The KTB boreholes that were drilled from 1989 to 1994 near Windischeschenbach, NE Bavaria, penetrated rocks of the Zone Erbendorf-Vohenstrauss (ZEV). The upper and lower sections of the 9101m-deep profile are mainly composed of gneiss units (former greywacke sediments) and variegated units (alternating gneisses and metabasites, probably of volcano-sedimentary origin). Metabasic units (amphibolites, metagabbros) constitute mainly the middle section. Geochemical data suggest their derivation from oceanic crust. All rocks are probably pre- to early Ordovician in protolith age. They underwent early Devonian MP metamorphism. Relics of preceeding HP metamorphism are preserved in metagabbros. Felsic to mafic dykes (dominantly Upper Carboniferous lamprophyres) crosscut the metamorphic rocks. The ZEV units are steeply inclined down to the final depth. The main structures (foliation, folds, faults) trend NW–SE. Semibrittle to brittle deformation plays an unexpected important role. In contrast to the conditions of the Mesozoic (mainly Cretaceous/Tertiary) faulting the P–T conditions of MP metamorphism, the cooling ages and the late-Variscan graphite-bearing prehnite–actinolite facies mineralization show no significant depth-dependent gradients. This can be explained by Mesozoic supracrustal stacking. A bundle of reverse faults between 6850 and 7300 m belongs to the Franconian lineament and can be correlated with the most prominent seismic reflector (SE1) in the area.

The German Continental Deep Drilling Program (KTB) drilled two holes through crystalline rocks which are rich in both high-salinity fluids and graphite accumulated along shear zones. Analyses of a large number of borehole measurements yield models for the electrical resistivity of the upper and middle crust in the vicinity of the KTB holes. High observed resistivity, of more than 10(5) Omega m in the lowermost part of the 9000 m deep main hole, in a rather ''wet'' crust, indicates that effective mechanisms exist to cut down connections between fluid accumulations and therefore that fluids are not the likely cause of high-conductivity anomalies. On the other hand, graphite accumulations appear to be connected along shear lineaments over hundreds of meters or more. Structural, mineralogical, and geochemical studies suggest a tectonic model which explains the deposition of graphite as the relic and witness of a shearing process that occurred during the late Variscan (Upper Carboniferous) thrusting. This process took place while this part of the crust resided at temperatures between 240 degrees and 380 degrees C, Subsequent independent reverse faulting lifted this part to the Earth's surface. Our conclusion is that the KTB case indicates how high electrical conductivities in the upper crust, which originated from the middle to lower crust, are caused by graphite accumulations, rather than by fluids, and that these anomalies are related to shearing processes, Such graphite accumulations may exist elsewhere and may be of relevance in the context of present-day midcrustal conductors.

For almost 10 years the KTB superdeep drilling project has offered an excellent field laboratory for adapting seismic techniques to crystalline environments and for testing new ideas for interpreting seismic reflections in terms of lithological or textural properties of metamorphic rock units. The seismic investigations culminated in a three‐dimensional (3‐D) reflection survey on a 19×19 km area with the drill site at its center. Interpretation of these data resulted in a detailed, structural model of the German Continental Deep Drilling Program (KTB) location with dominant, steep faults in the upper crust. The 3‐D reflection survey was part of a suite of seismic experiments, ranging from wide‐angle reflection and refraction profiles to standard vertical seismic profiles (VSP) and more sophisticated surface‐to‐borehole observations. It was predicted that the drill bit would meet the most prominent, steeply dipping, crustal reflector at a depth of about 6500–7000 m, and indeed, the borehole penetrated a major fault zone in the depth interval between 6850 and 7300 m. This reflector offered the rare opportunity to relate logging results, reflective properties, and geology to observed and modeled data. Post‐Variscan thrusting caused cataclastic deformation, with partial, strong alterations within a steeply dipping reverse fault zone. This process generated impedance contrasts within the fault zone on a lateral scale large enough to cause seismic reflections. This was confirmed by borehole measurements along the whole 9.1 km deep KTB profile. The strongest, reflected signals originated from fluid‐filled fractures and cataclastic fracture zones rather than from lithological boundaries (i.e., first‐order discontinuities between different rock types) or from texture‐ and/or foliation‐induced anisotropy. During the interpretation of seismic data at KTB several lessons were learned: Conventional processing of two‐dimensional (2‐D) reflection data from a presite survey showed predominantly subhorizontal layering in the upper crust with reflectivity striking in the Variscan direction. Drilling, however, revealed that all rock units are steeply dipping. This confirms that surface common depth point (CDP) seismics strongly enhances subhorizontal reflectivity and may thus produce a very misleading crustal image. Although this was shown for synthetic examples earlier, the KTB provides the experimental proof of how crucial this insight can be.

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.

In 1989 comprehensive seismic studies were carried out at the German deep drilling location (KTB) in the Oberpfalz (NE-Bavaria) by the DEKORP group. The survey is known as Integrated Seismics Oberpfalz 1989 (IS089). The aims were to predict events ahead of the drill bit, to investigate the structures and to connect the more or les small-scale results from the KTB-drillhole with the large-scale geological/tectonical environment, to investigate parameters as seismic velocities and anisotropy and to learn about the nature of seismic-reflections in crystalline area.