Building the Pamir-Tibetan Plateau-Crustal stacking, extensional collapse, and lateral extrusion in the Central Pamir: 1. Geometry and kinematics

Abstract

Asian deep crust exposed in the Pamir permits determination of the amount, sequence, and interaction of shortening, extension, and lateral extrusion over ~30 km of crustal section during the India-Asia collision. In the Central Pamir, gneiss domes and their hanging walls record Paleogene tripling of the 7–10 km thick Phanerozoic upper crustal strata; total crustal thickness may have amounted to 90 km. Two thrust sheets, comprising Cambro-Ordovician, respectively, Carboniferous to Paleogene strata, straddle the domes. Amphibolite-facies metamorphic rocks within the domes—equivalent to lower grade rocks outside the domes—form fold nappes with dome-scale wavelengths. E-W stretching occurred contemporaneously with top-to- ~ N imbrication and folding. At ~22–12 Ma, bivergent (top-to-N and top-to-S), normal-sense shear zones exhumed the crystalline rocks; most of the extension occurred along the northern dome margins. Shortening resumed at ~12 Ma with opposite-sense thrusting and folding focused along the dome margins. Throughout the building of the Central and South Pamir, dominant ~N-S shortening interacted with ~E-W extension along mostly dextral shear/fault zones. In the Neogene, shear is concentrated along a dextral wrench corridor south of the domes. We interpret the Paleogene shortening to record thickening and northward growth of the Pamir-Tibetan Plateau and short-lived Miocene crustal extension as gravitational adjustment, i.e., collapse, of the thickened Asian crust to Indian slab breakoff. Synconvergent Paleogene lateral extrusion thickened the Afghan Hindu Kush crust west of the India-Asia collision, and the Miocene-Recent dextral shear and ~E-W extension have accommodated collapse of the Pamir Plateau into the Tajik depression.