Tectonics of the central Swiss Molasse Basin: Post-Miocene transition to incipient thick-skinned tectonics?

In this paper we investigate the tectonics of the central Swiss Molasse Basin (SMB), situated between the central European Alps and the Jura fold-and-thrust belt. Based on (re)interpreted 2-D reflection seismic data and geological 3-D modeling, we present new insights into the structural configuration and evolution of the central SMB. During the thin-skinned, main deformation phase of the Jura fold-and-thrust belt (similar to 12-4Ma), the central SMB was riding passively above a major detachment horizon within Triassic evaporites, which roots beneath the External Crystalline Massifs of the Central Alps. In the SMB, low-amplitude detachment folds and detachment-rooted WSW-ENE striking thrust and reverse faults are related to this event. Younger, major NNE-SSW and NNW-SSE trending strike-slip fault zones cross the basin over a length of several kilometers, affecting the Cenozoic and Mesozoic cover units and extending into the pre-Mesozoic crystalline basement. In cases, the fault zones apparently reactivate inherited, Permo-Carboniferous trough (PCT) bounding, Paleozoic normal faults. Seismic data further reveal mild inversion of WSW-ENE striking PCTs. These structures clearly cut the basement-cover contact, thus postdating the main deformation phase of the Jura fold-and-thrust belt. Present-day earthquake focal mechanisms and epicenter distribution show strike-slip faulting and basement-seated seismicity, respectively, thus corroborating our observations from seismic interpretation. Our data suggest a change from late Miocene thin-skinned to Pliocene to present-day incipient thick-skinned tectonics. This major change reflects the establishment of a midcrustal detachment and may be related to changes in slab dynamics of the subducting European lithosphere.