The present-day crustal stress field of the Molasse Basin in Switzerland

Abstract We present a completely revised and extended database of the crustal present-day orientation of the maximum horizontal stress S Hmax for northern Switzerland and neighbouring regions. We analysed 32 deep boreholes located in the Molasse Basin of Switzerland from which we interpreted > 30 km of image and caliper logs and picked from these the stress-induced borehole breakouts, drilling induced tensile fractures, and petal centerlines. We also used 105 induced fractures from 139 microhydraulic fracturing tests in 8 boreholes. The interpretation of the data resulted in 64 data records of the S Hmax orientation. In addition, we also used an extended and revised dataset of 704 earthquake focal mechanisms computed by the Swiss Seismological Service and derived S Hmax orientations from these as well. Combining these data with data records from the World Stress Map database release 2016 resulted in a dataset with 948 data records. The overall regional pattern of the S Hmax orientation in the Alpine foreland of Switzerland shows a counterclockwise rotation of approximately 50° from N–S in the northeast to NW–SE in the southwest supporting earlier interpretations that the S Hmax orientation is primarily controlled by the gravitational potential energy of the Alpine topography. Beyond this large-scale pattern, no further lateral changes of the S Hmax orientation imposed by structural elements or rock property contrasts are resolved due to the resolution limit of the dataset. On average, there is only one reliable data record in Switzerland per 140 km 2 with predominant standard deviations of ± 20–25° for individual data records. Thus, small S Hmax rotations as well as rotations on lateral scales of a few 10s of km are typically not resolved. We also investigated changes of the S Hmax orientation with depth with an emphasis on a potential impact of a mechanical decoupling horizon between the Mesozoic sediments and the underlying basement, which is located in the Middle Muschelkalk (Triassic). For this we used a sub-dataset of 88 S Hmax orientations located in northern Switzerland where we have a good coverage of data records in the sediments (n = 43) and in the basement (n = 45). The mean S Hmax orientation in the sediments is 166 ± 12° and in the basement 159 ± 22° indicating that a potential decoupling horizon either does not leave an imprint in the S Hmax orientation or that decoupling is not acting on a regional scale.