Supracrustal intraplate thickening of Variscan basement due to Alpine foreland compression: Results from the superdeep well KTB (Bohemian Massif, Germany)

Until its final depth of 9101 m the superdeep well KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) drilled through Variscan basement rocks which suffered strong brittle‐ductile and brittle deformation during late‐ and post‐Variscan times. Investigations of faults and mineralized tension gashes revealed the following deformation stages: (1) Upper Carboniferous subvertical tension gashes, (2) Upper Carboniferous reverse faults, (3) Cretaceous subhorizontal tension gashes and reverse faults, (4) ?Neogene normal faults. Clear depth‐dependent variations of metamorphic conditions of the post‐Variscan deformation structures are obvious, whereas the Upper Carboniferous structures show almost no variations with depth. This refers to the metamorphic index minerals as well as to the deformation fabrics in quartz. We explain this considerable lack of gradients as being the result of reactivative reverse faulting that led to considerable vertical thickening of the upper crust. The fault geometry indicates an antiformal stack, the frontal ramp of which was drilled by KTB at 7000 m depth. There is strong evidence that this frontal ramp, referred to as the Franconian Lineament at the surface, rises from a subhorizontal detachment at about 10 km depth which corresponds to the brittle‐ductile boundary layer of quartz‐bearing rocks. The considerable amount of supracrustal vertical thickening above the Franconian Lineament results from repeated movements along this detachment since Upper Carboniferous times. It seems reasonable to suppose that in intraplate tectonic settings the brittle‐ductile boundary layer is prone to form a long‐lived decoupling horizon along which the upper brittle crust is detached from the viscous middle and lower crust.