Structure of the Saxonian Granulites: Geological and geophysical constraints on the exhumation of high-pressure/high-temperature rocks in the mid-European Variscan belt

The Saxonian Granulites represent a major exposure of high-pressure rocks w ithin the mid-European Variscan belt. The granulites emerge in an extension al dome structure The boundary between beneath a low-grade Paleozoic cover. The boundary between the granulites and their cover is a crustal-scale she ar zone with transport top to the SE, juxtaposing high-pressure (WP) granul ites against greenschist-grade rocks. Seismic reflection and refraction pro filing reveal that the granulite dome and its western continuation up to th e SW margin of the Bohemian Massif are underlain by a reflective layer up t o 1 s two-way time (TWT) thickness (similar to 3.5 km), with P wave velocit ies V-p generally above 6.0 and up to 7.0 km/s (probably a sheet of metabas ic rocks). This layer exhibits a NE trending antiformal structure, in line with the granulite antiform, with an apex at similar to 1.2 s TWT. The outc rop of felsic granulite forms a local cap on the NE part of this high-veloc ity layer. A magnetotelluric survey has revealed high resistivity in the up per crust and a zone of high conductivity under the high-velocity layer, in the middle and lower crust, terminating similar to 10 km to the south of t he granulite outcrop. Similar high-grade racks occur in the Erzgebirge anti form SE of the Saxonian Granulites, but their exhumation was later followed by grossly westdirected stacking with medium-pressure and low-pressure roc ks, followed by backthrusting toward the SE and late open folds. Isotopic d ata both from the Saxonian Granulites and the Erzgebirge indicate HF metamo rphism similar to 360-370 Ma, followed by a granulite stage at 350-340 Ma. This is entirely incompatible with the record of low-grade sediments overly ing the crystalline rocks, which document subsidence and marine sedimentati on lasting until similar to 330 Ma. This paradox is explained by tectonic u nderplating, differential thinning of the high-grade rocks derived firom on e of the subduction zones The granulites emerge in an extensional dome stru cture adjacent towards the NW and SE. Tectonic underplating and beneath a l ow-grade Paleozoic cover. The boundary between exhumation of the granulites must have occurred under the floor ofa marine basin.