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

The Tajik basin archives the orogenic evolution of the Pamir hinterland.

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

“…Figure S2 shows that the single‐grain ages of the four samples from the Neogene rocks are consistent with their sample ages (Table 1; ~24.8–16.4 Ma, median = 21.1 Ma). The sample ages are somewhat higher than or close to the upper limit of the depositional age range (three samples from the Khingou Formation with a stratigraphic age of ~18–12 Ma and one sample from the Baljuvon Formation with a stratigraphic age of ~28–18 Ma; see discussion on the difficult stratigraphic age assignment for these Formations in Klocke et al, 2017 and Dedow et al, 2020). The single‐grain ages of the samples from the Cr 1 rocks are also consistent with their sample ages.…”

“…Furthermore, several thrust sheets, in particular those bounding the central depression of the Tajik basin and the Darvaz fault zone, have matching young AHe ages, suggesting reactivation. Given that growth strata indicate that also the eastern Tajik FTB started to shorten before ~11 Ma (Dedow et al, 2020; Gągała et al, 2020), the clustering of young AHe ages in the eastern Tajik FTB (Vakhsh anticlinorium to eastern Pamir margin) suggests that the young Tajik FTB reactivation is concentrated there. The scarcity of young reactivation in the Uzbek Gissar indicates that after regional synchronous initiation, shortening concentrated in the Tajik basin, where the evaporitic décollement facilitated thin‐skinned deformation; this is particularly true for the eastern Tajik basin, where the widespread and thick salt has enabled related tectonics (e.g., Gągała et al, 2020; Nikolaev, 2002).…”

“…The synchronous foundering of the Pamir deep crust at ~14–11 Ma, documented in the xenoliths of the southeastern Pamir and related to the indentation process beneath the Pamir (Shaffer et al, 2017), provides an additional driving force by elevating the gravitational potential energy stored in the Pamir‐plateau crust. The northward and westward progression of rollback of Asian lithosphere may have caused the young deepening of the southeastern (internal) Tajik‐basin depocenter, the back‐stepping of the upper Miocene‐lower Pliocene synorogenic strata onto the North Pamir (Dedow et al, 2020; Klocke et al, 2017; Lyoskind et al, 1963), the young AHe ages, and the T‐t models suggesting burial and rapid exhumation over the last few Myr in the internal Tajik FTB and the adjacent Pamir.…”

“…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.…”

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

“…The Tajik basin is the eastern extension of the greater Amu Darya basin, situated in a retrowedge position with respect to the India‐Asia collision zone (Figure 1a). It evolved from a Triassic‐Eocene epicontinental sag basin into an Oligocene‐Recent foreland basin of the Pamir and Tian Shan (e.g., Brookfield & Hashmat, 2001; Carrapa et al, 2015; Chapman et al, 2019; Dedow et al, 2020; Klocke et al, 2017; Nikolaev, 2002). At present, the Tajik basin links to the Amu Darya basin through a narrow corridor between the southwestern termination of the Tian Shan and the Afghan platform (Figure 1b).…”