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

Abstract

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.

Kinematics of the Alpenrhein-Bodensee graben system in the Central Alps: Oligocene/Miocene transtension due to formation of the Western Alps arc

“…Simplified tectonic map of Alpine nappe stack across Alpenrhein Graben; map modified and simplified from 1:200,000 map of Vorarlberg [ Oberhauser and Rataj , ; Ortner et al ., ; Sala et al ., ; Pomella et al ., ]; refer to Figure for location of map. Nappe contacts west of Alpenrhein Graben are south dipping; geology east of Alpenrhein valley is characterized by large‐wavelength syncline/anticline pair cored by Austroalpine and Helevtic Säntis Nappe; Säntis Nappe shows drastic change in outcrop pattern across Alpenrhein Graben with apparent sinistral offset; note also sinistral faults in vicinity of Alpenrhein Graben (Iller Fault, Ostergunden fault system, Sax‐Schwende Fault); contacts of imbricated Molasse with Säntis Nappe above and undeformed Molasse below are hardly offset.…”

Active Seismotectonic Deformation in Front of the Dolomites Indenter, Eastern Alps

“…1029 data are already available for the Swiss part and adjacent regions (see Kastrup et al, 2004;Marschall et al, 2013, and references therein). For the Austrian and Italian sectors, focal mechanisms are available only for some stronger events (e.g., Pondrelli et al, 2004;Slejko et al, 1987) and for earthquakes near the BF Brandner (1980b) and Bousquet et al (2012), with supplementary information from Decker et al (1994), Eisbacher and Brandner (1995), Ortner et al (2015), Pomella et al (2016), and published geologic and tectonic maps (Autonome Provinz Bozen, 2017;Bigi et al, 1990;Bundesamt für Landestopografie, 2005;Geologische Bundesanstalt, 2017a, 2017b. The Dolomites indenter (DI, the boundary is shown with a thick black line) is the South Alpine segment bounded by the Giudicarie Fault (GF) and the Pustertal-Gailtal Fault (PGF Müller (1981).…”

Relating orogen width to shortening, erosion, and exhumation during Alpine collision

“…Shortening of the TALP in this section, excluding the amounts accommodated by transpressional deformation within the Insubric mylonitic belt, was estimated to 41 km, equally distributed between the Aar Massif and the Cressim antiform [ Rosenberg and Kissling , ]. However, recent investigations [ Ortner et al ., ] point to some 50 km of shortening accommodated by a series of thrusts that must have been rooted in the Aar Massif. As a consequence, bulk shortening of the TALP in this section, excluding transpressive deformation along the Insubric mylonites, is probably represented by 71 km, resulting from 50 km accommodated in the external part of the chain and approximately 21 km [ Rosenberg and Kissling , ] in the Cressim antiform.…”