The coupling of Indian subduction and Asian continental tectonics

• Global tomography evidences Indian continent subduction and breakoff.
• After complete breakoff at the OCT, Indian continent subduction resumes.
• Mantle flow generates by oceanic subduction drags the Indian continent in the mantle.
• Indian slab breakoff generated localized stress pulses in Asian continent.

In order to understand the potential controls on Asian tectonics during the subduction of the Tethys and Indian lithospheres, we reconstruct the coupled subduction-continent deformation history using tomographic imaging, kinematics constraints and numerical modeling.The global P-waves tomographic images of the mantle below the India-Asia collision zone provide constraints on the deep structure of continents and subduction history. Linking the slab positions in the mantle to the Asian tectonics reconstructions and the Indian plate kinematics, we reconstruct the timing and location of successive subduction and breakoff events, showing one major breakoff occurred between India and the Tethys Ocean ~ 45 Ma. In the western syntax, a vertical slab continuous to the continent is shown to override the deeper detached Tethys slab. In the central region similar structure is found with a detached slab, yet closer to the Tethys slab. In the eastern syntax, no slab is imaged. It is inferred that after Tethys slab had broke off, subduction only resumed in the center of the margin, while underthrusting took place at both extremities of the convergent margin. During following convergence, a second breakoff event detached the central Indian slab from the margin ~ 15 Ma ago, which renewed Indian lithosphere underthrusting below Asia. This most probably occurred when the Tibetan Plateau was already uplifted, implying that uplift is not a direct result of underthrusting.Numerical models of breakoff during subduction illustrate the controls of slab detachment on the complexities of the Indian margin. In these models the subduction of continental lithosphere resumes after breakoff only where this is entrained by the mantle flow associated with the long lasting oceanic slab sinking, that is in the center of the margin, while converging continent edges underthrusts the upper plate. Furthermore, the breakoff during subduction has profound implications on the Asian intra-plate tectonics. In the models, the breakoff is rapidly followed by large stresses in the upper plate interiors, propagating at large distance from the margin, along a belt oriented at ~ 45° from the trench. The long-term evolution of the Asian continental tectonics shows drastic changes in the fault pattern, with successive strike-slip faulting across the Asian continent, which are in agreement with the mechanisms illustrated by the models. Transient large coupling at the trench caused by the breakoff events during India-Asia convergence offers an explanation for episodic nucleation of lithospheric faults within the Asian continent and their link to deep processes.

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