KTB - The structure of a Variscan terrane boundary: Seismic investigation - Drilling - Models

On October 12, 1994, the Hauptbohrung (main hole) of the German Continental Deep Drilling Program (KTB) in the Oberpfalz/NE Bavaria reached its final depth of 9101 m. The drilling results provide a test of the validity of the initial structural concept for the KTB location in the boundary zone between the Saxothuringian and Moldanubian terranes of the Variscan orogen. This paper outlines some of the geological results as well as additional information from seismic investigations, and discusses briefly alternative structural models. The interpretation of local and regional seismic structures leads, in connection with the drilled profile, to the conclusion that the ZEV (Zone Erbendorf-Vohenstrauss, with the KTB drill-site) is derived from and linked with the 'Erbendorf Body' and equivalents of the Marianske Lazne Complex and/or the Bohemicum in the middle and lower crust. Instead of a rootless klippe resting on the Saxothuringian-Moldanubian suture, the ZEV appears to be a part of the suture itself. The present position of the ZEV is tied to the junction of the ENE-WSW suture with the NNW-SSE Oberpfalz Block as a part of a large wrench system, and developed, probably, by a two-stage transpressive exhumation process prior to and during the Variscan collision.