Geometry, amount, and sequence of thrusting in the Subalpine Molasse of western Austria and southern Germany, European Alps

• Published in: Tectonics (Washington, D.C.) D.C.), 2015-01, Vol.34 (1), p.1-30
• Main Authors: Ortner, Hugo, Aichholzer, Silvia, Zerlauth, Michael, Pilser, Roland, Fügenschuh, Bernhard
• Format: Article
• Language: English
• Subjects: Eastern European Alps; foreland basin; Miocene; Miocene shortening; Plate tectonics; Seismology; Tectonics; thrust belt
• ISSN: 0278-7407; 1944-9194
• DOI: 10.1002/2014TC003550

In this paper we review the structure of the most external thrust belt of the Alps between the Rhein valley and Salzburg based on a new tectonic map and the (re)interpretation of seismic sections. Specifically we address the correlation between deformation in the Subalpine Molasse and the Alpine thrust belt in general and focus on the control of sedimentary facies on the structural style. A dramatic change in architecture from a ramp‐flat structure to buckle folding is related to a change from coarse‐grained fans to fine‐grained deposits within the Subalpine Molasse. Additionally the interaction of escape tectonics with postcollisional shortening controls the decrease of late Early Miocene and younger shortening within the Subalpine Molasse from 50 km near the Rhine valley to almost zero near Salzburg. Transfer of shortening into the hinterland, which is the zone of lateral escape, ended foreland propagation of the Alpine thrusts and initiated a general break‐back sequence of thrusting. Throughout this time the thrusts remained active. In such a scenario, tectonic units on top of the Subalpine Molasse are expected to undergo clockwise rotation around vertical axes. As thrusting in the Subalpine Molasse is closely related to contemporaneous transport and shortening within the tectonically higher Helvetic thrust sheets, amounts of Miocene differential shortening and related clockwise vertical axis rotation are minimum amounts. True clockwise vertical axis rotation is probably larger than the 12° deduced from the Subalpine Molasse thrust belt. Key Points Break‐back sequence of thrusting due to interaction with escape tectonics Differential Miocene shortening causes clockwise vertical axis rotation Foreland propagation of thrusting ends with onset of escape tectonics