Outcrop-scale fracture systems in the Alpine foreland of central northern Switzerland: kinematics and tectonic context

This article summarizes the results of an extensive fieldwork-based analysis of outcrop-scale fractures across central northern Switzerland in the distal northern foreland of the Central Alps. The study focuses on the delineation of a relative fracture formation chronology, the fracture kinematics and their tectonic interpretation with the aim of enhancing the understanding of the study area's brittle deformation characteristics and history. The most commonly observed fractures within the analyzed outcrops of Middle and Upper Jurassic limestones are bedding-orthogonal joints, which show only minor increments of displacement (mode-II fractures). These structures formed relatively early during the tectonic evolution of the region and were repeatedly reactivated during later deformation events. Faults showing measurable displacements were observed comparatively rarely. Together with the results of fault-slip analyses, the outcrop-scale fracture patterns allow clear distinction of three major Cenozoic deformation steps in the study area: (1) formation of bedding-orthogonal joints lacking offsets and most likely related to regional uplift in Latest Mesozoic to Eocene times; (2) an only locally recognized phase of extension manifested by normal faults in the vicinity of the Hegau-Lake Constance Graben that onset after the Early Miocene; (3) shortening of the Alpine foreland from the Late Miocene onwards. Regionally recognizable NW-SE to NNW-SSE shortening, manifested by the strike-slip reactivation of bedding-orthogonal joints and normal faults, started prior to the comparatively late formation of mapscale folds in the easternmost Jura Mountains but continued to be active throughout this deformation event. Only locally recognizable NNE-SSW to NE-SW directed shortening is also interpreted in the context of Late Miocene Alpine shortening and does not represent an independent regional stress field. The latter has apparently changed little from Late Miocene to recent times.