CRUSTAL EVOLUTION OF THE KTB DRILL SITE - FROM OLDEST RELICS TO THE LATE-HERCYNIAN GRANITES

The crustal unit penetrated during the German Continental Deep Drillin g Program (KTB), the Zone of Erbendorf-Vohenstrauss (ZEV), comprises a n association of metabasites and paragneisses and orthogneisses that u nderwent high-and medium-pressure metamorphic cycles during the early Palaeozoic. In this summary of the structural, petrological, geochemic al and geochronological information from borehole and surface rocks, w e show that geological models proposed prior to drilling have had to b e significantly modified. Features of the ZEV, such as the dominant NW -SE trending structures, Devonian (>370 Ma) medium-pressure, amphiboli te facies, metamorphism (0.6-0.8 GPa, similar to 720 degrees C), and e arlier eclogite stage, are directly comparable with those of the nearb y Bohemicum unit in western Bohemia. Intervening units, in contrast, e xhibit NE-SW trending structures and Carboniferous (315-325 Ma), low-p ressure, metamorphism: all units are cut by predominantly posttectonic granites (mostly <320 Ma). Earlier models to explain this contrast by means of a nappe emplacement of the ZEV; in the narrow time interval between the younger regional metamorphism and granite intrusion, could not be substantiated because, contrary to predictions, the same rock association was present in the whole of the drilled 9.1 lan and theref ore no nappe boundary was drilled. Instead, it is interpreted that sha llow crustal levels reached by the ZEV, subsequent to the Devonian eve nt, shielded most of it from the effects of the Carboniferous metamorp hism. A lack of expected thermal and baric gradients within the drille d metamorphic pile is a result of repeated stacking of an original sim ilar to 3 km depth section in Mesozoic times. An important result of t he geological investigations of the KTB project is the recognition tha t the present-day geometry of the ZEV is a result of cold, shallow-lev el, thin-skinned tectonics that, in the absence of the view into the t hird dimension offered by the drill hole, could not have been accurate ly predicted from the fossil, high-grade assemblages and structures se en at the surface.