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Tectonic pseudotachylytes are thought to be unique to certain water–deficient seismogenic environments and their presence is considered to be rare in the geological record. Here, we present field and experimental evidence that frictional melting can occur in hydrothermal fluid–rich faults hosted in the continental crust. Pseudotachylytes were found in the > 40 km–long Bolfín Fault Zone of the Atacama Fault System, within two ca. 1 m–thick (ultra)cataclastic strands hosted in a damage–zone made of chlorite–epidote–rich hydrothermally altered tonalite. This alteration state indicates that hydrothermal fluids were active during the fault development. Pseudotachylytes, characterized by presenting amygdales, cut and are cut by chlorite–, epidote– and calcite–bearing veins. In turn, crosscutting relationship with the hydrothermal veins indicates pseudotachylytes were formed during this period of fluid activity. Rotary shear experiments conducted on bare surfaces of hydrothermally altered rocks at seismic slip velocities (3 m s-1) resulted in the production of vesiculated pseudotachylytes both at dry and water–pressurized conditions, with melt lubrication as the primary mechanism for fault dynamic weakening. The presented evidence challenges the common hypothesis that pseudotachylytes are limited to fluid–deficient environments, and gives insights into the ancient seismic activity of the system. Both field observations and experimental evidence, indicate that pseudotachylytes may easily be produced in hydrothermal environments, and could be a common co–seismic fault product. Consequently, melt lubrication could be considered one of the most efficient seismic dynamic weakening mechanisms in crystalline basement rocks of the continental crust.

Pseudotachylytes (solidified friction melts produced during seismic slip) are considered to be rare in the geological record because they should be typical of particular seismogenic environments characterized by water-deficient cohesive rocks. Here we present field and experimental evidence that frictional melting can occur in “fluid-rich” faults hosted in the continental crust. Pseudotachylytes were found in the >40 km long Bolfin Fault Zone of the Atacama Fault System (Northern Chile). The pseudotachylytes (1) are associated with a ~1 m thick ultracataclastic fault core which accommodated > 5 km of strike-slip displacement at 6-8 km depth and 280-350°C ambient temperature, (2) cut a ca. 50 m thick damage zone made of sub-greenschists facies hydrothermally altered diorites and gabbros, (3) cut and are cut by epidote+chlorite+calcite bearing veins. The microstructure of the pseudotachylytes include (1) tabular microlites of feldspar hosted in a glassy-like matrix and (2) vesicles filled by post-seismic sub-greenschist facies minerals hosted in a strongly altered matrix of albite, chlorite, and epidote crystals. Experiments reproducing seismic slip in the presence of pressurized water and conducted with the rotary shear apparatus SHIVA on experimental faults made of the sub-greenschists (hydrothermally altered) facies damage zone rocks from the Bolfin Fault Zone, resulted in the production of vesiculated pseudotachylytes. In these experiments, fault weakening mainly occurred by melt lubrication rather than by pore fluid thermal pressurization. The identification of pseudotachylytes and its association with intense pre- and post-seismic hydrothermal alteration challenges the common belief that pseudotachylytes are rare. Consistent with the experimental evidence, pseudotachylytes (1) could be a common coseismic fault product in the continental crust, (2) may easily be produced in fluid-rich hydrothermal environments but, (3) are easily lost from the geological re

Nanoparticles and amorphous materials are common constituents of the shallow sections of active faults. Understanding the conditions at which nanoparticles are produced and their effects on friction can further improve our understanding of fault mechanics and earthquake energy budgets. Here we present the results of 59 rotary shear experiments conducted at room humidity conditions on gouge consisting of mixtures of smectite (Ca-montmorillonite) and quartz. Experiments with 60, 50, 25, 0 wt.% Ca-montmorillonite, were performed to investigate the influence of variable clay content on nanoparticle production and their influence on frictional processes. All experiments were performed at a normal stress of 5 MPa, slip rate of 0.0003≤V≤1.5 ms−1, and at a displacement of 3 m. To monitor the development of fabric and the mineralogical changes during the experiments, we investigated the deformed gouges using scanning and transmission electron microscopy combined with X-ray powder diffraction quantitative phase analysis. This integrated analytical approach reveals that, at all slip rates and compositions, the nanoparticles (grain size of 10–50 nm) are partly amorphous and result from cataclasis, wear and mechanical solid-state amorphization of smectite. The maximum production of amorphous nanoparticle occurs in the intermediate slip rate range (0.0003≤V≤0.1 ms−1), at the highest frictional work, and is associated to diffuse deformation and slip strengthening behavior. Instead, the lowest production of amorphous nanoparticles occurs at co-seismic slip rates (V≥1.3 ms−1), at the highest frictional power and is associated with strain and heat localization and slip weakening behavior. Our findings suggest that, independently of the amount of smectite nanoparticles, they produce fault weakening only when typical co-seismic slip rates (>0.1 ms−1) are achieved. This implies that estimates of the fracture surface energy dissipated during earthquakes in natural faults might be extr

Tectonic pseudotachylytes (solidified friction melts produced during seismic slip) are considered to be rare in the geological record because they should be typical of particular seismogenic environments characterized by water-deficient cohesive rocks. Here we present field, microstructural and experimental evidence that frictional melting can occur in “wet” faults hosted in the continental crust. Pseudotachylytes were found in the >40 km long Bolfin Fault Zone of the Atacama Fault System (Northern Chile). The pseudotachylytes (1) are associated with a ~1 m thick ultracataclastic fault cores which accommodated > 5 km of strike-slip displacement at 6-8 km depth and 280-350°C ambient temperature, (2) cut a ~50 m thick damage zone made of sub-greenschists facies hydrothermally altered diorites and gabbros, (3) cut and are cut by veins filled by epidote, chlorite, quartz and calcite. The microstructure of the pseudotachylytes include (1) tabular microlites of feldspar hosted in a glassy-like matrix and (2) vesicles filled by post-seismic sub-greenschist facies minerals hosted in a strongly altered matrix of albite, chlorite, and epidote crystals. Experiments reproducing seismic slip in the presence of pressurized water and conducted with the rotary shear apparatus SHIVA on experimental faults made of the sub-greenschists (hydrothermally altered) facies damage zone rocks from the Bolfin Fault Zone, resulted in the production of vesiculated pseudotachylytes. In these experiments, fault weakening mainly occurred by melt lubrication rather than by pore fluid thermal pressurization. The identification of pseudotachylytes, the first so far to our knowledge in South America, and its association with intense pre- and post-seismic hydrothermal alteration challenges the common belief that pseudotachylytes are rare. Consistent with the experimental evidence, pseudotachylytes (1) could be a common coseismic fault product in the continental crust, (2) may easily be produced in

Seismological, tsunami and geodetic observations have shown that subduction zones are complex systems where the properties of earthquake rupture vary with depth as a result of different pre-stress and frictional conditions. A wealth of earthquakes of different sizes and different source features (e.g. rupture duration) can be generated in subduction zones, including tsunami earthquakes, some of which can produce extreme tsunamigenic events. Here, we offer a geological perspective principally accounting for depth-dependent frictional conditions, while adopting a simplified distribution of on-fault tectonic pre-stress. We combine a lithology-controlled, depth-dependent experimental friction law with 2D elastodynamic rupture simulations for a Tohoku-like subduction zone cross-section. Subduction zone fault rocks are dominantly incohesive and clay-rich near the surface, transitioning to cohesive and more crystalline at depth. By randomly shifting along fault dip the location of the high shear stress regions (“asperities”), moderate to great thrust earthquakes and tsunami earthquakes are produced that are quite consistent with seismological, geodetic, and tsunami observations. As an effect of depth-dependent friction in our model, slip is confined to the high stress asperity at depth; near the surface rupture is impeded by the rock-clay transition constraining slip to the clay-rich layer. However, when the high stress asperity is located in the clay-to-crystalline rock transition, great thrust earthquakes can be generated similar to the Mw 9 Tohoku (2011) earthquake.

Recent estimates of fracture energy G′ in earthquakes show a power-law dependence with slip u which can be summarized as G′ ∝ ua where a is a positive real slightly larger than one. For cracks with sliding friction, fracture energy can be equated to Gf: the post-failure integral of the dynamic weakening curve. If the dominant dissipative process in earthquakes is friction, G′ and Gf should be comparable and show a similar scaling with slip. We test this hypothesis by analyzing experiments performed on various cohesive and non-cohesive rock types, under wet and dry conditions, with imposed deformation typical of seismic slip (normal stress of tens of MPa, target slip velocity > 1 m/s and fast accelerations ≈ 6.5 m/s2). The resulting fracture energy Gf is similar to the seismological estimates, with Gf and G′ being comparable over most of the slip range. However, Gf appears to saturate after several meters of slip, while in most of the reported earthquake sequences, G′ appears to increase further and surpasses Gf at large magnitudes. We analyze several possible causes of such discrepancy, in particular, additional off-fault damage in large natural earthquakes.