COMPLEXITIES OF AN OROGEN-PARALLEL FAULT SYSTEM: THE MIOCENE ENNS VALLEY BASIN (AUSTRIA) AND THE NORTH ENNS VALLEY FAULT

Late-stage strike-slip faults occur in many mountain belts and guide the orogen-parallel drainage system. Here, we show an example of the SEMP fault system in the central Eastern Alps, which shows a distinct pattern of deformation.

The Miocene Enns Valley basin initiated along the ENE-trending Salzach-Enns-Mariazell-Puchberg fault and this fault separates the exhumed Hohe and Niedere Tauern blocks from the Miocene basin fill. Deposits of the Miocene Enns Valley basin occur in a number of dispersed exposures along the northern valley margin and are nearly exclusively derived from the southern Ennstal Quartzphyllite, Wölz Micaschist and Schladming/Bösenstein complexes. We also recognized a very specific, unique contributor, the Hochgrößen serpentinite massif. The Enns Valley basin fill is confined and disrupted along its northern margin by the North Enns Valley fault, a hitherto unidentified fault, which separates the Miocene Enns Valley basin from the Northern Calcareous Alps. The North Enns Valley fault postdates the deposition of the Miocene Enns Valley basin fill and likely extends to the WSW into the Mandling fault and, to the ENE, into the Pyhrn fault. If this interpretation is correct, then a ca. 20 km dextral offset and ca. 1–1.2 km northern block up displacement occurred along this fault, mostly during the Late Miocene/Early Pliocene inversion during E–W shortening as postulated by previous models. Dextral displacement along the North Enns Valley fault could also explain the Weyer Arc, a specific feature within the eastern Northern Calcareous Alps. This arc could be explained by accommodating a dextral displacement at the eastern termination of the North Enns Valley fault by counterclockwise rotation.