Deformation band formation as a function of progressive burial: Depth calibration and mechanism change in the Pannonian Basin (Hungary)

Deformation bands (DB) are ubiquitous structural elements that can be found in Miocene pre- and syn-rift sediments of the extensional Pannonian Basin, central Europe. Across the field sites we see examples of disaggregation to cataclastic DB sets and evidence of frequent reactivation by discrete faulting. Thin section analysis, cross-cutting relationships and well-defined time constraints of analysed brittle structures demonstrate that with increased burial depth, DB deformation mechanisms progressed from granular flow to cataclasis. The DB sets were classified into 10 deformation phases based on formerly published independent fault-slip analysis and structural mapping. Subsidence curves were constructed for each stratigraphic level involved in deformation and were used along with the intersection of deformation episodes to calculate the depth intervals of DB generations and subsequent structural elements. The DB formation depths obtained were transferred to the depth range of the related mechanisms. This combined methodology permits a more quantitative approach to determine the changes in the deformation mechanisms with depth. Our results show that granular flow (disaggregation bands) dominates down to 100–150 m as the earliest deformation structure, followed by weak then moderate cataclasis. The transition between weak and moderate cataclasis is at approximately 300 ± 100 m for host rock rich in feldspar or fragile tuffitic components and from around 900 ± 100 m in quartz-rich sediments. In addition, deformation by frictional sliding concentrates on discrete fault planes at the margin of cataclastic bands or on new fracture planes from ∼500 ± 100 m in volcanoclastic or feldspar rich host rocks, and 1000 ± 100 m in quartz-rich host rocks. We suggest that burial-induced diagenetic processes is dependent on subsidence history, and partly in connection with regional fluid migration path, control the transition from moderate or advanced cataclasis to discrete fault slip. All these changes affect the pore structure and porosity that contribute to rheological changes, and hence change in deformation mechanism of coeval fracturing events.