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<p>Active faults are characterized by creation/destruction of secondary (tectonic) permeability in response to a continuous interplay between deformation and fluid pressure fluctuations during the seismic cycle. The study of the paleofluid circulation in fault rocks can thus provide insights into the hydraulic and mechanical behavior of the seismogenic crust.</p> <p>This work integrates data from field geology with geochemical and geochronological constraints to understand the spatio-temporal evolution of the paleofluid circulation in the Mount Morrone Fault System (MMFS), a ~25 km-long tectonic structure activated during the extensional Quaternary phase of the central Apennines (Italy). The MMFS cuts through a Mesozoic-Cenozoic multilayer carbonate succession for a cumulative stratigraphic offset of about 2 km. Fluvio-lacustrine and slope deposits (Middle-Late Pleistocene) occur at its hanging wall and are variably involved by faulting. The MMFS is currently classified as a silent seismic fault, with an estimated Mw= 6.5-7.0 potential magnitude and recurrence time at 2.4 ka for an expected earthquake.</p> <p>The structural survey focused on the western strand of the MMFS cutting through a succession of Sinemurian dolomitized limestones. A composite network of NW-SE-striking, SW-dipping fault surfaces defines the structural architecture of the MMFS in the study outcrops, with high angle (dip > 55°) faults that systematically cut and displace medium-to-low angle (dip in the order of 30°-50°) faults. Both fault systems are characterized by dominant dip-slip movement and normal kinematics. Lenses of cm-thick cataclasites often occur along the slip surfaces. Cataclasites are made by sub-angular to sub-rounded carbonate clasts (up to 1 cm-wide) dispersed in a very fine-grained matrix. Layers of cm-thick carbonate concretions occur associated with the cataclasites, testifying for pulses of fluid discharge along the fault surface during the tectonic activity of the MMFS. Microstructural investigations document that: (i) carbonate concretions show an internal texture of fibrous vein having fiber growth direction roughly perpendicular to the vein wall, and (ii) the basal portions of the carbonate concretions are fractured and incorporated within the underlying cataclasites through the deposition of a new calcite cement. The geochemical (δ<sup>13</sup>C and δ<sup>18</sup>O stable isotope) analyses on selected samples attest for a progressive chemical shift of the mineralizing fluid from marine (in host rock and in cataclasites) to meteoric waters (in carbonate concretions). The U-Th dating of carbonate concretions and calcite slickenfibers constrains the fault-controlled fluid circulation to the Middle Pleistocene, with ages spanning from 270 to 180 ka. Significantly, the dating of carbonate concretions documents a 12-kyr cyclicity of the fluid infiltration in the fault zone.</p> <p>The development of the secondary permeability in the MMFS thus corresponds to a combination of faulting and tensile fracturing, in response to a cyclic increasing of the shear stress and the pore pressure during the seismic cycle. The polyphasic deformation system of the MMFS constitutes a record of fault activation and reactivation episodes that could contribute to define the recurrence model of seismic events on regional-scale faults.</p>

A fault gouge forms at the core of the fault as the result of a slip in the upper brittle crust. Therefore, the deformation mechanisms and conditions under which the fault gouge was formed can document the stages of fault movement in the crust. We carried out a microstructural analysis on a fault gouge from a hanging-wall branch fault of the Simplon Fault Zone, a major low-angle normal fault in the Alps. We use thin-section analysis, together with backscattered electron imaging and X-ray diffractometry (XRD), to show that a multistage history from ductile to brittle deformation, together with a continuous exhumation history from high to low temperature, took place within the fault gouge. Because of the predominance of pressure solution and veining, we associated a large part of the deformation in the fault gouge with viscous-frictional behaviour that occurred at the brittle-ductile transition. Phyllosilicates and graphite likely caused fault lubrication that we suggested played a role in the formation of this major low-angle normal fault.

<p>For thorough understanding of the dynamics of mountain building processes, it is crucial to reconstruct the youngest crustal deformation history over time. Low-angle normal faults are features caused by orogen-parallel extension, which occurs in the last stage of collision. Low-angle normal faults play a key role in the exhumation of the lower crust and they are the reason for most of the seismicity within the chain.</p><p>We carried out microstructural analyses on an outcrop in the footwall of one of the major normal faults of the Alpine chain, the Simplon Fault Zone. This low-angle normal fault extended the crust by tens kilometers and it caused exhumation of its footwall, the deeper lower crust of the Alps, i.e. the Penninic nappes. The Simplon Fault Zone itself consists of a thick mylonitic zone overprinted by a narrow cataclastic zone, with the same kinematics. Its timing evolution history from ductile to brittle deformation is still under discussion. This study shows a new microstructural analysis from a fault gouge within the footwall of the northern part of the Simplon Fault Zone, and how it can reconstruct the different stages of exhumation history of this shear zone.</p><p>Results from micro-structural analyses show grain boundary migration features on folded quartz veins. This suggests that the protolith of the fault zone was at high temperature conditions, T>600°C, during dynamic deformation. This folding belongs to extension-parallel folds that affect only the ductile shear zone. The presence of greenschist facies minerals suggests that the rock was exposed to low temperature and pressure conditions (T=300-400°C, P=0.2GPa). Pressure-solution mechanisms affect both quartz and greenschist paragenesis, indicating formation in a shallow position of the shear zone. The last deformation was purely brittle, as shown by vertical calcite veins or fractures in quartz. It suggests a near-surface position of the fault.</p><p>Altogether, these multiple deformation phases within the gouge samples indicate a continuous exhumation history from high to low temperatures, with clear cross-cutting relationships. However, the lack of cataclasite features can be related to an involvement of the rocks within the fault core in a subsequent stage of deformation. To explain this we suggest a model in which the footwall maintained a high temperature over a long time, which inhibited cataclastic processes.</p>