Stepwise Lithospheric Delamination Leads to Pulsed Cenozoic Uplifts of Central Tien Shan

Abstract

The Tien Shan provides an ideal site to study mechanism of intracontinental orogeny due to distant effect of Indo-Asian collision. We investigate lithospheric structures, in particular the lithosphere-asthenosphere boundary (LAB), of Central Tien Shan (CTS) using S wave receiver functions. The results show distinct structures across the orogen. Under the southern CTS, the LAB is shallower than that of the Tarim Basin; a 50 km vertical offset implies that part of the lithosphere has been delaminated. Under the middle CTS, two phases of negative velocity gradient are obtained, which may indicate a new LAB and an ongoing delamination underneath. Under the northern CTS and Kazakh Shield northward, the lithosphere is stable although the LAB inclines southward slightly. The two periods of lithospheric delamination under the southern and middle CTS account well for pulsed uplifts of the Tien Shan at ~11-8 Ma and ~5-0 Ma, respectively.

The Mantle Transition Zone Structure Beneath the Pamir Plateau and Western Tian Shan and Adjacent Regions

“…One end‐member model invokes active deep subduction of the Tarim lithosphere into deep mantle beneath the western Tian Shan, as suggested by ambient noise tomography (W Sun et al., 2022), body wave travel‐time tomography (Koulakov, 2011; Z Li et al., 2009; Zabelina et al., 2013) and Pn travel‐time tomography (Hapaer et al., 2022). Another end‐member model favors subhorizontal indentation of the rheologically strong Tarim lithosphere into the weak lithospheric mantle under the western Tian Shan accompanied by lithospheric delamination, as evidenced by recent P‐ and S‐wave RF studies (W. Li et al., 2022; Zhang et al., 2020; Zhao et al., 2022), seismic anisotropy derived by shear wave splitting measurements and harmonic fitting based on Pms (Moho P‐to‐S converted phases) arrival times (Zhang et al., 2022), and full‐wave ambient noise tomography (Lü et al., 2019).…”

“…Thus, we prefer the underthrusting or subhorizontal indentation of the Tarim lithosphere below the western Tian Shan. As a result, the delamination of thickened Tian Shan lithospheric root has occurred to accommodate the large convergence (e.g., ∼20 mm/yr, Zubovich et al., 2010) between the Kazakh Shield and Tarim Basin (Zhang et al., 2022; Zhao et al., 2022), which has greatly contributed to the rapid uplift of the western Tian Shan since 20–25 Ma in addition to its crustal shortening. The segments of the delaminated lithosphere might rest at the base of the MTZ, as inferred from the depressed d660 beneath the western Tian Shan (Area C in Figures 5b and 5e), in accordance with the previous RF study (Tian et al., 2010).…”

“…The continuous deep subduction of the Tarim lithosphere into deep mantle beneath the western Tian Shan has been favored by the high‐velocity structures in the upper mantle imaged by ambient noise tomography (W Sun et al., 2022), body wave travel‐time tomography (Koulakov, 2011; Z Li et al., 2009; Zabelina et al., 2013) and Pn travel‐time tomography (Hapaer et al., 2022). Alternatively, the Tarim lithosphere indenting northward has been suggested by the fast orientations that are parallel to the orogen in the south‐western Tian Shan derived from seismic anisotropy and the Moho and lithosphere‐asthenosphere boundary (LAB) topography from receiver function (RF) researches (W. Li et al., 2022; Zhang et al., 2020; Zhang et al., 2022; Zhao et al., 2022). The thinned LAB and thickened MTZ beneath the western Tian Shan further indicate that parts of the thickened lithosphere might drop down into the MTZ (Tian et al., 2010; Yu et al., 2017).…”