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

The north-western 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 unprecedented detail. Here, basement-rooted normal faults were not only 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.