THRUST ZONE KINEMATICS IN A BASEMENT COVER IMBRICATE STACK - EASTERN PELVOUX MASSIF, FRENCH ALPS

Thrust imbrication is now widely accepted as a major process in controlling basement-cover relationships in the external parts of the Western Alps. Complications in the simple geometries expected from thrusting alone arise from Alpine deformation affecting pre-existing extensional faults inherited from the old Mesozoic basins. This contribution describes the structural geology of part of the Eastern Pelvoux massif, centred on the Rocher de L'Yret, where Alpine deformation has masked any pre-existing structures. A series of major thrusts carry basement onto cover. These lie west of, and in the footwall to, the Pennine Front, a major zone of thrusting which carries the Internal Alps onto the external zones. Stretching lineations in calc-mylonites derived from Eocene limestones deposited on the basement, indicate a tectonic transport axis of WNW-ESE (plunging ca N100-degrees-E). This direction is supported by the orientation of striae and corrugations on faults which cut basement rocks. Shear criteria in the calc-mylonites imply top-to-the-WNW shearing. Despite this simple kinematic picture, basement-cover relationships cannot be explained by thrusting alone. A series of faults cut down through the thrust stack towards the WNW and show net extensional offsets with respect to the tectonic pile. These faults and associated shears in the calc-mylonites cannot be traced to depth, rather they appear to branch onto earlier thrust surfaces. This geometry is interpreted as due to distributed shearing in the hanging-wall to the local floor thrust, perhaps reflecting late reactivation of the Pennine Front.