INITIATION AND DEVELOPMENT OF A FAULT-CONTROLLED, OROGEN-PARALLES OVERDEEPENED VALLEY: THE UPPER ENNS VALLY, AUSTRIA.

The paper focuses on modelling the development of the orogen-parallel fault-controlled overdeepened Upper Enns Valley, Eastern Alps. A decrease in relief in the Middle Miocene, an increasing relief from Pliocene to recent times and the interplay between tectonically controlled differential uplift and denudation resulted in a morphology that is reflected by planation surfaces on valley slopes at elevations of about 1100 m. The Upper Enns Valley separates the crystalline basement of the Niedere Tauern in the south from the Greywacke Zone and the Northern Calcareous Alps in the north. The Upper Pleistocene Ramsau Conglomerate covers the Greywacke Zone on the northern valley slope and is suggested to have important information for reconstruct the history of overdeepening of the Upper Enns Valley in several steps. The formation of the Enns Valley started in Early to Middle Miocene and continued to recent times. Valley formation occurred under uplift along the ENE-trending, orogen-parallel transtensional Salzach-Enns strike-slip zone. The southern block suffered a much higher exhumation than the northern block. We interpret the asymmetric drainage pattern, with short northern tributaries to the Enns Valley and long southern ones, to indicate northward tilting of the entire region. Furthermore, Quaternary processes, such as uplift and cyclic glaciations, interfered with neotectonic activity. The provenance of the Upper Pleistocene Ramsau Conglomerate from the southern side of the valley and the presence of a terrace at 1100 m indicates that the Enns Valley was filled from the present base at ca. 820 m above sea level up to that elevation during Late Pleistocene times. Post-Ramsau Conglomerate glacial overdeepening scoured the Enns Valley and resulted in epigenetic and regressive incision of southern tributaries. The channels of the southern tributaries have knick zones, reflecting gradient changes between flat and steep reaches. The suggested continuity of the southern planation surfaces and the top of the Ramsau Conglomerate (at ca. 1100 m) is interpreted to represent a relic former valley bottom. Although close to the Salzach-Enns fault, pronounced knick zones along southern tributaries are interpreted to result from glacial overdeepening rather than from neotectonic activity. [ABSTRACT FROM AUTHOR]