Model of syn-convergent extrusion of orogenic lower crust in the core of the Variscan belt: implications for exhumation of high-pressure rocks in large hot orogens

International audience; Reflection seismic section, field structural analysis and gravimetric modelling of orogenic lower crust in the core of a Carboniferous orogenic root reveal details of the polyphase process of exhumation. Subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the NW. The fabrics are developed in both mid-crustal and lower crustal high-pressure granulite units as a result of intensive NW-SE intraroot horizontal shortening driven probably by the west-directed collision. In granulites, the steep fabrics originated as a result of extrusion of orogenic lower crust in a similar to 20 km wide vertical ascent channel from lower crustal depths at 350-340 Ma. The large granulite bodies preserve older granulite facies fabrics documenting a two-stage evolution during the exhumation process. Surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth, suggesting preservation of the similar to 20 km wide subvertical tabular structure reaching Moho depths. Horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units. This structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid-crustal levels (765 degrees C and 0.76 GPa) at 342-337 Ma. Large massifs of extruded and progressively dismembered felsic granulites disturbed mid-crustal fabrics in the surrounding horizontally flowing partially molten crust. The horizontal mid-crustal flow resulted in collapse of the supra-crustal Tepla-Barrandian Unit (interpreted as the orogenic lid) along a large-scale crustal detachment above the extruded lower crustal dome. The presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens.