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Zürich, Switzerland

Heuberger S.,Proseis AG | Roth P.,Proseis AG | Zingg O.,GeoEnergie Suisse AG | Naef H.,Geosfer AG | Meier B.P.,Proseis AG
Swiss Journal of Geosciences

The St. Gallen Fault Zone (SFZ) is a system of major NNE–SSW striking normal faults within the North Alpine Foreland Basin (NAFB), just west of the city of St. Gallen. It used to be only roughly known from 2D seismic data, locally displaying offsets of up to 300 m at the level of the Mesozoic strata. We present a detailed structural interpretation of a recently acquired 3D seismic dataset that reveals the occurrence of multiphase tectonic activity along the SFZ from at least the Late Paleozoic to the early Oligocene, and possibly even later. We can show that the SFZ roots in extensional basement structures that bound small Permo-Carboniferous grabens. Thickness changes in the younger sediments above these Paleozoic grabens indicate several phases of tectonic subsidence during the Triassic and the Jurassic. The Lower Cenozoic units in the northernmost part of the 3D seismic area are also offset by the SFZ. No offsets can be identified in the overlying, shallower part of the Cenozoic units. Most faults constituting the SFZ are favourably oriented in the present-day stress field (SHmax NNW–SSE) to be reactivated in strike-slip mode. The seismic events induced by testing operations at the geothermal exploration borehole “St. Gallen GT-1” (SG GT-1) in July 2013 revealed that, even though the seismicity of northeastern Switzerland is considered to be low and diffuse, parts of the SFZ have to be regarded as critically stressed. Combining the interpretation of geological and seismic data, we conclude that the SFZ represents a reactivated basement-rooted normal fault, which was active during several phases in Permo-Carboniferous and Mesozoic times and that is still active today in strike-slip mode. © 2016 Swiss Geological Society Source

Malz A.,Friedrich - Schiller University of Jena | Madritsch H.,National Cooperative for the Disposal of Radioactive Waste | Meier B.,Proseis AG | Kley J.,University of Gottingen

Triangle zones represent typical structural elements of thin-skinned foreland fold-and-thrust belts. Here, we report 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 ineffective 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 deformation 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. Source

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