Building the Pamir-Tibet Plateau: Crustal Stacking and Orogen Parallel Evasion of Upper and Middle Crustal Material in the Pamir
Daniel Rutte, Earth and Planetary Sciences, UC Berkeley, USA
Lothar Ratschbacher, Geowissenschaften, TU Bergakademie Freiberg, Germany
Konstanze Stübner, Geowissenschafen, Eberhard Karls Universität Tübingen, Germany
Michael Stearns, Geology and Geophysics, UU Salt Lake City
The gneisses of the Central Pamir Domes and (meta-)sediments in their cover document crustal stacking of a ~10 km thick Ediacaran–Paleogene succession to a thickness of >35 km and their exhumation along bi-vergent, top-to-N and top-to S, normal-sense shear zones. The giant South Pamir Shakhdara-Alichur gneiss-dome system formed similarly by N-S extension along bivergent detachments. Prograde amphibolite-facies metamorphism in the domes and low-grade metamorphism in their hanging wall is dated at ~40 Ma (Lu-Hf garnet, U-Pb titanite) [Smit et al., 2014; Stearns et al., 2015] and ~33 Ma (K/Ar sericite). Rapid retrograde metamorphism―driven by crustal extension―started at ~22 Ma and ended at ~12 Ma (multi-method thermochronology; Stearns et al.[2013]). These Gneiss Domes offer a unique window into the Eocene-Miocene state of the Asian middle crust of the Pamir-Tibet Plateau. We suggest that ~22 Ma initiation of N–S extension is the result of gravitational collapse forced by Indian slab breakoff. What caused the termination of N–S gravitational collapse at ~12 Ma? We integrate Indias northward underthrusting and Asian crustal shortening and show that Indian mantle lithosphere was likely underthrusting the Pamir by ~12 Ma; Coupling with the crust ended the N–S extension. After further northward underthrusting, the Indian mantle lithosphere collided with Asia's cratonic root at ~10 Ma and forced its delamination which is continuing today and represented by the Pamir slab [Kufner et al., 2015].
Prior to doming, top-to-N thrust stacking accommodated thickening in the upper crust, with displacements of single thrust sheets of >30 and >19 km. At depth, ductile flow formed km-scale recumbent fold nappes. We reconstruct their geometry by structural mapping and U-Pb zircon dating, documenting repetition of metatuffite, and paragneiss layers. In the interior of the domes, amphibolite-facies deformation fabrics with prograde kyanite define an E-W stretching lineation. Associated microstructures indicate top-to-E and top-to-W shear senses. Chocolate tablet boudinage indicate vertical flattening during bulk crustal thickening. We suggest that prograde E-W stretching relates to an early orogen-parallel flow component in the middle crust, contemporaneous with crustal stacking during bulk top-to-N convergence prior to ~22 Ma. Material likely evaded laterally out of the Pamir, contributing to >60 km thick crust in the Hindu Kush, west of the India-Asia frontal collision. In the Neogene, crust extruded laterally from the Pamir Plateau to the west by dextral wrenching and E-W extension; this component of deformation is accommodated by E-W shortening in the Afghan-Tajik Depression.