Subduction and break-off controls on Indentation tectonics and Western Syntaxis formation during India-Asia convergence

Abstract HKT-ISTP 2013 A

Amount of Asian lithospheric mantle subducted during the India/Asia collision

Body wave seismic tomography is a successful technique for mapping lithospheric material sinking into the mantle. Focusing on the India/Asia collision zone, we postulate the existence of several Asian continental slabs, based on seismic global tomography. We observe a lower mantle positive anomaly between 1100 and 900 km depths, that we interpret as the signature of a past subduction process of Asian lithosphere, based on the anomaly position relative to positive anomalies related to Indian continental slab. We propose that this anomaly provides evidence for south dipping subduction of North Tibet lithospheric mantle, occurring along 3000 km parallel to the Southern Asian margin, and beginning soon after the 45 Ma break-off that detached the Tethys oceanic slab from the Indian continent. We estimate the maximum length of the slab related to the anomaly to be 400 km. Adding 200 km of presently Asian subducting slab beneath Central Tibet, the amount of Asian lithospheric mantle absorbed by continental subduction during the collision is at most 600 km. Using global seismic tomography to resolve the geometry of Asian continent at the onset of collision, we estimate that the convergence absorbed by Asia during the indentation process is ~ 1300 km. We conclude that Asian continental subduction could accommodate at most 45% of the Asian convergence. The rest of the convergence could have been accommodated by a combination of extrusion and shallow subduction/underthrusting processes. Continental subduction is therefore a major lithospheric process involved in intraplate tectonics of a supercontinent like Eurasia.The project was funded by the Integrated Actions Program HF2008-0089 by the Spanish and French Foreign Affairs Ministries (Egide-Picasso).Peer Reviewe

Reconstruction of the deformed collision zone Between India and Asia by backward motion of lithospheric blocks

International audienceOn the basis of a synthesis of tectonic data available on the India-Asia collision, we present a first attempt to reconstruct the evolution of the collision zone. Assuming that the deformation of the lithosphere is localized along narrow shear zones and that the interiors of mantle blocks in between remain relatively undeformed, we define block contours from the fault pattern and move back the blocks along their boundary faults. Along convergent or extensional boundaries, the crust is assumed to shorten or stretch coherently. Step-by-step, we go backward in time to finally reach the collision onset. For each time step, we find a solution compatible with the data set available and the position of the adjacent blocks for each block. The search for compatibility at the scale of the entire collision zone allows for solving the kinematics of regions with fewer data and suggests plausible scenarios for regions where data is lacking. For each step, we calculate large-scale displacement maps, and determine Euler poles for each block. For the most recent time step, the map proposed is compared to GPS motions. The deformation budget implies that extrusion absorbed ∼30% of the convergence between India and Siberia during the entire collision span, but varied with time, accounting for as little as 3% or as much as 60% of this convergence at different epochs

The coupling of Indian subduction and Asian continental tectonics

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.This work has been supported by a grant from Labex OSUG@2020 (Investissements d’avenir – ANR10 LABX56) and the ANR DSP-Tibet.Peer reviewe

Large river offsets and Plio-Quaternary dextral slip rate on the Red River fault (Yunnan, China)

International audienceUsing multispectral SPOT images and 1/100,000 topographic data, we present an improved map of the active Red River fault zone between Midu (Yunnan, China) and Hanoi (Vietnam). The fault zone is composed of parallel strands, one of which, the Yuanjiang fault was previously undetected. There also appears to be a component of extension all along the fault zone. Such extension increases toward the SE, from Yunnan to the south China sea coast, and the vector describing the motion of south China relative to Indochina points within the N45 ø-135øE quadrant. We attempt to assess the Plio-Quaternary dextral slip rate on the Red River fault (RRF) by restoring large river offsets and searching for the largest, plausible one. Across much of Yunnan, the fault is perpendicular to local catchments that drain into the Red River. From precise mapping of the river courses on SPOT satellite images and on 1/100,000 topographic maps, numerous multiple offsets along the fault can be detected and reconstructed. The lack of correlation between the apparent offsets and the lengths of the rivers upstream from the fault suggests either that the drainage system was in large part established prior to the onset of dextral slip along the fault or that frequent captures have occurred. We thus try to find the best fit between series of river channels upstream and downstream from the fault by progressively restoring the dextral displacement in increments of 500 m, up to an offset of 50 km. For each increment we measure the misfits (root mean squares, RMS) between the upstream and downstream channels. The best fit and smallest RMS are obtained for an offset of 25 + 0.5 km that we interpret to represent the clearest, large right-lateral displacement recorded in the geomorphology along the active Red River fault. Since dextral motion is likely to have started around 5 Myr, the most probable average Plio-Quaternary slip rate on the fault is of order of 5 mm/yr. We attribute the apparent lack of seismic activity on a large stretch of the fault to millennial recurrence times between great earthquakes. Our study shows that relatively small drainage systems can keep a good record of fairly large cumulative fault offsets

Indian continental subduction and slab break-off during Tertiary collision

International audienceHigh wavespeed seismic anomalies in the transition zone and uppermost lower mantle beneath the India-Asia collision zone, imaged by body-wave seismic tomography, have been interpreted as subducted fragments of continental material. In this study, we focus on the prominent anomaly located beneath India between depths of about 450 and 900 km. By combining the location of this anomaly with palaeogeographical positions of India, we constrain the timing of the subduction event probably related to this anomaly. We infer that a large portion of the north-western margin of India initiated subduction at 35 ± 5 Ma along a 1500-km-long WNW–ESE striking zone and ended with a progressive slab break-off process. This break-off started most probably around 25 Ma at the western end of the slab and propagated eastwards until complete break-off around 15 Ma. This study helps to constrain better the amount of convergence between India and Asia absorbed by continental subduction

Modeling Deep Rooted Thrust Mechanism of Crustal Thickening in Eastern Tibet

International audienceTo test Eastern Tibet crustal thickening modes, we compare 2‐D numerical models of two emblematic end‐member models, with either an obstacle in the low viscosity lower crust or a thrust embedded in the high viscosity one. We show that the obstacle halts the viscous lower crustal flow potentially initiated by the weight of the high Central Tibet, generating a smooth exhumation gradient at the edge of the plateau, not observed in Eastern Tibet. On the contrary, including a low viscosity discontinuity in the upper crust, mimicking a shallow steep listric fault as inferred in the region, reproduces a sharper exhumation profile, as constrained from thermo‐kinematic inversions of thermochronological data, and the lack of foreland basin, as observed in the field. Moreover, such fault drives deformation throughout the entire crust, suggesting a deep crustal ductile shear zone limiting the more ductile deformation in the lower crust although no discontinuity is imposed

4-D evolution of SE Asia mantle structure from geological reconstructions and seismic tomography

How the collision between India and Asia is related to processes deeper in the mantle is unclear. Here we compare geological reconstructions of block motions within Asia since W50 Ma with the tomographically imaged threedimensional (3-D) morphology of subducted lithosphere to obtain insight into the spatiotemporal evolution of mantle structure. Past positions of the convergent margin show remarkable similarities with slab geometry at specific depths. The striking change in slab geometry from a linear structure beneath 1100 km to an increasingly distorted shape at depths of less than 700 km results from collision. The slab contours match the progressive deformation of Asia’s margin, including India’s indentation and Sundaland’s extrusion. Ever since the onset of collision, the Indian plate appears to have overridden its own sinking mantle and it does not seem, at present, to underthrust Tibet significantly north of the Zangbo suture. If correct, this observation would provide further evidence against models of plateau build-up involving Indian lithosphere. The tomographic images beneath India confirm that Asian deformation has absorbed at least (circa) 1500 km of convergence since collision began. From the match between the southeastward motion of Sundaland between 40 and 20 Ma and the principal change in slab structure between 700 and 1100 km depths, we infer that lateral advection in the mantle is small and that the sinking rate beneath Sunda was circa 2 cm/yr in the lower mantle and circa 5 cm/yr above the transition zone