Displacement localization and palaeo-seismicity of the Rencurel Thrust Zone, French Sub-Alpine Chains

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The palaco-seismicity and history of strain accumulation within the Rencurel Thrust Zone, French Sub-Alpine Chains, has been investigated by examining fracture-filling cements. Two generations of fracture-filling cement with distinct petrographic characteristics, cation geochemistries and 13C and 18O stable isotopic compositions have been distinguished within the Rencurel Thrust Zone. The hanging-wall rocks contain fracture-filling calcite cements, whereas ferroan calcite fills fractures within the Central Gouge Zone. Calcitecemented faults include frontal, oblique and lateral ramps, as well as faults dipping in the movement direction, whereas, ferroan calcite-cemented faults include only frontal ramps and faults dipping in the movement direction. Fragments of calcite cement occur as clasts within the Central Gouge Zone, indicating that the calcite cements formed prior to precipitation of ferroan calcite in the Central Gouge Zone. Lineation data indicate that precipitation of both generations of cement occurred during a single phase of thrusting. The lack of ferroan calcite cements in the hanging-wall rocks suggests that the hanging-wall of the Rencurel Thrust was not fractured during displacements within the Central Gouge Zone. This contrasts with seismogenic faults where rocks surrounding major faults are fractured during fault slip, and it is inferred that the exposed portion of the Rencurel Thrust may have experienced aseismic fault displacements. The aseismic fault slip may have been the consequence of the shallow burial depths (<3 km) experienced by the exposed portion of the Rencurel Thrust during faulting.