Post‐Variscan thermal and tectonic evolution of the KTB site and its surroundings

Abstract

The post‐Carboniferous crustal evolution of the German Continental Deep Drilling Program (KTB) area, as summarized in this paper, could not be predicted from surface observations: deep drilling was essential for its revelation. The most conspicuous and unexpected feature discovered in the drill hole is the absence of marked gradients with respect to 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°C from the Carboniferous onward. The late to post‐Carboniferous deformation is essentially confined to several fault zones. A major fault zone encountered in the drill hole at 7000 m depth is linked by a prominent seismic reflector to the Franconian Lineament, the surface boundary between Variscan basement and Mesozoic cover. This fault zone probably 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, and the sedimentary record in the adjacent basins. Upper crustal slices were detached at a specific level, corresponding to the approximate position of the brittle‐ductile transition in post‐Variscan times, and form an antiformal stack that was penetrated by the KTB throughout its entire depth range.