3D-seismic evidence for thick-skinned tectonics in a classic thin-skinned tectonics region (external Alpine foreland, Switzerland)

The northwestern Alpine foreland in Switzerland and France comprises the Late Miocene Jura Mountains, considered a type example for thin-skinned thrusting where deformation of the sedimentary cover is decoupled from the basement along a regional basal detachment. To what extent basement faults were involved during its deformation is a matter of debate. We use 3D seismic data to investigate the deformation style along the easternmost tip of the Jura range in an unprecedented detail. Here, basement-rooted normal faults were repeatedly reactivated before thrust belt formation but also contemporaneously active as reverse/transpressional faults. They either propagated up into the Mesozoic succession without interruption (hard linkage) or apparently controlled the localisation of Mesozoic faults via smaller-scale shear zones (soft linkage). Our analysis of the resulting fault geometries questions the existence of a large-scale basal detachment in this area and points out the importance of thick-skinned fault reactivation.