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

Abstract

The Saxonian Granulites represent a major exposure of high‐pressure rocks within the mid‐European Variscan belt. The granulites emerge in an extensional dome structure beneath a low‐grade Paleozoic cover. The boundary between the granulites and their cover is a crustal‐scale shear zone with transport top to the SE, juxtaposing high‐pressure (HP) granulites against greenschist‐grade rocks. Seismic reflection and refraction profiling reveal that the granulite dome and its western continuation up to the SW margin of the Bohemian Massif are underlain by a reflective layer up to l s two‐way time (TWT) thickness (∼3.5 km), with P wave velocities Vp generally above 6.0 and up to 7.0 km/s (probably a sheet of metabasic rocks). This layer exhibits a NE trending antiformal structure, in line with the granulite antiform, with an apex at ∼1.2 s TWT. The outcrop of felsic granulite forms a local cap on the NE part of this high‐velocity layer. A magnetotelluric survey has revealed high resistivity in the upper crust and a zone of high conductivity under the high‐velocity layer, in the middle and lower crust, terminating ∼10 km to the south of the granulite outcrop. Similar high‐grade rocks occur in the Erzgebirge antiform SE of the Saxonian Granulites, but their exhumation was later followed by grossly westdirected stacking with medium‐pressure and low‐pressure rocks, followed by backthrusting toward the SE and late open folds. Isotopic data both from the Saxonian Granulites and the Erzgebirge indicate HP metamorphism ∼360–370 Ma, followed by a granulite stage at 350–340 Ma. This is entirely incompatible with the record of low‐grade sediments overlying the crystalline rocks, which document subsidence and marine sedimentation lasting until ∼330 Ma. This paradox is explained by tectonic underplating, differential thinning of the hanging wall lithosphere, and extensional unroofing of the high‐grade rocks derived from one of the subduction zones adjacent towards the NW and SE. Tectonic underplating and exhumation of the granulites must have occurred under the floor of a marine basin.