Session 7: Orogenic processes

David C. Tanner1

(tanner@perm.geologie.uni-freiburg.de)

Jan H. Behrmann1

(behrmann@perm.geologie.uni-freiburg.de)

Onno Oncken2

(oncken@gfz-potsdam.de)

Klaus Weber3

(kweber@gwdg.de)

1 Geologisches Institut, Universität Freiburg, Albertstraße 23b, D-79104 Freiburg, Germany

2 GeoForschungsZentrum, Telegrafenberg C2, D-14472 Potsdam, Germany

3 IGDL, Universität Göttingen, Goldschmidtstraße 3, D-37077 Göttingen, Germany

A wealth of data on the post-Variscan deformation of the western margin of the Bohemian Massif was obtained from the German continental deep drilling project 9 km borehole (KTB). Three major intracontinental tectonic events have been recognised: Neogene to present day uplift, Upper Cretaceous tectonic upper crustal thickening and Upper Carboniferous-Permian late-Variscan thrusting and extension. A three-dimensional database has been constructed using all available geological and geophysical data from the Zone of Erbendorf-Vohenstrauß (ZEV), the tectonometamorphic unit drilled by the KTB. In a series of retro-scenarios, deformations have been removed sequentially in reverse order using a computer program capable of performing three-dimensional kinematic restoration. Paleostress analyses have shown that the Upper Cretaceous deformation event was clearly not only compressive but also contained a large dextral strike-slip component. The tectonic effects of this displacement have therefore been studied by retro-deforming the ZEV with a sinistral transtensional sense. The hanging wall was deformed using an inclined shear algorithm, and the consequences of different parameters such as shear vectors and fault detachment projection at depth were also analysed. Both the Frankonian and Luhe Lineaments (the latter is perpendicular to the former), together with a basal detachment at 10 km, are used simultaneously as the major faults. The retro-scenario has shown that uplift and crustal thickening are very localised in the ZEV. Only 5 km displacement on the major fault system can produce up to 40% thickening of the upper crust above the detachment. The 30° change in strike of the main fault causes considerable "fault-bend" effects, promoting strong local uplift in the vicinity of the KTB location. Strain is heterogeneously distributed throughout the hanging wall. Zones of high strain in this restoration correlate with probable faults in the subsurface of the ZEV (such as the Fichtelnaab fault) and prove the existence of others.