Faults in carbonate rocks show both seismic and aseismic deformation processes, leading to a wide range of slip velocities. We deformed two centimeter-scale cores of Carrara marble at 25°C, under in-situ conditions of stress of 2-3 km depth, and imaged the nucleation and growth of creeping faults using dynamic synchrotron X-ray microtomography with micrometer spatial resolution. The first sample was under a constant confinement of 30 MPa and no pore fluid. The second sample was under a confinement in the range 35-23 MPa, with 10 MPa pore fluid pressure. We increased the axial stress by steps until creep deformation occurred and imaged deformation in 4D during creep. The samples deformed with a steady-state strain rate when the differential stress was constant, a process called creep. However, for both samples, we also observed transient events that include the acceleration of creep, i.e., creep bursts, phenomena similar to slow slip events that occur in continental active faults. During these transient creep events, strain rates increase and correlate in time with strain localization and the development of system-spanning fault networks. In both samples, the acceleration of opening and shearing of microfractures accommodated creep bursts. Using high-resolution time-lapse X-ray micro-tomography imaging, and digital image correlation, during triaxial deformation allowed quantifying creep in laboratory faults at sub-grain spatial resolution, and demonstrates that transient creep events (creep bursts) correlate with the nucleation and growth of faults.

We quantify the evolving spatial distribution of fracture networks throughout six in situ X-ray tomography triaxial compression experiments on monzonite and granite at confining stresses of 5-35 MPa. We first assess whether one dominant fracture continually grows at the expense of others by tracking the proportion of the maximum fracture volume to the total fracture volume. This metric does not increase monotonically. We next examine if the set of the largest fractures continually dominates deformation by tracking the proportion of the cumulative volume of fractures with volumes >90th percentile to the total fracture volume. This metric indicates that the fracture networks tend to increase in localization toward the largest set of fractures for up to 80% of the experimental time (differential stress), consistent with observations from southern California of localizing and delocalizing seismicity. Experiments with higher confining stress tend to have greater localization. To further assess the fracture networks localization, we compare the geometry of the set of the largest fractures to a plane. We find the best fit plane through the fractures with volumes >90th percentile immediately preceding failure, and calculate the distance between these fractures and the plane, and the r2 score of the fractures and the plane throughout each experiment. The r2 scores and the distance indicate greater localization in the monzonite experiments than in the granite experiments. The smaller mean grain size of the minerals in the granite may produce more sites of fracture nucleation and termination, leading to more delocalized fracture networks that deviate further from a plane. The higher applied confining stress in the monzonite experiments (25-35 MPa) relative to the granite experiments (5-10 MPa) may also contribute to the more localized fracture networks in the monzonite experiments. The evolution of the clustering the fractures toward the plane and the Gini coefficient, which measures the deviation of a population from uniformity, closely match each other. Tracking these metrics of localization also reveals that macroscopic yielding appears to occur when the rate of fracture network localization increases.

Surge-type glaciers are present in many cold environments in the world. These glaciers experience a dramatic increase in velocity over short time periods, the surge, followed by an extended period of slow movement, the quiescence. The detailed processes that control this intermittent behaviour remain elusive. We develop a machine learning framework to classify surge-type glaciers, based on their location, exposure, geometry, surface mass balance and runoff. We apply this approach to the Svalbard archipelago, a region with a relatively homogeneous climate. We compare the performance of logistic regression, random forest, and extreme gradient boosting (XGBoost) machine learning models that we apply to a newly combined database of glaciers in Svalbard. Based on the most accurate model, XGBoost, we compute surge probabilities along glacier centerlines and quantify the relative importance of several controlling features. Results show that the surface and bed slopes, ice thickness, glacier width, surface mass balance and runoff along glacier centerlines are the most significant features explaining surge probability for glaciers in Svalbard. A thicker and wider glacier with a low surface slope has a higher probability to be classified as surge-type, which is in good agreement with the existing theories of surging. Finally, we build a probability map of surge-type glaciers in Svalbard. Our methodology could be extended to classify surge-type glaciers in other areas of the world.

We report here a slow-moving landslide revealed by Sentinel-1 interferometric time-series analysis. Located along the western coast of the Aral Sea, with a >80-km length and 4-km width, this is the world’s largest active landslide complex reported so far with a constant velocity of 40 mm/yr. Systematic subsidence up to 5 mm/yr, is also observed along narrow strips of terraces that appear to result from rotations of fault-bounded blocks. The horizontal deformation does not correlate with the annual variations of the water level in the Aral Sea over the same period, indicating a long-term forcing of this landslide that might be caused by the long-term sea-level drop. The lateral spreadings involve the competent limestone beds lying horizontally on plastic clay- and evaporite-rich layers. We propose a conceptual model for the mechanism of landslides that appear to be controlled by the attitude of bedding, lithological sequence, hydrogeology, and low angle faults.

Sustained serpentinization of peridotite within the oceanic lithosphere requires effective supply of water to systems that experience continuous expansion of the solid volume. Hence, serpentinization preferentially occurs along ridge axes and in subduction zones where tectonic activity is intense and fracturing helps generating and sustaining the permeability required to connect seafloor-near environments to depth. The slowest mid-oceanic ridges produce little melt leading to discontinuous magmatic activity with very thin to no crust along most of the ridge length and up to 8 km thick crust focused around local magmatic centers. Three types of ultra-slow ridge sections can be distinguished: i) amagmatic, characterized by scarce basaltic crust and deep seismic activity, ii) magmatic, characterized by a thin basaltic crust and intermediate depth seismic activity, and iii) volcanic, characterized by a thick basaltic crust and shallow seismic activity. At amagmatic and magmatic ridge types, aseismic zones are identified above the seismic zone. The lower limit of the aseismic zone along amagmatic sections is thermally controlled and follows a 400-500˚C isotherm corresponding to the upper temperature limit for the onset of serpentinization. This observation suggests that the aseismic zone is significantly serpentinized with ample supply of water to the peridotite-serpentine interface. Based on recorded seismic activity, we estimate the associated rock volume affected by brittle damage for the different ultra-slow ridge types. We show that damage produced by seismic activity sustains pervasive serpentinization along amagmatic and magmatic types, while it is limited in the case of volcanic sections.

The process of primary migration, which controls the transfer of hydrocarbons from source to reservoir rocks, necessitates the existence of fluid pathways in formations with inherently low permeability. Primary migration starts with the maturation of organic matter that produces fluids which increase the effective stress locally. The interactions between local fluid production, microfracturing, stress conditions, and transport remain difficult to apprehend in shale source rocks. Here, we analyze these interactions using a coupled hydro-mechanical model based on the discrete element method. The model is used to simulate the effects of fluid production emanating from kerogen patches contained within a shale rock alternating kerogen-poor and kerogen-rich layers. We identify two microfracturing mechanisms that control fluid migration: i) propagation of hydraulically driven fractures induced by kerogen maturation in kerogen-rich layers, and ii) compression induced fracturing in kerogen-poor layers caused by fluid overpressurization of the surrounding kerogen-rich layers. The relative importance of these two mechanisms is discussed considering different elastic properties contrasts between the rock layers, as well as various stress conditions encountered in sedimentary basins, from normal to reverse faulting regimes. Results show that the layering causes local stress redistribution that controls the prevalence of each mechanism over the other. When the vertical stress is higher than the horizontal stress in kerogen-rich layers, microfractures propagate from kerogen patches and rotate toward a direction perpendicular to the layers. Microfracturing in kerogen-poor layers is more pronounced when the confinement in these layers is higher. Those mechanisms were shown to be representative of Draupne formation.