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

We report fault slip data from exhumed fault surfaces along the NNE trending Alpenrhein valley and its intersection with the WNW striking Bodensee Graben near the Alpine thrust front of the Central Alps in the Swiss-Austrian-German border region. This conjugated graben system straddles the boundary between the Alps and its foreland and allows comparing the kinematics of graben formation between the two different tectonic domains. Our data show sinistral transtension along the Alpenrhein Graben and dextral transtension along the Bodensee Graben. Both transtensional graben systems resulted from the same kinematic regime of NW directed shortening and NE oriented extension. The graben faults are not older than NW striking tear faults associated with NW directed Oligocene nappe emplacement in the Helvetics starting at 35-30 Ma and ending by 25-20 Ma. Compatible with this are six U-Pb ages of calcite fibers from four samples yielding consistent ages ranging from 25.3 +/- 5.6 Ma to 21.8 +/- 3.4 Ma (2 sigma errors). Earthquake data since 1996 show that kinematic directions persisted until the Recent. Our data broadly fit with the kinematic evolution of the Oberrhein Graben, which shows significant E-W extension in the Oligocene. We suggest that Oligocene extension in the Alps and its foreland resulted from the increased curvature of the Western Alps arc and associated moderate tangential stretching in the internal parts of the Central Alps. We discuss a tectonic model of eastward rollback of the west Mediterranean subduction zone associated with counterclockwise rotation of Adria, the latter of which aided the formation of the Western Alps arc.