Late to post-Variscan deformation phases and palaeostresses in the KTB pilot research well (Bohemian Massif, Germany)

Abstract

Brittle faults and veins are very widespread in the crystalline rocks of the northern Oberpfalz, particularly in the 4000 m deep Kontinentale Tiefbohrung (KTB) pilot well. In the late Variscan (Late Carboniferous), subvertical tension gashes were formed under NE-SW extension. A genetic relationship between the vein development and late-phase magmatic activity of the late Variscan granites is obvious. Subsequently, but still during the Late Carboniferous, graphite-enriched reverse faults developed under E-W and NE-SW compression. In the deeper part of the KTB pilot hole these faults were formed within the brittle-ductile transition regime of the paragneisses. Since, during the late Variscan, the geothermal gradient was remarkably elevated, the brittle-ductile regime was situated at a relatively high crustal level where cataclasis, crystal plasticity and diffusion-controlled deformation mechanisms were active simultaneously. In post-Variscan time, probably during the Cretaceous, a further generation of reverse faults developed in addition to subhorizontal tension gashes and minor strike-slip faults. During these events the temperature was much lower compared with the conditions during the late Variscan deformations. The reverse faults evolved primarily under N-S compression, which is probably related to the Cretaceous subduction and collision events of the Alpine orogeny further to the south. The youngest, normal, faults are probably connected to the subsidence of the nearby Eger Graben which evolved during the Neogene uplift of the investigated area. The stress data, derived by using faults and striations from surface outcrops and from several oriented core intervals from the KTB pilot well, display weak depth-dependent variations. Although the deformation was polyphase, the “inversion method” has proved to be a suitable mode for determining reliable palaeostress data. However, if the brittle-ductile transition regime or broad shear zones are reached, the data become more and more invalid.