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 to ~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.