Role of extraction faulting and out-of-sequence thrusting in collisional orogeny - the base of the Dent Blanche Nappe in the Alps as an example

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Low-angle tectonic contacts, both thrusts and normal faults, play a key role in the tectonics of collisional mountain belts and in the exhumation of high-pressure metamorphic rocks. Results of previous work suggest that a major low-angle tectonic contact in the western Alps (Combin Fault), where ultrahigh-pressure metamorphic rocks are overlain by lower-pressure ones, represents an extraction fault, formed by the coalescence of faults with opposite shear sense when the intervening volume of rocks was extracted in a downward direction. The Combin Fault is overlain by one of the most prominent thrust faults of the Alps, the Dent Blanche Basal Thrust (DBBT), along which continental basement rocks where emplaced on oceanic rocks. We hypothesize that the formation of this out-of-sequence thrust is closely related to the extraction process.The role of the DBBT in the Alpine orogeny is not clear due to conflicting observations and interpretations regarding its kinematics and the paleogeographical position of the Dent Blanche nappe. We will study it by a combination of structural field work, microstructural and textural analyses, as well as petrological and geochronological methods. This research will further the understanding of the tectonic processes that allow high-pressure rocks to be exhumed and that control the formation and reactivation of large-scale shear zones during continental collision. In combination with petrological and geochronological data we will be able to deduce a comprehensive view of the tectonic processes during early continent collision.