Faults and veins in the superdeep well KTB: constraints on the amount of Alpine intra-plate thrusting and stacking of Variscan basement (Bohemian Massif, Germany)

Abstract

The crystalline rocks of the superdeep well KTB (9101 m depth) display abundant late- to post-Variscan brittle–ductile and brittle deformation structures. Clear depth-dependent variations in metamorphic conditions are obvious concerning Cretaceous subhorizontal tension gashes and reverse faults, whereas the Upper Carboniferous structures (subvertical tension gashes and reverse faults) show almost no variations with depth. The lack in depth-dependent gradients can be explained by strong Cretaceous reverse faulting that led to an intra-plate antiformal stack. The frontal ramp of this stack, a broad fault bundle referred to as the Franconian lineament, was drilled at 7300 m depth. It rises from a flat-lying detachment at approximately 9 km depth which corresponds to the brittle–ductile transition zone of quartz-bearing rocks. Supracrustal stacking and thickening along and above this ramp was particularly active during Alpine (Upper Cretaceous) foreland compression. Close to the Franconian lineament the Alpine impact strongly affected the Variscan crustal edifice disturbing the Variscan and late-Variscan depthdependent gradients in KTB.