Thrusting and extension in the southern dora-maira ultra-high-pressure massif (Western alps): View from below the coesite-bearing unit

The Dora-Maira ultra-high-pressure (UHP) unit, exposed in the internal Western Alps, is a similar to1-km-thick continental slice sandwiched between lower-pressure rock units. The contact at the base of the UHP unit is a major contractional shear zone that thrust the UHP unit and the overlying continental and oceanic units onto the Sanfront-Pinerolo unit. The latter, metamorphosed under blueschist-greenschist conditions, is the lowest structural unit exposed in the Western Alps. Our analysis indicates that all rock units of the Dora-Maira massif were sheared and juxtaposed together within a top-to-the-SW, large-scale, medium-temperature/low-pressure ductile shear zone in the middle crust. Thrust faulting was accompanied and followed by vertical thinning that progressively reduced the thickness of the orogenic wedge and brought the ultra-high-pressure (UHP) unit into the brittle crust. Contractional tectonic contacts in the southern Dora-Maira were concealed by late extensional structures. The majority of the extensional structures postdated the emplacement of the UHP unit over the Sanfront-Pinerolo unit, implying that extension lagged behind much of the exhumation of the UHP unit. Extensional shear zones in the Dora-Maira and elsewhere in the Western Alps are parallel to the strike of the orogen; their overall effect was to widen the orogenic belt in the (north) east-(south) west direction. The sense of extensional movement was toward the frontal part of the orogen so that rather than defining tectonic wedges formed during extrusion, the extensional structures represent lateral spreading of the mid- to upper crust of the internal Western Alps toward the foreland. Rapid compression of the Dora-Maira was coeval with the transition from marine flysch deposits to molasse sedimentation in the foreland of the Western Alps. The switchover from burial to exhumation and the lateral spreading of the upper crust in the internal Western Alps were coupled by readjustment of the entire orogenic wedge.