Unravelling the spatio-temporal nature of rock deformation using 4D X-ray tomography

• McBeck, Jessica Ann; Mair, Karen & Renard, Francois (2019). Linking macroscopic failure with micromechanical processes in layered rocks: How layer orientation and roughness control macroscopic behavior. Tectonophysics. ISSN 0040-1951. 750, p. 229–242. doi: 10.1016/j.tecto.2018.11.016. Full text in Research Archive Show summary

  To constrain the impact of preexisting mechanical weaknesses on strain localization culminating in macroscopic shear failure, we simulate triaxial compression of layered sedimentary rock using three-dimensional discrete element method simulations. We develop a novel particle packing technique that builds layered rocks with preexisting weaknesses of varying orientations, roughness, and surface area available for slip. We quantify how the geomechanical behavior, characterized by internal friction coefficient, μ0, and failure strength, σF, vary as a function of layer orientation, θ, interface roughness, and total interface area. Failure of the simulated sedimentary rocks mirrors key observations from laboratory experiments on layered sedimentary rock, including minima σF and μ0 for layers oriented at 30° with respect to the maximum compressive stress, σ1, and maxima σF and μ0 for layers oriented near 0° and 90° to σ1. The largest changes in σF (66%) and μ0 (20%) occur in models with the smoothest interfaces and largest interface area. Within the parameter space tested, layer orientation exerts the most significant impact on σF and μ0. These simulations allow directly linking micromechanical processes observed within the models to macroscopic failure behavior. The spatial distributions of nucleating microfractures, and the rate and degree of strain localization onto preexisting weaknesses, rather than the host rock, are systematically linked to the distribution of failure strengths. Preexisting weakness orientation more strongly controls the degree and rate of strain localization than the imposed confining stress within the explored parameter space. Using the upper and lower limits of μ0 and σF obtained from the models, estimates of the Coulomb shear stress required for failure of intact rock within the upper seismogenic zone (7 km) indicates that a rotation of 30° of σ1 relative to the weakness orientation may reduce the shear stress required for failure by up to 100 MPa.

• Petley-Ragan, Arianne Juliette; Ben-Zion, Yehuda; Austrheim, Håkon Olaf; Ildefonse, Benoit; Renard, Francois & Jamtveit, Bjørn (2019). Dynamic earthquake rupture in the lower crust. Science Advances. 5(7). doi: 10.1126/sciadv.aaw0913. Full text in Research Archive Show summary

  Earthquakes in the continental crust commonly occur in the upper 15 to 20 km. Recent studies demonstrate that earthquakes also occur in the lower crust of collision zones and play a key role in metamorphic processes that modify its physical properties. However, details of the failure process and sequence of events that lead to seismic slip in the lower crust remain uncertain. Here, we present observations of a fault zone from the Bergen Arcs, western Norway, which constrain the deformation processes of lower crustal earthquakes. We show that seismic slip and associated melting are preceded by fracturing, asymmetric fragmentation, and comminution of the wall rock caused by a dynamically propagating rupture. The succession of deformation processes reported here emphasize brittle failure mechanisms in a portion of the crust that until recently was assumed to be characterized by ductile deformation.

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

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

• McBeck, Jessica Ann; Cordonnier, Benoit; Vinciguerra, Sergio & Renard, Francois (2019). Volumetric and Shear Strain Localization in Mt. Etna Basalt. Geophysical Research Letters. ISSN 0094-8276. 46(5), p. 2425–2433. doi: 10.1029/2018GL081299. Full text in Research Archive Show summary

  To examine the impact of preexisting weaknesses on fracture coalescence during volcanic edifice deformation, we triaxially compressed Mount Etna basalt while acquiring in situ dynamic X‐ray microtomograms and calculated the internal strain tensor fields using image correlation. Contraction localization preceded dilation and shear strain localization into the protofault zone. This onset of strain localization preceded macroscopic yielding and coincided with increases in the magnitude and volume of rock experiencing dilation, and spatial clustering of the strain populations. The exploitation of weaknesses by propagating fractures enabled the dominant shear strain to switch senses as propagating fractures lengthened along 30–60° from σ1. Scanning electron microscopy images reveal pore‐emanated fractures, and fractures linking pores. These experiments provide evidence of internal contraction preceding dilation and shear, consistent with inferences from field and laboratory observations. The transition from contraction to dilation may provide a precursory signal of volcanic flank eruption.

• Jamtveit, Bjørn; Petley-Ragan, Arianne Juliette; Incel, Sarah; Dunkel, Kristina G; Aupart, Claire Olga Maryse & Austrheim, Håkon Olaf [Show all 9 contributors for this article] (2019). The effects of earthquakes and fluids on the metamorphism of the lower continental crust. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 124(8), p. 7725–7755. doi: 10.1029/2018JB016461. Full text in Research Archive Show summary

  Rock rheology and density have first‐order effects on the lithosphere's response to plate tectonic forces at plate boundaries. Changes in these rock properties are controlled by metamorphic transformation processes that are critically dependent on the presence of fluids. At the onset of a continental collision, the lower crust is in most cases dry and strong. However, if exposed to internally produced or externally supplied fluids, the thickened crust will react and be converted into a mechanically weaker lithology by fluid‐driven metamorphic reactions. Fluid introduction is often associated with deep crustal earthquakes. Microstructural evidence, suggest that in strong highly stressed rocks, seismic slip may be initiated by brittle deformation and that wall‐rock damage caused by dynamic ruptures plays a very important role in allowing fluids to enter into contact with dry and highly reactive lower crustal rocks. The resulting metamorphism produces weaker rocks which subsequently deform by viscous creep. Volumes of weak rocks contained in a highly stressed environment of strong rocks may experience significant excursions toward higher pressure without any associated burial. Slow and highly localized creep processes in a velocity strengthening regime may produce mylonitic shear zones along faults initially characterized by earthquake‐generated frictional melting and wall rock damage. However, stress pulses from earthquakes in the shallower brittle regime may kick start new episodes of seismic slip at velocity weakening conditions. These processes indicate that the evolution of the lower crust during continental collisions is controlled by the transient interplay between brittle deformation, fluid‐rock interactions, and creep flow.

• Huang, Lingcao; Baud, Patrick; Cordonnier, Benoit; Renard, Francois; Liu, Lin & Wong, Teng-fong (2019). Synchrotron X-ray imaging in 4D: Multiscale failure and compaction localization in triaxially compressed porous limestone. Earth and Planetary Science Letters. ISSN 0012-821X. 528. doi: 10.1016/j.epsl.2019.115831. Full text in Research Archive Show summary

  Understanding failure and strain localization in porous rock is of fundamental importance in rock physics. Confined compaction experiments on porous rocks have revealed a broad spectrum of failure modes. Techniques such as acoustic emission location and velocity tomography provide kinematic information on the partitioning of damage and localization of strain. Complementary observations on deformed samples using microscopy and microcomputed tomography (μCT) can also be used to image microscale damage and its distribution. Only by synthesizing such measurements on multiple scales could one infer the multiscale dynamics of compaction localization and similar rock failure phenomena. Located at the European Synchrotron Radiation Facility, the HADES rig allows direct in situ 3D imaging of the whole rock sample as it is triaxially compressed. The μCT data provide an integrated perspective of the spatiotemporal evolution of damage and strain localization on scales ranging from grain to continuum. We conducted an experiment on Leitha limestone (initial porosity of ∼22%) at a confining pressure of 20 MPa. With increasing differential stress, the sample strain hardened and two distinct yield points were identified in the stress-strain curve. The spatiotemporal evolution of local porosity and damage were analyzed at multiple scales. At a mesoscopic scale of 10 voxels (65 μm), the time-lapse μCT images reveal the strain partitioning associated with the first yield point and development of strain localization with the second. The latter development of five discrete compaction bands is the first unambiguous observation of such a bifurcation phenomenon in a porous carbonate rock, with geometric attributes comparable to compactions bands observed in porous sandstones. The μCT data on the voxel-scale elucidate in refined details the nucleation and propagation of discrete compaction bands under quasi-static loading, as well as the micromechanical processes, which in the past could only be inferred from a synthesis of kinematic observations of acoustic emissions activity and post-mortem observations of microstructure and damage.

• Zheng, Xiaojiao; Cordonnier, Benoit; McBeck, Jessica Ann; Boller, Elodie; Jamtveit, Bjørn & Zhu, Wenlu [Show all 7 contributors for this article] (2019). Mixed‐mode strain localization generated by hydration reaction at crustal conditions. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 124(5), p. 4507–4522. doi: 10.1029/2018JB017008. Full text in Research Archive Show summary

  Hydration reactions influence rock density and rheology. For example, volume increases produced in hydration reactions may generate sufficient tensile and shear stress to fracture both the rock undergoing the reaction and the surrounding host rock. We performed in situ dynamic X‐ray synchrotron microtomography experiments to investigate reaction‐induced fracturing. Two experiments on hydration of periclase were performed at 180 or 190 °C, under a confinement of 10 or 80 MPa, a pore fluid pressure of 5 or 75 MPa, and with or without differential stress. The sample assembly consists of a periclase cylinder inserted into a central hole within a serpentinite cylinder. The reaction from periclase to brucite results in a large volume increase (110%), pushing the periclase/brucite against the serpentinite and ultimately breaking it. Using time‐resolved three‐dimensional imaging, we quantify the spatial and temporal distribution of the reaction‐induced fractures. We perform digital volume correlation analysis to obtain the incremental strain tensors throughout the hydration and fracturing process. We use numerical models to assess the distribution of stress within the serpentinite. The digital volume correlation results show mixed‐mode strain localization. The von Mises strain, indicative of shear, increases by a larger percentage than the contractive or dilatative strain components as the reaction‐induced fractures grow. The distribution of von Mises strain follows a power law relationship in the cumulative frequency‐magnitude domain, indicative of long‐range elastic stress interactions during fracturing. This experimental finding sheds insights on the mechanisms of microseismicity measured in areas undergoing active serpentinization.

• Renard, Francois; McBeck, Jessica Ann; Kandula, Neelima; Cordonnier, Benoit; Meakin, Paul & Ben-Zion, Yehuda (2019). Volumetric and shear processes in crystalline rock approaching faulting. Proceedings of the National Academy of Sciences of the United States of America (PNAS). ISSN 0027-8424. 116(33), p. 16234–16239. doi: 10.1073/pnas.1902994116. Full text in Research Archive Show summary

  Understanding the approach to faulting in continental rocks is critical for identifying processes leading to fracturing in geomaterials and the preparation process of large earthquakes. In situ dynamic X-ray imaging and digital volume correlation analysis of a crystalline rock core, under a constant confining pressure of 25 MPa, are used to elucidate the initiation, growth, and coalescence of microfractures leading to macroscopic failure as the axial compressive stress is increased. Following an initial elastic deformation, microfractures develop in the solid, and with increasing differential stress, the damage pervades the rock volume. The creation of new microfractures is accompanied by propagation, opening, and closing of existing microfractures, leading to the emergence of damage indices that increase as powers of the differential stress when approaching failure. A strong spatial correlation is observed between microscale zones with large positive and negative volumetric strains, microscale zones with shears of opposite senses, and microscale zones with high volumetric and shear strains. These correlations are attributed to microfracture interactions mediated by the heterogeneous stress field. The rock fails macroscopically as the microfractures coalesce and form a geometrically complex 3D volume that spans the rock sample. At the onset of failure, more than 70% of the damage volume is connected in a large fracture cluster that evolves into a fault zone. In the context of crustal faulting dynamics, these results suggest that evolving rock damage around existing locked or future main faults influences the localization process that culminates in large brittle rupture events.

• Renard, Francois; McBeck, Jessica Ann; Cordonnier, Benoit; Zheng, Xiaojiao; Kandula, Neelima & Sanchez, Jesus Rodriguez [Show all 12 contributors for this article] (2019). Dynamic In Situ Three-Dimensional Imaging and Digital Volume Correlation Analysis to Quantify Strain Localization and Fracture Coalescence in Sandstone. Pure and Applied Geophysics (PAGEOPH). ISSN 0033-4553. 176, p. 1083–1115. doi: 10.1007/s00024-018-2003-x. Full text in Research Archive Show summary

  Advances in triaxial compression deformation apparatus design, dynamic X-ray microtomography imaging, data analysis techniques, and digital volume correlation analysis provide unparalleled access to the in situ four-dimensional distribution of developing strain within rocks. To demonstrate the power of these new techniques and acquire detailed information about the micromechanics of damage evolution, deformation, and failure of porous rocks, we deformed 3-cm-scale cylindrical specimens of low-porosity Fontainebleau sandstone in an X-ray-transparent triaxial compression apparatus, and repeatedly recorded three-dimensional tomograms of the specimens as the differential stress was increased until macroscopic failure occurred. Experiments were performed at room temperature with confining pressure in the range of 10–20 MPa. Distinct grayscale subsets, indicative of density, enabled segmentation of the three-dimensional tomograms into intact rock matrix, pore space, and fractures. Digital volume correlation analysis of pairs of tomograms provided time series of three-dimensional incremental strain tensor fields throughout the experiments. After the yield stress was reached, the samples deformed first by dilatant opening and propagation of microfractures, and then by shear sliding via grain rotation and strain localization along faults. For two samples, damage and dilatancy occurred by grain boundary opening and then a sudden collapse of the granular rock framework at failure. For the third sample, a fault nucleated near the yield point and propagated in the sample through the development of transgranular microfractures. The results confirm findings of previous experimental studies on the same rock and provide new detailed quantifications of: (1) the proportion of shear versus dilatant strain in the sample, (2) the amount of dilatancy due to microfracture opening versus pore opening when a fault develops, and (3) the role of grain boundaries and pore walls in pinning microfracture propagation and slowing down the rate of damage accumulation as failure is approached. Our study demonstrates how the combination of high-resolution in situ dynamic X-ray microtomography imaging and digital volume image correlation analysis can be used to provide additional information to unravel brittle failure processes in rocks under stress conditions relevant to the upper crust.

• Zhang, Lingran; Nasika, Christina; Donzé, Frédéric-Victor; Zheng, Xiaojiao; Renard, Francois & Scholtès, Luc (2019). Modeling Porosity Evolution Throughout Reaction‐Induced Fracturing in Rocks With Implications for Serpentinization. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 124(6), p. 5708–5733. doi: 10.1029/2018JB016872. Full text in Research Archive
• Kandula, Neelima; Cordonnier, Benoit; Boller, Elodie; Weiss, Jérôme; Dysthe, Dag Kristian & Renard, Francois (2019). Dynamics of microscale precursors during brittle compressive failure in Carrara marble. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 124(6), p. 6121–6139. doi: 10.1029/2019JB017381. Full text in Research Archive Show summary

  Microscale heterogeneities influence failure mechanisms in the crust. To track the microstructural changes in rock samples when loaded until failure, we employed a novel experimental technique that couples dynamic X‐ray microtomography imaging with a triaxial deformation apparatus. We studied the brittle failure of Carrara marble under triaxial compression. Dynamic tomographic data revealed the spatial organization of microfractures and damage increments characterizing the precursory activity toward catastrophic failure. We quantified the emergence of scaling relationships between microstructural parameters, including total damage volume, incremental damage volume, the largest connected microfracture, and the applied differential stress. The total volume of microfractures accumulated from the beginning of the experiment as well as the incremental damage showed power law increase. The growth of the largest connected microfracture was related to differential stress as a power law with divergence at failure. The microfracture volume increments were distributed according to a power law with an upper cutoff that itself spanned the entire volume toward failure. These characteristic features of brittle failure in Carrara marble under compression are in agreement with theoretical models that consider failure as a critical phase transition. We also observed that, very close to failure, several power law relationships broke down, which we interpret to be related to the coalescence of the largest microfractures in a finite size volume. Scaling laws and associated exponents computed from our data are compared with predictions made from theoretical and numerical models. Our results show that precursors of macroscopic brittle failure in Carrara marble follow predictable trends.

• McBeck, Jessica Ann; Mair, Karen & Renard, Francois (2019). How Porosity Controls Macroscopic Failure via Propagating Fractures and Percolating Force Chains in Porous Granular Rocks. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 124(9), p. 9920–9939. doi: 10.1029/2019JB017825. Full text in Research Archive
• Incel, Sarah; Schubnel, Alexandre; Renner, Jörg; John, Timm; Labrousse, Loic & Hilairet, Nadège [Show all 10 contributors for this article] (2019). Experimental evidence for wall-rock pulverization during dynamic rupture at ultra-high pressure conditions. Earth and Planetary Science Letters. ISSN 0012-821X. 528, p. 1–10. doi: 10.1016/j.epsl.2019.115832. Full text in Research Archive Show summary

  The mechanisms triggering intermediate and deep earthquakes have puzzled geologists for several decades. There is still no consensus concerning whether such earthquakes are triggered by brittle or ductile mechanisms. We performed a deformation experiment on a synthetic lawsonite-bearing blueschist at a confining pressure of 3 GPa and temperatures from 583 to 1,073 K. After deformation, the recovered sample reveals conjugated shear fractures. Garnet crystals are dissected and displaced along these narrow faults and reveal micro- and nanostructures that resemble natural pulverization structures as well as partial amorphization. Formation of such structures at low confining pressures is known to require high tensile stresses and strain rates and is explained by the propagation of a dynamic shear rupture. The absence of shearing in the pulverized wall rock is taken as evidence that these structures pre-date the subsequent heat-producing frictional slip. In analogy to observations at low pressure we infer that the garnet structures in our experiment result from rapid propagation of a shear fracture even at the high pressure exerted on the sample and thus suggest that brittle deformation is possible at lower crustal to upper mantle depths.

• Aslan, Gokhan; Lasserre, Cécile; Cakir, Ziyadin; Ergintav, Semih; Ozarpaci, Seda & Dogan, Ugur [Show all 8 contributors for this article] (2019). Shallow Creep Along the 1999 Izmit Earthquake Rupture (Turkey) From GPS and High Temporal Resolution Interferometric Synthetic Aperture Radar Data (2011-2017). Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 124(2), p. 2218–2236. doi: 10.1029/2018JB017022. Full text in Research Archive
• Aslan, G; Cakir, Ziyadin; Lasserre, Cécile & Renard, Francois (2019). Investigating subsidence in the Bursa Plain, Turkey, using ascending and descending sentinel-1 satellite data. Remote Sensing. 11(1). doi: 10.3390/rs11010085. Full text in Research Archive Show summary

  We characterize and monitor subsidence of the Bursa Plain (southern Marmara region of Turkey), which has been interpreted as resulting from tectonic motions in the region. We quantify the subsidence using Interferometric Synthetic Aperture Radar (InSAR) time-series analysis. The Stanford Method for Persistent Scatterers InSAR package (StaMPS) is employed to process series of Sentinel 1 A-B radar images acquired between 2014 and 2017 along both ascending and descending orbits. The vertical velocity field obtained after decomposition of line-of-sight velocity fields on the two tracks reveals that the Bursa plain is subsiding at rates up to 25 mm/yr. The most prominent subsidence signal in the basin forms an east-west elongated ellipse of deformation in the east, and is bounded by a Quaternary alluvial plain undergoing average vertical subsidence at ~10 mm/yr. Another localized subsidence signal is located 5 km north of the city, following the Bursa alluvial fan, and is subsiding at velocities up to 25 mm/yr. The comparison between temporal variations of the subsiding surface displacements and variations of the water pressure head in the aquifer allows estimation of the compressibility of the aquifer, α . It falls in the range of 0.5 × 10 − 6 − 2 × 10 − 6 Pa−1, which corresponds to typical values for clay and sand sediments. We find a clear correlation between subsidence patterns and the lithology, suggesting a strong lithological control over subsidence. In addition, the maximum rate of ground subsidence occurs where agricultural activity relies on groundwater exploitation. The InSAR time series within the observation period is well correlated with changes in the depth of the ground water. These observations indicate that the recent acceleration of subsidence is mainly due to anthropogenic activities rather than tectonic motion.

• McBeck, Jessica Ann; Cordonnier, Benoit; Mair, Karen & Renard, Francois (2019). The evolving energy budget of experimental faults within continental crust: Insights from in situ dynamic X-ray microtomography. Journal of Structural Geology. ISSN 0191-8141. 123, p. 42–53. doi: 10.1016/j.jsg.2019.03.005. Full text in Research Archive Show summary

  We investigate the evolving distribution of strain produced by a sliding fault within intact crystalline rock, and the energetics of deformation that occur both on- and off-fault. We slid precut faults of differing roughness oriented at 45° to while acquiring in situ X-ray microtomograms. Digital volume correlation of tomograms provide estimates of the 3D displacement and strain fields. This characterization of the strain tensor field reveal that the differing fault roughness produced distinct slip behavior, degree of strain localization and accumulation, and energy budget partitioning. The rougher fault slipped more episodically, hosted a wider and more asymmetric damage zone, and accommodated less normal and shear strain. This fault consumed more energy in off-fault deformation (Wint) per volume and more energy in frictional slip (Wfric) as portions of the total energy input to the system (Wext) than the smoother fault. In both experiments, Wfric consumed the largest portion of the energy budget (50–100%), while Wint consumed smaller percentages (5–20%). Tracking the temporal variability of energy partitioning revealed how evolving fault architecture determined the energetic dominance of particular deformational processes, and so highlighted the importance of tracking energy partitioning through time.

• Kaduri, Maor; Gratier, Jean-Pierre; Lasserre, Cécile; Cakir, Ziyadin & Renard, Francois (2019). Quantifying the Partition Between Seismic and Aseismic Deformation Along Creeping and Locked Sections of the North Anatolian Fault, Turkey. Pure and Applied Geophysics (PAGEOPH). ISSN 0033-4553. 176(3), p. 1293–1321. doi: 10.1007/s00024-018-2027-2. Full text in Research Archive Show summary

  Shallow aseismic creep is a key deformation component along plate boundaries that contributes to the energy budget during the seismic cycle. Several major active continental faults show spatial alternation of creeping and locked sections. The present study focuses on the evaluation of the aseismic part of the total displacement along the North Anatolian Fault in Turkey. Detailed microstructural analyses of finite strain were performed using various methods, based on change of length or angle, on six representative samples collected over 32 outcrops along locked and creeping sections of the fault. Chemical analyses were used to map mineral composition of fault rocks and to calculate relative volume changes associated with creep. The relationship between finite strain and volume change allowed quantifying the evolution of the penetrative pressure solution cleavage mechanism of creep. In volcanic and analogous creeping rocks, finite strain measurements revealed two spatial scales of strain that correspond to the alternation of two types of shear zones, with cleavages either oblique or sub-parallel to the fault displacement. Using geodetic and geologic data, cumulative aseismic displacement was calculated in the range 9–49% of the total 80-km displacement in the creping sections and was negligible in locked sections. The large uncertainty in the kilometer-width creeping sections was related to the difficulty of quantifying the high strain values associated with high shear displacement and for which measurement uncertainties are large. A promising way to improve such quantification would be to develop reliable statistical analysis of cleavage orientation in the field.

• Renard, Francois; Weiss, Jérôme; Mathiesen, Joachim; Ben-Zion, Yehuda; Kandula, Neelima & Cordonnier, Benoit (2018). Critical Evolution of Damage Toward System-Size Failure in Crystalline Rock. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 123(2), p. 1969–1986. doi: 10.1002/2017JB014964. Full text in Research Archive Show summary

  Rock failure under shear loading conditions controls earthquake and faulting phenomena. We study the dynamics of microscale damage precursory to shear faulting in a quartz‐monzonite rock representative of crystalline rocks of the continental crust. Using a triaxial rig that is transparent to X‐rays, we image the mechanical evolution of centimeter‐size core samples by in situ synchrotron microtomography with a resolution of 6.5 μm. Time‐lapse three‐dimensional images of the samples inside the rig provide a unique data set of microstructural evolution toward faulting. Above a yield point there is a gradual weakening during which microfractures nucleate and grow until this damage span the whole sample. This leads to shear faults oriented about 30° to the main compressive stress in agreement with Anderson's theory and macroscopic failure. The microfractures can be extracted from the three‐dimensional images, and their dynamics and morphology (i.e., number, volume, orientation, shape, and largest cluster) are quantified as a function of increasing stress toward failure. The experimental data show for the first time that the total volume of microfractures, the rate of damage growth, and the size of the largest microfracture all increase and diverge when approaching faulting. The average flatness of the microfractures (i.e., the ratio between the second and third eigenvalues of their covariance matrix) shows a significant decrease near failure. The precursors to faulting developing in the future faulting zone are controlled by the evolving microfracture population. Their divergent dynamics toward failure is reminiscent of a dynamical critical transition. © 2018 Wiley

• Aslan, G; Cakir, Ziyadin; Ergintav, Semih; Lasserre, Cécile & Renard, Francois (2018). Analysis of secular ground motions in istanbul from a long-term InSAR time-series (1992-2017). Remote Sensing. 10(3). doi: 10.3390/rs10030408. Full text in Research Archive Show summary

  The identification and measurement of ground deformations in urban areas is of great importance for determining the vulnerable parts of the cities that are prone to geohazards, which is a crucial element of both sustainable urban planning and hazard mitigation. Interferometric synthetic aperture radar (InSAR) time series analysis is a very powerful tool for the operational mapping of ground deformation related to urban subsidence and landslide phenomena. With an analysis spanning almost 25 years of satellite radar observations, we compute an InSAR time series of data from multiple satellites (European Remote Sensing satellites ERS-1 and ERS-2, Envisat, Sentinel-1A, and its twin sensor Sentinel-1B) in order to investigate the spatial extent and rate of ground deformation in the megacity of Istanbul. By combining the various multi-track InSAR datasets (291 images in total) and analysing persistent scatterers (PS-InSAR), we present mean velocity maps of ground surface displacement in selected areas of Istanbul. We identify several sites along the terrestrial and coastal regions of Istanbul that underwent vertical ground subsidence at varying rates, from 5 ± 1.2 mm/yr to 15 ± 2.1 mm/yr. The results reveal that the most distinctive subsidence patterns are associated with both anthropogenic factors and relatively weak lithologies along the Haramirede valley in particular, where the observed subsidence is up to 10 ± 2 mm/yr. We show that subsidence has been occurring along the Ayamama river stream at a rate of up to 10 ± 1.8 mm/yr since 1992, and has also been slowing down over time following the restoration of the river and stream system. We also identify subsidence at a rate of 8 ± 1.2 mm/yr along the coastal region of Istanbul, which we associate with land reclamation, as well as a very localised subsidence at a rate of 15 ± 2.3 mm/yr starting in 2016 around one of the highest skyscrapers of Istanbul, which was built in 2010.

• Clerc, Adriane; Renard, Francois; Austrheim, Håkon Olaf & Jamtveit, Bjørn (2018). Spatial and size distributions of garnets grown in a pseudotachylyte generated during a lower crust earthquake. Tectonophysics. ISSN 0040-1951. 733, p. 159–170. doi: 10.1016/j.tecto.2018.02.014. Full text in Research Archive Show summary

  In the Bergen Arc, western Norway, rocks exhumed from the lower crust record earthquakes that formed during the Caledonian collision. These earthquakes occurred at about 30–50 km depth under granulite or amphibolite facies metamorphic conditions. Coseismic frictional heating produced pseudotachylytes in this area. We describe pseudotachylytes using field data to infer earthquake magnitude (M ≥ ~6.6), low dynamic friction during rupture propagation (μd < 0.1) and laboratory analyses to infer fast crystallization of microlites in the pseudotachylyte, within seconds of the earthquake arrest. High resolution 3D X-ray microtomography imaging reveals the microstructure of a pseudotachylyte sample, including numerous garnets and their corona of plagioclase that we infer have crystallized in the pseudotachylyte. These garnets 1) have dendritic shapes and are surrounded by plagioclase coronae almost fully depleted in iron, 2) have a log-normal volume distribution, 3) increase in volume with increasing distance away from the pseudotachylyte-host rock boundary, and 4) decrease in number with increasing distance away from the pseudotachylyte -host rock boundary. These characteristics indicate fast mineral growth, likely within seconds. We propose that these new quantitative criteria may assist in the unambiguous identification of pseudotachylytes in the field.

• Jamtveit, Bjørn; Ben-Zion, Yehuda; Renard, Francois & Austrheim, Håkon Olaf (2018). Earthquake-induced transformation of the lower crust. Nature. ISSN 0028-0836. 556(7702), p. 487–491. doi: 10.1038/s41586-018-0045-y. Full text in Research Archive Show summary

  The structural and metamorphic evolution of the lower crust has direct effects on the lithospheric response to plate tectonic processes involved in orogeny, including subsidence of sedimentary basins, stability of deep mountain roots and extension of high-topography regions. Recent research shows that before orogeny most of the lower crust is dry, impermeable and mechanically strong1. During an orogenic event, the evolution of the lower crust is controlled by infiltration of fluids along localized shear or fracture zones. In the Bergen Arcs of Western Norway, shear zones initiate as faults generated by lower-crustal earthquakes. Seismic slip in the dry lower crust requires stresses at a level that can only be sustained over short timescales or local weakening mechanisms. However, normal earthquake activity in the seismogenic zone produces stress pulses that drive aftershocks in the lower crust2. Here we show that the volume of lower crust affected by such aftershocks is substantial and that fluid-driven associated metamorphic and structural transformations of the lower crust follow these earthquakes. This provides a ‘top-down’ effect on crustal geodynamics and connects processes operating at very different timescales.

• Zheng, Xiaojiao; Cordonnier, Benoit; Zhu, Wenlu; Renard, Francois & Jamtveit, Bjørn (2018). Effects of Confinement on Reaction-Induced Fracturing During Hydration of Periclase. Geochemistry Geophysics Geosystems. 19(8), p. 2661–2672. doi: 10.1029/2017GC007322. Full text in Research Archive Show summary

  Hydration of a nominally dry rock can cause expansion of the solid volume, resulting in reaction‐induced fracturing and an associated increase in the porosity and permeability of the rock. We study the effect of confinement on the coupling between stress generation, reaction‐induced fracturing, and porosity evolution during the hydration of periclase (MgO) into brucite (Mg(OH)2). Samples of a microporous MgO ceramic were hydrated at 170–210 °C, 5–80 MPa confining pressure, 6–95 MPa differential stress, and 5–75 MPa pore fluid pressure in a purpose‐designed triaxial load cell. Hydration‐induced changes were recorded in situ by X‐ray microtomographic imaging at 5‐min intervals. Below 30 MPa effective mean stress, the fraction of periclase replaced by brucite is a sigmoidal function of time. After a slow start, the replacement rate picks up with concomitant intense fracturing. The porosity increase resulting from the reaction‐induced fractures is transient (pulse‐like). Following the porosity pulse the rate of replacement declines until the replacement is almost complete. Above 30 MPa, the reaction rate is slow, porosity decreases monotonically without any observable fracturing during the time of the experiment. At these stress conditions, the lack of fracturing cannot be limited by the thermodynamic affinity of the reaction. A possible interpretation is that the stress generated by the reaction may overcome the disjoining pressure at the grain‐grain interface, expelling the water film trapped there and thereby dramatically reducing the reaction rate.

• Lafay, Romain; Montes-Hernandez, German; Renard, Francois & Vonlanthen, Pierre (2018). Intracrystalline reaction-induced cracking in olivine evidenced by hydration and carbonation experiments. Minerals. 8(9). doi: 10.3390/min8090412. Full text in Research Archive Show summary

  In order to better understand the microtextural changes associated with serpentinization reactions, natural millimeter-sized olivine grains were experimentally reacted with alkaline NaOH and NaHCO3 solutions at a temperature of 200 °C and for durations of 3 to 12 months. During hydration experiments, dissolution and precipitation were intimately correlated in time and space, with reaction products growing in situ, either as layered veins or as nearly continuous surface cover. In contrast, carbonation experiments showed a strong decoupling between both processes leading to essentially delocalized precipitation of the reaction products away from dissolution sites. Textural analyses of the samples using scanning electron microscopy, Raman spectroscopy, and X-ray synchrotron microtomography provided experimental evidence for a cause-and-effect relationship between in situ precipitation and intracrystalline reaction-induced cracking in olivine. Juvenile cracks typically nucleated at the tip of dissolution notches or on diamond-shaped pores filled with reaction products, and propagated through the olivine crystal lattice during the course of the reaction. The occurrence of new cracks at the tip of diamond-shaped pores, but also of tiny subspherical pores lining up along microcracks, indicated that fracturation and porosity networks were mutually driven, making serpentinization an extremely efficient alteration process over time. Alternatively, our data suggested that some form of porosity also developed in absence of fracturation, thus further highlighting the remarkable efficiency and versatility of serpentinization processes.

• Toussaint, Renaud; Aharonov, Einat; Koehn, Daniel; Gratier, Jean-Pierre; Ebner, Martin & Baud, Patrick [Show all 8 contributors for this article] (2018). Stylolites: A review. Journal of Structural Geology. ISSN 0191-8141. 114, p. 163–195. doi: 10.1016/j.jsg.2018.05.003. Full text in Research Archive Show summary

  Stylolites are ubiquitous geo-patterns observed in rocks in the upper crust, from geological reservoirs in sedimentary rocks to deformation zones, in folds, faults, and shear zones. These rough surfaces play a major role in the dissolution of rocks around stressed contacts, the transport of dissolved material and the precipitation in surrounding pores. Consequently, they play an active role in the evolution of rock microstructures and rheological properties in the Earth's crust. They are observed individually or in networks, in proximity to fractures and joints, and in numerous geological settings. This review article deals with their geometrical and compositional characteristics and the factors leading to their genesis. The main questions this review focuses on are the following: How do they form? How can they be used to measure strain and formation stress? How do they control fluid flow in the upper crust? Geometrically, stylolites have fractal roughness, with fractal geometrical properties exhibiting typically three scaling regimes: a self-affine scaling with Hurst exponent 1.1 ± 0.1 at small scale (up to tens or hundreds of microns), another one with Hurst exponent around 0.5 to 0.6 at intermediate scale (up to millimeters or centimeters), and in the case of sedimentary stylolites, a flat scaling at large scale. More complicated anisotropic scaling (scaling laws depending of the direction of the profile considered) is found in the case of tectonic stylolites. We report models based on first principles from physical chemistry and statistical physics, including a mechanical component for the free-energy associated with stress concentrations, and a precise tracking of the influence of grain-scale heterogeneities and disorder on the resulting (micro)structures. Experimental efforts to reproduce stylolites in the laboratory are also reviewed. We show that although micrometer-size stylolite teeth are obtained in laboratory experiments, teeth deforming numerous grains have not yet been obtained experimentally, which is understandable given the very long formation time of such geometries. Finally, the applications of stylolites as strain and stress markers, to determine paleostress magnitude are reviewed. We show that the scalings in stylolite heights and the crossover scale between these scalings can be used to determine the stress magnitude (its scalar value) perpendicular to the stylolite surface during the stylolite formation, and that the stress anisotropy in the stylolite plane can be determined for the case of tectonic stylolites. We also show that the crossover between medium (millimetric) scales and large (pluricentimetric) scales, in the case of sedimentary stylolites, provides a good marker for the total amount of dissolution, which is still valid even when the largest teeth start to dissolve – which leads to the loss of information, since the total deformation is not anymore recorded in a single marker structure. We discuss the impact of the stylolites on the evolution of the transport properties of the hosting rock, and show that they promote a permeability increase parallel to the stylolites, whereas their effect on the permeability transverse to the stylolite can be negligible, or may reduce the permeability, depending on the development of the stylolite.

• Dunkel, Kristina G; Austrheim, Håkon Olav; Renard, Francois; Cordonnier, Benoit & Jamtveit, Bjørn (2017). Localized slip controlled by dehydration embrittlement of partly serpentinized dunites, Leka Ophiolite Complex, Norway. Earth and Planetary Science Letters. ISSN 0012-821X. 463, p. 277–285. doi: 10.1016/j.epsl.2017.01.047. Full text in Research Archive Show summary

  Dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure, a process referred to as dehydration embrittlement. However the extents of strain localization and unstable frictional sliding during deserpentinization are still under debate. In the layered ultramafic sections of the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde metamorphism of serpentinite veins led to local fluid production and to the growth of Mg-rich and coarse-grained olivine with abundant magnetite inclusions and δ18O values ‰ below the host rock. Embrittlement associated with the dehydration caused faulting along highly localized (<10 μm-wide) slip planes near the centers of the original serpentinite veins and pulverization of wall rock olivine. These features along with an earthquake-like size distribution of fault offsets suggest unstable frictional sliding rather than slower creep. Structural heterogeneities in the form of serpentinite veins clearly have first-order controls on strain localization and frictional sliding during dehydration. As most of the oceanic lithosphere is incompletely serpentinized, heterogeneities represented by a non-uniform distribution of serpentinite are common and may increase the likelihood that dehydration embrittlement triggers earthquakes.

• Aben, F.M.; Doan, ML; Gratier, JP & Renard, Francois (2017). Experimental postseismic recovery of fractured rocks assisted by calcite sealing. Geophysical Research Letters. ISSN 0094-8276. 44(14), p. 7228–7238. doi: 10.1002/2017GL073965. Full text in Research Archive Show summary

  Postseismic recovery within fault damage zones involves slow healing of coseismic fractures leading to permeability reduction and strength increase with time. To better understand this process, experiments were performed by long‐term fluid percolation with calcite precipitation through predamaged quartz‐monzonite samples subjected to upper crustal conditions of stress and temperature. This resulted in a P wave velocity recovery of 50% of its initial drop after 64 days. In contrast, the permeability remained more or less constant for the duration of the experiment. Microstructures, fluid chemistry, and X‐ray microtomography demonstrate that incipient calcite sealing and asperity dissolution are responsible for the P wave velocity recovery. The permeability is unaffected because calcite precipitates outside of the main flow channels. The highly nonparallel evolution of strength recovery and permeability suggests that fluid conduits within fault damage zones can remain open fluid conduits after an earthquake for much longer durations than suggested by the seismic monitoring of fault healing.

• Renard, Francois; Cordonnier, Benoit; Kobchenko, Maya; Kandula, Neelima; Weiss, Jérôme & Zhu, Wenlu (2017). Microscale characterization of rupture nucleation unravels precursors to faulting in rocks. Earth and Planetary Science Letters. ISSN 0012-821X. 476, p. 69–78. doi: 10.1016/j.epsl.2017.08.002. Full text in Research Archive Show summary

  Precursory signals, manifestations of microscale damage that precedes dynamic faulting, are key to earthquake forecasting and risk mitigation. Detections of precursors have primarily relied on measurements performed using sensors installed at some distance away from the rupture area in both field and laboratory experiments. Direct observations of continuous microscale damage accumulated during fault nucleation and propagation are scarce. Using an X-ray transparent triaxial deformation apparatus, we show the first quantitative high resolution three-dimensional (3D) information about damage evolution of rocks undergoing brittle failure. The dynamic microtomography images documented a spectrum of damage characteristics and different fault growth patterns. The interplay between various deformation mechanisms can result in either a positive, negative, or constant net volume change. Consequently, changes in rock density and acoustic wave velocities before faulting are expected to vary in different tectonics settings, hence making failure forecasting intrinsically dependent on rock type at depth.

• Linga, Gaute; Mathiesen, Joachim & Renard, Francois (2017). Self-similar distributions of fluid velocity and stress heterogeneity in a dissolving porous limestone. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 122(3), p. 1726–1743. doi: 10.1002/2016JB013536. Full text in Research Archive Show summary

  In a porous rock, the spatial distribution of the pore space induces a strong heterogeneity in fluid flow rates and in the stress distribution in the rock mass. If the rock microstructure evolves through time, for example, by dissolution, fluid flow and stress will evolve accordingly. Here we consider a core sample of porous limestone that has undergone several steps of dissolution. Based on 3‐D X‐ray tomography scans, we calculate numerically the coupled system of fluid flow in the pore space and stress in the solid. We determine how the flow field affects the stress distribution both at the pore wall surface and in the bulk of the solid matrix. We show that during dissolution, the heterogeneous stress evolves in a self‐similar manner as the porosity is increased. Conversely, the fluid velocity shows a stretched exponential distribution. The scalings of these common master distributions offer a unified description of the porosity evolution, pore flow, and the heterogeneity in stress for a rock with evolving microstructure. Moreover, the probability density functions of stress invariants (mechanical pressure or von Mises stress) display heavy tails toward large stresses. If these results can be extended to other kinds of rocks, they provide an additional explanation of the sensitivity to failure of porous rocks under slight changes of stress. This research was originally published in Journal of Geophysical Research. Solid Earth. © 2017 American Geophysical Union

• Aben, F.M.; Doan, M.-L.; Gratier, JP & Renard, Francois (2017). High strain rate deformation of porous sandstone and the asymmetry of earthquake damage in shallow fault zones. Earth and Planetary Science Letters. ISSN 0012-821X. 463, p. 81–91. doi: 10.1016/j.epsl.2017.01.016. Full text in Research Archive Show summary

  In contrast to coseismic pulverization of crystalline rocks, observations of coseismic pulverization in porous sedimentary rocks in fault damage zones are scarce. Also, juxtaposition of stiff crystalline rocks and compliant porous rocks across a fault often yields an asymmetric damage zone geometry, with less damage in the more compliant side. In this study, we argue that such asymmetry near the sub-surface may occur because of a different response of lithology to similar transient loading conditions. Uniaxial unconfined high strain rate loadings with a split Hopkinson pressure bar were performed on dry and water saturated Rothbach sandstone core samples. Bedding anisotropy was taken into account by coring the samples parallel and perpendicular to the bedding. The results show that pervasive pulverization below the grain scale, such as observed in crystalline rock, does not occur in the sandstone samples for the explored strain rate range (60–150 s−1). Damage is mainly restricted to the scale of the grains, with intragranular deformation occurring only in weaker regions where compaction bands are formed. The presence of water and the bedding anisotropy mitigates the formation of compaction bands and motivates intergranular dilatation. The competition between inter- and intragranular damage during dynamic loading is explained with the geometric parameters of the rock in combination with two classic micromechanical models: the Hertzian contact model and the pore-emanated crack model. In conclusion, the observed microstructures can form in both quasi-static and dynamic loading regimes. Therefore caution is advised when interpreting the mechanism responsible for near-fault damage in sedimentary rock near the surface. Moreover, the results suggest that different responses of lithology to transient loading are responsible for sub-surface damage zone asymmetry.

• Toy, Virginia G; Niemeijer, Andre; Renard, Francois; Morales, Luiz & Wirth, Richard (2017). Striation and slickenline development on quartz fault surfaces at crustal conditions: Origin and effect on friction. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 122(5), p. 3497–3512. doi: 10.1002/2016JB013498. Full text in Research Archive Show summary

  Fragments of optically flat silica discs embedded in synthetic gouge were deformed to examine the relationship between the development of striations and slickenlines, and deformation mechanisms, conditions, and fault rheology. Experiments were performed under hydrothermal conditions in a rotary shear apparatus at 100°C or 450°C, to shear strains of 2.02 < γ < 8.25. Slip hardening and softening prevail at low and high temperatures, respectively. In recovered samples, disc fragment surfaces are decorated by fine gouge, sometimes arranged in trails, pits, and scratch marks. Prominent grooves—inferred slickenlines—with constant orientation, wavelength <10 μm, and amplitude <0.7 μm are only observed on disc fragments deformed at 450°C. Some parts of the grooves below the original disc fragment surface contain scattered rounded beads of silica ~200 nm diameter. Conversely, close examination of pits in 100°C experiments reveals they contain angular particles <2 μm diameter. The 200 nm diameter crystalline quartz can precipitate at 450°C in only 250 s, well within the time frame of the experiments but precipitation at 100°C would take at least 8 years. No systematic dislocation arrays were observed in the quartz disc fragments, but microfractures are sporadically present. This indicates that at both temperatures brittle failure generated microfractures and microcomminution occurred where gouge particles impacted disc fragment surfaces. These observations suggest formation of silica beads by precipitation from amorphous silica facilitates slip weakening, smoothing of the fault surface parallel to the slip vector, development of undulations perpendicular to the slip vector by a pressure solution creep mechanism, weakening, and maintenance of a constant slip direction. This research was originally published in Journal of Geophysical Research: Biogeosciences. © 2017 American Geophysical Union

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