The architecture of the Trofaiach pull-apart Basin (Eastern Alps): An integrated geophysical and structuralstudy

Numerous sedimentary basins including the Trofaiach Basin were formed along wrench corridors during the Miocene lateral extrusion of the Eastern Alps. Because of its rhomboidal outline, a pull-apart mechanism was proposed for the Trofaiach Basin already in the 1980s. However, the internal basin architecture is still widely unknown. To get a better insight into basin formation during continental extrusion, the Trofaiach Basin was studied integrating different geophysical techniques (gravity, seismics, magnetics), digital elevation models, microtectonic and maturity data. Basin formation is related to the E-W trending Trofaiach strike slip fault, which enters the basin at its eastern tip. The northern basin margin is controlled by a terminating branch of this fault, while the main movement was transferred through the basin along subvertical faults in the central basin and along its southern rim. (Oblique) normal faults define the western basin margin. The basin depth reaches a maximum of 800 to 900 m. A fluvial and shallow lacustrine environment was interpreted from seismic facies and borehole data. Clinoform geometries and petrographic evidence indicate sediment supply mainly from the South. Localized coal seams developed in different stratigraphic positions. Water depth probably did not exceed 50 m. Deep lacustrine environments resulting from high subsidence rates are characteristic for many pull-apart basins, but were not established in the Trofaiach Basin. Several erosional events are part of the evolution of the basin. An early erosional phase followed southward tilting of the oldest basin fill and uplift of basement rocks north-west of the basin. A second event caused a major erosional unconformity in the central basin. Finally, related to post-Middle Badenian compression, more than 1 km of strata have been eroded.