Crustal evolution of the KTB drill site: From oldest relics to the Late Hercynian granites

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Abstract
The crustal unit penetrated during the German Continental Deep Drilling Program (KTB), the Zone of Erbendorf‐Vohenstrauss (ZEV), comprises an association of metabasites and paragneisses and orthogneisses that underwent high‐ and medium‐pressure metamorphic cycles during the early Palaeozoic. In this summary of the structural, petrological, geochemical and geochronological information from borehole and surface rocks, we show that geological models proposed prior to drilling have had to be significantly modified. Features of the ZEV, such as the dominant NW‐SE trending structures, Devonian (>370 Ma) medium‐pressure, amphibolite facies, metamorphism (0.6–0.8 GPa, ∼720°C), and earlier eclogite stage, are directly comparable with those of the nearby Bohemicum unit in western Bohemia. Intervening units, in contrast, exhibit NE‐SW trending structures and Carboniferous (315–325 Ma), low‐pressure, 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 km and therefore no nappe boundary was drilled. Instead, it is interpreted that shallow crustal levels reached by the ZEV, subsequent to the Devonian event, shielded most of it from the effects of the Carboniferous metamorphism. A lack of expected thermal and baric gradients within the drilled metamorphic pile is a result of repeated stacking of an original ∼3 km depth section in Mesozoic times. An important result of the geological investigations of the KTB project is the recognition that the present‐day geometry of the ZEV is a result of cold, shallow‐level, thin‐skinned tectonics that, in the absence of the view into the third dimension offered by the drill hole, could not have been accurately predicted from the fossil, high‐grade assemblages and structures seen at the surface.
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