Hans Jørgen Kjøll

• Senger, Kim; Ammerlaan, Fenna; Betlem, Peter; Gaina, Carmen; Galland, Olivier & Grundvåg, Sten-Andreas [Show all 21 contributors for this article] (2026). A Field Guide to the High Arctic Large Igneous Province in and Around Isfjorden, Svalbard. Tektonika. 3(2), p. 176–216. doi: https:/tektonika.online/index.php/home/article/view/124/197. Full text in Research Archive Show summary

  The High Arctic Large Igneous Province (HALIP) is a major tectono-magmatic element that affected large parts of the Arctic during the Cretaceous (magmatism occurred from 126 to 78 million years ago [Ma]). In Svalbard, HALIP intrusive and extrusive rocks are grouped and classified as the Diabasodden Suite. Sills and dykes are present throughout Svalbard, emplaced in units stratigraphically ranging from crystalline basement to Jurassic-Cretaceous siliciclastics. Time-equivalent lava flows are exposed on Kong Karls Land in eastern Svalbard. In this contribution, we firstly define and introduce major features of the circum-Arctic HALIP. Secondly, we zoom into Svalbard where we document and summarize the attributes of HALIP outcrops (i.e., the Diabasodden Suite) located in and around Isfjorden, the most accessible location that is reachable on day trips using boats from Longyearbyen. For each of the 15 sites we focus on specific aspects of HALIP characterization, including geochronology, geochemistry and geometry of volcanic and intrusive rocks. We complement field descriptions of the sites with digital geoscientific data, including digital outcrop models and photospheres. Finally, we synthesize the Isfjorden study area in the context of data and knowledge gaps for the HALIP in general. We hope that this contribution will stimulate further HALIP research in Isfjorden and beyond.

• Johannesen, Rakul Maria Ingunardóttir; Galland, Olivier; Kjøll, Hans Jørgen; Ólavsdóttir, Jana; Eidesgaard, Óluva & Planke, Sverre (2026). Impact of fractures on basaltic reservoirs - Field characterisation of fracture distribution in lava flows, Faroe Islands. International Journal of Greenhouse Gas Control. ISSN 1750-5836. 151. doi: 10.1016/j.ijggc.2026.104620. Full text in Research Archive
• Kjøll, Hans Jørgen; Scheiber, Thomas & Galland, Olivier (2025). Rapid viscous flow of crustal rocks controls dyke emplacement in the ductile crust. Nature Communications. 17(1). doi: 10.1038/s41467-025-67464-3. Full text in Research Archive Show summary

  Abstract Magmatic dykes are the main pathways for magma through Earth’s crust. They are often assumed to be magma-filled fractures that propagate as thin, tapered sheet intrusions through mode I tensile opening at the fracture tip and elastic bending of the host rock along the dyke walls. Here, we present field evidence from northern Sweden, showing that deep-crustal dyke emplacement was, despite high strain rates, associated with significant ductile deformation of the host rocks. On average, 25% of the dyke thickness was accommodated by ductile flow of the immediate host rock. Modelling magma cooling times suggests average ductile strain rates of 10 −3 s −1 to 10 −6 s −1 , i.e. 6 to 10 orders of magnitude faster than typical tectonic ductile strain rates in the middle crust. These results have major implications for understanding ductile crustal strength and the interpretation of geophysical signals used to mitigate geohazards in volcanically active areas.

• Sartell, Anna Marie Rose; Söderlund, U.; Senger, Kim; Kjøll, H. J. & Galland, Olivier (2025). A Review of the Spatiotemporal Evolution of the High Arctic Large Igneous Province, and a New U‐Pb Age of a Mafic Sill Complex on Svalbard. Geochemistry Geophysics Geosystems. 26(12). doi: 10.1029/2024gc011842. Full text in Research Archive Show summary

  The High Arctic Large Igneous Province (HALIP) formed in the circum‐Arctic during the Cretaceous. The timing and duration of emplacement of these mafic magmas are important for understanding the climatic and environmental effects, yet many uncertainties remain. The dating methods used vary greatly between different regions. For example, the mafic intrusions in Svalbard have mainly been dated using the 40 K/ 40 Ar method, which is more sensitive to overprinting at lower temperatures. This is problematic especially in the Arctic, where the Eocene Eurekan orogeny has impacted the intrusions post‐emplacement. Meanwhile, in the Canadian Arctic, 206 Pb/ 238 U dating on zirconium minerals has been the most common method employed, which requires much higher temperatures to be reset. We present a new compilation of ages for HALIP igneous and volcanic rocks in the circum‐Arctic, derived from a thorough review and reassessment of previously reported data. This compilation applies rigorous, method‐specific criteria to evaluate the reliability of existing HALIP age determinations, ensuring traceability and applicability for future data sets. By establishing a robust framework for assessing age data, this approach enhances the reliability of geological interpretations of HALIP magmatism, and highlights, for example, the spatial migration of peak magmatic activity through time in the High Arctic. To improve our understanding of the temporal evolution of the HALIP, we also present a new 206 Pb/ 238 U baddeleyite isotopic dilution thermal ionization mass spectrometry age from Svalbard. The new weighted mean 206 Pb/ 238 U age from Svalbard, 123.3 ± 1.6 Ma, is based on six samples belonging to one large sill. This age is in perfect agreement with existing published 206 Pb/ 238 U and 40 Ar/ 39 Ar ages, and suggests magma emplacement on Svalbard between 124.7 ± 0.3 and 120.2 ± 1.9 Ma ago.

• Kjøll, Hans Jørgen; Midtkandal, Ivar; Manton, Ben & Planke, Sverre (2025). Seismic Geomorphology of an Early Eocene Canyon‐Channel‐Lobe System North of Shetland. Basin Research. ISSN 0950-091X. 37(3), p. 78–105. doi: 10.1111/bre.70031. Full text in Research Archive Show summary

  Submarine canyon and channel systems are the primary paths for sediment transport from platforms to deep‐sea terminal depositional fans and are present on all continental margins. The temporal morphological expression of such systems should be documented in order to fully understand the development of these important sediment pathways. We utilise a 3D seismic dataset from the northern Shetland Platform to the southern Møre Basin region and map two regionally continuous seismic horizons. The dataset includes an early Eocene northward‐oriented canyon‐channel system that allows high‐resolution mapping of erosion and sedimentation. The morphology of the system allows a subdivision of platform, upper, middle, and lower slope, and basin floor. It is further subdivided from east to west according to the degree of erosive maturity. The platform contains channel shapes and their fill stretching from the south towards the platform edge and upper slope, where dendritic canyon heads developed. Scars in the canyon sidewalls indicate slope failure as an important widening mechanism. The slope canyons progressively shallow and widen basinward. At the canyon exits, deepwater submarine channels developed and continued onto the basin floor. We document progressive filling and overspilling of channels towards the terminal fan. The filling of the channels that resulted in overbank deposition is likely due to a decrease in slope gradient leading to an increased deposition rate that exceeds the channel capacity. Post‐depositional differential compaction of the clastic channel fill and the surrounding muddy sediments causes the channel fills to form elongated ridges that lead towards the terminal fan. It is further shown that the morphological elements observed in the canyon‐channel system adjust towards an equilibrium profile with a shallower gradient. These detailed observations provide an enhanced understanding of how canyon‐channel systems develop on active margins and how sediment transport pathways are influenced by the submarine equilibrium profile and post‐depositional processes.

• Galland, Olivier; Sartell, Anna Marie Rose; Horota, Rafael Kenji; Kjøll, Hans Jørgen; Runge, Jonathan J. S. & Midtkandal, Ivar [Show all 7 contributors for this article] (2025). Stratigraphic Influence on Emplacement and 3‐Dimensional Structure of a Large Mafic Sill in Sedimentary Strata. Basin Research. ISSN 0950-091X. 37(4). doi: 10.1111/bre.70047. Full text in Research Archive Show summary

  Sills are fundamental elements of volcanic plumbing systems emplaced in sedimentary basins. Even though sills are commonly considered simple, planar concordant igneous sheets, they are actually complex 3‐dimensional objects. Detailed knowledge of the 3D structure of sills and their host rocks is of primary relevance to better constrain the emplacement mechanisms and the impacts of sills on sedimentary basins. This study describes the results of 3‐dimensional geological mapping of a large (~14 × 9 km), well‐exposed Early Cretaceous dolerite sill in Central Spitsbergen, Svalbard, Arctic Norway, using a combination of digital outcrop modelling and field mapping. The sill was emplaced within Upper Palaeozoic sedimentary formations of Svalbard. It is made of distinct segments emplaced at different stratigraphic levels of the host rock stratigraphy. The mapping shows a clear stratigraphic control on the intrusion morphology. Sill segments emplaced at the boundary between two formations, which mark a strong lithological and rheological boundary, are straight and concordant. Conversely, segments emplaced within a more homogeneous formation exhibit more irregular, locally discordant shapes. The sill segments emplaced at distinct stratigraphic levels are connected by steeply dipping steps, which formed through dilatant shearing between the tips of the sill segments. The preferred NW‐SE orientation of the steps and the thinning of the sill towards the SE suggests a propagation direction of the magma towards the SE. Our study shows how 3‐dimensional knowledge of igneous intrusions is key for revealing their emplacement mechanisms.

• Kjøll, Hans Jørgen; Midtkandal, Ivar; Planke, Sverre; Millett, John; Manton, Ben & Anderskouv, Kresten (2023). The interplay between siliciclastic and carbonate depositional systems: Maastrichtian to Danian basin-floor sediments of the mid-Norwegian Møre Basin. Basin Research. ISSN 0950-091X. 36(1). doi: 10.1111/bre.12827. Full text in Research Archive Show summary

  Abstract Source‐to‐sink sedimentary systems associated with volcanic rifted margins serve as important archives for basin development by recording lithospheric changes affecting the depositional systems. Distinguishing between sediment transport processes and their sediment source(s) can guide the interpretation of a basin's history, and thereby inform regional paleogeographic reconstructions. In this contribution, we integrate and utilize wireline geophysical logs, detailed petrographic observations from side‐wall cores, and seismic analysis to describe and decipher a Maastrichtian to Danian‐aged basin‐floor depositional system in the deep outer Møre Basin, mid‐Norwegian margin. Well 6302/6‐1 (Tulipan) is a spatially isolated borehole drilled in 2001 that penetrates Maastrichtian and younger strata. A succession of hitherto undescribed carbonates and sandstones in the outer Møre Basin was discovered. It is investigated for sediment transport, provenance, and depositional processes on the basin floor surrounded by structural highs and ridges. The strata from the lower parts form a basin‐floor apron consisting of redeposited carbonate sourced from a westerly sub‐aerial high. The apron transitions vertically from mixed siliciclastic and carbonate into a purely siliciclastic fan with intercalated sandstone and mudstone, providing a rare high‐resolution record of how depositional environments experience a complete shift in dominant processes. The development coincides with similar latest Cretaceous‐earliest Palaeocene sequences recorded south of this region (e.g., well 219/20‐1) and may have been influenced by regional uplift associated with the onset of magmatism in the Northeast Atlantic. This study improves our understanding of a late, pre‐breakup source‐to‐sink sedimentary system developed near the breakup axis of an infant ocean, and documents what is possibly the northernmost chalk deposit in the Chalk Group.

• Rosenqvist, Marija Plahter; Meakins, Max William John; Planke, Sverre; Millett, John M.; Kjøll, Hans Jørgen & Voigt, Martin J. [Show all 7 contributors for this article] (2023). Reservoir properties and reactivity of the Faroe Islands Basalt Group: Investigating the potential for CO<inf>2</inf> storage in the North Atlantic Igneous Province. International Journal of Greenhouse Gas Control. ISSN 1750-5836. 123. doi: 10.1016/j.ijggc.2023.103838. Full text in Research Archive Show summary

  Offshore injection of CO2 into volcanic sequences of the North Atlantic Igneous Province may present a large-scale, permanent storage option through carbonate mineralization. To investigate this potential, onshore studies of reservoir properties and reactivity of the subaerially erupted Faroe Islands Basalt Group have been conducted. Outcrop and borehole samples reveal that the lava flow crusts commonly contain vesicles that have been filled with secondary minerals due to hydrothermal fluid circulation, however, unmineralized and highly porous layers do occur. Bulk density measurements, micro-computed tomography (µ-CT) image analysis, and microscope studies of samples from onshore boreholes give present-day porosities ranging from 0.5% to 36.2% in the volcanic sequences. The unmineralized brecciated lava flow crusts contain the largest estimated porosity and simulated absolute permeability (reaching up to 10−12 m2). µ-CT studies of the mineralized, brecciated flow crusts indicate initial porosities reaching up to 45%, before clogging. Kinetic experiments of rock dissolution show that the reactivity of the basalt and volcaniclastic sediments depends on the alteration state with more altered basalt being less reactive. However, the presence of reactive, high porosity, and high permeability flow crusts prior to clogging indicate the existence of promising and very large CO2 reservoirs in less altered offshore sequences.

• Turino, V.; Magni, Valentina; Kjøll, Hans Jørgen & Jakob, Johannes (2023). The Effect of Magma Poor and Magma Rich Rifted Margins on Continental Collision Dynamics. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 128(12). doi: 10.1029/2023JB027173. Full text in Research Archive Show summary

  Abstract The transition between non‐rifted continental lithosphere and oceanic lithosphere in rifted margins can display a wide range of characteristics, depending on the regional tectonic evolution. The velocity and duration of the rifting process as well as the geodynamic setting influence the properties and geometry of the margins, which are often grouped into two main categories: magma‐poor and magma‐rich. We show how different types of rifted margins can influence the dynamics of continental collision, focusing on the time and depth of slab break‐off after collision and the fate of margin material. We find that rifted margins have a noticeable impact on subduction dynamics, as we observe large variability in slab break‐off times and depths. In particular, the presence of a rifted margin can delay slab break‐off to up to 60 Myr after the onset of collision. Our results show that a large portion of the weak crust of magma‐poor margins is likely to detach from the subducting plate and accrete to the upper plate, while the dense and strong mafic and ultramafic component of magma‐rich margins causes most of the margin to subduct and be lost into the mantle, leaving only a small fraction of transitional and oceanic crust at the surface. Therefore, the volume of accreted material is much larger when the margin is magma‐poor than magma‐rich, which is consistent with geological observations that fossil magma‐poor rifted margins are preserved in many mountain ranges, whereas remnants of magma‐rich rifted margins are scarce.

• 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.

• Kjøll, Hans Jørgen (2020). Late Neoproterozoic basin evolution of the magma rich Iapetus margin of Baltica. Norwegian Journal of Geology. ISSN 2387-5844. 100(1), p. 1–32. doi: 10.17850/njg100-1-6. Full text in Research Archive Show summary

  The Särv and Seve nappe complexes (NC) in the central and northern Scandinavian Caledonides locally display well-preserved, mafic dyke-intruded, sedimentary successions commonly interpreted to have been deposited in sag- to rift basins along the margin of the Iapetus Ocean. The sedimentary successions are generally interpreted to have been deposited prior to or during the Late Neoproterozoic opening of the Iapetus Ocean. They were later incorporated into the Scandinavian Caledonides during the Silurian-aged Scandian orogeny. Whereas the minimum depositional age is constrained by the dated mafic dyke swarm at c. 596?608 Ma, a maximum depositional age for the sedimentary successions is poorly constrained. No fossils or diamictite units have hitherto been reported from the sedimentary successions found in the Seve NC. This contribution presents new geological observations and geochronological data from allochthonous, dyke-intruded, rift-related basins. Key elements in the sedimentary succession, such as carbonates with meta-evaporitic domains, diamictite and stromatolites are described from the Särv and Seve NC. Evidence presented here suggests that the diamictite is of glaciogenic origin. It is cut by the dykes and is therefore older than 608 Ma and could be related to the Marinoan or Sturtian glaciations. The stromatolite resembles Eleonora laponica and is found below the diamictite. Two field areas have young detrital zircons of c. 700?750 Ma providing possible upper maximum depositional ages and thus bracket the deposition within a c. 100 M.yr. time interval between 700 and 608. This is corroborated by a Palaeoproterozoic orthogneiss with a 631 Ma Pb-loss event, possibly reflecting the development of a top-west, ductile extensional shear fabric and frictional co seismic deformation. This extensional event generated accommodation space at the surface and thus basin formation. Similarities between the separate basins, like the highly dyke-intruded nature, the similarities in contact and regional metamorphism as well as the detrital zircon age distributions, suggest that the basins are related.

• 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.

• 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.

• 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.

• Kjøll, Hans Jørgen; Viola, Giulio; Menegon, Luca & Sørensen, Bjørn Eske (2015). Brittle-viscous deformation of vein quartz under fluid-rich lower greenschist facies conditions. Solid Earth (SE). ISSN 1869-9510. 6(2), p. 681–699. doi: 10.5194/se-6-681-2015. Full text in Research Archive Show summary

  We studied by Electron BackScatter Diffraction (EBSD) and optical microscopy a coarse-grained (ca. 0.5–6 mm) quartz vein embedded in a phyllonitic matrix to gain insights into the recrystallization mechanisms and the processes of strain localization in quartz deformed under lower greenschist facies conditions, broadly coincident with the brittle–viscous transition. The vein deformed during faulting along a phyllonitic thrust of Caledonian age within the Porsa Imbricate Stack in the Paleoproterozoic Repparfjord Tectonic Window in northern Norway. The phyllonite hosting the vein formed at the expense of a metabasaltic protolith through feldspar breakdown to form interconnected layers of fine, synkinematic phyllosilicates. In the mechanically weak framework of the phyllonite, the quartz vein acted as a relatively rigid body. Viscous deformation in the vein was initially accommodated by quartz basal slip. Under the prevailing deformation conditions, however, dislocation glide- and possibly creep-accommodated deformation of quartz was inefficient, and this resulted in localized strain hardening. In response to the (1) hardening, (2) progressive and cyclic increase of the fluid pressure, and (3) increasing competence contrast between the vein and the weakly foliated host phyllonite, vein quartz crystals began to deform by brittle processes along specific, suitably oriented lattice planes, creating microgouges along microfractures. Nucleated new grains rapidly sealed these fractures as fluids penetrated the actively deforming system. The grains grew initially by solution precipitation and later by grain boundary migration. We suggest that the different initial orientation of the vein crystals led to strain accommodation by different mechanisms in the individual crystals, generating remarkably different microstructures. Crystals suitably oriented for basal slip, for example, accommodated strain mainly viscously and experienced only minor fracturing. Instead, crystals misoriented for basal slip hardened and deformed predominantly by domainal fracturing. This study indicates the importance of considering shear zones as dynamic systems wherein the activated deformation mechanisms may vary through time in response to the complex temporal and spatial evolution of the shear zone, often in a cyclic fashion.

View all works in NVA

• Kjøll, Hans Jørgen & Kaste, Anders Moen (2026). Derfor er steinen i kong Haralds sarkofag så spesiell. [Internet]. forskning.no. Full text in Research Archive Show summary

  Kommer den til å holde på fargen over tid?

• Kjøll, Hans Jørgen & Grosvold, Roger (2026). Vulkanfrykt etter jordskjelv: - Urovekkende. [Journal]. www.dagbladet.no. Show summary

  Fredagens kraftige jordskjelv ved Napoli skjedde i et av Europas farligste vulkanområder. - Følgene av et massivt utbrudd er rett og slett katastrofale, sier norsk ekspert.

• Tjoflot, Gunn Kristin; Kjøll, Hans Jørgen; Galland, Olivier & Scheiber, Thomas (2026). The pressure from magma can fold solid rock in less than one day. [Internet]. partner.sciencenorway.no. doi: https:/partner.sciencenorway.no/geology-mineralogy-natural-sciences/the-pressure-from-magma-can-fold-solid-rock-in-less-than-one-day/2658836. Full text in Research Archive Show summary

  The consequences of volcanic eruptions can be severe. Researchers have now made a surprising discovery. They found the evidence on a mountainside.

• Tjoflot, Gunn Kristin; Kjøll, Hans Jørgen; Galland, Olivier & Scheiber, Thomas (2026). Magmatrykket kan folde solid fjell på under én dag. [Internet]. forskning.no. doi: https:/www.forskning.no/geofag-geologi-partner/magmatrykket-kan-folde-solid-fjell-pa-under-en-dag/2659015. Full text in Research Archive Show summary

  Konsekvensene av vulkanutbrudd kan være enorme. Nå har UiO-forskere gjort en overraskende oppdagelse. Beviset fant de i en fjellside.

• Kjøll, Hans Jørgen (2026). Hvor kommer lyden fra i et jordskjelv? [Internet]. forskning.no. Full text in Research Archive Show summary

  Jordskjelvet på Østlandet søndag den 27. april 2026 traff klokken 9.25 nær Jessheim, bare noen mil fra Oslo, ifølge jordskjelvovervåkerne i NORSAR. Mange beskriver en voldsom lyd over et stort område etter jordskjelvet på Østlandet. Dette er nær området i Norge som er tettest befolket, så svært mange mennesker både følte og hørte lyden.

• Kjøll, Hans Jørgen; Scheiber, Thomas & Galland, Olivier (2026). Magma på vei mot overflaten knøvler fjell - Institutt for geofag. Full text in Research Archive Show summary

  Vulkanutbrudd viser naturens rå krefter, fra historiske kjempeutbrudd til ferske lavastrømmer på Island. En ny studie fra UiO-forskere i prosjektet «Beyond Elasticity» ved Institutt for geofag og Njord-senteret gir ny kunnskap om de enorme kreftene som kan oppstå dypt nede i jordskorpen.

• Kjøll, Hans Jørgen; Scheiber, Thomas & Galland, Olivier (2026). Rapid viscous flow of crustal rocks controls dyke emplacement in the ductile crust. doi: 10.5194/egusphere-egu26-19107. Full text in Research Archive Show summary

  In the upper crust, dykes are commonly assumed to propagate as elastic fractures, exhibiting thin, tapered geometries, and propagating through mode I, tensile opening at the fracture tip. In the lower ductile crust, which is often assumed to have a Maxwell-type rheology, the fast strain rates associated with dyke emplacement are thought to embrittle the host rock. This reasoning has led to the assumption that ductile deformation of the host rock is negligible during dyke emplacement in the deep crust. In Sarek National Park, northern Sweden, a dyke complex was emplaced during a ca 608 Ma continental rifting event at depths of 10–15 km and temperatures reaching ca 650 °C. The dyke swarm was emplaced into carbonate and sandstone host rocks. Detailed field observations from glacially polished outcrops demonstrate that significant ductile deformation of the host rock accommodated dyke emplacement. We quantify that approximately 25% of the dyke thickness is accommodated by ductile folding of the host rock. Thermal modelling is used to estimate magma crystallization times, which in turn allow estimating ductile strain rates on the order of 10-3 to 10-6 s-1. These strain rates are 6 to 10 orders of magnitude higher than typical tectonic ductile strain rates in the middle crust (10-12 - 10-15 s-1). Furthermore, we document how the weak rheology of the host rock influenced the shape and geometry of the mafic dykes. These results have implications for our understanding of dyke emplacement as well as general deformation and rock strength in the ductile crust, which constitute a significant part of the pathway for magma to reach the surface.

• Turino, Valeria; Magni, Valentina; Kjøll, Hans Jørgen & Jakob, Johannes (2026). The effect of magma poor and magma rich rifted margins on continental collision dynamics. doi: 10.5194/egusphere-egu26-13121. Full text in Research Archive Show summary

  Rifted margins form when continents rift apart and are commonly characterized by a thinned transition zone between the continental crust and the oceanic crust. This transition zone can display a wide range of characteristics, which primarily depend on the regional tectonic evolution. The velocity and duration of the rifting process as well as the geodynamic setting influence the properties and geometry of the margins, which are often grouped into two main categories: magma-poor and magma-rich. Magma-rich margins are characterized by large input of mafic melt, while magma-poor margins are characterized by much less magma production during the rifting process, resulting in variations in geometry and rheology of rifted margins worldwide.Using the finite elements code Citcom, we show how different types of rifted margins can influence the dynamics of continental collision, focusing on the time and depth of slab break-off after collision and the fate of margin material. We compared these models as a function of various parameters (e.g., margin length, density, and viscosity), in order to understand how the architecture of a passive margin affects the dynamics of continental collision.We find that rifted margins have a noticeable impact on subduction dynamics, as we observe large variability in slab break-off times and depths. In particular, the presence of a rifted margin can delay slab break-off to up to 60 Myr after the onset of collision. Our results show that a large portion of the weak crust of magma-poor margins is likely to detach from the subducting plate and accrete to the upper plate, while the dense and strong mafic and ultramafic component of magma-rich margins causes most of the margin to subduct and be lost into the mantle, leaving only a small fraction of transitional and oceanic crust at the surface. Therefore, the volume of accreted material is much larger when the margin is magma-poor than magma-rich, which is consistent with geological observations that fossil magma-poor rifted margins are preserved in many mountain ranges, whereas remnants of magma-rich rifted margins are scarce.Importantly, our results show that rifted margin type controls the architecture of the subsequent collisional phase of the Wilson cycle.

• Kjøll, Hans Jørgen; Galland, Olivier & Scheiber, Thomas (2025). Fast ductile deformation during magma emplacement – Evidence for inelastic deformation of host rock surrounding dykes. Full text in Research Archive Show summary

  In the upper crust, dykes and are often assumed to propagate as elastic fractures exhibiting thin, tapered sheet shapes, and propagating through mode I, tensile opening at the fracture tip. In the lower ductile crust, which is often assumed to have a Maxwell-type rheology, the fast strain rates associated with dyke emplacement are envisaged to embrittle the host rock. This argument has been used to assume that ductile deformation of the host rock is negligible during dyke emplacement in the deep crust. However, recent work we conducted in Sarek national park, northern Sweden, describe spectacular outcrops of dykes emplaced in the ductile crust. Field observations show that significant ductile deformation of the host rock has accommodated the emplacement of the dykes. We quantify that c. 15% of the dyke thickness is accommodated by ductile folding of the host rock. Thermal modelling is used to estimate crystallization time for the magma, which allows estimating ductile strain rates on the order of 10-8 s-1. This strain rate estimate is orders of magnitude higher than typical tectonic ductile strain rates (10-12 - 10-15 s-1) in the middle crust. Furthermore, we document how the weak rheology of the host rock influenced the shape and geometry of the mafic dykes. These new results have implications for how we envisage dyke emplacement as well as general deformation and rock strength in the ductile crust, which constitute a significant part of the pathway for magma to reach the surface.

• Kjøll, Hans Jørgen; Galland, Olivier & Scheiber, Thomas (2025). Fast ductile rock deformation accommodating dyke emplacement in the middle crust. Full text in Research Archive Show summary

  Growth of magmatic sheet intrusions is generally assumed to occur through initial tensile fracturing and elastic bending of the host rock resulting in intrusion geometries characterized by straight segments and sharply tapered tips. In addition, most models assume that rates of dyke emplacement are too fast to be accommodated by ductile flow of the host rock, even at mid-crustal levels. There is, however, increasing field evidence that suggests that inelastic processes such as plastic deformation and/or partial melting of the host rock may play an important role in emplacement dynamics. We have studied a mafic dyke complex emplaced in the middle crust of an Ediacaran-aged magma-rich rifted margin in Northern Sweden. The host rocks in the field area are composed primarily of dolostone intercalated with silicic beds. The peak ambient temperature during dyke emplacement was approximately 500-700°C. Local partial melting of the host rock occurred contemporaneously with emplacement of the mafic dykes. Field observations show that the host rock beds are tightly folded (up to 30% shortening) close to the contacts of the surrounding dykes, and strain, i.e., the wavelength of the folds, decreases rapidly with increasing distance from the dykes. The axial planes of the folds next to the contacts are sub-parallel to the dyke contacts themselves, suggesting that the folds formed as a consequence of dyke emplacement and inflation. Estimates of dykes cooling times suggest that the magma reached its solidus temperature within one year. Our results suggest that the strain rate accommodating dyke emplacement was very rapid (on the order of 10-7 s-1) compared to typical tectonic strain rate estimates (ca 10-12 - 10-15s-1). Our data show that inelastic deformation is an important aspect during dyke emplacement in the middle crust, and that fast strain rates are possible.

• Johannesen, Rakul Maria Ingunardóttir; Galland, Olivier; Ólavsdóttir, Jana; Kjøll, Hans Jørgen; Eidesgaard, Óluva & Planke, Sverre (2025). Fractured basalts as reservoirs for permanent CO2 storage on the Faroe Islands. doi: 10.5194/egusphere-egu25-12801. Full text in Research Archive Show summary

  During the last decade, the pilot CarbFix project in the Iceland rift zone has provided highly promising results for permanently storing CO2 as carbonates in basalts. A key question is whether this method can be applied in other basaltic formations worldwide where the rocks are older, the porosities are lower, the chemical compositions are different, and/or the temperatures are lower. Storing CO2 in the subsurface necessitates injectivity, where two essential properties are permeability and porosity. In basaltic formations, porous volumes are typically found as vesicle zones, near lava flow boundaries and in flow tops, or fractures. Currently, it appears that the permeability necessary for injecting fluids in basalts is dominated by fractures. The Faroe Islands, in the North Atlantic, consist of layered basalts of Paleogene age. The aim of this study is to examine the potential of the Faroe Island Basalt Group (FIBG) as a CO2 reservoir. To evaluate the reservoir properties of the FIBG we interpret and integrate new and existing data at multiple scales, including satellite images, UAV photogrammetric surveys, and field mapping. We have determined the distribution of large-scale lineaments (faults and dykes), flow-scale fractures, and the interaction between them in three dimensions. Here, we focus on the Malinstindur Formation, a compound basalt sequence, which is a potential pilot injection sequence because of its permeability potential. Large-scale mapping of the FIBG volcanostratigraphy is used to map the depth of the Malinstindur Formation and to determine the presence of large strike-slip faults across the archipelago. In addition, large-scale structural lineaments exhibiting a preferred EW-trending orientation are mapped. In addition, fracture analyses on different scales (from islands to outcrops), have been conducted to investigate how lava flow architectures, regional stress, and faults control fracture distribution and connectivity. The results of these analyses show that (1) the fracture density decreases with flow thickness, (2) the orientation of the internal fractures follows the same as those of the sets of regional scale lineaments, and (3) there is no link between the fracture intensity in the basalts and the distance to the regional scale lineaments. This study will serve as a foundation for determining the subsurface distribution of the Malinstindur Formation and the fracture distribution and connectivity at potential injection sites in order to estimate the potential for permanent CO2 storage within the Faroe Islands.

• Galland, Olivier; Kjøll, Hans Jørgen & Scheiber, Thomas (2024). Beyond Elasticity project - Fieldwork 2024 in Sarek National Park, northern Sweden. [Internet]. YouTube. doi: https:/youtu.be/3dpzyvERjqo. Full text in Research Archive Show summary

  August 2024, Prof. Olivier Galland, Drs. Hans Jørgen Kjøll and Thomas Scheiber conducted a geological field expedition at Sarek National Park, northern Sweden. The expedition was funded through the "Beyond Elasticity" project, a large research project funded through the very competitive FRIPRO program, Research Council of Norway. The aspiration of "Beyond Elasticity" is to revisit our understanding of propagation and emplacement of magmatic sheet intrusions, such as dykes, sills and cone sheets. The primary objective of this project is to establish novel mechanical understanding of dyke and sill emplacement that reconciles the distinct magma emplacement models. We aim to show that, and quantify how much, inelastic deformation contributes to dyke and sill propagation and emplacement. The field work is one part of this multidisciplinary project, which integrates laboratory modelling, numerical modelling and theory, and geophysics. The project gathers scientists from the Departments of Geosciences, Physics and Mathematics at the University of Oslo, as well as scientists from Iceland, Sweden, and UK. Music credits: Song: Shine On Music by: CreatorMix.com

• Kjøll, Hans Jørgen (2024). Bulldozering dykes in the ductile crust – Observations from Sarek national park in northern Sweden. Full text in Research Archive Show summary

  For a volcano to erupt, magma must propagate through the earth’ crust from a magma reservoir at depth to the surface. The most fundamental pathway for transport of magma is through sheet intrusions referred to as dykes. These sheet intrusions are generally thought to propagate like a fracture by tensile opening of their tips and elastic bending of the host rock. A growing number of observations in the geological rock record, however, suggest that other mechanisms contribute to how sheet intrusions are emplaced. One such mechanism assumes that the tip of the sheet intrusion pushes the host rock, which in turn deforms by inelastic processes. This mechanism appears to be particularly important where the host rock is weak. To document the contribution from inelastic processes during dyke emplacement we conducted fieldwork in Sarek national park in Sweden, which displays a now eroded deep section of a rifted Ocean-Continental transition intruded by hundreds of dykes. This presentation will show observations from fresh outcrops in high alpine glacial cirques, which provide nearly 100% exposure over relatively large distances of the dyke complex and its host rock. The contact between the sheet intrusions and the host has been mapped in detail at several localities and for different types of host rock and show that the host rock is locally deformed in a manner which is non-compatible with large scale regional deformation trends.

• Kjøll, Hans Jørgen (2024). Bulldozering dykes in the ductile crust. Full text in Research Archive
• Svensen, Henrik Hovland; Callegaro, Sara; Kjøll, Hans Jørgen; Midtkandal, Ivar; Whattam, Jack William & Dalslåen, Bjørgunn Heggem [Show all 10 contributors for this article] (2024). Rhomb porphyry lavas from the Oslo Rift revisited: New insights from construction-related boreholes and cores. Full text in Research Archive
• Callegaro, Sara; Svensen, Henrik Hovland; Heimdal, Thea H.; Kjøll, Hans Jørgen; Harstad, Andreas Olaus & Augland, Lars Eivind [Show all 9 contributors for this article] (2024). Geochemistry of sills from the early stage of the Oslo Rift. Full text in Research Archive
• Sartell, Anna Marie Rose; Beier, Christoph P.; Söderlund, Ulf; Senger, Kim; Shephard, Grace & Kjøll, Hans Jørgen [Show all 7 contributors for this article] (2024). Geochemistry and geochronology of the High Arctic Large Igneous Province on Svalbard. Full text in Research Archive
• Nygaard, Madeleine Skov; Planke, Sverre; Berndt, Christian; Zastrozhnov, Dmitry; Faleide, Jan Inge & Jones, Morgan Thomas [Show all 24 contributors for this article] (2024). PALMAR – Paleogene Basin Development on the Vøring and Møre Margins. Full text in Research Archive
• Nygaard, Madeleine Skov; Planke, Sverre; Berndt, Christian; Zastrozhnov, Dmitry; Faleide, Jan Inge & Jones, Morgan Thomas [Show all 24 contributors for this article] (2024). IODP Exp. 396 Drilling and High-Resolution 3D Seismic on the Mimir High. 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

• Callegaro, Sara; Svensen, Henrik Hovland; Heimdal, Thea Hatlen; Kjøll, Hans Jørgen; Harstad, Andreas Olaus & Neumann, Else Ragnhild (2022). New insights on shallow intrusions from the early stage of the Oslo Rift. 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.

• Kjøll, Hans Jørgen (2020). Langs dette fjellet går geologene rett ned i jordas indre. [Internet]. forskning.no. Full text in Research Archive Show summary

  Når geologene går bortover Pårte-fjellet i Sarek nasjonalpark i Nord-Sverige, beveger de seg samtidig nedover og bakover i jordskorpas historie. Det gir enkel tilgang til dype innsikter om havenes livssyklus. Forskningen i dette spennende området har vært fokus i doktoravhandlingen til Hans Jørgen Kjøll ved Institutt for geofag, Universitetet i Oslo og CEED-senteret.

• 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
• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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
• 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?

• 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; 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
• Fellman, Tuva & Kjøll, Hans Jørgen (2016). Jordskjelv. [Radio]. NRK P3. Full text in Research Archive
• Kjøll, Hans Jørgen (2016). Modums geologiske historie. Full text in Research Archive
• Andersen, Torgeir Bjørge; Jakob, Johannes; Kjøll, Hans Jørgen; Corfu, Fernando; Tegner, Christian & Alsaif, Manar [Show all 8 contributors for this article] (2016). Hyperextension, micro-continents, magma-poor and magma-rich segments in the pre-Caledonian margin of Baltica: research in progress. Full text in Research Archive
• Kjøll, Hans Jørgen (2016). Dette er de fem vakreste bergartene. [Journal]. https://titan.uio.no/node/1773. Full text in Research Archive
• Kjøll, Hans Jørgen; Andersen, Torgeir Bjørge; Tegner, Christian; Corfu, Fernando & Planke, Sverre (2016). Wilson-cycle “kick-off”: Constraining the influence of a Large Igneous Province during the Neoproterozoic evolution of the pre-Caledonian margin of Baltica. Full text in Research Archive
• Tegner, Christian; Andersen, Torgeir Bjørge; Corfu, Fernando; Planke, Sverre; Kjøll, Hans Jørgen & Torsvik, Trond Helge (2016). The pre-Caledonian Large Igneous Province and the North Atlantic Wilson Cycle. Full text in Research Archive
• Kjøll, Hans Jørgen; Andersen, Torgeir Bjørge; Corfu, Fernando & Tegner, Christian (2016). Constraining the influence of a LIP on the Neoproterozoic break-up of Baltica and Laurentia. Full text in Research Archive
• Kjøll, Hans Jørgen; Torgersen, Espen & Viola, Giulio (2014). Caledonian shortening by combined folding and thrusting in the immediate footwall of the Caledonian sole thrust: The example of the Repparfjord Tectonic Window, northern Norway. Full text in Research Archive Show summary

  The Repparfjord Tectonic Window (RTW) is a window through the Caledonian nappes in northern Norway that exposes a package of greenschist facies metasedimentary and metavolcanic rocks of Paleoproterozoic age. These were deformed and metamorphosed during the Svecofennian orogeny producing km-wavelength upright NE-SW folds. Pervasive effects of a later Caledonian overprint, caused by the emplacement of the Kalak Nappe Complex during the Silurian, are limited and confined to the northwestern edge of the window, where NW-SE shortening caused the development of a compressional imbricate stack. Individual imbricates exploit preexisting, progressively tightening upright to overturned folds and are bound by generally very steep to sub-vertical discrete faults. One of these structures, the Skinnfjellet Fault Zone (SFZ), truncates the large doloarenite-hosted Nussir Cu deposit and has a present-day orientation that makes reverse displacement mechanically difficult. This study aims at a better understanding of the mechanical and temporal evolution of these steep thrusts. The SFZ strikes roughly NS, is sub-vertical and bears dip-slip lineations. It separates greenstones in the west from arcosic sandstones, conglomerates and the Nussir Cu deposit in the east. Kinematic indicators give east block up. Faulting occurred mostly under brittle conditions producing an approximately eight meter thick damage zone and a 40 cm thick fault gouge core. A second prominent fault is the Nussirjavrri Fault Zone (NFZ). The main fault plane dips moderately toward the NNE, bears a NW plunging lineation and a number of kinematic indicators indicate top-to-the-SSE thrusting. The fault trace is mappable for c. one km, but high-resolution geophysics indicates an ENE-WSW continuation. Mapping shows that the fault zone is folded openly with a fold axis trending NNE/SSW, consistent with the geometry of a subregional folding phase of inferred Caledonian age. The fault affects greenstones, graphitic slates and dolomites. The mylonitic thrust core thickness varies from 10 to 40 cm and is composed by alternating dark and light bands of chlorite, muscovite and graphite, together with quartz and carbonates. Interspersed within the foliation are interstitial euhedral pyrite and cm to dm scale dolomite clasts. Dolomite decarbonation is locally observed. Synkinematic quartz veins occur subparallel to the tectonic foliation. Quartz is dynamically recrystallized by SGRR and has an average grain size of c. 35um. Thin sections show two distinct and strong LPO’s. One is preserved within the core grains whereas the other is found within arrays of the recrystallized subgrains, whose distribution appears to be controlled by healed brittle fractures. Mapping indicates that the NFZ is cut by the SFZ and its overall transport direction to the SSE, in addition to its structural location at the front of the imbricate, strengthen its interpretation as a Caledonian structure (also consistent with dated and similarly oriented faults from the RTW). We suggest that the SFZ formed as an antithetic back thrust within the Caledonian imbricate stack, possibly exploiting the limbs of a preexisting large-scale antiform. During Caledonian shortening this Paleoproterozoic megafold was tightened, leading to a progressive steepening of the fold limbs and of the SFZ, while NFZ was progressively folded around the fold hinge.

• Kjøll, Hans Jørgen & Viola, Giulio (2015). Structural Evolution of the Porsa Imbricate Stack (Finnmark, northern Norway) - Based on field mapping and microstructural analysis. Norges teknisk-naturvitenskapelige universitet. Full text in Research Archive Show summary

  This thesis reports the results from detailed lithological and structural mapping of the northwestern part of the RTW, which consists of a large-scale compressional imbricate stack that formed during the Caledonian Orogeny. Detailed mesoscopic and microstructural analysis of fault rocks reveal that several different processes facilitated strain localization within the stack. Early Caledonian NW SE shortening caused tightening of inherited NE SW-trending Paleoproterozic folds. Flexural slip led to ingress of fluids that promoted alteration of the host rocks and the production of phyllosilicates through extensive phyllonitization processes leading to strain localization and the formation of thrusts parallel to the folds limbs. Further deformation was accommodated by localized slip along these thrusts. A weak layer consisting of metavolcanic tuffs may have acted as a relatively strong décollement promoting the formation of primarily foreland verging thrusts. At late stages of PIS formation, a shift in deformation mechanisms caused folding of the PIS thrusts. Extensive normal reactivation of the inherited thrusts occurred during two phases of extension during the Paleozoic and the Mesozoic. Microstructural investigations by optical microscopy and Electron Backscatter Diffraction (EBSD) of a pre- to syntectonic quartz vein deformed within the Nussirjávri Fault Zone are presented. Viscous deformation in the vein was initially accommodated by quartz basal slip. Under the prevailing deformation conditions, however, dislocation glide- and possibly creep-accommodated deformation of quartz was inefficient, and this resulted in localized strain hardening. In response to the 1) hardening, 2) progressive and cyclic increase of the fluid pressure, and 3) increasing competence contrast between the vein and the weakly foliated host phyllonite, vein quartz crystals began to deform by brittle processes along specific, suitably oriented lattice planes, creating microgouges along microfractures. Nucleated new grains rapidly sealed these fractures as fluids penetrated the actively deforming system. The grains grew initially by solution-precipitation and later by grain boundary migration. We suggest that the different initial orientation of the vein crystals led to strain accommodation by different mechanisms in the individual crystals, generating remarkably different microstructures. Crystals suitably oriented for basal slip, for example, accommodated strain mainly viscously and experienced only minor fracturing. Instead, crystals misoriented for basal slip hardened and deformed predominantly by domainal fracturing.

View all works in NVA