3D thermo-mechanical model of the orogeny in Pamir constrained by geological and geophysical observations

Abstract:

The Pamir-Hindu Kush orogenic belt is the result of the indentation of the western corner of the India plate into Eurasia. It has accommodated the highest strain over the shortest meridional distance of the India-Eurasia collision zone. Recent high-resolution seismic tomographic and receiver function studies confirm the presence of a south-dipping continental lithosphere, which has been traced to the depth of more than 200-300 km beneath the northern Pamir (Pamir slab). Balanced cross-section document crustal shortening associated with the corresponding frontal thrust system of a few tens of km, which is much less than the expected (>200-300 km) for intra-continental subduction zone, based on the seismically imaged length of the Pamir slab. Another unique feature of Pamir is the presence of large gneiss domes with the exhumation reaching a depth of 30-40 km.

We employ the finite-element thermomechanical modelling technique SLIM3D to simulate the evolution of the Pamir orogeny during the last 25 Myr. The technique is using advanced non-linear elasto-visco-plastic rheology with parameters based on laboratory experiments. Our 3D model extends 1100 km N-S, 800 km E-W, and 300 km deep, and replicates indentation of the western half of India promontory into Eurasia. The moving modelling-window technique allows focusing at the most extensively deforming domains, and a 3D model setup and boundary conditions allow lateral material flow to the west, i.e. perpendicular to the direction of tectonic shortening.

The model replicates major features of the lithospheric structure and geological history. For instance, we demonstrate that large part of the roll back of the Pamir slab may be caused by the delamination of the mantle lithosphere together with the lower crust, rather than classical intra-continental subduction involving thrust-imbrication of the uppermost crust. The resulting shortening at the Pamir frontal thrust system appears to be lower than 50 km in accord with geological observations. The model also replicates evolution of surface topography including the collapse of high Pamir Plateau in N-S and E-W directions, resulting in exhumation of gneiss domes. We demonstrate that extensive westward outflow of material and the relatively small initial width of Pamir are the key factors that controlled its evolution.