Paleo-stress fields in the Harz Mountains and surrounding areas between the Variscan convergent period and the Upper Cretaceous

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This proposed project is based on numerous investigations of the applicants conveming fault kinematics and paleo-stress fields in the area between the Subhercynian Basin and the Franconian Line (see list of publications H.J. Franzke). Most recent investigations have been conducted in a DFG-granted project about the subsidence history of the Subhercynian Basin and the synchronous uplift of the Harz block (Applicants: R. Gaupp & H. J. Franzke, final report Fr 874/4, in 2004, see appendix 5). The objective of this project is to determine the paleo-stress fields in the Harz Mountains and surrounding areas based on field studies of synchronously activated fault populations in different stratigraphic units and under several tectonic environments. In the Harz Mountains a continuous pile of Paleozoic sedimentary rocks is well exposed, surrounded by more than two kilometres thick Permian to Upper Cretaceous rocks. Meanwhile lots of radiometric and paleontological age data on the sedimentary development, the widespread Late Variscan intrusive/volcanic rocks and the countless fault-bound hydrothermal vein structures are available. Therefore a detailed time-control of fault activities can be checked over a long period of about 200 Ma between the Variscan convergent phase in the Carboniferous and several brittle faulting periods in Mesozoic and Cenozoic time. Results to these topics have already been published by different authors, including the applicants (see references), but up to now a summarising investigation over a larger area, using modern methods of structural geology, is missing. To evaluate the distribution of the regional paleo-stress pattern in the Harz Mountain area and its forelands the orientation of the principal stress directions is calculated by fault-slip data inversion. The displacement vector on fault planes recorded by slickenside striae is considered to correspond to the direction and sense of the shear-stress tensor. Based on this assumption, the deviatoric stress tensor is calculated using quantitative computeraided inversion methods. The results are obtained in terms of a reduced stress tensor, consisting of (1) orientations of the three principal stresses en, (1, (2 and (3 as with ((1 ((2 ((3 and (2) the ratio of principal stress differences,((((2((3(/((1((3(/ with 1((((. In the case of polyphase tectonics, the chronology of successive events is deduced and the total fault population from each site is qualitatively divided into different sub-sets, each being consistent with one specific temporal stress regime. Since stratigraphy and evolution of the proposed area are well known, temporal and spatial correlations of paleo-stress orientations are possible. A network of local determinations related to each tectonic event will allow to reconstruct the regional paleo-stress patterns in time. Finally they will be evaluated with respect to the control they exerted on the structural evolution of the Harz black and its forelands. Our proposed study may complete the project Mesozoic fault systems (grant Sehe 674/1- 1) and can be also useful for the other two proposed projects investigating the paleo-stress evolution (Bayer and Littke; Urai and Tanner). That means, the formulated aims are relevant to other projects of the SPP 1135: Sedimentary basin dynamics under fluctuating stress fields .