Extensional structures and their tectonic inversion in the Western Alps

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Abstract
During Alpine collision the western margin of Tethys was uplifted on large-scale thrust zones, to preserve and give excellent 3D exposures of Mesozoic fault geometries. The faults are particularly well exposed along the margins of the Belledonne and Pelvoux basement massifs, east of Grenoble. Jurassic normal faults were in places reworked and the basins partially inverted during Cretaceous to Miocene compression. They were subsequently locally reworked by an important phase of late Alpine extension. Both listric and straight (rotational) normal faults occur in Variscan granitic basement and Mesozoic cover. Several orders of synthetic and antithetic faults can be mapped, recording strains which are probably related to bends or kinks in the surfaces of the master faults. The extension directions vary, apparently related to variations in anisotropy of the Variscan basement, in particular to the trends of Variscan mylonites. In some regions there was apparently simultaneous extension on two orthogonal fault orientations. On the northern margin of the Pelvoux Massif, late Alpine extensional faults rework both Variscan fabrics in the basement and Alpine mylonites in the cover, producing a large extensional duplex whose roof is a low-angle fault zone in schistose Lias. The late Alpine extension may be caused by gravitational collapse of the thickened crust, or by regional continental extension as part of the opening of the Gulf of Lyon and the Rhone—Bresse Basins. Pulses of compressional tectonics occurred in the Middle Cretaceous, latest Cretaceous—Early Palaeogene, Eocene—Oligocene and Miocene. During each pulse the deformation occurred principally on foreland-propagating fold and thrust sequences but there are important break-back thrusts, e.g. along the Frontal Pennine Zone, where the thrusts slice across already-sheared and back-tilted rocks to give apparent extensional cut-off relationships. The Alpine compression reactivated Mesozoic faults in basement and syn-rift sediments, deforming the syn-rift graben fill by tight upright folds and back-thrusts. NW-trending Mesozoic lateral ramps gave the basin margin a tooth-like geometry, which on compression did not fit back into the sockets of the opposite margin, causing material to be squeezed out laterally as well as vertically. Such variations in thrust displacement, a form of continental escape similar to large scale indentation tectonics, were partially responsible for the development of the Alpine arc.