STRUCTURAL EVOLUTION OF AN EXTRUSIONAL WEDGE: CONSTRAINTS FROM A FORGOTTEN FAULT SYSTEM IN THE EASTERN ALPS

Structural inheritance controls the development of internal structures within extruding wedges. Distinct crustal-scale fault systems are generally reactivated instead of forming new faults. We exemplify this hypothesis by the Upper Oligocene–Miocene Central East Alpine east-directed extrusional wedge. This wedge is characterized by N–S extension across the motion direction of the extrusional wedge, high-angle normal faults opposite to the motion direction (Görtschitz Valley fault system - GVFS, which has not been considered in previous models of extrusion tectonics), and block tilting towards the motion direction. The Miocene high-angle normal faults reactivate an area of strong Late Cretaceous low-angle normal faulting. The GVFS is the most prominent fault system within the central Eastern Alps and juxtaposes superimposed Late Cretaceous, latest Paleocene–Middle Eocene, Lower Miocene and Quaternary sedimentary basins to the uplifted Saualpe basement block with its Cretaceous-aged eclogites. The basins were deposited onto the uppermost structural level of the Austroalpine nappe complex and allow to define the succession of tectonic events of the extruding wedge since Late Cretaceous: (1) Extreme Late Cretaceous ESE–WNW extension resulted in exhumation of eclogites along ductile low-angle normal faults and formation of the collapse-type Krappfeld Gosau basin. (2) After Paleocene deep subtropical weathering, (3) an Eocene marine transgression occurred due to N–S extension forming the Sonnberg–Krappfeld basin, which is deformed by gentle Late Eocene or Oligocene folds. (4) During subsequent Late Oligocene-Early Miocene eastwards extrusion, the GVFS was activated and at least 3 km of surface uplift of the Saualpe block occurred. The fluvial Waitschach Gravel of likely Karpathian age (Early Miocene) was deposited along the western branch of the GVFS with clastic input from the hangingwall units of the Saualpe block. (5) Incision of the Görtschitz Valley started in post-Karpathian times and resulted in a modified regional drainage system. (6) During Quaternary times, the unglaciated Krappfeld basin subsided again and was filled with ca. 200 m fluviatile gravels. We attribute Quaternary subsidence to still ongoing eastward extrusion and E–W shortening.