A buried fold and thrust belt: the structural geometry of the central part of the Tisza Unit, East Hungary

The basement of the south-eastern part of the Miocene Pannonian back-arc basin is represented by the Tisza Unit. The deep structure of the Tisza unit is poorly studied, despite its significant geothermal and CH potential.  This work is a first step in our structural mapping project, which investigates the structures within the basement of the Pannonian Basin. 

The Tisza unit is composed of Proterozoic to Early Paleozoic poly-metamorphic basement rocks, and Late Paleozoic to Mesozoic sedimentary cover. The Tisza Unit is built up by three main nappes, the Mecsek, the Villány-Bihar and the Codru subunits. The Tisza Unit is exposed in inselbergs (Mecsek, Villány, Apuseni Mts.), however, most of it is covered by several km thick Miocene succession. The pore space containing energy source materials is located in the Miocene Pannonian Basin cover sediments, and in the fractured basement rocks near its surface and in their deeper part, especially in the Cretaceous sedimentary formation. Our research targets the better understanding of the Alpine shortening tectonics and structure of the Tisza Unit, with special attention to the structures of these tectonically buried sedimentary basement patches.

In this study we use modern 3D seismic data sets and well data to investigate the central part of the Tisza Unit. Based on that, the Tisza Unit is a Late Cretaceous fold and thrust belt, which can be characterized by major thick-skinned nappes, and second-ordered thin-skinned structures. Such second-ordered structures are the active and passive roof-duplexes below the Villány nappe (Derecske), and out-of-the-syncline thrusts in the front of the Codru nappe (Vésztő). The basal thrust of the Villány nappe cuts across pre-existing normal faults and associated half-grabens, demonstrating the presence of the early Alpine rift-related structures. Major nappes are unconformably overlain by Santonian to Maastrichtian beds, nevertheless, the presence of growth-synclines in this succession indicates ongoing shortening after major nappe emplacement during the latest Cretaceous. The Cretaceous fold and thrust belt of the Tisza Unit is strongly overprinted by Miocene extensional and transtensional structures, which are related to the rifting of the Pannonian back-arc basin.