Full text in Research Archive
• Corfu, Fernando & Andersen, Torgeir Bjørge (2016). Proterozoic magmatism in the southern Scandinavian Caledonides, with special reference to the occurrences in the Eikefjord Nappe. GFF. ISSN 1103-5897. 138(1), p. 102–114. doi: 10.1080/11035897.2015.1077886. Full text in Research Archive Show summary

  Caledonian nappes of southern Norway comprise basement slices and sedimentary cover, and generally have close affinity with autochthonous terranes in the Fennoscandian foreland. The nap- pes can be distinguished in those having Gothian (1700–1600 Ma) character and those of Telemarkian (1550–1480 Ma) type. There were also several periods of variously intense magmatism between these early accretionary events and the end of the Sveconorwegian Orogeny. Most prominent features are the anorthositic-noritic massifs, which formed mainly during the Telemarkian period and in the late stages of the Sveconorwegian Orogeny between 980 and 920 Ma. In this paper, we review the main events affecting the crystalline parts of the nappes and report new results from the Eikefjord Nappe. An augen gneiss yields a crystallization age of 1191 ± 2 Ma, which is an event rarely seen in nappes or the au- tochthon. An amphibolitic gneiss formed at about 1500 Ma and underwent high-grade metamorphism at 986 ± 3 Ma, followed by lower grade overprints as reflected by titanite dates ranging to 915 Ma. The Sveconorwegian metamorphism was ubiquitous in the nappes, although its intensity was locally highly variable. The association of metamorphism with the second main anorthosite generation event points to intense subcrustal magmatism, heating and deformation of the lithosphere. The various crustal forming events have different geneses, some are clearly related to subduction processes, others more consistent with localized mantle melting. Those closer in character to classical Large Igneous Provinces are the events at 1470–1420, 1280–1240 and 1220–1180 Ma.

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• Andersen, Torgeir Bjørge (2024). The Geology of the Hornelen Devonian Basin and its sourroundings, oral presentation and excursion leader. Full text in Research Archive
• Andersen, Torgeir Bjørge (2024). Exhumation of subcontinental mantle by hyper-extension in passive margins: the Caledonian example. Full text in Research Archive
• Andersen, Torgeir Bjørge & Austrheim, Håkon Olaf (2024). Discovery and significance of pseudotachylytes in the Corsican Subduction Complex. Full text in Research Archive
• Andersen, Torgeir Bjørge (2024). Ekskursjonsledele og presentasjon av geologi for UNESCO i forbindelse med søknad om status som internasjonal UNESCO geopark. Full text in Research Archive
• Andersen, Torgeir Bjørge (2024). Geology of the Hornelen Basin and surrounding high-pressure rocks. Full text in Research Archive
• Tegner, Christian; Andersen, Torgeir Bjørge; Kjøll, Hans Jørgen; Brown, Eric L.; Graham-Hagen, Peter & Corfu, Fernando [Show all 8 contributors for this article] (2024). Dyke Complexes of the Scandinavian Caledonides. Full text in Research Archive Show summary

  Dyke Complexes of the Scandinavian Caledonides Christian Tegnera, Torgeir B. Andersenb, Hans Jørgen Kjøllb, Eric L. Browna, Graham Hagen-Peterc, Fernando Corfub, Sverre Plankeb, and Trond H. Torsvikb aDepartment of Geoscience, Aarhus University, Denmark, christian.tegner@geo.au.dk; bDepartment of Geosciences, University of Oslo, Norway; cEarth Sciences, Vrije Universiteit Amsterdam, Netherlands Dyke complexes (c. 615 – 560 Ma) exposed in the Scandinavian Caledonides are part of the Central Iapetus Magmatic Province, a large igneous province related to the opening of the Iapetus Ocean and an earlier phase of the North Atlantic Wilson Cycle. These include a >1000 km long dyke complex preserved in the Särv, Seve and Corrovarre nappes (Andréasson et al., 1998; Hollocher et al., 2007; Tegner et al., 2019) and dykes of the Seiland Igneous Province preserved in the Kalak nappe (Robins and Takla, 1979; Reginiussen et al., 1995). The compositions of the >1000 km long dyke complex is mainly tholeiitic and displays lateral geochemical zonation from enriched to depleted basaltic compositions from south to north. In addition, the central part of this dyke complex (in Trøndelag, Norway and Jämtland, Sweden) displays alkali basalt compositions. In contrast, the dykes of the Seiland Igneous Province are entirely composed of alkali basalts including ankaramite, picrite and lamprophyres. In this talk we will review the geochemical details of the basaltic magmas in the Scandinavian dyke complexes and discuss their origin from heterogenous and most likely multiple mantle plumes, and from enriched subcontinental lithospheric mantle. References Andréasson, P.G., Svenningsen, O.M., Albrecht, L. (1998). Dawn of Phanerozoic orogeny in the North Atlantic tract; Evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides. Journal of the Geological Society of Sweden GFF 120, 159–172. doi:10.1080/11035899801202159 Hollocher, K., Robinson, P., Walsh, E., Terry, M.P. (2007). The Neoproterozoic Ottfjället dike swarm of the Middle Allochthon, traced geochemically into the Scandian Hinterland, Western Gneiss Region, Norway. American Journal of Science 307, 901–953. doi:10.2475/06.2007.02 Reginiussen, H., Ravna, E.J.K., Berglund, K. (1995). Mafic Dykes from Øksfjord, Seiland Igneous Province, northern Norway: geochemistry and palaeotectonic significance. Geological Magazine 132, 667–681. Robins, B., Takla, M.H. (1979). Geology and geochemistry of a metamorphosed picrite-ankaramite dyke suite from the Seiland province, northern Norway. Norsk Geologisk Tidsskrift 59, 67-95. Tegner, C., Andersen, T.B., Kjøll, H.J., Brown, E.L., Hagen-Peter, G., Corfu, F., Planke, S., and Torsvik, T.H. (2019). A mantle plume origin for the Scandinavian Dyke Complex: a “piercing point” for 615 Ma plate reconstruction of Baltica? Geochemistry, Geophysics, Geosystems 20, 1075-1094. doi:10.1029/2018GC007941

• Austrheim, Håkon Olaf & Andersen, Torgeir Bjørge (2024). Extreme enrichment of Arsenic and Antimony during formation of Ni-Cr- rich jasper and quartzite from serpentinized peridotite. Full text in Research Archive Show summary

  Extreme enrichment of Arsenic and Antimony during formation of Ni-Cr- rich jasper and quartzite from serpentinized peridotite Håkon Austrheima and Torgeir B. Andersenb aDepartment of Geosciences, University of Oslo,Oslo, Norway, e-mail: h.o.austrheim@geo.uio.no bDepartment of Geosciences, University of Oslo, Oslo, Norway, e-mail: t.b.andersen@geo.uio.no Alteration of serpentinised peridotites of Highland Border Complex in Scotland take place in two steps. Listvenite-like dolomite-quartz rocks are formed by addition CaO and CO2 at constant MgO and SiO2 involving a mass-increase of ca 140 %. Stage-two involves dissolution of dolomite evinced by the abundant pores and rhombohedral-shaped grains of quartz to form Cr- and Ni-rich jasper and quartzites. Formation of the jasper-quartzites involve a mass-reduction of ca 80%. The listvenite-like and jasper-quartzite rocks have enrichment in the fluid mobile elements Ba, Sr, Cs, As and Sb. The As is present in the Aluminium Phosphate-Sulphate group of minerals formed during alteration of Cr- spinel. Cr-spinel also alters to porous hematite and ferrihydrite with patches containing up to 5 wt% As2O3. Enrichment of As, related to alteration of chromite, is previously unknown from natural rocks, but strongly resembles efficient methods used for remediation of this potential toxic element. Formation of quartzite and jasper from peridotite and their common presence as pebbles both in the Devonian Old Red conglomerates, in the Highland Border Complex and in Devonian Basins in the Scandinavian Caledonides, highlights their importance and potential for provenance- and tectono- stratigraphic correlations.

• Andersen, Torgeir Bjørge (2023). Foredrag: Bergrunnsgeologien i Fjordkysten Regional og Geopark. Full text in Research Archive
• Austrheim, Håkon Olaf & Andersen, Torgeir Bjørge (2023). Extreme enrichment of Arsenic and Antimony in altered peridotite; mechanism and consequences. Full text in Research Archive
• Svendby, Anne Kathrine; Osmundsen, Per Terje; Andersen, Torgeir Bjørge & Andresen, Arild (2023). Progressive faulting evolution in a transtensional basin; the Grunnevatnet fault, Western Norway. Full text in Research Archive Show summary

  The Devonian basins in western Norway developed in a contintal environment above a crustal‐scale detachment in a strain field dominated by sinistral transtension. The observatons from the Kvamshesten basin are used to conceptualize scenarios for the basin evoluon. In the southwest, the unconformity below the basin is offset by synsedimentary faults, which show normal/ oblique separtiaon in the present configuraon. The largest is the SW‐NW striking Grunnevatnet Fault (GF), which is characterized by an apparent sinistral offset of >2 km offset along the SW basin‐margin, and just a few 10`s of meters in the north, where alluvial midfan lithofacies can be traced across the fault. This is interpreted as the most important evidence for syndeposional slip (Svendby 2011). The large apparent displacement gradient indicates a development different from many normal faults. The fault terminates against the top of the extensional Nordord Sogn Detachment Zone (NSDZ), providing addional evidence that the main acvity of the fault pre‐dated the last movements on the brile detachment. How can such a large change in fault displacement over such a short lateral distance be explained? We discuss the evoluon of the Grunnevatnet Fault in terms of scenarios where the GF is interpreted as active during different stages of the basin evoluon. Observations in the NW part of the basin indicates a pattern where the oldest part of the basin fill was rotated onto the flank of a syncline that grew laterally as units progressively onlapped basement towards the northeast. The Grunnevatnet fault may represent early deformation related to growth of the main detachment and associated core complex.

• Andersen, Torgeir Bjørge (2023). En Wilson-syklus og sporene av platetektonikk i norsk geologi. Full text in Research Archive
• Andersen, Torgeir Bjørge & Austrheim, Håkon Olaf (2023). Scotland revisited: Ekstreme enrichment of Arsenic and Antimon in alteret peridotites in The Highland Border Complex. Full text in Research Archive
• Andersen, Torgeir Bjørge; Jakob, Johannes; Beyssac, Olivier & Mohn, Geoffroy (2022). Tectonostratigraphy of the Scandinavian Caledonides and its role for understanding the Scandian Orogenu. Full text in Research Archive
• Tegner, Christian; Andersen, Torgeir Bjørge; Kjøll, Hans Jørgen; Brown, Eric L.; Hagen-Peter, Graham & Corfu, Fernando [Show all 8 contributors for this article] (2021). A mantle plume origin for the Scandinavian Dyke Complex: a “piercing point” for 615 Ma plate reconstruction of Baltica? Full text in Research Archive Show summary

  The origin of Large Igneous Provinces (LIPs) associated with continental breakup and the reconstruction of continents older than c. 320 million years (pre-Pangea) are contentious research problems. Here we study the petrology of a 615 - 590 Myr dolerite dyke complex that intruded rift-basins of the magma-rich margin of Baltica and now is exposed in the Scandinavian Caledonides. These dykes are part of the Central Iapetus Magmatic Province (CIMP), a LIP emplaced in Baltica and Laurentia during opening of the Caledonian Wilson Cycle. The >1000 km long dyke complex displays lateral geochemical zonation from enriched to depleted basaltic compositions from south to north. Geochemical modelling of major and trace elements shows these compositions are best explained by melting hot mantle 75-250°C above ambient mantle. Although the trace element modelling solutions are non-unique, the best explanation involves melting a laterally zoned mantle plume with enriched and depleted peridotite lithologies, similar to present-day Iceland and to the North Atlantic Igneous Province. The origin of CIMP appears to have involved several mantle plumes. This is best explained if rifting and breakup magmatism coincided with plume generation zones at the margins of a Large Low Shear-wave Velocity Province (LLSVP) at the core mantle boundary. If the LLSVPs are quasi-stationary back in time as suggested in recent geodynamic models, the CIMP provides a guide for reconstructing the paleogeography of Baltica and Laurentia 615 million years ago to the LLSVP now positioned under the Pacific Ocean. Our results provide a stimulus for using LIPs as piercing points for plate reconstructions.

• Labrousse, Loic; Incel, Sarah; Zertani, Sascha; Baisset, Marie; Kaatz, Lisa & Schubnel, Alexandre [Show all 14 contributors for this article] (2020). Up-scaling eclogitization: from experimental and natural aggregates behaviours to seismological signatures. Full text in Research Archive Show summary

  Eclogite formation in the subducting crust was the first metamorphic transformation to be acknowledged as important in the dynamics of convergent plate boundaries. It is indeed expected to affect the mass distribution via density change, but it also influence the fluid content of crustal and possibly lithospheric wedges; both density and fluids being first order in values measured by passive geophysical imaging such as tomography of receiver functions. Recent high accuracy focal mechanism solutions showing singular signatures in deep orogens actually imply that eclogitization could also have a signature in the seismological source signals, and hence have an impact at much shorter time-scales. This presentation aims at bridging what we know from the field and the lab at smaller time and space scales, to what we observe at larger scales in collision zones. Field-based studies show the ways a pristine rock can evolve from metastable to fully eclogitized from the thin section to the kilometre scale. More than the contrast between eclogitized and non-eclogitized domains, the eclogitization front itself is expected to be detected in the geophysics, especially when driven by strain. Indeed strain-assisted eclogitization develops a characteristic shear zone network pattern with a significant anisotropy. This network itself evolves with the eclogitization progress. The observed progressive widening and increasing connectivity of eclogite-facies shear zones with increasing fluid availability could actually be controlled by the transient properties of the newly formed assemblages, inducing fluid pressure gradients for instance. In this context it appears that the competition between reaction kinetics and strain-rate is a key factor. This is also the case at shorter time scales. Experimental studies show that strain of metastable assemblages in the eclogite facies is more likely to lead to mechanical instabilities for intermediate reaction kinetics, implying again that not the eclogite but the eclogitization rate is the smoking gun. Eclogitization of plagioclase-bearing rocks is the finite result of a large set of reactions involving different chemical subsystem (Na or Ca end-members, with or without fluid available), not reacting at the same pace. Further work is therefore needed on the kinetics of the different reactions and their interactions to distinguish the one(s) that controls the eclogitization front signature, and hence improve the seismological imaging acuity.

• Zertani, Sascha; Vrijmoed, Johannes Christiaan; Tilmann, Frederik; Andersen, Torgeir Bjørge & Labrousse, Loic (2020). P wave anisotropy caused by partial eclogitization of descending crust demonstrated by modeling effective petrophysical properties. Full text in Research Archive Show summary

  Eclogitization occurs deep in subduction and collision zones inaccessible to direct observation. Field-based studies dealing with crustal material previously transformed at eclogite-facies conditions and exhumed to the surface provide information from the micro scale up to a few kilometers. On the other hand, geophysical methods aim at imaging the ongoing processes in-situ. However, these methods are limited by the achievable resolution and typically only sensitive to structures a few kilometers in size, leaving a large gap between the scales at which observations are interpreted. In this study we try to discern the implications of structures mapped in field-based studies to interpretations of geophysical imaging. We therefore calculated effective anisotropic P wave velocities for a suite of representative structural associations using the finite element method. The structural associations are directly extracted from observations of partially eclogitized assemblages on the island of Holsnøy in the Bergen Arcs of western Norway. Physical properties of the constituting lithologies are taken from laboratory measurements of the same rocks and the calculations are performed on a variety of scales, from the 20-m scale up to the kilometer scale to be able to predict how the effective seismic properties change with varying scale. Our results show that the P wave velocity of the effective medium is solely controlled by the volumetric fraction of the constituting lithologies and their elastic properties. We find that the structural relationship of the different lithologies has no significant influence on the resulting seismic velocities. P wave anisotropy, however, is controlled by the constituting lithology with the highest initial anisotropy and to a lesser extent by the modal abundance of the different lithologies. Further, our results show that seismic anisotropy is largely transferable across scales validating the assumptions often made when measuring seismic velocities on centimeter-sized sample volumes. On the kilometer scale, a scale that is potentially resolvable by geophysical methods, our results show that an eclogite-facies shear zone network such as the one exposed on Holsnøy would indeed produce a significant P wave anisotropy on a crustal scale. This anisotropy is produced by the eclogite-facies shear zones themselves even though eclogites are typically considered to be low-anisotropy rocks. Comparison of our results with active settings of continental collision and subduction zones reveals that eclogite-facies shear zones have the potential to produce a significant backazimuthal bias of the retrieved signal in geophysical imaging and underline the significance of seismic anisotropy as a tool to further increase the sensitivity of seismological methods to lithological variations.

• Kjøll, Hans Jørgen; Galland, Olivier; Labrousse, Loic & Andersen, Torgeir Bjørge (2020). Emplacement mechanisms of a dyke swarm across the Brittle- Ductile transition. Full text in Research Archive Show summary

  Dykes are the main magma transport pathways through the Earth’s crust and, in volcanic rifts, they are considered the main mechanism to accommodate tectonic extension. Most models consider dykes as hydro-fractures propagating as brittle tensile, mode I cracks opening perpendicular to the least principal stress. This implies that dykes emplaced in rifts are expected to be sub-vertical and accommodate crustal extension. Here we present detailed field observations of a well-exposed dyke swarm that formed near the brittle-ductile transition at a magma-rich rifted margin during opening of the Iapetus Ocean. It was related to a ca 600 million year-old large igneous province. Our observations show that dykes were not systematically emplaced by purely brittle deformation and that dyke orientation may differ from the typical mode 1 pattern. Distinct dyke morphologies related to different emplacement mechanisms have been recognized including: 1) Brittle dykes that exhibit straight contacts with the host rock, sharp tips, and en-echelon segments with bridges exhibiting angular fragments; 2) Brittle-ductile dykes with undulating contacts, rounded tips, folding of the host rock and contemporaneous brittle and ductile features; 3) Ductile “dykes” with rounded shapes and mingling between partially molten host rock and the intruding mafic magma. The brittle dykes exhibit two distinct orientations separated by ~30° that are mutually cross-cutting, demonstrating that the dyke swam did not consist of only vertical sheets oriented perpendicular to regional extension, as expected in rifts. By using the host-rock layers as markers, a kinematic restoration to quantify the average strain accommodating the emplacement of the dyke complex was performed. This strain estimate shows that the dyke swarm accommodated >100% horizontal extension, but also 27% vertical thickening. This suggests that the magma influx rate was higher than the tectonic stretching rate, which imply that magma was emplaced forcefully, as supported by field observations of the host-rock deformation. Finally, observations of typical “brittle” dykes that were subsequently deformed by ductile mechanisms as well as dykes that were emplaced by purely ductile mechanisms suggest that the fast emplacement of the dyke swarm triggered a rapid shallowing of the brittle-ductile transition. The abrupt dyke emplacement and associated heating resulted in weakening of the crust that probably facilitated the continental break-up, which culminated with opening of the Iapetus Ocean.

• Kjøll, Hans Jørgen; Galland, Olivier; Labrousse, Loic & Andersen, Torgeir Bjørge (2020). Emplacement Mechanisms of a Dyke Swarm Across the Brittle-Ductile Transition. Full text in Research Archive
• Andersen, Torgeir Bjørge (2019). Tectonic setting of the Bergen Arcs. Full text in Research Archive
• Andersen, Torgeir Bjørge (2019). Corsican blueschist facies Earthquakes. Full text in Research Archive
• Jakob, Johannes; Corfu, Fernando & Andersen, Torgeir Bjørge (2019). Evidence pre-Scandian deformation and metamorphism in the Caledonian margin of Baltica: U-Pb zircon ages from the Tronfjell Gabbro and related mafic intrusives in the Hummelfjell Group, Central Scandinavian Caledonides. Full text in Research Archive Show summary

  Evidence pre-Scandian deformation and metamorphism in the Caledonian margin of Baltica: U-Pb zircon ages from the Tronfjell Gabbro and related mafic intrusives in the Hummelfjell Group, Central Scandinavian Caledonides. Johannes Jakob, Fernando Corfu, Torgeir B. Andersen The Centre for Earth Evolution and Dynamics, University of Oslo, Norway It is commonly accepted that main Scandian thrusting in the Scandinavian Caledonides commenced in the Mid-Silurian, shortly before or during the continental collision of Baltica and Laurentia. Because the Baltican margin was extremely wide (>1000 km) before the collision (Jakob et al. in review), deformation and metamorphism was diachronous and distal parts were deformed earlier than proximal parts and the foreland. As shown by the abundant high and ultra-high-pressure rocks, Baltica constituted the lower plate during the collision. Subduction/collision-related bimodal Late Ordovician to Early Silurian intrusives in the Scandinavian Caledonides are therefore interpreted to occur only in nappes derived from the upper plate, i.e. in the outboard terranes. The Neoproterozoic Hummelfjell Gp. comprises foliated, amphibolite facies metamorphic, metasandstones and schists. These metasediments host a number of foliated mafic plutons and volcanics that have been correlated with the Ottfjället dykes in the Särv Nappe in Jämtland, Sweden. The rocks of the Hummelfjell Gp. are interpreted to be Baltican pre- to syn-rift, proximal, sediments intruded by syn-rift mafic magmas belonging to the Ediacaran Scandinavian Dyke Complex (~615 to 595 Ma). The Tronfjell Gabbro near Alvdal, has previously been interpreted to have intruded the Hummelfjell Gp. between 500 and 425 Ma (Wellings, 1996). In contrast to the deformed and metamorphosed metasediments and mafic intrusives of the Hummelfjell Gp, that show a shared tectono-metamorphic history, the Tronfjell Gabbro is only affected by greenschist facies metamorphism and a weaker deformation than the Hummelfjell rocks. U-Pb zircon geochronology from the Tronfjell Gabbro and from mafic intrusives within the Hummelfjell metasediments in the contact zone immediately below the gabbro yield U-Pb crystallisation ages of 437.0 ± 0.5 Ma and 437.1 ± 1.1 Ma, respectively. Because, the Hummelfjell Group is interpreted to be derived from the margin of Baltica, and because the amphibolite facies fabric in the Hummelfjell group apparently predates the emplacement of the Tronfjell gabbro, the U-Pb ID-TIMS results suggest that the main deformation and metamorphism of the Baltican margin commenced well before 437 Ma; probably already in the Middle to Late Ordovician as indicated by HP metamorphism in the Seve Nappe Complex (446 Ma, Root & Corfu, 2012) and mica by cooling ages and the Middle Silurian unconformity below the Høyvik Gp in the Dalsfjord Nappe Complex of western Norway (449 Ma, Andersen et al., 1998).

• Kjøll, Hans Jørgen; Galland, Olivier; Labrousse, Loic & Andersen, Torgeir Bjørge (2019). Dyke emplacement mechanisms across the brittle-ductile transition. Full text in Research Archive Show summary

  Dyke emplacement mechanisms across the brittle-ductile transition Hans Jørgen Kjøll1, Olivier Galland2, Loic Labrousse3 & Torgeir Andersen1 1 Center for Earth Evolution and Dynamics (CEED), University of Oslo 2 Physics of Geological Processes, the Njord Center, Department of Geosciences, University of Oslo 3 Institut des Sciences de la Terre Paris, ISTeP, CNRS-INSU, Sorbonne Université Dyking is the main process of magma transport through the Earth’s lithosphere. Dykes are thin sheets exhibiting shapes similar to fractures, so that the main models of dyke emplacement assume that they form by mode I hydraulic fracturing following the σ1-σ2 plane. Because of the rapid strain rates accommodating dyking, it is assumed that dyke propagation and emplacement are only governed by brittle processes, even in the ductile crust. However, the contribution of ductile deformation in dyke emplacement has not been assessed. Here we report detailed and spectacular field observations from northern Sweden and Norway of a ~605 Ma old dyke complex emplaced near the brittle-ductile transition. The dyke complex formed during continental rifting and opening of the Iapetus Ocean, and is now exposed in the Scandinavian Caledonides. In northern Sweden, observations are made along a 1.5 km long continuously exposed cliff providing unique and exceptional overview images of the dyke complex. The detailed structural analysis of the dykes and of the structures related to their emplacement allows us to identify distinct dyke emplacement mechanisms, sub-divided into: 1) Brittle dykes that exhibit straight contacts with the host rock, sharp tips, en-echelon segments with either broken bridges or intact bridges between the segments. The dyke thicknesses follow a Weibull distribution, commonly applied to fracture mechanics; 2) Brittle-ductile dykes that exhibit ductile bridges with complex patterns. Both brittle-ductile and ductile-brittle features are observed, i.e. where ductile flow induced by inflating dykes overprint brittle structures associated with dyke emplacement and vice versa; 3) Ductile “dykes” that show mingling textures between the soft ductile host rock and the intruding mafic magma as well as irregular magmatic boudinage. The dykes exhibit two distinct orientations, and are mutually cross-cutting, suggesting that the dykes did not form as vertical sheets perpendicular to regional extension. Thanks to the well-exposed layering of the dykes’ host rock, we performed a kinematic restoration to quantify the strain induced by the dyke complex. As expected, the dyke complex accommodated >100% extension in agreement with the rifting. However, counter-intuitively it also accommodated 12% of crustal thickening, in agreement with local shortening structures near the dyke walls, showing the forceful mechanism of magma emplacement. Our observations underline the complexity of magma emplacement mechanisms near the brittle-ductile transition and show that dyke emplacement cannot be described as simple mode-I brittle fractures that are being passively filled and inflated by magma.

• Quintela, Orlando; Jakob, Johannes & Andersen, Torgeir Bjørge (2019). Serpentinite-bearing metasedimentary complex near Lesja and relationship with the hyperextended pre-Caledonian margin of Baltica. Full text in Research Archive Show summary

  Serpentinite-bearing metasedimentary complex near Lesja and relationship with the hyperextended pre-Caledonian margin of Baltica Orlando Quintela, Johannes Jakob and Torgeir B. Andersen CEED, University of Oslo In the Scandinavian Caledonides, a serpentinite-bearing metasedimentary complex has been investigated for its complex lithological association and enigmatic tectono-stratigraphic position. It comprises a large number of meta-peridotite bodies as well as detrital serpentinites. These ultramafic rocks are associated with meta-sandstones, mica schists and locally mafic lithologies. In the study area near Sjongsvatnet north of Lesja in south central Norway, the serpentinite-bearing metasedimentary complex is represented by the Sjongsæter Group. It is structurally positioned in the Blåhø Nappe, which overlies the Sætra Nappe, that is laterally correlated with the Seve Nappe Complex. The Sjongseter Gp. comprises a serpentinite sheet at its base, overlain by metasediments including serpentinite conglomerates, mica- and quartz-rich schists and phyllites, as well as amphibolite bodies and mafic schists. Some metaperidotites are truncated by mafic dykes. To the west, the Sjongseter Gp is infolded with the structurally lower basement of the Western Gneiss Region. The structural pattern is defined by map- to outcrop-scale east-west-trending folds that interfere with northwest-southeast-trending folds, and by east-northeast plunging mineral and stretching lineation. The structural evolution of the serpentinite-bearing complex is characterized by polyphase deformation and metamorphism during the Scandian phase of the Caledonian Orogeny and late- to post-orogenic extension, as observed from the interfering folds and lineation pattern. A similar serpentinite-bearing metasedimentary complex has previously been reported from the Bøverdalen, Stølsheimen and the Bergen Arcs. It has been suggested that this unit is the vestiges of a hyperextended segment of the Baltican margin, based on comparison with ancient and modern analogues such as the Alps, Pyrenees and the Iberian margin. It has also been suggested that this unit may be traceable at the same structural level towards Lesja, and the Røros area. The lithological resemblance between the serpentinite-bearing complexes to the SW and and the Sjongseter Gp is is evident. In the Lesja area, however, the original stratigraphy of sediments on exhumed mantle is preserved. The new observation presented here support the previous correlation of the serpentinite-bearing unit across southern Norway, and that this unit in the study area contains more mafic material compared the melange in the SW.

• Mohn, Geoffroy; Jakob, Johannes & Andersen, Torgeir Bjørge (2019). How are the Alpine ophiolites different from the Caledonian ophiolitic mélanges? Full text in Research Archive Show summary

  How are the Alpine ophiolites different from the Caledonian ophiolitic mélanges? Mohn,G., Jakob,J., and Andersen T.B. Both the Caledonian and Alpine belts preserve extensive exposures of ophiolitic sequences. These ophiolitic sequences have been variably interpreted to represent tectonic mélanges or vestiges of either supra-subduction zone spreading ridges or continent-ocean transition zones, among other hypotheses. In this regard, the Alpine ophiolites consist of serpentinised mantle, ophicalcites in relation with fluid circulation, minor magmatic intrusions, tectono-sedimentary breccias and deep-marine sedimentary systems. Such lithostratigraphic association is interpreted as the remnants of the former continent-ocean transition of the Alpine Tethys that formed during the final stage of the Mesozoic rifting. In comparison, several distinct ophiolitic belts can be identified in the Scandinavian Caledonides. The structurally uppermost ophiolitic assemblages are associated with magmatic arc complexes and have been related to an intra-oceanic subduction zone. Located in a structurally lower position, another ophiolitic belt has been recognized in between the continent-derived Jotun-Lindås-Dalsfjord nappe complexes and the Baltica continent. The structurally lower ophiolitic belt, previously recognized as a tectonic mélange, contains numerous solitary metaperidotite bodies embedded in micaschists that are locally associated with mafic rocks. This ophiolitic sequence also contains locally fossiliferous conglomeratic layers made of ultramafic materials. Recent studies already recognized the similarities with the Alpine ophiolites and reinterpreted these tectonic mélanges as the remnants of the former Continent-Ocean Transition of Baltica. In that perspective, the aim of this contribution is to compare and confront fundamental observations from the Alpine ophiolites with key localities of the Caledonides that are particularly well exposed in the Southern Caledonides (Vågåmo, Otta, Lesja areas). We will detail the similarities but also the differences between the Alpine and Caledonian ophiolites and critically discuss some implications for the architecture of the Pre-Baltica margin and its reactivation during the Caledonian orogeny.

• Andersen, Torgeir Bjørge; Jakob, Johannes & Kjøll, Hans Jørgen (2019). The pre-Caledonian Margin of Baltica. Full text in Research Archive
• Kjøll, Hans Jørgen; Andersen, Torgeir Bjørge; Labrousse, Loic & Galland, Olivier (2019). From rift to drift - Construction of the magma-rich pre-Caledonian Iapetus margin of Baltica. Full text in Research Archive Show summary

  The northern part of the pre-Caledonian Iapetus margin, which now rests within nappes in the Scandinavian Caledonides, is generally characterized as a fossil magma-rich rifted margin. The margin developed in the late Ediacaran during the opening of the Iapetus Ocean. It was deformed and metamorphosed during the Caledonian orogeny, but large parts of the margin escaped the penetrative strain and regional metamorphism related to this event, and are now preserved in kilometer-scale boudins. The boudins preserve a several kilometer-thick sedimentary package mostly composed of siliciclastic and carbonaceous rocks. A key unit in the stratigraphy is a stromatolite-bearing dolomitic marble containing pure magnesite lenses as well as Cl-rich scapolite, both indicative of evaporite deposits. This unit can be found at several localities along the Scandinavian Caledonides. A recently described glaciogenic diamictite directly overlies the carbonates, which in turn is overlain by a > 1 km thick shallow marine sandstone succession, possibly marking the onset of rifting. The youngest detrital zircon from the sandstone unit is 698 Ma, providing a useful maximum deposition age. The sedimentary succession rests on top of a basement slice with top-W extensional shear indicators. The deformation has been dated to 637 Ma and corroborates the interpretation that Baltica and Laurentia was rifting at this time. The entire sedimentary succession is dissected by a dense network of mafic dikes, which has been dated to range from 596-608 Ma, providing a minimum estimate for the deposition of the sedimentary succession. This minimum age also rules out the Gaskiers glacial event to be responsible for the deposition of the diamictite. The emplacement of the dike swarm was short and intense and led to local partial melting of the host rocks. The rapid influx of magma exceeded the tectonic stretching rate, which together with their conjugate geometry allowed the dike emplacement to also cause a vertical thickening of the crust synchronous with extension. The diking event led to the break-up of Baltica and Laurentia in the Late Ediacaran after at least 30 million years of rifting and show that the magmatic event started late in the overall rifting history.

• Andersen, Torgeir Bjørge; Jakob, Johannes; Kjøll, Hans Jørgen & Tegner, Christian (2019). A new look at the Scandinavian Caledonides and the significance of Hyper-extension in the pre-Caledonian passive margin of Baltica. Full text in Research Archive Show summary

  The pre-Caledonian Baltican rifted margin has been outlined as a tapering wedge with increasing magmatism towards the ocean-continent transition (OCT), and with a mostly similar structure along its length. It is, however, well-known that passive margins can be complex, with different and diachronous segment-evolutions along and across strike. The Baltican nappes in the Scandes preserve a complexity akin to modern margins, including an OCT, a micro-continent and continental slivers, hyperextended-to-embryonic oceanic basins as well as a major magma-rich segment. The margin was several hundred-, probably up to 1500 km wide, before distal parts were affected by plate-convergence between ~480 and 450 Ma. The margin vestiges, overprinted by Scandian and earlier fabrics as well as the late/post-orogenic extension, now occur at low to middle structural levels in the mountain belt. Proximal parts comprise continental to marine rift-basins with syn- to post-rift deposits of Late Cryogenian, Ediacaran and Lower Palaeozoic age. Its vestiges also comprise continental slivers, coarse- to fine-grained sediments as well as deep-marine deposits. A major change in the nappe-structure occurs across a NW-SE transverse zone parallel to a fundamental basement structure within Baltica, the Sveconorwegian (Grenville) deformation front. The most prominent changes across this lineament are: 1) the coincidence with the NE-termination of the Jotun-Lindås-Dalsfjord nappe complexes, interpreted as the Jotun-Microcontinent (JMC) and 2) the transition from a magma-rich segment in the NE to a hyperextended, magma-poor segment in the SW. The latter has numerous (>100) solitary meta-peridotites and detrital serpentinites (some fossiliferous). The mantle-fragments w/ophicalcite breccias were emplaced tectonically and are covered mostly by deep-basin sediments and local breccias and conglomerates. This ‘mixed’ (mélange) unit was locally affected by pre-Caledonian metasomatism and intruded by gabbros and granitoids (at ~487±1 to 471±2 Ma); also reflected by clastic zircons (>468 Ma) present in the sediments. Other important features are Baltican basement slivers (up to 40 x 1.5 km). The magma-poor SW segment is overlain by a huge basement/cover nappe complex, which after rifting, but prior to Scandian collision, was positioned outboard the hyperextended domain as the Jotun-Microcontinent (JMC). JMC´s most distal parts have mafic dykes and lavas and were affected by early-Caledonian deformation/metamorphism. The SW margin segment was ~400 km long, hyperextended- and magma-poor, and received sediments as late as the Middle Ordovician (and possibly until the onset of the mid-Silurian Scandian orogeny?). The NE magma-rich segment also has mantle peridotites and detrital serpentinites (locally w/fossils), but its most prominent characteristic is the Scandinavian Dyke Complex (SDC) forming parts of a ~615-595 Ma Large Igneous Province that probably assisted opening of the Iapetus Ocean. In the SW segment there is so far no evidence of Late Proterozoic magmatism, but Baltican basement was locally truncated by mafic dykes at ~850 to 830Ma and 615 Ma. The magma-poor SW segment, inboard the JMC, formed a transitional-crust basin opening to an ocean basin, similar to the present North Atlantic rift system between Ireland and the Hatton-Rockall ribbons. The margin of Baltica probably has its best modern analogue in the North-Atlantic and Norwegian-Greenland Sea margins.

• Andersen, Torgeir Bjørge (2019). Volcanoes and Earthquakes Interview with school kids at Manglerud skole International Classes in Oslo. Full text in Research Archive
• Osmundsen, Per Terje; Braathen, Alvar; Svendby, Anne Kathrine; Midtkandal, Ivar; More, Miquel Peyotas & Andersen, Torgeir Bjørge (2019). On fault growth and bulk constriction in transtensional supradetachment basins. Full text in Research Archive
• Kjøll, Hans Jørgen; Andersen, Torgeir Bjørge; Galland, Olivier; Corfu, Fernando; Labrousse, Loic & Tegner, Christian [Show all 7 contributors for this article] (2019). Deep section of a Neoproterozoic fossil magma rich rifted margin exposed. Full text in Research Archive Show summary

  Structures of rifted continental margins are the finite result of active processes when continents rift apart. During the convergent stages of Wilson cycle, remnants of rifted margins may be incorporated into orogens, especially the magma-poor end-member. The magma-rich margins, however, are commonly lost in subduction due to low buoyancy. The understanding of magma-rich margins is therefore mostly based on drill holes and geophysical observations. In this contribution, we explore the temporal evolution and the ambient conditions of a magma-rich rifted margin preserved within the Scandinavian Caledonides. The Scandinavian Dyke Complex was emplaced in a sedimentary basin during the opening of the Iapetus Ocean 615 to 590 million years ago. The dyke complex now constitutes 70-90% of the area and is locally well-preserved despite the complex Caledonian history. Five field seasons in northern Sweden and Norway provide new observations from regional to microscopic scale about the structural geometry, relative timing, and development of the margin. Jadeite in clinopyroxene geothermobarometry, titanium in biotite geothermometry and garnet isopleth modeling show that the ambient pressure and temperature conditions were similar for the entire dyke complex at 0.25 to 0.45 GPa, with contact metamorphic temperatures up to c. 700◦C. Using a photomosaic of a large and well-exposed cliff face we used layers in the metasediments as markers to restore the host-rock back to the pre-dike configuration, allowing us to quantify the average strain accommodated by the dyke swarm. It accommodated for >100% extension and for 27% crustal thickening. From this we infer that the magma influx rate was higher than the tectonic stretching rate, implying that magma was emplaced in a forceful manner, which is also supported by field observations. In the northern part of the study area, high precision dating of magmatic zircon shows that significant partial melting of the sedimentary host-rock, at relatively shallow levels, occurred at 613 ± 1 Ma. This shows that the crust was molten already 6 Ma before the dyke swarm was emplaced at 606 ± 2 Ma. We propose that the locally pervasive partial melting occurred due to high geothermal gradient and introduction of mafic melts in the lower crust. These processes caused a rapid shallowing of the brittle-ductile transition, which thereby significantly reduced the strength of the crust.

• Quintela, Orlando; Jakob, Johannes & Andersen, Torgeir Bjørge (2019). Geology of the metaperidotite-bearing Sjongsæter Group, South Central Norway. Full text in Research Archive Show summary

  In the Scandinavian Caledonides, a metaperidotite-bearing metasedimentary complex has been investigated for its intricate lithological association and enigmatic tectonostratigraphic position. In the study area north of Lesja, south central Norway, the unit is represented by the Sjongsæter Group, structurally positioned in the Blåhø Nappe. The latter overlies the Sætra Nappe, which is in turn laterally correlated with the Seve Nappe Complex in Sweden. The Sjongsæter Group comprises a metaperidotite sheet at its base, overlain by ultramafic conglomerates and sand- stones, mica- and quartz-rich schists, amphibolites and mafic schists. Some metaperidotites are truncated by mafic dykes. The Sjongsæter Group has been deformed and metamorphosed during the Scandian phase of the Caledonian Orogeny and late- to post-orogenic extension. To the west, the Sjongsæter Group is folded with the structurally lower basement of the Western Gneiss Region. The rocks of the Sjongsæter Group are folded by east-west as well as northeast-southwest-trending folds. The mineral stretching lineations plunge chiefly to the northeast. The metaperidotites crop out in the cores of NE-SW trending antiforms and are overlain by mica schists and mafic rocks that are exposed in synforms. Locally, the contacts of the metasediments/mafic magmatic rocks with the metaperidotites are overturned. To the northeast, the Sjongsæter Group is in contact with the WGR. Locally this contact is also overturned. We suggest that in the Sjongsæter Group an original stratigraphy of metasediments with a metaperidotite substratum is preserved.

• Zertani, Sascha; John, Timm; Vrijmoed, Johannes Christiaan; Tilmann, Frederik; Labrousse, Loic & Andersen, Torgeir Bjørge (2019). The variation of petrophysical properties during eclogitization of lower continental crust and their influence on geophysical imaging. Full text in Research Archive Show summary

  Studying the deep structure of active subduction zones is a difficult task, which is typically approached by large- scale geophysical methods. Particularly seismological methods such as the receiver function method are widely applied and produce many images of the structure at depth. In order to interpret these images in a quantitative way, not only a good understanding of the lithologies present at depth and their petrophysical properties is required, but also the geometrical framework of structures close or somewhat smaller than the resolution limit. There is no way to analyze rocks in active subduction zones directly but we can study fossil subduction zones, where previously subducted rocks are now exposed at the surface. The island of Holsnøy in the Bergen Arcs of western Norway provides an excellent example of a slice of continental crust that was deeply buried during Caledonian collision. The structures developed at eclogite-facies conditions are preserved with only minor alteration during exhumation. The complex consists of lower crustal granulites that were partly eclogitized both with and without associated deformation. In particular the eclogite-facies shear zones, which cut the unaffected pristine granulite-facies pro- tolith, have been studied extensively and provide an excellent example of structures that might be imageable using seismological methods. We use Holsnøy as a natural laboratory and through a combination of geological map- ping, thermodynamic modelling and laboratory measurements extract P- and S-wave velocities representative for the exposed rocks as well as the associated geometries. This provides first-order variations of the petrophysical properties that can be expected at depth in subduction settings. Our results show that eclogitization on Holsnøy produces increased seismic velocities and seismic anisotropy as well as decreased Vp/Vs-ratios. The sample-scale, however, is far below the wavelengths used in seismological studies. Therefore, we need to extract bulk seismic properties from intermediate-scale models. For our meso-scale approach we use geometries derived from geolog- ical mapping in a mechanical model using the Finite Elements method. This model is used to investigate how the complex mixtures of eclogites and granulites behave as an effective medium, and subsequently, to extract bulk properties that are representative of the signal that is recorded using seismological methods in active subduction zones.

• Jakob, Johannes & Andersen, Torgeir Bjørge (2019). The effects of Devonian extensional tectonics on the architecture of the allochthons of the Scandinavian Caledonides, Soth- and South-Central Norway. Full text in Research Archive Show summary

  The Scandinavian Caledonides were chiefly formed during the Silurian-Devonian continental collision of Baltica with Laurentia. During the collision the Baltican margin was subducted below the cratonic margin of Laurentia and a complex orogenic wedge was thrust over the Baltican margin. The thrusting was followed by extensional tectonics in the Devonian, which included the exhumation of UHP rocks of the WGR and other basement windows, back sliding of nappe stack to the NW, and brittle normal faulting in the nappes and basement. A metaperidotite-bearing metasedimentary complex can be traced below the large crystalline nappes of Southern Norway, i.e. the Lindås, Dalsfjord and Jotun nappes along strike the orogen and across the Gudbrandsdalen An- tiform into South-Central Norway, where it structurally underlies the Trondheim Nappe Complex. Whereas the metaperidotite-bearing unit occurs on the eastern as well as on the western flank of the Trondheim Nappe Com- plex, it has only been mapped along the western flank of the Jotun Nappe Complex, where it is in contact with rocks of the WGR. The unit cannot be traced towards the east across the Lærdal-Gjende Fault, which is a large late Scandian normal fault. Moreover, on the eastern flank of the Trondheim Nappe Complex, the metaperidotite- bearing unit is structurally underlain by at least three major thrust units, whereas in the west it is in contact with rocks of the WGR. Here we discuss, how the architecture of the South and South-Central Caledonian allochthons likely was devel- oped before the onset of extensional tectonics and how the orogenic collapse and in particular the exhumation of the WGR region might have altered the architecture of the nappe stack.

• Andersen, Torgeir Bjørge; Jakob, Johannes & Kjøll, Hans Jørgen (2019). The Jotun Microcontient. Full text in Research Archive
• Andersen, Torgeir Bjørge; Jakob, Johannes & Kjøll, Hans Jørgen (2019). Exhumation of continental Mantle by Hyperextension, in passive continental margins: the Caledonian example”. Full text in Research Archive Show summary

  Exhumation of Continental Mantle by Hyperextension in Passive Continental Margins:the Caledonian example The pre-Caledonian Baltican rifted margin has been outlined as a tapering wedge with increasing magmatism towards the ocean-continent transition (OCT), and with a similar structure along its entire length. It is, however, well-known that passive margins can be complex, with different and diachronous segment-evolutions along and across strike. The Baltican nappes in the Scandes preserve a complexity akin to modern margins, including an OCT, a micro-continent and continental slivers, hyperextended-to-embryonic oceanic basins with exhumed sub-continental mantle and a major magma-rich segment. The margin was probably up to 1500 km wide, before distal parts were affected by plate-convergence in the early to middle Ordovician. The margin vestiges, overprinted by Scandian and earlier fabrics now occur at low to middle structural levels in the mountain belt. Proximal parts comprise rift-basins with syn- to post-rift deposits of Cryogenian, Ediacaran and Lower Palaeozoic age. Its vestiges also comprise continental slivers, coarse- to fine-grained sediments as well as deep-marine deposits. A major change in the Caledonian structure occurs across a NW-SE transverse zone parallel to a fundamental basement structure within Baltica, the Sveconorwegian (Grenville) deformation front. The most prominent changes across this lineament are: 1) the coincidence with the NE-termination of the Jotun-Microcontinent (JMC) and 2) the transition from a magma-rich segment in the NE to a hyperextended, magma-poor segment inboard the JMC in the SW. The latter comprises numerous (>100) solitary meta-peridotites and detrital serpentinites. These mantle-fragments, mostly original harzburgites to dunites, were emplaced tectonically and were variably hydrated and carbonated prior to the Caledonian orogeny. Where exhumed to the seafloor some produced detrital serpentinites. The mantle fragments were covered by deep-basin fine-grained sediments and local breccias and conglomerates. Metasomatic alteration and ophicalcite breccias are commonly developed. Other important features in these OCT basins are Baltican basement slivers (up to 40 x 1.5 km). The magma-poor OCT segment is now overlain by a huge basement nappe complex (Jotun, Lindås, Dalsfjord, Eikefjord NC), which after rifting, but prior to Scandian collision, was positioned outboard the hyperextended OCT basin as the Jotun-Microcontinent (JMC). The JMC´s distal margin have mafic dykes and lavas and was affected by early-Caledonian deformation/metamorphism. The magma-rich segment (Seve-Kalak NC) also hosts exhumed mantle peridotites and detrital serpentinites (locally w/fossils), but its most prominent characteristic is the Scandinavian Dyke Complex (SDC) forming parts of a ~615-595 Ma Large Igneous Province that probably assisted opening of the Iapetus Ocean. The magma-poor SW segment inboard the JMC, formed an OCT basin similar to the present North Atlantic rifted margin system between Ireland and the Hatton-Rockall ribbons.

• Jakob, Johannes & Andersen, Torgeir Bjørge (2019). The rift-inherited architecture of the Scandinavian Caledonides and the significance of 437 Ma magmatism within the distal domain of the pre-Caledonian rifted margin of Baltica. Full text in Research Archive Show summary

  The Scandinavian Caledonides were finally assembled during the Silurian Devonian continental collision of Laurentia with Baltica/Avalonia. During the collision an intricate stack of nappes was thrust over the partly subducted continental margin of Baltica. From bottom to top, these nappes contain 1) Proterozoic continental margin successions and discontinuous sheets of Baltican basement; 2) orthogneisses of Baltican affinity; 3) Proterozoic continental margin successions with locally high volumes of plume-related mafic magmas that were emplaced during continental break-up; 4) Alpine-type metaperidotite-bearing metasedimentary complexes that host an island-type, lower Middle Ordovician fauna; 5) (in South Norway) large, mainly crystalline nappes of Baltican affinity (Jotun, Lindås, Dalsfjord nappes) 6) ophiolite and volcanic arc assemblages, locally with fossils of Laurentian affinity. We suggest that the differences in the lithological compositions as well as the structural sequence of the Baltican-derived allochthons is rift-inherited and originates from the highly irregular architecture of the pre-Caledonian rifted margin of Baltica. The rift-inherited elements include a change from an Ediacaran magma-poor to a magma-rich rifted margin along strike of the orogen, as well as a stack of proximal to distal rift domains. The transition from magma-poor to magma-rich occurs over a distance of about 200 km and coincides with the termination of the large crystalline nappes in Southern Norway. By unstacking the Caledonian nappe stack, the pre-Caledonian rifted margin of Baltica can be reconstructed. We suggest that the pre-Caledonian rifted margin of Baltica may have been very wide (≥ 1000 km) and may have included a magma-poor hyperextended domain as well as a microcontinent (Jotun Microcontinent). A ≥1000 km wide margin, however, would suggest that the initial collision between Baltica and Laurentia may had occurred considerably earlier than in traditional plate tectonic models. Here we show new geochronological evidence that suggest that the dyked Neoproterozoic continental margin successions as well as the metaperidotite-bearing units were in an upper plate-position before ~437 Ma.

• Andersen, Torgeir Bjørge (2019). How hyper-extension in the pre-Caledonian margin of Baltica affected the structure of Caledonides: the story of the Jotun Microcontinent. Full text in Research Archive
• Kjøll, Hans Jørgen; Galland, Olivier; Labrousse, Loic & Andersen, Torgeir Bjørge (2019). Emplacement Mechanisms of a Dyke Swarm Across the Brittle-Ductile Transition. Full text in Research Archive
• Andersen, Torgeir Bjørge; Jakob, Johannes; Kjøll, Hans Jørgen; Quintela, Orlando; Corfu, Fernando & Torsvik, Trond Helge [Show all 9 contributors for this article] (2019). Hyperextension in magma-poor and magma-rich domains along the pre-Caledonian passive margin of Baltica. Full text in Research Archive Show summary

  Hyperextension in magma-poor and magma-rich domains along the pre-Caledonian passive margin of Baltica Torgeir B. Andersen, Johannes Jakob, Hans-Jørgen Kjøll, Orlando Quintela, Fernando Corfu, Trond H. Torsvik, 1)Christian Tegner, 2)Loic Labrousse and 3)Geoffroy Mohn CEED, Univ. Oslo; 1)Århus Univ.; 2)Sorbonne Univ, ISTeP-Paris; 3)Univ. Cergy-Pontoise Email: t.b.andersen@geo.uio.no The pre-Caledonian Baltican rifted margin has been outlined as a tapering wedge with increasing magmatism towards the ocean-continent transition (OCT), and with a mostly similar structure along its length. It is, however, well-known that passive margins can be complex, with different and diachronous segment-evolutions along and across strike. The Baltican nappes in the Scandes preserve a complexity akin to modern margins, including an OCT, a micro-continent and continental slivers, hyperextended-to-embryonic oceanic basins as well as a major magma-rich segment. The margin was several hundred-, probably up to 1500 km wide, before its distal parts were affected by convergence between ~480 and 450 Ma. Its vestiges, overprinted by Scandian and earlier fabrics as well as the late/post-orogenic extension, now occur at low to middle structural levels in the mountain belt. Proximal parts comprise continental to marine rift-basins with syn- to post-rift deposits of Late Cryogenian, Ediacaran and Lower Palaeozoic age. Its vestiges also comprise continental slivers, coarse- to fine-grained sediments as well as deep-marine deposits. A major change in the nappe-structure occurs across a NW-SE transverse zone parallel to a fundamental basement structure within Baltica, the Sveconorwegian front. The most prominent changes across this lineament are: 1) the coincidence with the NE-termination of the Jotun-Lindås-Dalsfjord nappe complex, interpreted as the Jotun- Lindås-Dalsfjord microcontinent (JMC) and 2) the transition from the magma-rich segment in the NE to the hyperextended, magma-poor segment in the SW. The latter has numerous (>100) solitary meta-peridotites and detrital serpentinites (some fossiliferous). The mantle-fragments w/ophicalcite breccias were emplaced tectonically and are covered mostly by deep-basin sediments and local breccias and conglomerates. This ‘mixed’ (mélange) unit was locally affected by pre-Caledonian metasomatism and intruded by gabbros and granitoids (at ~487±1 to 471±2 Ma); this magmatism is also reflected by clastic zircons (>468 Ma) present in the sediments. Other important features are Baltican basement slivers (up to 40 x 1.5 km). The magma-poor SW segment is overlain by a huge basement/cover nappe complex, which after rifting, but prior to Scandian collision, was positioned outboard the hyperextended domain as the Jotun-Lindås-Dalsfjord Microcontinent (JMC). JMC´s distal parts at Atløy, have mafic dykes and lavas in the Høyvik Gp, and were affected by early-Caledonian event(s). The SW margin segment was ~400 km long, hyperextended- and magma-poor, and received sediments as late as the Middle Ordovician (and perhaps until the onset of the mid-Silurian Scandian orogeny?). The NE magma-rich segment also has mantle peridotites and detrital serpentinites (locally w/fossils), but its most prominent characteristic is the Scandinavian Dyke Complex (SDC) forming parts of a ~615-595 Ma Large Igneous Province that probably assisted opening of the Iapetus Ocean. In the SW segment there is so far no evidence of Late Proterozoic magmatism, but Baltican basement was locally truncated by mafic dykes at ~850 to 830Ma and 615 Ma. The magma-poor SW segment, inboard the JMC, formed a transitional-crust basin opening to an ocean basin, similar to the present North Atlantic rift system between Ireland and the Hatton-Rockall ribbons. The margin of Baltica probably has its best modern analogue in the North-Atlantic and Norwegian-Greenland Sea margins.

• Katz, Liza; Zertani, Sascha; Moulas, Evangelos; John, Timm; Labrousse, Loic & Schmalholz, Stefan Markus [Show all 7 contributors for this article] (2018). Evolution of hydrous shear zones during incipient eclogitization of metastable dry and rigid lower crust (Holsnøy, Western Norway). Full text in Research Archive Show summary

  Seismic imaging suggests that within subducting slabs eclogitization of dry crustal rocks often occurs at depths greater than 80 km, where eclogitization of other rocks is almost completed. This indicates a kinetically delayed slab densification. Recent studies show a slight kinking of subducting slabs accompanied by high seismic activity at the depths where eclogitization seems to occur. Hence, weakening of the subducting slab due to eclogitization reactions has a potentially strong impact on the deformation of the slab. Findings from the dry anorthositic granulites of Holsnøy (western Norway) indicate that the local eclogitization is an interplay between brittle deformation, ductile deformation, fluid infiltration and subsequent fluid-rock interactions. The finite result is an interconnected network of hydrous shear zones responsible for the eclogitization of the rigid lower crust. To decode the development of those shear zones with the main focus on the progressive interaction be- tween eclogitization and deformation, we combine detailed mapping and petrological investigations of natural outcrops with numerical modelling. We examined two key outcrops on Holsnøy, where unreacted granulites are cross-cut by partly to completely eclogitized shear zones on a meter scale. Both outcrops (A and B) are largely comprised of pristine granulite with minor eclogite-facies shear zones (sz) and adjacent regions of static eclogitic overprint. The sz differ in width and outcrop B contains internally folded shear zones. We assume outcrop A (lower sz-width, dextral bookshelf-style deformation, NE-SW-striking sz, no folds) developed from a parallel setting of N-S-striking sz and represents an early deformation stage prior to that of outcrop B (higher sz-width, E-W-striking and folded sz). The deformation facilitated eclogitization, which gradually progressed into the granulites until either stress relaxed or the trigger mechanism of fluid-induced reactions reduce their significance. We generated three numerical models to identify the key parameters that have the largest impact on the sz- widening and consequently, leading to sz-folding during ongoing deformation with: (i) considering a constant fluid content throughout the whole sz and the entire evolution of the sz system; (ii) applying a fixed value of free fluid to the sz that distributes while the sz evolves; (iii) implementing a constant fluid content combined with grain size coarsening within the sz center to obtain a viscosity contrast within the evolving sz system. By applying field- and laboratory-based data it is possible to decipher the geometrical development of the interconnected sz-network as a function of fluid-induced mineral reactions coupled with ongoing deformation. Therefore, 2D-modelling with the described setup provides a new mechanical view on the dynamic weakening processes affecting metastable dry and rigid lower crustal rocks.

• Kjøll, Hans Jørgen; Andersen, Torgeir Bjørge; Tegner, Christian; Labrousse, Loic; Corfu, Fernando & Planke, Sverre (2018). Defining a magma-rich rifted margin fossil analogue in the Scandinavian Caledonides. Full text in Research Archive Show summary

  The Iapetus Ocean opened ca 610 Ma ago possibly due to the interaction with a mantle plume at the base of the lithosphere. Stretching of the conjoined crystalline crust started prior to break-up and provided accommodation for continental and shallow marine syn-rift sediments. An early non-magmatic phase with discrete and localized deformation was followed by pervasive mafic magmatism where dyke-emplacement accounted for the bulk of the stretching. During the Caledonian orogeny, the Iapetan margin was thrusted onto Baltica as the Iapetus closed. To- day, vestiges of the magma-rich margin reside within nappes from central Sweden to northern Norway. Although overprinted by Caledonian deformation fabrics and metamorphism, there are localities where pre-Caledonian mag- matic and sedimentary structures are well-preserved, thereby allowing for detailed studies of deep to intermediate processes at magma-rich rifted margins. Three field seasons have yielded observations on the outcrop and micro- scopic scale that provide information about relative timing and development of the margin, both the early non- magmatic stage and the subsequent magmatic stage. In particular, dyke geometries and morphologies have been assessed from photogrammetric 3D models to constrain the active mechanisms during dyke emplacement. Jadeite in clinopyroxene geobarometry constrain the crystallization pressure of clinopyroxenes of the different intrusion levels and coupled with U/Pb geochronology constrain the timing of intrusion. Furthermore, dense sampling of mafic intrusions for geochemistry reveals a similar lateral geochemical signature as observed across the Iceland plume. Based on these new observations we argue that the well-preserved parts of the margin comprise: 1) Parts of an early lower crustal magmatic complex consisting of coronitic gabbros and granites; 2) Strongly stretched and attenuated crystalline basement intruded by mafic dykes; 3) Highly intruded pre- to syn-rift sediments, locally forming sheeted dyke complexes and 4) Extrusive mafic lavas, including pillow basalts, interlayered with metased- iments. Such a complete fossil magma-rich rifted margin analogue has never been recognized in the field before and provides unique opportunities to study processes occurring at deep levels in the crust even at the microscopic scales and may thus reform our understanding of the architecture, evolution, and processes at magma-rich rifted margins.

• Zertani, Sascha; John, Timm; Tilmann, Frederik; Labrousse, Loic & Andersen, Torgeir Bjørge (2018). The effect of eclogitization and associated deformation on the petrophysical properties of lower continental crust. Full text in Research Archive Show summary

  The subduction of crustal material is accompanied by fabric and petrology changes of the downgoing rocks. These changes cause modifications of the physical properties of the subducted material (e.g., seismic velocities and anisotropies). In geophysical images, such as receiver functions, the subducting crust is typically well imaged in the shallow parts of the subduction zone, while the deeper parts cannot be imaged at all. This is generally attributed to the eclogitization of the downgoing crust and the subsequent decrease of the contrast between subducted material and the surrounding mantle with respect to seismic velocities. The transition between theses zones is marked by a characteristic blurring and fading of the receiver function signal, possibly caused by partial eclogitization and inhomogeneously distributed deformation. In order to better understand the effect of deformation and eclogitization on P- and S-wave velocities and their respective anisotropies we conducted field work on the island of Holsnøy in western Norway with the goal of linking seismic properties with petrological and structural properties of the rocks. We performed direct measurements of P- and S-wave velocities (ultrasonic pulse emission technique) from selected samples related to the three principal fabric directions. The resulting seismic velocities and their anisotropy were then compared to seismic velocities derived from thermodynamic modelling and texture analysis using neutron diffraction. The resulting dataset links seismic with petrological/structural features of the investigated samples and was used for 2D seismic modelling. The resulting synthetic receiver function models show that eclogitization and deformation cause the same blurring or fading of receiver function signals as observed in currently active subduction zones, suggesting that the structures observed in the field are similar to those in subduction zones active today. Further, these synthetic signals may be used to further enhance our knowledge of the processes taking place during subduction and improve the visualization of the geometries within subduction zones.

• Broek, Joost Martijn van den; Gaina, Carmen; Buiter, Susanne & Andersen, Torgeir Bjørge (2018). Subduction-related continental rifting and microcontinent formation. Full text in Research Archive Show summary

  Continental rifting sometimes results in the formation of microcontinents and continental fragments. A micro- continent is a block of continental crust completely surrounded by oceanic lithosphere,. Classical examples are the Jan-Mayen Microcontinent in the NE Atlantic and the Seychelles in the Indian Ocean. A continental fragment is still attached to its parent continent by highly extended crust which failed to break-up. Examples are fragments off the west coast of Ireland associated with the opening of the NE Atlantic. Proposed scenarios for the formation of microcontinents and continental fragments are mostly associated with rifted continental margins, where rifting followed by seafloor spreading and mid-ocean-ridge relocation can result in the isolation of continental blocks and thus the formation of microcontinents. However, subduction processes can also result in microcontinent and continental fragment formation but the processes involved in their creation are not properly understood so far. Among microcontinents formed in subduction settings are the Corsica-Sardinia block in the Central Mediterranean, the Louisiade Plateau off the NE coast of Australia, and the Macclesfield and Reed Banks in the South China Sea. Using publicly available geophysical and geological data we first review the tectonic structure of micro- continents and conjugate margins. In particular, we analysed the structure of rifted margins by inspecting their geophysical signature from gravity and magnetic data. Observations of bathymetry profiles, where sediment thickness has been subtracted, and Bouguer gravity anomalies show an asymmetry in widths of the margin and the Continent Ocean Transition of these rifted conjugate margins. We then revisit the plate kinematics of regions where microcontinents formed in subduction-related set- tings with the aim to explain why microcontinents and continental fragments form in some areas and not in others. From our examples we find that microcontinents and continental fragments are associated with a component of rotation and/or oblique basin opening. The Central Mediterranean is characterised by a large 50◦ CCW rotation of the Corsica-Sardinia block due to Calabrian slab rollback which triggered rifting in the Liguro-Provençal Basin. In the South China Sea, rifting due to the southward subduction of the Proto-South China Sea propagated towards the SW resulting in a V-shaped basin. The evolution of the Coral Sea is thought to be controlled by the interplay between the northward motion of the Australian plate and the counterclockwise rotation of the Pacific plate. This triggered rifting and seafloor spreading NE of Australia resulting in the detachment of several microcontinents, including the Louisiade Plateau. Our preliminary analysis points towards the importance of small, enclosed basins as the locus of microcontinent formation in subduction settings and of rotational motions that allow separation of microcontinents from the parent continent.

• Moulas, Evangelos; Schmalholz, Stefan Markus; Katz, Liza; John, Timm; Andersen, Torgeir Bjørge & Labrousse, Loic (2018). Impact of weak zone geometry on pressure variations and potential implications for variation of mineral assemblages. Full text in Research Archive Show summary

  The formation of eclogites and high-pressure rocks has been routinely related to the depth of recrystallization during metamorphism. Although this is a good approximation for a non-deforming lithosphere, significant stress and pressure variations that develop during deformation may provide more complex relationships between mineral assemblages developing in deforming rocks and their burial depth. Results from mechanical modeling demonstrate that the stress distribution in natural deforming rocks is mainly function of the general stress level in the system and the spatial distribution of weak and strong lithologies. The general stress level depends on the boundary conditions and material properties such as effective viscosity and yield stress. However, the spatial distribution of strong and weak lithologies impose a non-trivial stress pattern, which can be responsible for significant pressure deviations from the lithostatic. In general, weak layers and shear zones will develop stress and pressure variations according to their orientation to the far-field tectonic stress and their strength contrast with the surrounding lithologies. To estimate the effect of weak-zone geometry on the stress and pressure distribution we developed numerical models based on continuum mechanics that take into account the natural geometry of the interconnected eclogite shear zones as they have been mapped on Holsnøy, Bergen Arcs, Norway. Our results demonstrate that pressure and stresses in the shear zones can strongly vary from the surrounding pressure and stresses. We perform a series of simulations with different viscosity ratios between shear zones and adjacent country rocks in order to calculate representative pressure values for both the shear zones and their country rock system. We finally compare our results with different models which have been recently proposed for the relationship between differential stress, tectonic pressure and metamorphic reactions and calculate the expected mineral assemblages using Perplex. Comparison of synthetic mineral assemblages with those actually documented in the natural shear zones from Holsnøy are discussed.

• Incel, Sarah; Labrousse, Loic; Hilaret, Nadege; John, Timm; Gase, Julien & Yanbin, Wang [Show all 11 contributors for this article] (2018). Reaction-induced faulting in granulite: New insights for the generation of intermediate-depth earthquakes in lower continental crust. Full text in Research Archive Show summary

  Pseudotachylite-networks in granulites on Holsnøy in the Bergen Arcs, SW Norway, and seismic tomography of the Tibetan plateau reveal that earthquakes were triggered even at the high temperature/ high pressure condi- tions of the lower continental crust. Both, field and geophysical observations, demonstrate a strong link between the nucleation of intermediate-depth earthquakes and areas of partial eclogitization within nominally anhydrous granulitic lower crust. This study presents four deformation experiments performed on granulite samples from Holsnøy. To accelerate reaction kinetics, which is very slow in dry rocks, we applied confining pressures of 2.5-3 GPa and temperatures in the range 995-1225 K, significantly higher than the expected eclogitization conditions of the Bergen Arcs (pressure= 1.5-2 GPa, temperature= 923-973 K). Based on the mechanical data, micro- and nanos- tructural observations, and recorded acoustic emissions, we were able to correlate the degree of eclogitization to the rheological behavior of the samples. Depending on the net eclogitization rate relative to the deformation rate (5 ×10−5s−1) the sample either behaved strong and ductile if no reaction occurred, mainly brittle when the rate of eclogitization was slow, or mostly weak ductile when the rate of eclogitization was fast. Our experimental re- sults emphasize that shear localization due to grain size reduction triggered by the breakdown of plagioclase under eclogite-facies conditions lead to brittle failure accompanied by acoustic emissions. These and other experiments on a variety of lithologies suggest that there could be one common mechanism that triggers intermediate and deep earthquakes.

• Jakob, Johannes & Andersen, Torgeir Bjørge (2018). The Rift-inherited structural architecture of the South and South-Central Scandinavian Caledonides. Full text in Research Archive Show summary

  The Rift-inherited structural architecture of the South and South-Central Scandinavian Caledonides Johannes Jakob and Torgeir B. Andersen University of Oslo, The Centre For Earth Evolution and Dynamics, Department of Geosciences, OSLO, Norway (johannes.jakob@geo.uio.no) The Caledonide Orogen was formed during the continental collision of Laurentia with Baltica/Avalonia in the Silurian and Lower-Devonian. During the collision, the pre-Caledonian margin of Baltica was partly subducted beneath Laurentia and a stack of nappes was transported southeast-wards hundreds of kilometres onto Baltica. The allochthons of the South-Central Caledonides can be described from bottom to top as: A) a series of nappes mainly composed of metamorphosed, Neoproterozoic, pre- to syn-rift, continental margin sequences that are mostly without syn-rift magmatism; B) a series of Baltican affine gneisses that also are mostly devoid of syn-rift magmatism, C) a second series of Neoproterozoic, metamorphosed, pre- to syn-rift, continental margin sequences that contain a large number of syn-rift plutons and volcanics, and which are interpreted as the magma-rich segment of the pre-Caledonian rifted margin of Baltica; D) a series of Cambrian-Ordovician, mostly fine-grained sedimentary units that also contain a number of mafic bodies and Alpine-type metaperidotites; E) outboard Iapetus ophiolites and island-arcs of Early Ordovician to Middle Silurian age. All are overlain by nappes of Laurentian origin. Unlike the Central Caledonides, the South Caledonian allochthons are dominated by the presence of large crystalline basement nappes, including the Bergsdalen, Jotun, Lindås, and Dalsfjord nappe complexes. The break in the structural architecture between the South-Central and South Caledonides occurs along a lineament parallel with the Gudbrandsdalen Antiform and the Sveconorwegian front in the Baltican basement. Notably, the number of syn-rift magmas in the second series of continental margin successions (C) strongly decreases towards this lineament. Crossing the Gudbrandsdalen Antiform to the southwest, the eastern flank of the Jotun Nappe overlies continental margin succession characterised by a paucity of syn-rift magmatism. The western flank of the Jotun Nappe Complex, however, overlies another series of Cambrian-Ordovician mostly fine-grained meta-sediments, containing Alpine-type metaperidotites. Petrographic and lithostratigraphic considerations suggest that the units below the western Jotun Nappe can be correlated across the Gudbrandsdalen Antiform with those in the South-Central Caledonides. We suggest that the decrease in the abundance and volume of mafic rocks in the continental margin successions north of the Gudbrandsdalen Antiform is linked to a transition zone between the magma-rich segments of the pre- Caledonian margin of Baltica and transitional magma-poor crust inboard of a “Jotun microcontinent/continental sliver”. Moreover, the correlation of the units in the South and South-Central Caledonides challenge the traditional across-strike correlations of the nappes. Therefore, we also suggest that the Baltican affine units in the South- Central Caledonides (A-D) represent nappe-stacks of rift-inherited domains, including the proximal, necking, hyperextended and distal domains of the rifted margin of Baltica. In the South Caledonides, remnants of the proximal-necking domains are structurally overlain by remnants of magma-poor transitional crust and by rocks of the “Jotun microcontinent/continental sliver”.

• Incel, Sarah; Hilaret, Nadege; Labrousse, Loic; John, Timm; Renner, Jørg & Deliccque, Damien [Show all 10 contributors for this article] (2018). Grain size reduction during metamorphic reactions as trigger for deep earthquakes. Full text in Research Archive Show summary

  Due to nucleation depths of 50-700 km in Earth’s lower crust and mantle, deep earthquakes cannot be explained by Byerlee friction as it is the case for their shallow counterparts. However seismic relocation and field observations clearly demonstrate embrittlement of Earth’s crust and mantle even under elevated pressure-temperature condi- tions. The observation that those events coincide with areas in which metamorphic reactions are expected to occur (e.g. eclogitization of oceanic crust during subduction or of continental crust during orogeny) lead to the hypothesis that mineral reactions could weaken the rock to enable brittle failure. The fact that deep earthquakes are recorded in various rock types in different geological settings points to a common weakening mechanism that is independent of rock’s mineralogical composition. To experimentally investigate this hypothesis, deformation experiments using a D-DIA apparatus were performed under eclogite-facies conditions on either hydrous lawsonite-bearing blueschist, representing the oceanic crust (cold subduction), or on nominally anhydrous mafic granulite as representative for the lower continental crust (continent-continent-collision). Experiments were conducted at either P= 1.5-3.5 GPa, T= 583-1121 K (blueschist samples) or at P=2.5-3 GPa, T= 1023-1225 K (granulite samples) and a strain rate of around 5 × 10−5s−1. The experimental results demonstrate faulting in both rock types accompanied by the record of acoustic emissions. Micro- and nanostructural observations reveal a link between the growth of nanocrystalline eclogite-facies minerals and strain localization. Based on our experimental results on crustal rocks in combination with previous experimental studies on the olivine-spinel transition, we suggest that shear localization due to grain size reduction during metamorphic reactions act as a viable common mechanism to trigger deep earthquakes.

• HOSSEINZADEHSABETI, Elham; Ferre, Eric C.; Geissman, John W; Friedman, Sarah A.; Andersen, Torgeir Bjørge & Spagnuolo, Elena [Show all 7 contributors for this article] (2018). THE KINEMATICS OF FRICTIONAL MELTS IN A SUBDUCTION ZONE:PILOT STUDY ON CORSICAN PSEUDOTACHYLYTES. Full text in Research Archive Show summary

  THE KINEMATICS OF FRICTIONAL MELTS IN A SUBDUCTION ZONE:PILOT STUDY ON CORSICAN PSEUDOTACHYLYTES Pseudotachylytes from the Schistes Lustrés in Corsica may preserve the record of large earthquakes in subduction zones. These rocks, formed along the oceanic crust – mantle boundary, offer opportunities to further our knowledge of subduction seismic processes. Our study on ultramafic pseudotachylytes uses the mini-AMS method and shows a consistent flow plane, flow direction and sense of shear. These fabrics result from coseismic viscous shear in frictional melts. The sense of shear recorded along slip planes is crucial to assess whether slip occurred during a reverse or normal kinematics. The kinematic analysis of the seismic rupture represented by these generation veins is problematic due to the paucity of external offset markers and the lack of internal shear sense criteria in these extremely fine grained materials. The absence of garnet or its relicts in the peridotites of this locality suggests a maximum pressure of equilibration for the host-rock of 1.2 GPa (≈40 km). Four representative oriented samples of these rocks display characteristic microstructures of frictional melts, including skeletal grains, microlites, vesicles, and corroded clast. The material consists of a crypto-crystalline matrix of olivine and pyroxenes, and a small percentage of clasts. The samples of this study are free of serpentinization. The magnetic properties of the pseudotachylytes indicate the presence of pseudo- single domain to single domain stoichiometric magnetite grains, along with a minor contribution of various paramagnetic silicates. The magnetic fabric of these rocks is very consistent within specimens (3.5 mm cubes) from the same vein. The magnetic assemblage results from breakdown of olivine, pyroxenes and chrome spinel, followed by rapid nucleation of magnetite. The magnetite grains form during coseismic viscous shear in a manner similar with those formed in frictional laboratory experiments on the same material. The AMS of these rocks arises from the SPO of elongated magnetite grains. Finally, the AMS fabric obliquity with respect to the seismic slip plane is used as a shear sense indicator. Preliminary results show the dominance of normal senses of slip, hence support that seismic slip took place during the late exhumation of the peridotitic units.

• Incel, Sarah; Labrousse, Loic; Hilairet, Nadège; John, Timm; Gasc, Julien & Shi, Feng [Show all 11 contributors for this article] (2018). Reaction-induced faulting in granulite causes earthquakes in the lower continental crust. Full text in Research Archive Show summary

  Pseudotachylite-networks in granulites on Holsnøy in the Bergen Arcs, SW Norway, and seismic tomography of the Tibetan plateau reveal that earthquakes were triggered even at the high temperature/ high pressure conditions of the lower continental crust. Both, field and geophysical observations, demonstrate a strong link between the nucleation of intermediate-depth earthquakes and areas of partial eclogitization within nominally anhydrous granulitic lower crust. This study presents four deformation experiments performed on granulite samples from Holsnøy. To accelerate reaction kinetics, which is very slow in dry rocks, we applied confining pressures of 2.5-3 GPa and temperatures in the range 995-1225 K, significantly higher than the expected eclogitization conditions of the Bergen Arcs (pressure= 1.5-2 GPa, temperature= 923-973 K). Based on the mechanical data, micro- and nanostructural observations, and recorded acoustic emissions, we were able to correlate the degree of eclogitization to the rheological behavior of the samples. Depending on the net eclogitization rate relative to the deformation rate (5∙10-5 s-1) the sample either behaved strong and ductile if no reaction occurred, mainly brittle when the rate of eclogitization was slow, or mostly weak ductile when the rate of eclogitization was fast. Our experimental results emphasize that shear localization due to grain size reduction triggered by the breakdown of plagioclase under eclogite-facies conditions lead to brittle failure accompanied by acoustic emissions. These and other experiments on a variety of lithologies suggest that there could be one common mechanism that triggers intermediate and deep earthquakes.

• Andersen, Torgeir Bjørge (2018). Interview with school kids at Manglerud skole International Classes in Oslo regarding Plate Tectonics, Geology and Geohazards. Full text in Research Archive
• Andersen, Torgeir Bjørge; Jakob, Johannes; Kjøll, Hans Jørgen; Corfu, Fernando; Torsvik, Trond Helge & Tegner, Christian [Show all 8 contributors for this article] (2018). Architecture and Palaeography of the per-Caldonian Margin of Baltica. Full text in Research Archive
• Andersen, Torgeir Bjørge; Jakob, Johannes; Kjøll, Hans Jørgen; Corfu, Fernando; Planke, Sverre & Torsvik, Trond Helge [Show all 9 contributors for this article] (2018). The Pre-Caledonian Margin of Baltica: overview and research in progress. Full text in Research Archive Show summary

  The Caledonian margin of Baltica formed by continental break-up of Rodinia in the Late Proterozoic to Ediacaran. With exception of the dike-swarm near Egersund in SW Norway, the Fennoscandian basement including the autochthonous basement windows along the axis of the mountain belt were little affected by the magmatism associated with the break-up. The distal parts, however, were strongly attenuated, hyper-extended and a 1000 km long segment, intensively intruded by a Large Igneous Province (LIP), the Pre-Caledonian LIP (PC-LIP). Here, we provide glimpses of our work in progress from the vestiges of the margin. More details on several aspects of the margin evolution are presented by co-authors. Here we present a regional model for the pre-Caledonian margin suggesting it was highly complex and included micro-continental sliver(s) and both a hyperextended, magma-poor domain with transition(s) to attenuated embryonic oceanic and magma-rich margin domains. The break-up related PC-LIP magmatism lasted from approximately 615 to 570 Ma, but the most intense activity appears to have been at ~600 Ma. Our ongoing work suggests that the impingement of a mantle plume on the Ediacaran continental lithosphere was associated with a temperature anomaly of ~100oC, causing widespread melting of the asthenosphere and dyke-intrusion of the continental crust and sediments of the margin. We suggest that the pre-Caledonian margin of the Iapetus preserved in the Scandes comprise most of the elements of passive continental margins, and that it probably represents one of the best exposed field analogue for the deeper and least known parts of passive margins.

• Tegner, Christian; Andersen, Torgeir Bjørge; Kjøll, Hans Jørgen; Brown, Eric L.; Hagen-Peter, Graham & Corfu, Fernando [Show all 8 contributors for this article] (2018). The pre-Caledonian Scandinavian Dyke Complex and 600 Ma plate reconstructions of Baltica. Full text in Research Archive Show summary

  A spectacular dyke complex is surprisingly well preserved along c. 1000 km in the Caledonian nappes of central and northern Scandinavia. This dyke complex was originally emplaced into continental sedimentary basins along the rifted margin of Baltica, it is part of the Central Iapetus Magmatic Province (CIMP), and it has U-Pb ages of 615-590 Ma. To constrain its origin and to potentially guide plate reconstructions of Baltica we: (1) re-visited the dyke complexes of the Ottfjället, Sarek, Kebnekaise, Tornetrask and Indre Troms mountains of Sweden and Norway; (2) produced new and compiled published geochemical data; (3) modeled mantle sources and melting dynamics; and (4) extended reconstructions of the paleo-position of Baltica back to 600 Ma. The compiled dataset includes c. 600 analyses that forms a coherent suite dominated by tholeiitic ferrobasalt, but including alkali basalts in the central portion. The tholeiitic dykes display lateral variations in geochemical enrichment (e.g. delta-Nb, La/Sm(N) and Sr isotopes) in the southern and central portions, grading to more depleted compositions in the north. Our petrological modeling suggests melting of asthenospheric mantle involving at least two source compositions at temperatures elevated about 100 °C above ambient mantle, consistent with melting of a zoned mantle plume originating from a plume generation zone at the core-mantle boundary. If the position of the present plume generation zone in the Pacific can be viewed as stationary back to 600 Ma, we entertain the idea that the Scandinavian Dyke Complex may be used to guide plate reconstructions.

• Jakob, Johannes; brook, Joost M. van den & Andersen, Torgeir Bjørge (2018). Remnants of the pre-Caledonian Baltica rifted margin preserved in a lithologically mixed unit between Bergen and Tynset, Scandinavian Caledonides, South and Central Norway. Full text in Research Archive Show summary

  Remnants of the pre-Caledonian Baltica rifted margin preserved in a lithologically mixed unit between Bergen and Tynset, Scandinavian Caledonides, South and Central Norway Johannes Jakob, Joost M. van den Broek, Torgeir B. Andersen The Centre for Earth Evolutions and Dynamics, Department of Geosciences, University of Oslo, Norway Remnants of the magma-poor and magma-rich pre-Caledonian rifted margin of Baltica are preserved in the allochthons of the Scandinavian Caledonides. The transition from magma-poor to the magma-rich domain corresponds to the northern termination of the Jotun Nappe Complex. Solitary metaperidotite bodies abound in the magma-poor segment and the transition zone. The metaperidotite bodies display a characteristic early flattening fabric. The flattening fabric is well-developed in the ultramafic bodies enclosed in the metasedimentary units structurally below the large crystalline basement nappes of Southern Norway including the Jotun, Lindås, and upper Bergsdalen nappe complexes, but can also be found in the metaperidotite lenses between Vågåmo and Tynset. Pre-Scandian contact relationships between those deformed metaperidotites and the metasediments have been obliterated during the Scandian Orogeny. However, well-recrystallised, pre-Scandian, extensional and sedimentary "ghost structures" within the metaperidotite bodies are locally preserved and can be found on favourably weathered surfaces. The interpretation of the lithologically mixed unit between Bergen and Tynset is challenging, because, no geochronological evidence of the early rift stages from within the unit have yet been reported. Moreover, the unit may have been reworked between the latest Cambrian and early Mid-Ordovician (Jakob et al. 2017). However, an original lithostratigraphic succession of the ancient Baltica rifted margin may be preserved in a thrust nappe north of Lesja (Jakob et al. this volume) and may help to shed light on the origin and pre-Scandian history of the mixed unit between Bergen and Tynset. Jakob, J., Alsaif, M., Corfu, F., & Andersen, T.B. 2017: Age and origin of thin discontinuous gneiss sheets in the distal domain of the magma-poor hyperextended pre-Caledonian margin of Baltica, southern Norway. Journal of the Geological Society 174, 557-571. Jakob, J., Mohn, G., Closset, P., Andersen, T.B. this volume: Indications for a magma-rich to magma-poor transition zone in the southernmost part of the Pre-Caledonian LIP, Scandinavian Caledonides, Norway

• Kjøll, Hans Jørgen; Andersen, Torgeir Bjørge; Labrousse, Loic & Corfu, Fernando (2018). A new tectonic model for the Seve Nappe Complex in Norrland, Sweden. Full text in Research Archive Show summary

  A new tectonic model for the Seve Nappe Complex in Norrland, Sweden Hans Jørgen Kjøll, Torgeir B. Andersen, Loic Labrousse, Fernando Corfu 1) CEED 2) Paris The Seve Nappe Complex (SNC) in Norrbotten, Sweden has classically been mapped as a series of nappes formed by the telescoping of the rifted margin of Baltica. Typically, the SNC is sub-divided into the lower, middle and upper nappe-series. The lower Seve generally constitute slivers of basement, such as the Akkajaure nappe and are overlain by meta-sedimentary and meta-igneous rocks representing both the middle and upper Seve nappes. The peak metamorphism varies considerably within the Seve from local eclogite facies to a more regional amphibolite facies and domains where original pre-Caledonian contact metamorphism related to dolerite intrusions are preserved. This has led to the norm of subdividing the SNC in different thrust nappes. We propose, however, a model where the Seve in Norrbotten represents a more continuous section through the magma-rich, attenuated margin of Baltica and that the metamorphic overprint, rather than representing decoupling and re-stacking, represent differential reactivity of individual lithologies with different reaction potential. High strain zones are present throughout Seve but the need for large internal thrusts juxtaposing older rocks on top of younger is not present internally in the Seve. This has dramatic consequences for, not only the understanding of the mountain building in the Caledonides, but may also open a new interest for the Seve as a place to study the deep processes of magma-rich passive margins, which are often lost in subduction and in general difficult to study in seismic lines due to masking of high velocity mafic intrusive and extrusive rocks.

• Kjøll, Hans Jørgen; Andersen, Torgeir Bjørge; Tegner, Christian; Labrousse, Loic & Planke, Sverre (2018). A fossil magma-rich margin revealed. Full text in Research Archive Show summary

  A fossil magma-rich rifted margin revealed Hans Jørgen Kjøll, Torgeir B. Andersen, Christian Tegner, Loic Labrousse and Sverre Planke 1) CEED 2) Aarhus Universite 3) Paris 4) CEED/VBPR The Iapetus opened ~610 Myr ago, when a plume impinged on the lower crust of Baltica and Laurentia. Stretching of the conjoined crystalline crust started in the Neoproteozoic, prior to break-up and provided accommodation for continental and shallow marine syn-rift sediments. An early a-magmatic phase with discrete and localized deformation was followed by pervasive mafic magmatism where dike-emplacement accounted for the majority of the stretching. During the Caledonian orogeny the Baltican margin was thrusted onto Baltica as the Iapetus closed. Now, vestiges of the magma-rich margin resides within nappes from central Sweden to northern Norway. Although, overprinted by Caledonian fabrics, there are localities where pre-Caledonian structures are well-preserved, thereby allowing for detailed studies of deep to intermediate processes at magma-rich rifted margins. We propose the architecture of the magma-rich margin of Baltica and magma-rich rifted margins in general can be studied in the Caledonides. The well-preserved parts of the margin comprise: 1) Parts of a lower crustal magmatic complex w/gabbros and mafic dykes intruding stretched crystalline basement; 2) Strongly stretched and attenuated crystalline basement intruded by mafic dikes; 3) Highly intruded pre- to syn-rift sediments and 4) Extrusive mafic lavas, including pillow basalts, interlayered with metasediments. Together these levels represent a nearly continuous section through a magma-rich rifted margin, with some never-before described levels, such as the lower crustal magmatic complex.

• Jakob, Johannes; Mohn, Geoffroy; Closset, Pierre & Andersen, Torgeir Bjørge (2018). The lithostratigraphy of a hyperextended domain in the magma-rich to magma-poor transition zone in the southern Pre-Caledonian LIP, Scandinavian Caledonides, Norway. Full text in Research Archive Show summary

  Remnants of the pre-Caledonian Baltica rifted margin preserved in a lithologically mixed unit between Bergen and Tynset, Scandinavian Caledonides, South and Central Norway Johannes Jakob, Joost M. van den Broek, Torgeir B. Andersen The Centre for Earth Evolutions and Dynamics, Department of Geosciences, University of Oslo, Norway Remnants of the magma-poor and magma-rich pre-Caledonian rifted margin of Baltica are preserved in the allochthons of the Scandinavian Caledonides. The transition from magma-poor to the magma-rich domain corresponds to the northern termination of the Jotun Nappe Complex. Solitary metaperidotite bodies abound in the magma-poor segment and the transition zone. The metaperidotite bodies display a characteristic early flattening fabric. The flattening fabric is well-developed in the ultramafic bodies enclosed in the metasedimentary units structurally below the large crystalline basement nappes of Southern Norway including the Jotun, Lindås, and upper Bergsdalen nappe complexes, but can also be found in the metaperidotite lenses between Vågåmo and Tynset. Pre-Scandian contact relationships between those deformed metaperidotites and the metasediments have been obliterated during the Scandian Orogeny. However, well-recrystallised, pre-Scandian, extensional and sedimentary "ghost structures" within the metaperidotite bodies are locally preserved and can be found on favourably weathered surfaces. The interpretation of the lithologically mixed unit between Bergen and Tynset is challenging, because, no geochronological evidence of the early rift stages from within the unit have yet been reported. Moreover, the unit may have been reworked between the latest Cambrian and early Mid-Ordovician (Jakob et al. 2017). However, an original lithostratigraphic succession of the ancient Baltica rifted margin may be preserved in a thrust nappe north of Lesja (Jakob et al. this volume) and may help to shed light on the origin and pre-Scandian history of the mixed unit between Bergen and Tynset. Jakob, J., Alsaif, M., Corfu, F., & Andersen, T.B. 2017: Age and origin of thin discontinuous gneiss sheets in the distal domain of the magma-poor hyperextended pre-Caledonian margin of Baltica, southern Norway. Journal of the Geological Society 174, 557-571. Jakob, J., Mohn, G., Closset, P., Andersen, T.B. this volume: Indications for a magma-rich to magma-poor transition zone in the southernmost part of the Pre-Caledonian LIP, Scandinavian Caledonides, Norway

• Andersen, Torgeir Bjørge (2017). The FORM Conference: Concluding Remarks. Full text in Research Archive
• Tegner, Christian; Andersen, Torgeir Bjørge; Brown, Eric L.; Corfu, Fernando; Planke, Sverre & Kjøll, Hans Jørgen [Show all 7 contributors for this article] (2017). The pre-Caledonian Scandinavian Dyke Complex and 600 Ma plate reconstructions of Baltica. Full text in Research Archive Show summary

  Magmatism of the first known rifting phase of the North Atlantic Wilson cycle is surprisingly well preserved in the Cale- donian nappes of central and northern Scandinavia. Along c. 1000 km the Särv and Seve Nappes are characterised by spectacular dyke complexes and other intrusive forms origi- nally emplaced into continental sedimentary basins along the rifted margin of Iapetus. We refer to this as the magma-rich segment of the pre-Caledonian margin of Baltica, in contrast to the hyper-extended and magma-poor segment exposed in southern Norway (Andersen et al., Jakob et al., this meeting). In the larger picture the Scandinavian Dyke Complex is part of the Central Iapetus Magmatic Province ( ). The intensity, volume and structure of the Scandinavian Dyke Complex is comparable to that of the present passive margins of the North Atlantic large igneous province (Kjøll et al., this meeting) and U-Pb ages suggest magmatism was short-lived at 610-590 Ma. To constrain the origin of the Scandinavian Dyke Complex and to potentially guide plate reconstructions of Baltica we: (1) re-visited the dyke complexes of the Särv, Sarek, Kebnekaise, Tornetrask and Indre Troms mountains of Sweden and Norway; (2) compiled new and published geochemical data for the entire dyke complex; (3) modeled mantle sources and melting dy- namics; and (4) extended reconstructions of the paleo-position of Baltica back to 600 Ma. Although the appearance of the dykes ranges from garnet amphibolite gneiss to pristine mag- matic intrusions, all bulk rock compositions largely reflect the original magmatic rock. The compiled dataset includes c. 600 analyses that essentially forms a coherent suite dominated by tholeiitic ferrobasalt akin to the North Atlantic large igneous province, but including alkali basalts in the central portion where meta-carbonatite is also reported. A few samples (<30) are significantly contaminated with crust, but most are largely uncontaminated. The tholeiitic dykes display systematic lateral variations over c. 1000 km in trace element compositions suggesting geochem- ical enrichment (e.g. delta-Nb and La/Sm(N) relative to mid- ocean ridge basalt) in the southern and central portions, grad- ing to more depleted compositions in the north. The most enriched tholeiites occur in the central portion that also in- cludes alkali basalts. The lateral geochemical zoning is com- parable to e.g. the North Atlantic large igneous province and around Iceland today. Our petrological modeling suggests melting of asthenospheric mantle involving at least two source compositions at temperatures elevated above ambiant mantle. We therefore speculate that the Scandinavian Dyke Complex formed by melting of a zoned mantle plume originating from a plume generation zone (margins of large low shear-wave velocity provinces) at the core-mantle boundary. If the posi- tion of the present plume generation zone in the Pacific can be viewed as stationary back to 600 Ma, we entertain the idea that the Scandinavian Dyke Complex may be used to guide plate reconstructions.

• Jakob, Johannes; Boulvais, Philippe; Beyssac, Olivier; Corfu, Fernando; Alsaif, Manar & Andersen, Torgeir Bjørge (2017). Transitional crust in the fossil ocean-continent transition zone of the pre-Caledonian rifted margin of Baltica and its link to the Scandinavian Dyke Complex. Full text in Research Archive Show summary

  Part of the pre-Caledonian passive margin of Baltica is pre- served in the nappes of the Scandinavian Caledonides. On a first order, the passive margin can be divided into: 1) a magma- rich segment, where mafic intrusives abound in metasediments and which is referred to as the Scandinavian Dyke Complex; 2) and a magma-poor segment, that is characterised by a lithos- tratigraphy typical for transitional crust as well as a paucity of rift-related intrusives. Crystallisation ages of metamorphosed gabbro, dolerite, and basalt in the metasediments of the magma-rich segment have been dated at ⇠ 616-596 Ma and are interpreted to have been emplaced as part of the Central Iapetus Magmatic Province during opening of the Iapetus ocean. Vestiges of magma-poor hyperextended crust in central South Norway are structurally below large crystalline nappes of Baltican origin, including the Jotun, Lindås, Dalsfjord, and upper Bergsdalen nappe com- plexes. The fossil transitional crust of the magma-poor hy- perextended margin is preserved as a lithological mixed unit, including detrital and solitary metaperidotites with a com- plex hydration and carbonation history, originally fine-grained metasediments, continent-derived sandstones/conglomerates, thin slivers of Proterozoic gneisses, and minor late Cambrian- Middle Ordovician and late Silurian metamorphosed felsic and mafic intrusive rocks. The mixed unit is characterised by a consistent along strike Scandian metamorphic overprint, which is estimated at 500±50°C and ⇠7±2kbar, as well as by homogeneous oxygen isotope compositions of the (ophi- )carbonates of about 11.5-15 ‰ (SMOW). The depositional age of the mixed unit is apparently early Middle Ordovician in age (Dapingian detrital zircon and fossil ages), suggesting that the magma-poor segment may have been reworked and rede- posited during a poorly-understood orogenic event in the late Cambrian-Middle Ordovician. Because of the young depositional ages of the metasediments and the paucity of rift-related intrusives in the mixed unit, a direct link of the magma-poor with the magma-rich segment is di cult. Moreover, associated with the change in lithology of the mixed unit from a magma-poor to a magma-rich pas- sive margin is a major structural break in the architecture of the Scandinavian Caledonides. In the vicinity of Vågåmo and Otta across Gudbrandsdalen, the large, crystalline, Baltican nappes terminate and the fossil ocean-continent transition zone is directly overlain by the exotic Trondheim Nappe Complex. However, the lithostratigraphic association of the magma-poor hyperextended segment continues across the structural break into the nappes structurally below the Trondheim Nappe Com- plex. In contrast to the fossil transitional crust below the Jotun Nappe Complex, the mixed unit structurally underlying the Trondheim Nappe Complex additionally comprises a progres- sively increasing number of yet undated elongate bodies of metamorphosed gabbro, dolerite, and basalt towards the north and northeast. These units may represent the vestiges of a tran- sition zone from the magma-poor to the magma-rich segments of the pre-Caledonian passive margin. Therefore, the mixed unit between Gudbrandsdalen and Essandsjøen may link the reworked, fossil, transitional crust that was originally formed in a narrow oceanic basin between the proximal Baltican mar- gin and an outboard lying microcontinent with the Ediacaran Scandinavian Dyke Complex.

• Zertani, Sascha; John, Timm; Thilmann, Fredrik; Motra, Hem B.; Labrousse, Loic & Andersen, Torgeir Bjørge (2017). Petrophysical properties of eclogite facies shear zones and their relationship to receiver function signals. Full text in Research Archive Show summary

  The visualization of subducting lithosphere is dependent on geophysical methods such as receiver functions. With this the shallow part of the slab can be imaged clearly while the deeper part often eludes detection due to its insufficient seismic velocity contrast compared to the earth’s mantle. Inbetween these two sections a blurry zone can be observed, which is often interpreted as the zone of progressive partial eclogitization of the subducting material. To investigate this blurry zone we chose the island of Holsnøy in the Bergen Arcs of western Norway as a natural analogue. This area is comprised of lower crustal granulite facies rocks, which were partly eclogitized during Caledonian subduction. Partial eclogitization is induced by fluids and can be observed in distinct shear zones as well as statically overprinted finger-like areas. In order to incorporate the structural geometry and relationships of this area into seismic models, the area was mapped and samples for petrophysical analysis were collected. To constrain the petrophysical properties of the rocks at depth seismic velocities were calculated using thermodynamic modelling (Perple_x) as well as Voigt-Reuss-Hill averaging. Both methods yield consistent results for p-wave and s-wave velocities and are compared to ultrasonic pulse measurements of the same samples. These results show clearly that the eclogite facies shear zones observed in the field constitute seismic boundaries with contrasting velocities at depth within subducting lithosphere. Using these results as an input for seismic modelling of different simple case studies, we can reproduce the signals obtained from real receiver function studies and help to interpret the structures at depth.

• Andersen, Torgeir Bjørge; Jakob, Johannes; Kjøll, Hans Jørgen; Corfu, Fernando; Planke, Sverre & Torsvik, Trond Helge [Show all 8 contributors for this article] (2017). The Pre-Caledonian Margin of Baltica. Full text in Research Archive Show summary

  The pre-Caledonian margin of Baltica formed by the rifting and eventual continental break-up of Rodinia in the Late Proterozoic to Ediacaran. With exception of the ca. 615 Ma dolerite dike-swarm near Egersund in SW Norway, the Fennoscandian basement including the mostly autochthonous basement windows in western Scandinavia (Lofoten, Western Gneiss Region, etc.) were surprisingly little affected by the rifting and the subsequent magmatism associated with the break-up. The distal parts of the margin, however, were strongly attenuated, hyper-extended and in a more than 1000 km long segment, intensively intruded by a break-up related Large-Igneous Province (LIP), here referred to as the Pre-Caledonian LIP (PC-LIP). In this presentation, we provide glimpses of new results and observations as well as work in progress from a large part of the nappe-stack of Scandinavian Caledonides containing the vestiges of the pre-Caledonian margin. More details on several aspects of the Pre-Caledonian margin evolution are presented by co-workers during this meeting. Based on our recent and some previous work, we propose a regional paleogeography model for the pre-Caledonian passive margin as it evolved from the Ediacaran into the late-Cambrian to the early-Ordovician, when the Iapetus Ocean started closing. We suggest that the distal margin architecture was highly complex and included micro-continental sliver(s) and a hyperextended, magma-poor domain with transition(s) to highly attenuated embryonic oceanic and magma-rich passive margin domains. The break-up related PC-LIP magmatism lasted from approximately 615 to 570 Ma, but the most intense dyke-emplacement appears to have been around 600 Ma. Our ongoing structural and metamorphic petrology studies, combined with geochronology, geochemical analyses and petrological modelling suggest that the impingement of a mantle plume on the Ediacaran continental lithosphere may have been associated with a temperature anomaly of up to 100oC, causing widespread melting of the asthenosphere as well as partial melt-generation in highly attenuated and intensely dike-intruded parts of the continental crust and sediments of the margin. It is our opinion that the pre-Caledonian distal margin rocks of the Iapetus as preserved in nappes of Scandinavian Caledonides preserve most of the elements of wide passive margins, and that it probably represents one of the best exposed field analogue for the deeper and least known parts of passive margins.

• Tegner, Christian; Andersen, Torgeir Bjørge; Brown, Eric L.; Corfu, Fernando; Planke, Sverre & Kjøll, Hans Jørgen [Show all 7 contributors for this article] (2017). The pre-Caledonian Scandinavian Dyke Complex and 600 Ma plate reconstructions of Baltica. Full text in Research Archive
• Jakob, Johannes; Alsaif, Manar; Andersen, Torgeir Bjørge & Corfu, Fernando (2017). U-Pb ID-TIMS geochronology of gneisses and metaintrusives in the pre-Caledonian magma-poor hyperextended margin of Baltica. Full text in Research Archive
• Andersen, Torgeir Bjørge; Jakob, Johannes; Kjøll, Hans Jørgen; Corfu, Fernando & Tegner, Christian (2017). A new look at the pre-Caledonian margin of Baltica. Full text in Research Archive Show summary

  From the long history of previous work, most recently compiled by Nystuen and co-workers (2008, Episodes), it is well-documented that the pre-Caledonian margin of Baltica must have constituted a several hundred km wide and a more than 2000 km long passive continental margin. The vestiges of the margin now occur in the nappes at low- to intermediate tectono-stratigraphic levels in the mountain belt. The lower and originally proximal parts of the margin are dominated by continental rift basins mostly filled with coarse-grained siliciclastic sediments deposited in the late Proterozoic through the Ediacaran and into the Lower Palaeozoic, where the youngest dated clastic zircons are ~470 Ma (Slama & Pedersen 2015 JGS). At higher levels, and originally in more distal positions of the original margin, the nappes commonly comprise both basement and cover as well as deep-marine basin deposits. A major change in the architecture of the tectonic units that originally formed in the passive margin takes place across a NW-SE trending zone, which is roughly parallel to the present-day Gudbrandsdalen of South Norway. The transition zone is also roughly parallel to the major basement lineament of the Sveconorwegian orogenic front in south Norway. The most important change across this transverse lineament is that the NE segment is magma-rich, characterized by abundant basaltic magmatism. The basalts, mostly now seen mostly as dykes and gabbroic intrusions can be identified from Corrovarre in the north (Zwaan & van Roermund 1990, NGT) to Otta in the south (e.g. Sturt et al. 1991). Where dated, they were apparently emplaced during a relatively short time span between ~615 and 595 Ma (e.g. Baird et al. 2015, GFF; Nystuen et al. 2016, & Kumpulainen et al. 2016, both Abstracts, Nordic Winter meeting). This magmatism constitutes a large igneous province (LIP see presentation by Tegner et al. and Kjøll et al. this abstract volume). The SW segment is magma-poor and largely devoid of magmatic rocks of Ediacaran age (~540-635 Ma), except for the important Egersund dolerites cutting in-situ Baltican basement at 616±3 Ma (Bingen et al. 1998, The Journal of Geology; Walderhaug et al. 2007, Geophysical Jl. International). Instead the SW segment is characterised by numerous (>100) solitary meta-peridotites, mostly meta-dunites and meta-harzburgites as well as a number of detrital serpentinites and soapstones. These are interpreted as fragments of exhumed mantle and their erosion products, respectively. The meta-peridotites were originally exhumed and emplaced structurally, and covered by dominantly deep-basin sediments, but also by coarse sedimentary breccias and conglomerates, as part of the rifted margin development. The mélange was intruded locally by Early Ordovician gabbro to granitic rocks and was affected by initial metasomatism before the main Scandian metamorphism and deformation (Jakob et al. JGS in press and this abstract volume). Another important feature of the sedimentary and ultra-mafic mélange unit is the common presence of elongate (up to ~40 km), mostly thin (<100 m) deformed basement slivers of Baltican affinity (Jakob et al. JGS, in press). The magma-poor SW segment is structurally overlain by huge crystalline basement nappes (Jotun, Bergsdalen, Lindås, Dalsfjord Nappes) of Baltican basement, which after rifting, but prior to the Scandian Caledonian shortening were positioned outboard of the hyperextended segment. These nappes probably constituted a micro-continental sliver comparable in size to the Jan Mayen microcontinent in the present Greenland-Norwegian Sea. The SW segment represented by the 'melange' is interpreted to have constituted an approximately 400 km long, hyperextended- and magma-poor basin, which received sediments into the Early Ordovician and perhaps until the onset of the Scandian orogeny?

• Svendby, Anne Katrine; Osmundsen, Per Terje; Andresen, Arild & Andersen, Torgeir Bjørge (2017). Transtensional basins from fault growth to bulk constriction: insights from the `Old Red´ basins of western Norway. Full text in Research Archive
• Kjøll, Hans Jørgen; Andersen, Torgeir Bjørge; Tegner, Christian; Corfu, Fernando & Planke, Sverre (2017). Wilson cycle-kick-off: Constraining the influence of a LIP during the Neoproterozoic evolution of the pre-Caledonian margin of Baltica and Laurentia. Full text in Research Archive
• Andersen, Torgeir Bjørge; Kjøll, Hans Jørgen; Jakob, Johannes; Corfu, Fernando & Tegner, Christian (2017). The pre-Caledonian margin of Baltica. Full text in Research Archive
• Kjøll, Hans Jørgen; Andersen, Torgeir Bjørge & Tegner, Christian (2017). Constraining dike emplacement conditions from virtual outcrop modelling. Full text in Research Archive
• Andersen, Torgeir Bjørge (2017). Corsican Adventures: discovery of subduction related earthquake fault-rocks. Full text in Research Archive
• Andersen, Torgeir Bjørge (2017). Geologisk Vandring på Holsnøy, Hordaland. Full text in Research Archive
• Andersen, Torgeir Bjørge; Jakob, Johannes; Kjøll, Hans Jørgen; Corfu, Fernando; Tegner, Christian & Mohn, Geoffroy [Show all 9 contributors for this article] (2017). The Magma-poor and Magma-rich settings; the onshore perspective: The Caledonian case. Full text in Research Archive
• Andersen, Torgeir Bjørge (2017). Trond Helge Torsvik turns 60; The Early Days. Full text in Research Archive
• Jakob, Johannes; Andersen, Torgeir Bjørge; Boulvais, Philippe & Beyssac, Olivier (2017). Scandian metamorphism of metapelites and serpentinites in the pre- Caledonian magma-poor hyperextended margin of Baltica. Full text in Research Archive
• Incel, Sarah; Hilairet, Nadège; Labrousse, Loic; Andersen, Torgeir Bjørge; John, Timm & Wang, Yanbin [Show all 7 contributors for this article] (2017). Eclogite-facies metamorphic reactions under stress and faulting in granulites from the Bergen Arcs, Norway: an experimental investigation. Full text in Research Archive Show summary

  Field observations from the Bergen Arcs, Norway, demonstrate a network of pseudotachylites quenched under eclogite-facies conditions in mafic granulites. In these nominally anhydrous high-pressure high-temperature (HP/HT) rocks the formation of pseudotachylites, believed to represent fossilized earthquakes, cannot be explained by processes akin to dehydration embrittlement. On the contrary, the transition to eclogite is expected to involve hydration of the initial rock. To experimentally investigate the underlying mechanisms leading to brittle failure in HP/HT rocks, we performed deformation experiments on natural granulite samples from the Bergen Arcs. The experiments were conducted under eclogite-facies conditions (2-3 GPa, 990-1220 K) to trigger the breakdown of plagioclase - the main constituent of granulite. For these experiments, both a D-DIA and a Griggs apparatus were used. The D-DIA press is mounted on a synchrotron beamline, enabling us to monitor strain, stress, and phase changes in-situ while contemporaneously recording acoustic emissions. The Griggs experiments were performed on a new device installed at ENS Paris, in which only stress-strain were recorded, and post-mortem microstructures investigated. The initial material consisted of a fine grain size granulite powder (< 38 µm) composed of mainly plagioclase and minor amount of pyroxene. Hydrous phases are phlogopite and epidote group minerals that make up less than 1 vol. % of the total bulk rock powder plus the adhesion water on grain surfaces. Mechanical data together with XRD observations and the record of acoustic emissions demonstrate a correlation between stress drops, the growth of plagioclase breakdown products and the onset of acoustic emissions during deformation of our specimen within the eclogitic field. Microstructural analysis show remarkable similarities with that of the natural ecoligitic pseudotachylites of the Bergen arcs. The plagioclase decomposition products form narrow conjugated shear bands, along which dissected and displaced crystals are found in the samples. The lack of microstructural evidence for macroscopic brittle failure in our microstructures could be due to plastic overprinting of early brittle structures. Nevertheless, our preliminary experimental results show a strong correlation between strain localization, dynamic fracture propagation (rapid enough to produce AEs) and the eclogitization of granulite.

• Incel, Sarah; Hilaret, Nadege; Labrousse, Loic; John, Timm; Deldicque, Damien & Ferrand, Thomas [Show all 11 contributors for this article] (2016). Laboratory earthquakes triggered during eclogite-facies metamorphic reactions. Full text in Research Archive Show summary

  Laboratory earthquakes triggered during eclogite-facies metamorphic reactions Sarah Incel1, Nadège Hilairet2, Loïc Labrousse3, Timm John4, Damien Deldicque1, Thomas Ferrand1, Yanbin Wang5, Jörg Renner6, Luiz Morales7, Torgeir B. Andersen8, Alexandre Schubnel1 1) Ecole Normale Supérieure, 24 Rue Lhomond, 75005 Paris, France, 2) Université Lille 1, 59655 Villeneuve d'Ascq, France, 3) Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris, France, 4) Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, Germany, 5) GeoSoilEnviroCARS, University of Chicago, Argonne, IL 60439, USA, 6) Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany, 7) GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany, 8) University of Oslo, 0371 Oslo, Norway The origin of intermediate-depth seismicity has been debated for decades. A substantial fraction of these events occur within the upper plane of Wadati-Benioff double seismic zones believed to represent subducting oceanic crust. We deformed natural lawsonite-rich blueschist samples under eclogite-facies conditions (1 < P < 3.5 GPa; 583 K < T < 1121 K), using a D-DIA apparatus installed at a synchrotron beam line continuously monitoring stress, strain, phase content, and acoustic emissions (AEs). Two distinct eclogitization paths were followed: i) a cold path (maximum temperatures of 762 to 927 K), during which lawsonite and glaucophane went gradually unstable at higher pressure; ii) a hot path (maximum temperatures of 1073 to 1121 K) during which the complete breakdown of lawsonite at high temperature was triggered, but glaucophane or amphibole in general remained stable. Brittle failure of the sample, accompanied by the radiation of AEs, occurred for the cold path. In-situ XRD and post-mortem microstructural analysis demonstrate that fractures are topologically related to the growth of omphacite. Amorphous material was detected along the fractures by transmission electron microscopy without evidence for free-water. Since the growth of omphacite is associated with grain-size reduction, we interpret the observed mechanical instability as a transformation-induced thermal runaway under stress (or transformational faulting) triggered during the transition from lawsonite-blueschist to lawsonite-eclogite. In contrast, we find no microstructural evidence that the breakdown of lawsonite, and hence the liberation of water leads to the fracturing of the sample along the hot path, although some AEs were detected during an experiment performed at 1.5 GPa. Our experimental results challenge the concept of “dehydration embrittlement”, which ascribes the genesis of intermediate-depth earthquakes to the breakdown of hydrous phases in the subducting oceanic plate. Instead our results demonstrate that grain-size reduction (transformational faulting) during the transition from lawsonite-blueschist to lawsonite-eclogite leads to the brittle failure of the samples.

• Corfu, Fernando & Andersen, Torgeir Bjørge (2016). Origin of the Gargia Nappe in the northernmost Scandinavian Caledonides: Pre-Caledonian hyperextension of a traditional basement-cover nappe? Full text in Research Archive Show summary

  Hyperextension stretches the lithosphere to the breaking point leading to the exhumation of serpentinized upper mantle peridotites, and the development of sedimentary basins and tectonic melange. The products of such processes are well documented along recent passive margins and in the Alps. Melange defines also a prominent nappe units extending along the southern Scandinavian Caledonides, where it is interpreted as the product of extension during formation of Iapetus (Andersen et al. 2012). In western Finnmark, northern Norway, a melange-type assemblage with km-size serpentinite bodies embedded in various metasedimentary rocks, schists and mylonitic units occurs in the Gargia nappe. The nappe overlies an autochthonous basement suite of Early Paleoproterozoic basalts and associated sedimentary rocks and its Neoproterozic sedimentary cover including a tillite horizon. The Gargia nappe is itself overlain by the Kalak Nappe Complex, in this region composed mainly of psammites, likely deposited around 1000 Ma. The Kalak Nappe Complex contains the record of a multistage tectonic evolution with several episodes of magmatism and metamorphism between 900 and 550 Ma, which paleogeographically do not fit the Archean to Paleoproterozoic evolution of the underlying autochthon of the Baltic Shield. Because of this recent new information, the postulation, in past decades, of an origin of the Kalak Nappe Complex from the Baltic margin has been problematic. The presence of exhumed mantle serpentinite bodies in the underlying Gargia Nappe, however, provides evidence for a truly allochthonus nature of the overlying nappes. The serpentinite mega-boudins of the Gargia Nappe are embedded in mica schists, hornblende schists, rare marbles, and mylonitic gneisses. Preliminary U-Pb geochronology indicates that the protoliths of the mylonitic gneisses are about 2830 Ma and probably were derived from the deformed basement gneisses, likely of Archean age, in the nappe. The time of extension is still uncertain and is presently under investigation. Andersen T.B., Corfu F., Labrousse L. and Osmundsen P.T. 2012 Evidence for hyperextension along the pre-Caledonian margin of Baltica. Journal of the Geological Society, London 169: 601-612

• Andersen, Torgeir Bjørge; Bråstein, Espen & Nesje, Atle (2024). The Geology of the Fjordkysten Geopark: The rise and fall of the Caledonian Mountain belt. Beskrivelse av geologien for søknad om status som UNESCO internasjonal geopark. Fjordkysten Geopark. Full text in Research Archive

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