POST-VARISCAN THERMAL AND TECTONIC EVOLUTION OF THE KTB SITE AND ITS SURROUNDINGS

The post-Carboniferous crustal evolution of the German Continental Dee p Drilling Program (KTB) area, as summarized in this paper, could not be predicted from surface observations: deep drilling was essential fo r its revelation, The most conspicuous and unexpected feature discover ed in the drill hole is the absence of marked gradients with respect t o the pre-Carboniferous record, There are no depth-related differences in K-Ar cooling ages of hornblende and white mica, in petrology or in lithology, All metamorphic rocks encountered, both at the surface as well as in the drill hole down to 9100 m depth, were below 300 degrees C from the Carboniferous onward. The late to post-Carboniferous defor mation is essentially confined to several fault zones, A major fault z one encountered in the drill hole at 7000 m depth is linked by a promi nent seismic reflector to the Franconian Lineament, the surface bounda ry between Variscan basement and Mesozoic cover, This fault zone proba bly formed in the late Paleozoic and reactivated as a reverse fault in the Mesozoic. Two important episodes of NE-SW directed shortening by movements along reverse faults took place in the early Triassic and in the late Cretaceous, as indicated by the distribution of apatite and titanite fission-track ages, the sericite K-Ar ages of fault rocks, an d the sedimentary record in the adjacent basins, Upper crustal slices were detached at a specific level, corresponding to the approximate po sition of the brittle-ductile transition in post-Variscan times, and f orm an antiformal stack that was penetrated by the KTB throughout its entire depth range.