Effects of reactivated extensional basement faults on structural evolution of fold-and-thrust belts: Insights from numerical modelling applied to the Kopet Dagh Mountains

Abstract

The structural evolution of basement-involved, or thick-skinned, fold-and-thrust belts is often affected by preexisting, inherited extensional faults within the basement. Here, a crustal-scale two-dimensional finite difference model with a visco-elasto-brittle/plastic rheology is applied to investigate the formation of fold-and-thrust belts as a result of tectonic inversion, from intracontinental extension to compression. We examine the influence of frictional strain weakening, varying surface process intensity and different crustal rheologies. Intense strain weakening results in narrow and deep basins with wide shear zone spacing during the extension phase and fault reactivation and localized uplift during inversion. Little weakening of shear zones results in thrusts cross-cutting pre-existing normal faults and the development of newly-formed thrusts in distal parts of the fold-and-thrust belt during inversion. Enhanced surface mass movement localizes deformation and uplift in the central part of mountain belts. Implementation of a weak upper crust (quartzite) leads to less localized deformation compared to a strong crust (quartz-diorite). We compare modelling results to the basement-involved Kopet Dagh Mountains, NE Iran, and discuss potential lateral variation of fault reactivation and erosion rates being responsible for deeper tectonic exhumation in the eastern part of the mountain belt, relative to the West and Central Kopet Dagh.