Tajik Basin and Southwestern Tian Shan, Northwestern India-Asia Collision Zone: 3. Preorogenic to Synorogenic Retro-foreland Basin Evolution in the Eastern Tajik Depression and Linkage to the Pamir Hinterland

Abstract

©2020. American Geophysical Union. All Rights Reserved. The Tajik basin archives the orogenic evolution of the Pamir hinterland. Stratigraphic-sedimentologic observations from Cretaceous-Pliocene strata along its eastern margin describe the depositional environment and basin-formation stages in reaction to hinterland exhumation and basin inversion. During the Late Cretaceous-Eocene (preorogenic stage: ~100–34 Ma), a shallow-marine to terrestrial basin extended throughout Central Asia. An alluvial plain with influx of conglomerate bodies (Baljuvon Formation) indicates a first pulse of hinterland erosion and foreland-basin formation in the late Oligocene-early Miocene (synorogenic stage Ia: ~34–23 Ma). Further hinterland exhumation deposited massive alluvial conglomerates (Khingou Formation) in the early-middle Miocene (synorogenic stage Ib: ~23–15 Ma). Westward thickening growth strata suggest transformation of the Tajik basin into the Tajik fold-thrust belt in the middle-late Miocene (synorogenic stage IIa: ~15–5 Ma). Increased water supply led to the formation of fluvial mega-fans (Tavildara Formation). Latest Miocene-Pliocene shortening constructed basin morphology that blocked sediment bypass into the central basin from the east (Karanak Formation), triggering drainage-system reorganization from transverse to longitudinal sediment transport (synorogenic stage IIb: < ~5 Ma). Accelerated shortening (~27–20 Ma) and foreland-directed collapse (~23–12 Ma) of Pamir-plateau crust loaded the foreland and induced synorogenic stages Ia and Ib. Coupling of Indian and Asian cratonic lithospheres and onset of northward and westward delamination/rollback of Asian lithosphere (i.e., lithosphere of the Tajik basin) beneath the Pamir at ~12–11 Ma transformed the Tajik basin into the Tajik fold-thrust belt (synorogenic stage IIa). The timing of the sedimentologically derived basin reconfiguration matches the thermochronologically derived onset of Tajik-basin inversion at ~12 Ma.