Spontaneous ductile crustal shear zone formation by thermal softening and related stress, temperature and strain rate evolution

Abstract

Ductile crustal shear zones play an important role during the formation of orogenic wedges but the softening mechanism controlling their formation and evolution are still debated. We perform two-dimensional thermo-mechanical numerical simulations of lithospheric shortening, including an underlying asthenosphere, to investigate the mechanism for ductile shear localization and to quantify the evolution of differential stress, temperature and strain rate inside shear zones that form in the ductile part of the upper crust. The simulations show the development of an intra-continental subduction zone whereby the lower crust and mantle lithosphere subduct beneath the adjacent mantle lithosphere. The upper crust forms a doubly-vergent wedge in which shear zones form subsequently. Shear zone formation is exclusively caused by thermal softening due to shear heating and temperature dependent viscosity, and is initiated by locally elevated shear stresses. The shear zone thickness is physics-controlled, hence mesh-insensitive and numerically resolved in the simulations. Temperature increase inside crustal shear zones is ca. 100 °C. The tectonic overpressure inside upper crustal shear zones is up to 250MPa and can be twice the value of the corresponding deviatoric stress. Stresses inside the shear zone decrease during its formation and are hence smaller inside the shear zone than outside. Surface processes influence the location and orientation of shear zones, but not their formation and propagation by thermal softening. The numerically calculated differential stress (30–260MPa), temperature (280–380 °C) and strain rate (∼ 10−13s−1) inside ductile crustal shear zones agree with corresponding estimates for natural shear zones. This agreement between modelled differential stress, temperature and strain rate with corresponding natural estimates supports previous results which indicate that thermal softening is a controlling softening mechanism for natural shear zone formation.