Formation of orogenic wedges and crustal shear zones by thermal softening, associated topographic evolution and application to natural orogens

The model of an orogenic wedge has been applied to explain the tectonic evolution of many orogens worldwide. Orogenic wedges are characterized by (1) a first-order shear zone which underthrusts the mantle lithosphere and lower crust beneath the adjacent mantle lithosphere and (2) a sequence of second-order upper crustal shear zones which form tectonic nappes. Shear zone formation in the lithosphere is, however, incompletely understood. We perform two dimensional thermo-mechanical numerical simulations of lithospheric shortening to study shear zone formation, propagation and associated wedge formation. The only perturbation in the model lithosphere is a different temperature at the left (1300° C) and right (1400° C) half of the model bottom. Despite this smooth and weak perturbation, simulations show self-consistent and spontaneous formation of first- and second-order shear zones generating an orogenic wedge. The shear zones are caused by thermal softening and temperature-dependent rock viscosity. Lateral spacing of upper crustal shear zones spans between 30 and 50km and is controlled by the depth of the boundary between upper and lower crust which acts as mechanical detachment level. Modelled upper crustal shear zones are active for ∼1 to ∼4My. Surface processes such as sedimentation and erosion influence shear zone orientation, spacing and duration but do not impact fundamental processes of shear zone formation and propagation. Simulations produce both singly-vergent and doubly-vergent wedges. Topographic uplift rates are controlled by the applied bulk shortening rate. The modelled surface uplift and subsidence associated with crustal shear zones could explain major consecutive thrusting events and related sedimentation within flexural basins during the formation of the Helvetic nappe system in Western Switzerland. Furthermore, results for shear zone propagation along a mid-crustal detachment and associated uplift of crustal basement could potentially explain foreland basement-cored uplifts in natural orogens such as the Laramide orogen, Taiwan or the Shillong Plateau. •Thermal softening causes spontaneous ductile shear zone formation and propagation.•Thermally-activated shear zones form orogenic wedge during lithospheric shortening.•Surface processes influence shear zone propagation but not their formation.•Applications of orogenic wedge model to natural examples are discussed.