Tajik Basin and Southwestern Tian Shan, Northwestern India-Asia Collision Zone: Timing of Basin Inversion and Associated Deformation-Field Evolution
Final Report Abstract
The project aimed on the understanding of the evolution of the deformation during the inversion of the Tajik basin into a fold-thrust belt; the establishment of the incremental shortening history used fault-slip data. The timing of the deformation during the inversion used a combination of lowtemperature (apatite fission-track, AFT) thermochronology and laser-ablation – inductively coupled plasma – mass spectrometry (LA-ICPMS) U-Pb geochronology of calcite on faults and fault-related tension gashes. The structural analysis was conducted on 53 stations distributed over the entire Tajik basin; 26 stations provided the anticipated information on the horizontal and/or vertical axis rotations and allowed the selection of samples for the geochronology part. 72 mounts from samples covering pre-, syn-, and post-folding stages of deformation, and the early and late stages of vertical axis rotation, were prepared and 35 selected for LA-ICPMS U-Pb calcite analysis. From this selection, only two samples from Bukhara limestone yielded interpretable results—the other samples had too little U to define an interpretable isochron. The fact that few additional samples from the successfully-dated Bukhara limestone with meaningful structural results were available resulted in a failure of the U-Pb calcite dating project. The two results demonstrate that—in principle—the U-Pb dating has great potential. Calcite fibers on a classic pre-folding layer-parallel shortening before buckling fault yielded a U-Pb calcite age of 10.61 ± 0.79 Ma; this age is within uncertainty (1s) identical to three AFT ages from the same part of the thrust belt (10.8, 10.8, and 7.5 Ma), which date the onset of fold-thrust belt development. In contrast, the sample from a latest syn- to post-folding fault from the hinterland of the fold-thrust belt yielded a U-Pb calcite age of 1.99 ± 0.67 Ma, and, thus, dates the late-stage reactivation of the salt-involved part of the fold-thrust belt. About 20 new dates, mostly from the— up-to-now—undersampled eastern Tajik basin confirmed the interpretation that the onset of Tajikbasin inversion occurred at 13-12 Ma. Figure 3. Geologic transects (~N-S) across the southwestern Tian Shan from the Dushanbe Trough of the Tajik basin in the south to the Fergana basin in the north. Averaged and projected into the transects are the thermochronologic data, paleotemperatures calculated from vitrinite maturity, and exhumation-depth estimates for particular units. Apatite fission-track (AFT) ages along the cross sections are color-coded to highlight age ranges. The failure in the U-Pb calcite dating prompted a shift in the project focus. Cooperating in an InSAR rate-map based study of the deformation of the Tajik fold-thrust belt, a fault-data base, structural cross-sections, and details on the kinematics of certain faults were provided. The InSAR data and the new AFT data derived in this project highlight for the Ilyak fault along the northern margin of the Tajik basin and the Vakhsh anticlinorium in the near-field of the Nurak reservoir (one of the highest man-made dams of the World) strong rate changes; the AFT ages are younger than the ages typical for the onset of the major deformation of the Tajik basin. This indicates that the Nurak-reservoir dam scale tectonics, halokinesis, and seasonal-driven near-surface effects. Abrupt ∼6 mm/yr horizontalwas built on active structure. The study also highlights that Tajik fold-thrust belt combines basin- rate changes occur across the kinematically linked dextral Ilyak strike-slip fault bounding the Tajik fold-thrust belt in the north, and the Babatag backthrust, the major thrust of the fold-thrust belt, located far west in the belt. As an additional task, we embarked on the re-analysis of the structural-thermochronologic evolution of the southwestern Tian Shan, its intra-montane basins, and the northern margin of the Tajik basin. In this research, we dated the reactivation of the southwestern Tian Shan (including the northern margin of the Tajik basin) and explored its temporal and spatial variations. Based on the structures and the thermochronologic results, the southwestern Tian Shan can be divided in three domains. The central domain records a 13‒10 Ma onset of shortening and >4 km exhumation. The northern domain exhumed from <4 km, but AHe ages suggest a similar reactivation history as in the central domain. The synchronous structural reactivation implies rapid shortening propagation from the Pamir indenter across the Tajik-basin fold-thrust belt into the Tian Shan. In the southern domain, 7‒9 Ma AFT and ~4 Ma AHe ages suggest southward shortening propagation from both northern domains and anew thrust generation. The refined timing supports the synchronous building of the Cenozoic Tian Shan and the India-Asia cratonic collision under the Pamir and western Tibet, and the instantaneous transfer of shortening across a ~1000-km-wide foreland, facilitated by structural reactivation.
Publications
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Tajik Depression and Greater Pamir Neotectonics From InSAR Rate Maps. Journal of Geophysical Research: Solid Earth, 126(12).
Metzger, Sabrina; Gągała, Łukasz; Ratschbacher, Lothar; Lazecký, Milan; Maghsoudi, Yasser & Schurr, Bernd
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Structure and Stress Field of the Lithosphere between Pamir and Tarim. California Digital Library (CDL).
Bloch, Wasja; Schurr, Bernd; Yuan, Xiaohui; Ratschbacher, Lothar; Reuter, Sanaa; Kufner, Sofia‑Katerina; Xu, Qiang & Zhao, Junmeng
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The 2015–2017 Pamir earthquake sequence: foreshocks, main shocks and aftershocks, seismotectonics, faultinteraction and fluidprocesses. Geophysical Journal International, 233(1), 641-662.
Bloch, Wasja; Metzger, Sabrina; Schurr, Bernd; Yuan, Xiaohui; Ratschbacher, Lothar; Reuter, Sanaa; Xu, Qiang; Zhao, Junmeng; Murodkulov, Shokhruhk & Oimuhammadzoda, Ilhomjon
