The forms of the margins of the Lhasa terrane and the Tethyan Himalaya prior to the collision of India and Eurasia as constrained by paleomagnetism are ambiguous due to the disordered Cretaceous paleomagnetic data from the central Lhasa terrane and the counterclockwise rotation of the Indian plate during the Cretaceous. This ambiguity has induced controversy over the processes of suturing of India and Eurasia and the closure of the Neo-Tethys Ocean. We obtained a set of high-quality Late Cretaceous paleomagnetic data from the central Lhasa terrane, which, integrated with reliable Cretaceous and Paleogene paleomagnetic data sets from the other parts of the Lhasa terrane and Tethyan Himalaya, confirmed that the southern margin of the Lhasa terrane and the northern margin of the Tethyan Himalaya were originally oriented ∼317° and ∼326°, respectively, prior to the collision of India and Eurasia. The margins of the Lhasa terrane and Tethyan Himalaya were almost consistent with the original straight fold axes of Cretaceous strata in the southern part of the Lhasa terrane, which were oriented 332.5° ± 8.5°, indicating that the subduction of the Neo-Tethys Ocean beneath Eurasia and the movement of the Tethyan Himalaya consistently maintained a stabilized direction of 62.5° ± 8.5° during the Late Cretaceous. The different kinematic characteristics of the Indian plate and Tethyan Himalaya and the overlap of the margins of the Tethyan Himalaya and Lhasa terrane during 59.0−56.0 Ma indicate that the Tethyan Himalaya was already rifted from the Indian plate prior to 62.5−59.2 Ma, and then it quasi-parallelly collided with the Lhasa terrane during 59.0−56.0 Ma, quasi-synchronously closing the Neo-Tethys Ocean.
The Cenozoic tectonic evolution of the southeastern edge of the Tibetan Plateau is interpreted in terms of two alterative dynamic mechanisms: lateral extrusion of coherent lithospheric blocks and viscous lower crustal flow. To contribute to this debate, we conducted a magnetostratigraphic study of the Early Miocene sedimentary strata of the Gengma Basin, in the northern part of the Shan‐Thai Block (STB). The results show that variations in the deposition rate were synchronous with the rate of clockwise rotation during ∼18.05–15.93 Ma, indicating that the crustal clockwise rotation of the northern STB was the driver of the tectonic transformation of the NE‐SW trending strike‐slip system within the STB by at least ∼18.05 Ma. The rapid decrease in the clockwise rotational rate of the Gengma Basin was also synchronous with the sharp decrease in the convergence rate between the Indian Plate (IDP) and Eurasia at ∼17.23 Ma, which is similar to that occurred in the interior of the Tibetan Plateau during the Paleocene and Eocene, demonstrating the fast response of the southeastern edge of the Tibetan Plateau to the convergence of the IDP and Eurasia, in the form of the continuous upper crustal ductile deformation of the Tibetan Plateau and its periphery. The lateral flow of the viscous lower crust beneath the southeastern edge of the Tibetan Plateau may be the predominant driver of the tectonic evolution of the southeastern edge of the Tibetan Plateau since the Early Miocene.
The Himalayan-Tibetan orogenic belt is the most spectacular and active continent-continent collision orogen on Earth (Yin & Harrison, 2000). Knowledge of the size of Greater India is essential for determining the timing of initial India-Asia collision and for calculating the magnitude of crustal shortening and subduction (Ding et al., 2017). The definition of present-day Greater India is reviewed by Ali & Aitchison. (2005), who proposed that the size of Greater India ranged from several hundred to around 3,000 km. At present, estimates of Greater India can be grouped into two categories, resulting in differences in the timing of the India-Asia collision and in the collision modes (single-stage collision vs. multi-stage collision). Estimates in the first category include the large Greater India continental model which suggests that Greater India consisted of continental crustal and with a north-south extent of 2,000-3,000 km (e.g., Meng et al., 2020), which was then consumed by continental subduction and tectonic shortening after the ∼55 Ma collision. The second category involves a smaller Greater India extent, which attempts to avoid the difficulty of subducting more than 2,000 km of continental crust.In this latter category, several geodynamic models have been proposed. One suggested that, during ∼60-50 Ma, India first collided with an arc that existed within the Neo-Tethys Ocean or was broken up from the southern margin of Asia, and then collided with Asia during ∼45-40 Ma (e.g., Kapp & DeCelles, 2019;Martin et al., 2020). Other models suggest that the Tibetan-Himalaya terrane (including the Tethyan Himalaya [TH] and Greater Himalaya) separated from "Greater India" in the Early Cretaceous (120 Ma) or mid-Late Cretaceous (75 Ma), respectively, to form an ocean basin, and then drifted northward before colliding with the Lhasa terrane between about 61 and 58 Ma (van Hinsbergen et al., 2018;Yuan et al., 2021). These two propositions all suggest a late India-Asia collision at ∼50 or ∼25 Ma, respectively, in the Main Central Thrust zone. Yuan et al. (2022) combined the above two models and proposed an "arc-continent collision and subsequent two-stage continent-continent collision" model.
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