Control of buoyancy forces and thermal softening on the emplacement of low-angle thrust sheets during continental collision

A characteristic feature of some collisional orogens, such as the Alps, are low-angle thrust sheets. The most prominent thrust sheet in the Alps is likely the Glarus nappe that has been displaced at least 30 to 40 km. The Glarus thrust has been studied for more than 150 years and these studies created much knowledge of, for example, rock deformation, strain localization, and softening mechanisms. However, the type of forces and the mechanisms that drive the tens of kilometers displacement at a low angle during continental collision remain unclear. Furthermore, the relative importance of softening mechanisms at the base of the thrust sheet is still disputed and proposed mechanisms include fluid overpressure, grain size reduction, or fluid release caused by shear heating.

In this study, we investigate the formation of low-angle thrust sheets and nappes with two-dimensional thermo-mechanical numerical models. We consider a lithosphere-mantle system with a continental crust that exhibits initially a horizontal variation in crustal thickness. The lithosphere is shortened by far-field convergence velocities. For simplicity, we apply thermal softening due to shear heating as the only softening mechanism since this mechanism requires the least assumptions in our thermo-mechanical model. The applied numerical algorithm is based on a staggered finite difference discretization and a matrix-free iterative pseudo-transient solver.

Preliminary numerical results indicate that buoyancy forces due to lateral crustal thickness variations can trigger the formation of sub-horizontal thrusting and a switch from a locally pure shear-dominated to a simple shear-dominated deformation. Without lateral thickness variations and associated buoyancy forces, thermal softening causes thrusting with 45-degree angles and, hence, no low-angle thrusting. We perform systematic numerical simulations and dimensional analysis to evaluate the conditions that are required to generate low-angle thrusting during lithospheric shortening.