Structural Evolution of Orogenic Wedges: Interplay Between Erosion and Weak Décollements

Abstract

Orogenic wedges commonly display an inner wedge, where crystalline units have been exhumed, and an outer wedge formed by imbricated sedimentary units detached from the basement. Analog experiments have shown that similar structures can emerge naturally in the presence of weak décollements due to the interplay between erosion and deformation. In this study, we further investigate this hypothesis using two-dimensional, visco-elasto-plastic numerical models. Our experiments assume a basal and an intermediate décollement within the wedge. Experiments with a frictional strength of the basal décollement lower or equal to that of the intermediate décollement show a structural evolution of fold-and-thrust belts dominated by out-of-sequence thrusting. Conversely, when the intermediate décollement is weaker than the basal décollement, distinct outer and inner wedges are formed. This process leads to episodic migration of midcrustal ramps, tectonic underplating, and antiformal stacking facilitated by erosion. Comparison between our models and the Himalayan wedge suggests a low effective friction (∼0.10), which is probably due to dynamic weakening during large (M8+) Himalayan earthquakes. The deeper décollement, along which the lower plate thrusts beneath the High Himalaya, must be a thermally activated ductile shear zone with an apparent friction of ∼0.18. Fold-and-thrust belts worldwide exhibit various architectures in which different décollement levels might be activated. Thus, our study provides a framework to help assess under which conditions a variety of structures observed in orogenic systems can arise. © 2020. American Geophysical Union. All Rights Reserved.