An unusual triangle zone in the external northern Alpine foreland (Switzerland): Structural inheritance, kinematics and implications for the development of the adjacent Jura fold-and-thrust belt Alexander Malz a, ⁎, Herfried Madritsch b , Beat Meier c , Jonas Kley d a Friedrich-Schiller-University Jena, Institute of Geosciences, Burgweg 11, D-07749 Jena, Germany b Nationale Genossenschaft für die Lagerung radioaktiver Abfälle (Nagra), Hardstraße 73, CH-5430 Wettingen, Switzerland c Proseis AG, Schaffhauserstrasse 418, CH-8050 Zürich, Switzerland d Georg-August-University Göttingen, Centre of Geosciences, Goldschmidtstr. 3, D-37077 Göttingen, Germany abstract article info Article history: Received 17 April 2015 Received in revised form 10 December 2015 Accepted 23 December 2015 Available online 8 January 2016 Triangle zones represent typical structural elements of thin-skinned foreland fold-and-thrust belts. Here, we re- port the results of an in-depth structural analysis of a rather unusual triangle zone at the front of the easternmost Jura fold-and-thrust belt in the otherwise only very mildly deformed Alpine foreland of Northern Switzerland. The investigation is based on the interpretation of recently reprocessed and depth-migrated 2D reflection seismic sections. Classical bed-length and area cross-section balancing methods were used to validate the interpretation and unravel the tectonic evolution of the triangle zone. According to our interpretation the analyzed triangle zone formed along the Baden–Irchel–Herdern-Lineament (BIH-Lineament), a regional Paleozoic normal fault that shows evidence of Cenozoic reactivation. The triangle zone is composed of one major foreland-directed thrust rooting in Triassic evaporites and a back-thrust splaying from it in the Middle Jurassic Opalinus Clay, pointing to the importance of secondary detachments. Steeply dipping secondary reverse faults next to the triangle zone suggest reactivation of pre-existing normal faults. The formation of the thrust triangle is considered to relate to thin-skinned foreland deformation in Late Miocene time. Strain estimations of the thrust triangle along-strike show a laterally very uniform amount of shortening, which is in contrast to the southward adjacent Jura fold-and- thrust belt. We interpret this constant shortening to represent the maximum contractional strain attainable by the specific geometry of the BIH triangle zone. At this point, the complex structure became mechanically ineffec- tive and further shortening led to the formation of new contractional structures in its hinterland. This kinematic hypothesis suggests an early-stage formation of the BIH triangle zone followed by back stepping of the deforma- tion front. As such, it challenges the classical view of a purely forward-breaking sequence for the Jura fold-and- thrust belt in the northwestern foreland of the Alps. © 2016 Elsevier B.V. All rights reserved. Keywords: Triangle zone Thin-skinned tectonics Fold-and-thrust belt Structural inheritance Jura Mountains 1. Introduction The structural styles and kinematics of foreland fold-and-thrust belts have been and still are intensively studied, partly due to their im- portant role as hydrocarbon play (Nemcok and Henk, 2006). Thin- skinned fold-and-thrust belts form along a basal décollement horizon typically constituted by mechanically weak lithological units (salt, evap- orites and shale). They are found in the forelands of many orogens worldwide, for example the Rocky Mountains (Price, 1981), the Andes (Allmendinger et al., 1983; Baby et al., 1997; Jordan et al., 1993; Roeder, 1988) the Pyrenees (Puigdefabregas et al., 1992; Verges et al., 1992; Williams, 1985) and the Alps (Burkhard, 1990; Laubscher, 1961). Thin-skinned foreland fold-and-thrust belts are considered to obey the laws of critical wedges (Chapple, 1978). The surface slope and the dip of the basal décollement define the geometry of a critical wedge. Both depend on the basal friction and the material of the wedge, its den- sity and shear strength (Chapple, 1978; Dahlen, 1990; Davis et al., 1983). As a consequence the tectonic evolution and kinematics of thin-skinned fold-and-thrust belts are strongly controlled by the thick- ness of the sedimentary cover overlying the basal décollement eventu- ally modified by syntectonic erosion or sedimentation, as well as aspects influencing the basal friction of the décollement such as fluid pressure and fracture strength (Hindle, 2008; Kley et al., 1999; Sommaruga, 1997; Uba et al., 2009). In addition to these principal controlling mech- anisms, the localization and development of contractional structures in thin-skinned foreland fold-and-thrust belts is known to be commonly influenced by pre-existing structures (cf. Butler et al., 2006; Giambiagi et al., 2003). Deformation events predating thrust-belt initiation affect Tectonophysics 670 (2016) 127–143 ⁎ Corresponding author at: Georg-August-University Göttingen, Centre of Geosciences, Goldschmidtstr. 3, D-37077 Göttingen, Germany. E-mail address: amalz@geo.uni-goettingen.de (A. Malz). http://dx.doi.org/10.1016/j.tecto.2015.12.025 0040-1951/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto