DISPLACEMENT LOCALIZATION AND PALAEO-SEISMICITY OF THE RENCUREL THRUST ZONE, FRENCH SUB-ALPINE CHAINS

The palaeo-seismicity and history of strain accumulation within the Re ncurel Thrust Zone, French Sub-Alpine Chains, has been investigated by examining fracture-filling cements. Two generations of fracture-filli ng cement with distinct petrographic characteristics, cation geochemis tries and C-13 and O-18 stable isotopic compositions have been disting uished within the Rencurel Thrust Zone. The hanging-wall rocks contain fracture-filling calcite cements, whereas ferroan calcite fills fract ures within the Central Gouge Zone. Calcite-cemented faults include fr ontal, oblique and lateral ramps, as well as faults dipping in the mov ement 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, indi cating that the calcite cements formed prior to precipitation of ferro an calcite in the Central Gouge Zone. Lineation data indicate that pre cipitation of both generations of cement occurred during a single phas e of thrusting. The lack of ferroan calcite cements in the hanging-wal l rocks suggests that the hanging-wall of the Rencurel Thrust was not fractured during displacements within the Central Gouge Zone. This con trasts with seismogenic faults where rocks surrounding major faults ar e fractured during fault slip, and it is inferred that the exposed por tion of the Rencurel Thrust may have experienced aseismic fault displa cements. The aseismic fault slip may have been the consequence of the shallow burial depths (<3 km) experienced by the exposed portion of th e Rencurel Thrust during faulting.