Tajik Basin and Southwestern Tian Shan, Northwestern India‐Asia Collision Zone: 1. Structure, Kinematics, and Salt Tectonics in the Tajik Fold‐and‐Thrust Belt of the Western Foreland of the Pamir

Abstract

Surface, seismic, and borehole data characterize the Neogene-Recent Tajik fold-and-thrust belt of the Tajik basin. The basin experienced little sub-detachment basement deformation, acting as a rigid foreland plate during the Pamir orogeny. The Tajik fold-and-thrust belt contains variable thin-skinned structural styles, changing along and across strike as a function of the thickness and facies of Upper Jurassic evaporites, which constitute the basal detachment, and the influence of the surrounding thick-skinned belts. The southern Tajik fold-and-thrust belt shows regularly spaced, salt-cored, thrusted detachment anticlines that transition northward into imbricated thrust sheets grouped in oppositely verging stacks facing each other across a common footwall syncline. The width of the fold-and-thrust belt decreases northeastward accommodated by the Ilyak fault, a lateral ramp developed over a seismically active dextral basement fault. The southeastern Tajik fold-and-thrust belt contains massive subaerial salt sheets, formed by squeezing of preexisting salt diapirs. The salt-tectonic domain originates from a local depocenter within the Late Jurassic Amu Darya-Tajik evaporitic basin. Serial cross sections, integrating the structural geometries, yielded minimum thin-skinned shortening oriented at~90°to the India-Asia convergence direction, increasing from~93 km in the south to~148 km in the center, and dropping tõ 22 km in the northeast; total shortening-including the foreland buttress-is ≥170 km. Most of the shortening in the central-southern Tajik fold-and-thrust belt occurred by hinterland-vergent, high-displacement back thrusts. The Pamir played a dominant role in the transfer of shortening to the sedimentary infill of the Tajik basin with the Tian Shan acting as a semi-passive buttress.

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

“…(c) New fold‐fault map of Cenozoic structures of the Tajik basin and surrounding thick‐skinned fold‐thrust belts. Interpreted from 1:200,000 geological maps and Soviet seismic and drill‐hole data (compilation in Gągała et al, 2020), the Geological and Mineral Resource Map of Afghanistan (Doebrich & Wahl, 2006), satellite images (Google Earth®), and fieldwork between 2004 and 2014.…”

“…Figure 2b sketches the constraints imposed by the foreland shortening of the Asian (Tajik) crust on the geometry of the Tajik‐basin lithosphere (Pamir slab) beneath the Pamir (cf. Gągała et al, 2020). Sippl, Schurr, Yuan, et al, 2013) interpreted the subvertical cluster of seismicity between ~100‐ and 200‐km depth as representing eclogitized Asian (Tajik) lower crust, overlain between 60 and 100 km by subducted middle crust.…”

“…This geometry defines the curved upper interface of the rolling back Tajik‐basin lithosphere and is mimicked by the Tajik Moho down to ~90 km (Schneider et al, 2019). The ≥175‐km shortening estimates in the Tian Shan foreland and the Tajik FTB (Gągała et al, 2020) and the yet unquantified amounts of accretion and deep subduction (Sippl, Schurr, Yuan, et al, 2013) of Tajik (Asian) upper and middle crust in and beneath the North Pamir are considered a proxy for the amount of convergence since ~12 Ma. The deep‐reaching seismic zone in this scenario constitutes an incomplete Tajik lithosphere, deprived of its middle and upper crustal layer, the middle crust partly subducted to 80–100 km, and—together with the upper crust—accreted to the hanging‐wall and forming the North Pamir and the Tajik FTB.…”

“…(b) Cross‐sectional sketch of the constraints imposed by the foreland shortening of the Asian (Tajik) crust on the geometry of the Tajik‐basin lithosphere (Pamir slab) beneath the Pamir (cf. Gągała et al, 2020). The seismicity cluster and subducted Tajik middle crust approximate the down‐dip extension of the detachment(s) of the Tajik fold‐thrust belt and its basement.…”

“…Herein, our major goal is to extend the “northward indentation‐westward flow” tectonic model of the active deformation field (Figures 2a and 2b) in time and investigate the onset and progression of the inversion of the Tajik basin, and the age of the involvement of the western (Uzbek Gissar) and northern (Tajik Gissar) foreland buttresses of the Tajik FTB in the southwestern Tian Shan (Figure 1c). In part 1 of this paper series, Gągała et al (2020) detail the structures, kinematics, and amount of shortening during the evolution of the Tajik FTB and its foreland; in part 3, Dedow et al (2020) specify the preorogenic and synorogenic tectono‐sedimentary evolution of the eastern Tajik basin. Here, we provide estimates of burial temperatures from vitrinite‐reflectance maturity data of the Jurassic to Upper Cretaceous (Cr 2 ) strata of the Tajik basin and the Uzbek and Tajik Gissar, and zircon (U,Th)/He (ZHe), AFT, and AHe thermochronologic dates from samples of crystalline basement rocks of the Tajik and Uzbek Gissar, and the Jurassic to Lower Cretaceous (Cr 1 ) clastic rocks of the Tajik basin, exhumed along the thrusts of the Tajik FTB.…”