Present-day and future tectonic underplating in the western Swiss Alps: reconciliation of basement/wrench-faulting and decollement folding of the Jura and Molasse basin in the Alpine foreland

摘要

The western Alps from a geodynamically active mountain belt showing the typical features of an evolving orogenic wedge with its pro-wedge geometry to the NNW and its retro-wedge structures to the SSE. Renewed tectonic underplating of European continental crust occurred after the orogenic wedge underwent major dynamic disequilibrium following the break-off of the southward subducting also of the European passive margin. The most important of these basement imbricates are the Mont-Blanc-Aiguilles Rouges and Gastern-Aar crystalline massifs, also forming the Alps' highest mountains. The upper plate-present-day orogenic wedge of the western Alps includes the Molasse basin and the Jura fold-and-thrust belt, both decoupled from the basement over a basal decollement surface. The overall geometry of this wedge appears to be strongly unstable according to simple wedge models. In its attempt to regain stability, out-of-sequence thrusts form in the existing basement nappes; but also new basement nappes should develop beneath the southern portion of the Molasse basin. New out-of-sequence thrusts in the cover, trigger higher than averaged uplift rates concentrated around the newly forming structures and are accompanied by a concentration of earthquakes. Tectonic underplating is further corroborated by neotectonics and the tectonic structures observed in the Prealpes, Molasse basin and Jura. Similarly, uplift rates, and earthquakes along the southern edge on the Jura mountains seem to witness the development of a new/incipient basement nappe at depth (partial inversion of former Permo-Carboniferous grabens in the basement). A possible spatial coincidence of areas with strong earthquake activity and zones with uplift rates above surrounding values, suggest a common mechanism for their origin in the western Swiss Alpine foreland. Combined with information from basement geometry and wedge dynamics it is proposed that the common mechanism is the development of basement imbricates by tectonic underplating. The proposed model for ongoing and possible future tectonic underplating beneath and active Alpine orogenic wedge also allows to reconcile the models of basement/wrench-faulting in the Molasse basin and Jura with the distant push theory, where the Molasse basin and Jura develop over a basal decollement horizon.