Formation of the Bohemian Massif high-grade rocks

The Bohemian Massif located in central Europe occupying the territory of central Czech Republic, eastern Germany, and northern Austria is the largely exhumed and well-preserved part of the collision zone of two supercontinents Laurasia (in the north) and Gondwana (in the south). The accretion of different terranes during this collision (the Variscan Orogeny) occurred in the Paleozoic Era (300-500 million years ago). Uncertainties in the number, structures, and forms of the involved microplates make it very difficult to reconstruct tectono-magmatic evolution of the Variscan Orogeny, in particular the Bohemian Massif. The most interesting rocks in the Bohemian Massif are gneisses and granulites, which represent the continental crust being buried to mantle depths (> 60 km). The geological data predict that after the burial the continental rocks underwent deformation and metamorphism at highultrahigh- pressure, then been exhumed, shortly underwent ultrahigh-temperature metamorphism at lower crustal depth, and finally been exhumed to middle-to-upper crustal depths. The geologists proposed the variety of the tectonic scenarios being able to produce such rocks within the Bohemian Massif, including a continental subduction (where the rocks of continental crustal margin are brought to the big depths due to the heavy attached oceanic plate, and then after the detachment of the oceanic plate are exhumed towards the surface because of their low density) and crustal underthrusting (where the continental rocks do not sink deep into the mantle during collision event, but penetrate between the overriding crust and the mantle, being deformed at the high temperatures and finally exhumed within the overriding continental crust). Unfortunately none of these mechanisms have been tested numerically. In this project we will use a numerical modeling approach for the evaluation of the theoretical models for the Bohemian Massif rocks formation and will compare the results with the variety of the available geological data. The project is supposed to introduce further (after a successful project of the applicant funded by Lise-Meitner Program by FWF) one of the most modern plate tectonics modeling tools (in cooperation with Prof. T. Gerya, ETH Zurich) to the Austrian research landscape and will be carried out at Graz University where there is a strong research group on mountain building processes. The numerical experiments will be tested against voluminous structural and petrological datasets for the Bohemian Massif available through cooperation with Prof. S. W. Faryad (Charles University, Prague), and Prof. H. Fritz (Graz University), the two worldwide-recognized experts in petrology, geodynamics, and origin of the orogenic complexes.

In this project, we investigated possible scenarios for the Bohemian Massif formation. Despite long years of research, there is no definite tectonic concept for that. There are some interesting rocks in the massif, which attract geologists from all over the world. These crustal rocks were brought during collision to the depth of up to 150 km, then exhumed to the crustal level of 50-60 km, where they were intensively heated. In the project we took these data as the reference point and performed a series of numerical experiments, where we simulated continental collision, to find a possible model which is able to produce these particular changes recorded in the rocks of the Bohemian Massif. Finally, we found one model which provided us with very similar results observed in nature. The main mechanism in this model is a short stage of slab rollback (where the subducting part of the continental plate is moving backwards) during continental collision. This mechanism has two effects on the subducted rocks: 1) it provides a weak channel along the contact with the mantle wedge that allows a rapid exhumation from the big depths; 2) it generates an exhumation of hot mantle, which lead to the intensive heating of the exhumed rocks. This mechanism can be applicable not only for the Bohemian Massif, but also for other collisional orogens where we deal with similar two-stages record in metamorphic evolution of the crustal rocks. In the second part of the project we have focused on the mantle rocks recorded very high pressures and temperatures and appearing now within the crustal rocks which show much lower pressures and temperature. We have performed a series of experiments with distinct styles of the exhumation of the continental crust and analyzed all mantle rocks particles occurring within the crustal rocks by the end of collision. We detected a big variety of mantle rocks there: some materials came from the mantle wedge located above the subducting plate, some materials came from the very far regions of the mantle and came close to the subduction channel due to convection in the mantle, overcoming hundreds of kilometers. Besides the fact that this is a pioneer work in this field and could help in general to reconstruct the orogeny based on this analysis, that allowed us to conclude, that some high-grade mantle rocks in the Bohemian Massif were most likely extracted by the vertically or diapirically exhuming crustal materials. Based on the model from the first part of the project with slab rollback, where the vertical crustal extrusion was the prevailed mechanism, we believe, that this scenario could be indeed considered as a possible mechanism for continental crust exhumation in the Bohemian Massif.