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

Abstract

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.