Parametric mechanical analysis of thin- versus thick-skin tectonics applied to the Jura belt

Field observations and seismic interpretations testify that the front of the Jura fold-and-thrust belt is still submitted to compressive deformation, but whether the basement is deforming (thick-skin) or not (thin-skin) remains an open question. We propose a mechanical point of view using the Kinematic approach of the Limit Analysis theory (KLA). We first draw cross-sections containing a major shallow décollement level in the triassic evaporites, including the Alps up to the topographic maximum and including the whole crust. We submit the cross-sections to a compressive force at their southern end, and the KLA determines the location and geometry of the incipient ruptures by optimisation of the associated compressive force, accounting for mechanical balance and the rock strength (Coulomb criterion). Five cross-sections span the whole Jura from west to East, allowing us to explore the lateral variations. From the analysis of five hundred simulations (one hundred for each cross section), varying the friction angles on the Triassic décollement and of the lower crust between 1 deg and 10 deg, we have identified five types of tectonics at the Jura front depending on the emergence of a basement thrust beyond the Jura front (type 1), at the Jura front (type 2) with simultaneous activation of the shallow décollement (type 3), or south of the Jura front (type 5), with activation of the shallow décollement at the Jura front (type 4). The analysis allows us to draw two conclusions. First, the transitions between the various tectonic styles occur abruptly upon continuous changes in the friction parameters, revealing a threshold behaviour that we interpret as an extension of the concept of wedge criticality in the Critical Coulomb Wedge theory: at criticality, several tectonic types may occur within a narrow, critical, range of parameter values. Second, the critical range evolves sytematically between cross-sections, in such a way that the front of the thick-skinned deformation crosses laterally the Jura belt. The two most western cross-section exhibit only thin skin or no tectonics at the Jura front (types 1, 4 and 5), the central one hosts all five styles, and the two Eastern ones show thick-skin solutions (1, 2, 3), for all values tested. We also show that a thick-skinned tectonic style can be accompanied by a simultaneous activation of the shallow triassic décollement (type 4), complicating the interpretation of apparent thin-skin field structures. Modifications of our cross-sections to explore the effect of a bumpy upper/lower crust interface, or of a major décollement at the upper/lower crust interface show the robustness of our conclusions. They only modify slightly the critical ranges at which the tectonic changes occur. These findings may serve as guides, or first order questions, for more sophisticated mechanical approaches including temperature and rate dependent rheologies and the three dimensions of space.