Determining the timing of formation of the Rawil Depression in the Helvetic Alps by palaeomagnetic and structural methods

Abstract

Anisotropy of magnetic susceptibility, palaeomagnetism and structural methods are used in order to test the relative timing of antiform updoming and formation of the Rawil Depression in the Helvetic Alps. Samples were collected from all nappes currently exposed in the study region. The magnetic fabric is consistent with extension oblique and parallel to the regional fold trend and with palaeostress reconstructions from fault planes and veins. Palaeomagnetic analyses show a stable characteristic remanence (ChRM), with samples recording both normal and reverse polarity. A successful fold test performed across the antiformal dome structure suggests that the palaeomagnetic signal was acquired prior to doming. By comparison with thermochronometric data, the ChRM was acquired between 25 and 10 Ma and is pre- to synfolding. A secondary post-doming palaeomagnetic component (A), whose magnetization is likely to have occurred between 10 and 3.5 Ma, appears to be too steep with regards to the inclination of the Earth's field, suggesting recent large-scale tilting has occurred in the region. These combined analyses indicate that widespread orogen-parallel extension occurred prior to the formation of the Rawil Depression, which is finally interpreted as the result of a stepover structure at the curvature between Central and Western Alps.