Tectonic evolution around the Mont Terri rock laboratory, northwestern Swiss Jura: constraints from kinematic forward modelling

摘要

We propose a geometrically, kinematically, and mechanically viable thin-skinned kinematic forward model for a cross section intersecting the Mont Terri rock laboratory in the frontal-most part of the Jura fold-and-thrust belt, Switzerland. In addition to the available tunnel, borehole, and surface data, initial boundary conditions are crucial constraints for the forward modelling scenarios, especially the inherited topography of the basement and any pre-compressional offset within the Mesozoic sediments. Our kinematic analysis suggests an early-stage formation of the Mont Terri anticline located above ENE-trending, Late Paleozoic extensional faults, followed by back-stepping of the deformation developing the Clos du Doubs and Caquerelle anticlines further south. In this model, the thrust sequence was dictated by the inherited basement faults, which acted as nuclei for the ramps, detached along the basal decollement within the Triassic evaporites. The mechanical viability of both the thrust angles and thrust sequence was demonstrated by applying the limit analysis theory. Despite numerous subsurface geological data, extrapolation of structures to depth remains largely under-constrained. We have tested an alternative model for the same cross section, involving an upper detachment at the top of the Staffelegg Formation that leads to duplication of the sub-Opalinus Clay formations, prior to detachment and thrusting on the Triassic evaporites. This model is geometrically and kinematically viable, but raises mechanical questions. A total displacement of 2.9 and 14.2 km are inferred for the classical and the alternative scenarios, respectively. In the latter, forward modelling implies that material was transported 10.8 km along the upper detachment. It is not yet clear where this shortening might have been accommodated. Despite the differences in structural style, both models show that pre-existing basement structures might have interfered in time and space. Both styles may have played a role, with lateral variation dictated by basement inherited structures.