RAPID BURIAL AND EXHUMATION DURING OROGENY: THICKENING AND SYNCONVERGENT EXHUMATION OF THERMALLY WEAKENED AND THINNED CRUST (VARISCAN OROGEN IN WESTERN EUROPE)

摘要

We present a model demonstrating that the presence of a thinned and thermally weakened crustal domain is a prerequisite for the formation of an orogenic root in mountain belts. Furthermore this mechanism permits burial to 50 to 70 km in time periods of less than 20 Myr and exhumation from these depths in less than 10 Myr. Our example from the Variscan Belt is based on field geology, petrology of metamorphic rocks and detailed geochronology. It describes an evolution from (Ⅰ) sedimentation and magmatism during back-arc extension, (Ⅱ) compression leading to burial, thickening of the lithosphere and granulite-facies metamorphism, followed by (Ⅲ) exhumation, formation of dehydration melts, migmatites and late intrusive granites. The existing U-Pb, Ar-Ar data constrain this evolution to no more than 20 to 25 Myr. Thermorheological modelling starts from a hot thinned crust forming a narrow (50 km wide) orogenic root within 15 Myr at a maximum of 50 km depth, and subsequent exhumation by extrusion within 7 Myr. The model explains exhumation rates of 3 to 4 mm/year by compressional extrusion and shows that extension cannot do the same job within the same time. Extrusion transported the internal thermal structure into the upper crust within a short time, followed by rapid cooling, whereas exhumation through extension would cause mantle upwelling and the formation of a long-lived thermal anomaly. The latter would contradict the geochronological and petrological database. The model explains orogeny as a thermorheological process without needing to involve an external heat source from the mantle, and presents a new explanation of the high-temperature and low-pressure conditions very typical of the Variscan orogenic crust.