Thesis #354

This thesis is devoted to the analysis of faults that were repeatedly reactivated under changing stress fields. The investigations were carried out at the junction between the Upper Rhine Graben and the Jura Mountains. This area represents the intersection between part of the western European Cenozoic rift system and the northern Alpine foreland. It has been shaped by an interplay between extensional and compressional tectonic forces and involved the repeated reactivation of crustal-scale faults since the Late Palaeozoic. At present, it is characterised by increased seismicity, giving proof of ongoing tectonic activity. Therefore, studying the tectonic evolution of this area can improve the understanding of various tectonic precesses that shape the earth's crust.
Extension in the Upper Rhine Graben initiated in the Late Eocene under W-E- to WNW-ESE-oriented extension. Its southern end connects into the Rhine-Bresse transfer zone. This transfer zone linked the simultaneous extension in both the Upper Rhine Graben and the Bresse Graben, another branch of the European Cenozoic rift system. The transition from mainly rift-perpendicular extension in the Upper Rhine Graben to sinistral transtensive movements in the Rhine-Bresse transfer zone was predetermined by ENE-trending basement faults delimiting a Late Palaeozoic trough system in the subsurface. Rifting involved the reactivation of these ENE-trending basement faults as sinistral transtensive strike-slip faults, simultaneously with normal faulting along NNE-oriented faults. In the sedimentary cover, this was manifested in simultaneous extensional flexuring above ENE-trending faults and growth faulting above NNE-oriented faults. The interaction of these differently oriented faults led to cumulative throw and localised depocenters in the Late Eocene to Early Oligocene.
The formation of the thin-skinned Jura Mountains in the Late Miocene to Early Pliocene initiated under NW-SE- to N-W-oriented compression and was related to Alpine orogeny. In the study area, the detached sediments encountered a fault pattern inherited from Palaeogene rifting. While ENE- to E-trending faults led to the nucleation of frontal thrusts and anticlines, NNE-trending faults paralleling the Rhine Graben served as transfer zones. Along these transfer zones, sinistral oblique ramps developed and allowed the detached sediments to be transferred sinistrally to the north. As a result, the northern Jura Mountains reveal a geometry that largely mimics the structural pattern inherited from Palaeogene extensional tectonics.
Thin-skinned Jura folding was followed by post-Late Pliocene thick-skinned fault reactivation, as evidenced in the spatial coincidence between subsurface faults and surface anticlines. This youngest tectonic phase characterised by horizontal shortening rates below 0.1 mm/a, is most presumably ongoing at present, as evidenced by faulted Late Pliocene gravels and deflected rivers.
Dynamically scaled sand-silicone models showed that within wrench systems, the reactivation of faults in a sand cover, separated from a basal discontinuity by a viscous décollement layer, is strongly controlled by the mechanical coupling between "basement" and "cover" across a décollement layer. The coupling in turn depends on the displacement rates applied to the basal plate. The experimental setup was inspired from the Rhine-Bresse transfer zone (RBTZ) that connected the Upper Rhine Graben with the Bresse Graben. This transfer zone bears evidence for the reactivation of basement-rooted structures in Neogene times. The experimental results suggest not only that fault reactivation i the RBTZ has occurred under low displacement rates, but they also provide an explanation for partial stress decoupling between cover and basement.