Geodynamics of the Pamir-Tien Shan: Cenozoic continental subduction, crustal bucking and orocline formation
Final Report Abstract
Under the Pamir–Hindu Kush, unusual deep earthquakes and seismic velocity anomalies suggest subduction of Asian and Indian lithosphere. We distinguished a narrow sliver of Indian lithosphere beneath the deepest Hindu Kush earthquakes and a broad, arcuate slab of Asian lithosphere beneath the Pamir, and suggest that this double subduction zone arises by contrasting modes of convergence under the Pamir and Hindu Kush, imposed by the different mechanical properties of the three types of lithosphere involved. While the buoyant northwestern salient of Cratonic India bulldozes into Cratonic Asia, forcing delamination and rollback of its lithosphere, India’s thinned western continental margin separates from Cratonic India and subducts beneath Asia. Along its northern margin, where GPS measurements show significant shortening, thrust and dextral strike-slip faulting along W to NW trending planes; this indicates slip partitioning between N-ward thrusting and W-ward extrusion. An active, N-NE trending, sinistral transtensional fault system dissects the Pamir’s interior, connecting the lakes Karakul and Sarez, and extends by distributed faulting into the Hindu Kush of Afghanistan. East of this lineament, the Pamir moves N-ward en bloc, showing little seismicity and internal deformation. The western Pamir exhibits a higher amount of seismic deformation; sinistral strike-slip faulting on NE-trending or conjugate planes and normal faulting indicate E-W extension and N-S shortening. We explain this deformation pattern by the gravitational collapse of the western Pamir Plateau margin and the lateral extrusion of Pamir rocks into the Tajik-Afghan depression, where it causes thin-skinned shortening of basin sediments above an evaporitic décollement. Superposition of Pamir’s bulk northward movement and collapse and westward extrusion of its western flank likewise causes the gradual change of surface velocity orientations from N-NW to due W observed by GPS geodesy. The distributed shear deformation of the western Pamir and the activation of the Sarez-Karakul fault system may ultimately be caused by the NE-ward propagation of India’s western transform margin into Asia, thereby linking deformation in the Pamir all the way to the Chaman fault in the S in Afghanistan. We present the crustal resistivity structure of the Pamir and Southern Tian Shan orogenic belts measured at 178 sites, whereof 26 combine broad band and long period recordings. The most intriguing features of the 2-D and 3-D inversion results is a laterally extended zone of high electrical conductivity below the Pamir Plateau, with resistivities below 1 Ωm, starting at a depth of ∼10–15 km. The high conductivity can be explained with the presence of partially molten rocks at middle to lower crustal levels, possibly related to ongoing migmatization and/or middle/lower crustal flow underneath the Southern Pamir. This interpretation is consistent with a low velocity zone found from local earthquake tomography, relatively high vp/vs ratios, elevated surface heat flow, and thermomechanical modelling suggesting that melting temperatures are reached in the felsic middle crust. Asian deep crust exposed in the Pamir gneiss domes permits determination of the amount, sequence, and interaction of shortening, extension, and lateral extrusion over ~30-50 km of crustal section during the India- Asia collision. The gneiss domes and their hanging walls record Paleogene tripling of the 7‒10 km thick Phanerozoic upper crustal strata. E-W stretching occurred contemporaneously with top-to-~N imbrication and folding. At 22−2 Ma, normal-sense shear zones exhumed the crystalline rocks. Shortening resumed at ~12 Ma in the Central Pamir domes. 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 Central Pamir domes. We interpret the Paleogene shortening to record thickening and northward growth of the Pamir-Tibet Plateau, and short-lived Miocene crustal extension as gravitational adjustment, i.e., collapse, of the thickened Asian crust to Indian slab breakoff. Prolonged extension in the South and East Pamir domes record lateral extrusion that accommodated westward collapse of the Pamir Plateau into the Tajik depression.
Publications
- 2013. The giant Shakhdara migmatitic gneiss dome, Pamir, India–Asia collision zone, I: Geometry and kinematics. Tectonics, 32, 948-979 [Outstanding Publication Award 2015 der Geological Society of America]
Stübner, K., L. Ratschbacher, Rutte, V., K. Stanek, D. Minaev, M. Wiesinger, R. Gloaguen, et al.
(See online at https://doi.org/10.1002/tect.20057) - 2013. The giant Shakhdara migmatitic gneiss dome, Pamir, India–Asia collision zone, II: Timing of dome formation. Tectonics, 32, 1404-1431 [Outstanding Publication Award 2015 der Geological Society of America]
Stübner, K., L. Ratschbacher, C. Weise, J. Chow, J. Hofmann, J. Khan, D. Rutte, B. Sperner, J.A. Pfänder, B.R. Hacker, I. Dunkl, M. Tichomirowa, M.A. Stearns, et al.
(See online at https://doi.org/10.1002/tect.20059)
