Tajik Basin and Southwestern Tian Shan, Northwestern India‐Asia Collision Zone: 2. Timing of Basin Inversion, Tian Shan Mountain Building, and Relation to Pamir‐Plateau Advance and Deep India‐Asia Indentation

Sanaa Abdulhameed,Sofia‐Katerina Kufner,Lothar Ratschbacher,Adam Szulc,Mustafo Gadoev,Alexandra Käßner,Ilhomjon Oimahmadov,Myriam A C Kars,Jean‐Claude Ringenbach,Eva Enkelmann,Raymond Jonckheere,Bernd Schurr,Łukasz Gągała,Jahanzeb Khan,Mykhaylo Nakapelyukh

doi:10.1029/2019tc005873

Abstract

AbstractThe Tajik basin and southwestern Tian Shan constitute the northwestern tip of the India‐Asia collision zone. Basin inversion formed the thin‐skinned Tajik fold‐thrust belt, outlined by westward convex fold trains, underlain by a décollement in Jurassic evaporites. The belt's leading edge—the Uzbek Gissar—and its transpressional northern lateral margin—the Tajik Gissar—constitute the thick‐skinned foreland buttresses. Apatite fission‐track data indicate ~40‐ to 15‐Ma reheating by sediment burial in the Tian Shan. In the Gissar and the Tajik fold‐thrust belt, apatite fission‐track and (U,Th)/He ages date the major phase of shortening/erosion between ~12 and 1 Ma, with exhumation to 2‐ to 3‐km crustal depths within a few Myr after onset of shortening. Shortening spread immediately across the fold‐thrust belt, typical for belts floored by a detachment in ductile rocks, and into the foreland buttresses. Reactivation concentrated in the internal (eastern) fold‐thrust belt with the thickest evaporates. The youngest ages (~6.6–1.6 Ma) occur along the Vakhsh thrust, the active erosional front of the fold‐thrust belt in the northeastern Tajik basin, where it narrows between the converging Tian Shan and Pamir. Our study links major events in the Pamir hinterland with the Tajik basin and Tian Shan foreland. In the late Eocene–early Miocene, the advancing Pamir‐plateau crust loaded the foreland, inducing subsidence, reheating, and early shortening. Basin inversion and major shortening/transpression in the foreland buttresses from ~12 Ma onward were synchronous with the subcrustal indentation of Indian lithosphere into the Tajik‐Tarim basin lithosphere and the onset of its rollback beneath the Pamir.

Highlights

The Tajik basin of Central Asia is a retro‐foreland basin with ~7–10 km, southward and eastward thickening, Mesozoic‐Cenozoic strata at the northwestern tip of the India‐Asia collision zone (Figure 1a; e.g., Burtman & Molnar, 1993; Carrapa et al, 2015; Chapman et al, 2019; Klocke et al, 2017; Nikolaev, 2002)
The Tajik basin and southwestern Tian Shan constitute the northwestern tip of the India‐Asia collision zone
We investigated eight Cr1 and four Oligocene‐Neogene sandstones from the north‐central Tajik fold‐thrust belt (FTB), six Cr1 sandstones from the northern Tajik‐basin margin and the footwalls of thrusts bounding the intramontane Hakimi basin of the Tajik Gissar, three Cr1 and three Oligocene‐Neogene sandstones from the eastern part of the Tajik FTB, three metamorphic basement rocks from the western Pamir at the eastern margin of the Tajik basin (Darvaz‐Visharv Pamir), 28 Cr1 sandstones and crystalline basement rocks from the Uzbek Gissar, and eight crystalline rocks from a basement spur at the western edge of the southwestern Tian Shan

Summary

Introduction

The Tajik basin of Central Asia is a retro‐foreland basin with ~7–10 km, southward and eastward thickening, Mesozoic‐Cenozoic strata at the northwestern tip of the India‐Asia collision zone (Figure 1a; e.g., Burtman & Molnar, 1993; Carrapa et al, 2015; Chapman et al, 2019; Klocke et al, 2017; Nikolaev, 2002). Schurr et al (2014) and Kufner et al (2018) explained the active deformation field by a combination of bulk northward displacement (Pamir orocline growth) and synchronous collapse and westward lateral material flow (“lateral extrusion,” cf Ratschbacher et al, 1991) of the Pamir plateau (~4 km average elevation) into the Tajik‐basin depression (~1 km average elevation); there, extrusion causes thin‐skinned, ~NW‐SE shortening of the Tajik‐basin strata above an evaporitic décollement and thick‐skinned shortening along the frontal and lateral foreland buttresses (Figure 1c). The northward movement and westward extrusion combine to cause the progressive rotation of the GNSS surface velocities from ~NNW to ~WNW from the eastern Pamir to the Tajik basin (Figure 1b)

Results

Discussion

Conclusion