Influence of Inherited Structures on Deformation Patterns in the Eastern Jura Fold and Thrust Belt: Insights from 3D Seismic Data

Abstract

The Jura fold and thrust belt is characterised by dominant thin-skinned thrusting of the Mesozoic-Cenozoic sedimentary cover and variable interaction with inherited structures and deep-seated (i.e., sub-detachment) faults. In its central parts, this fold-and-thrust belt is detached from an underlying mechanically stiffer basement, along Triassic evaporites and displaced up to 30 km. The eastern termination of the fold and thrust belt is characterised by (i) a much lower amount of shortening (on the order of the hundreds of meters), (ii) a significant thickness reduction of evaporites of the Muschelkalk Group, and (iii) interaction of the décollement with steps associated with faults bounding an underlying Permo-Carboniferous graben (Constance-Frick Trough; CFT). Using recently processed seismic data (Nördlich Lägern 3D), we document the role exerted by the inherited structures rooted in the Permo-Carboniferous basin fill and basement in the development of the different styles of deformation and structures in an area located northwest of the city of Zürich. The ENE-striking master normal fault bounding the CFT to the south displays evidence of reactivation, during both extensional and compressional episodes, illustrated by apparent normal steps, alignments with no apparent displacement, and gentle folding of the Permo-Carboniferous basin fill. The investigated seismic volume indicates that contractional deformation is concentrated in two major ENE trending fold-thrust zones involving the Triassic-Jurassic epicontinental platform succession and the stratigraphically overlying Cenozoic Molasse Basin deposits. The northern fold-thrust zone (NFTZ), manifested at the surface by the Siglisdorf anticline, consists of an up to 2 km wide pop-up structure linked to thrusting in the detached cover series. The structure is located in correspondence with steps in the topography of the base Mesozoic unconformity produced by south dipping ENE striking minor faults rooted in the Permo-Carboniferous sequence without affecting the detachment integrity. The southern fold-thrust zone (SFTZ), corresponding to the Baden-Irchel-Herdern Lineament, is located above a north dipping major fault of the Late Paleozoic half-graben. The overlying deformation structures in the SFTZ, comprising multiple thrusts and backthrusts, that form fish-tail structures in a narrow, steep zone involving the Mid-Triassic-Jurassic series, are interpreted as the result of layer-parallel shortening associated with buckling of the Mesozoic multilayer. In contrast, the Mesozoic succession is characterized by low deformation and absence of major faults in the area over the central part of the graben (comprised between the Weiach – Glattfelden – Eglisau Lineament – WGEL – to the north and the south fold-thrust zone). The WGEL separates a sub-horizontal to gently folded Mesozoic rock panel to the south from a gently dipping panel to the north. This suggests that fault reactivation may have been accompanied by mild basin shortening and inversion. In conclusion, different modes of interaction between the Mesozoic multilayer (including the weak evaporite level at its base) and underlying Upper Paleozoic basin fill with inherited basement faults have produced a marked contrast in structural style among the detachment-dominated NFTZ, the buckling-dominated SFTZ, and a low deformation central area.