GEOMETRY, SEQUENCE AND AMOUNT OF THRUSTING IN THE SUBALPINE MOLASSE OF AUSTRIA AND BAVARIA

The Subalpine Molasse (SM) is the most external tectonic unit of the Alps extending from Geneva (Switzerland) to west of Salzburg (Austria), where it disappears into the subsurface. From western Austria eastward, continental, coarse-grained conglomerates grade into deep marine marls. We investigate the influence of this facies transition on tectonic style and observed amount of shortening in four cross sections.

1 In western Austria thick coarse-grained continental conglomerates (>3km) overlie marine marls, which form the regional detachment. Ramp-flat geometries formed in the SM. The frontal structure is a triangle zone at depth with a hinterland-facing fold compensating movement on the passive roof thrust of the triangle. All structures were steepened during late contraction.

2 In western Bavaria the continental unit is sandstone-dominated, and overlies thick marine marls. An upright detachment fold takes up the movement of the passive backthrust of the frontal triangle zone. The perfectly round detachment folds of the SM are crosscut by several straight out-of-sequence thrusts, that postdate the triangle zone and folding.

3 In central Bavaria (TRANSALP section) thick marine marls (>1km) are overlain by brackish sandstones and marls. The frontal structure is a thrust that splays toward the surface. Detachment folds of the SM are crosscut by younger thrusts. To prevent 6km of marine marls in the section below the detachment folds, a Helvetic/Flysch thrust wedge is introduced.

4 East of the last surface outcrops of the SM, sandy to conglomeratic channel fills are intercalated into deep marine marls. The competent units form a duplex buried by Miocene sediments.

The age of shortening is constrained by growth strata above the frontal triangle to Late Burdigalien and younger, and postdates the youngest sediments (9Ma). Shortening decreases from 32km (64%) in the west to the east 11km (55%) in the SM. In the easternmost section young shortening is insignificant, whereas Oligocene shortening is 8km (46%).

The end of foreland propagation of the Alpine wedge might be related to onset of major orogen-parallel extension. As break-back thrusting starts earlier in the east, also orogen-parallel extension should have progressed from east to west. However, the sections 1 and 2 are outside the area affected by Miocene stretching.