A revisit to continental collision between India and Asia

Abstract

The India-Asia continental collision and the uplift of the Tibetan Plateau have long been attributed to continuous Cenozoic convergence with two generic assumptions: the ongoing India-Asia collision and the underthrusting of the Indian continent beneath the Tibet hinterland. This study presents a challenge to the two assumptions through an integrative analysis of geological, geophysical and geochemical data, mainly focusing on the duration of syn-collisional processes, the spatial range of syn-collisional effects, and the distinction in structure and composition between syn-collisional and post-collisional products. It turns out that the India-Asia collisional orogeny was short-lived in the Early Cenozoic without significant underthrusting of the Indian continent beneath the Tibet hinterland, and that the plateau uplift was governed by post-collisional mantle dynamics in the Late Cenozoic rather than the ongoing collision/subduction during the whole Cenozoic. The geological architecture of the Tibetan Plateau is highlighted as a mosaic of terranes that were accreted northwards from the Early Paleozoic through the Late Paleozoic to the Mesozoic. These terranes and their sutures were variably reactivated during the short-lived collision in the Early Cenozoic rather than forming as a singular collisional entity throughout the entire Cenozoic. The India-Asia collisional effects are the most prominent in the Himalayan and Gangdese orogens.

Inspection of key findings in the literature dismantles the traditional paradigms based on the two generic assumptions. Seismic tomography and helium isotope data constrain subduction of the Indian continent to depths of 200–300 km mainly beneath the Yarlung-Zangpo Suture that marks the southernmost margin of the Tibetan Plateau. Paleomagnetic discrepancies regarding the width of Greater India are resolved by emphasizing the cover-basement decoupling during soft collision and shallow subduction, limiting its underthrusting to short distances of ≤300 km. This reconciles geological shortening estimates with exaggerated paleomagnetic-derived convergence distances. A critical examination of petrological, structural and geochronological data partitions the formation of the Himalayan orogen into two stages. The first is the continental suturing from soft collision through hard collision to deep subduction, leading to crustal shortening through slice thrusting mainly at 55–45 Ma in the Early Cenozoic. The second is the post-collisional reworking due to upwelling of the asthenospheric mantle induced by foundering of the lithospheric mantle, resulting in crustal anatexis, emplacement of leucogranites and metamorphic core complexes, and domical uplift mainly at 30–10 Ma in the Late Cenozoic. The Himalaya-Tibet tectonic collage has behaved in a intracontinental setting in the Late Cenozoic when it uplifts due to the asthenospheric upwelling in response to the lithospheric foundering.

The present study demonstrates that the two generic assumptions for the India-Asia collision do not stand any more under close scrutiny and thus are essentially specious in previous studies. This has great bearing on the evolution of the Himalaya-Tibet tectonic collage in the Cenozoic. It casts doubt on the tectonic interpretation of paleomagnetic and seismic data with respect to the continental convergence and underthrusting between India and Asia. Many of geological and geochemical observations were interpreted under the two assumptions, overlooking a series of differences in temporal sequence, dynamic regime and geothermal gradient between the syn-collisional and post-collisional processes. Therefore, it is necessary to examine the rationality of geodynamic models for the processes, mechanisms and effects of the India-Asia collision and plateau uplift through prioritizing of the post-collisional processes over the syn-collisional effects. As such, this study offers transformative insights into the nature of continental tectonics at precedingly converged plate margins globally.