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Multiple stages of continental subduction during India/Asia convergence; insight from seismic tomography and tectonic reconstructions


, : Multiple stages of continental subduction during India/Asia convergence; insight from seismic tomography and tectonic reconstructions. Congres Geologique International, Resumes 33

We use tomographic images of mantle structure down to 1600 km depth beneath the India/Asia collision zone to identify several slab fragments of Indian or Asian continental lithosphere. Their positions and their lateral extent are properly estimated, giving a better insight into the complex 3D mantle structure. At a given depth, a high-wavespeed anomaly zone is interpreted as the record of a subduction process, which occurred at the age associated with that depth. The reconstructed location of major faults through time allows us to discuss the tectonic context associated with these subduction processes. We infer a major slab breakoff process occurring at the early stage of the collision, probably around 45 Ma. We were able to draw the geometry of India after the breakoff, and to follow its evolution in the following collision process. We propose that India/Asia convergence between approximately 45 and 30 Ma was absorbed by southward subduction of the Asian lithosphere along the proto-Red River fault, whereas Indian lithosphere was moving horizontally, without subducting. In contrast, convergence from approximately 30 Ma to at least 15 Ma was likely accommodated by subduction along the proto-Karakorum fault of a portion of the northwestern margin of India. During the same period, the northeastern portion of India was probably cut and extruded eastwards with the Indochina block. The central part was likely affected by a combination of both subduction and extrusion mechanisms that caused a forward motion of the convergent boundary, resulting in an overturned geometry of the slab. We therefore infer a very heterogeneous behaviour of northern India from west to east along the collision zone. This lateral heterogeneity implies that the Indian continent was torn perpendicularly to its northern boundary during the collision process. Along most convergent margins where oceanic plates subduct into the mantle, the upper plate deforms little. Relating the dynamics of the subducting plate to tectonic shortening within the upper plate is then a second order problem. This is not the case, however, for the India/Asia collision zone, where the crust of upper plate shortened several hundreds of kilometres and extruded eastward. We used our knowledge of the crustal deformation during the collision to relate surface deformations to the dynamics of sinking lithospheric material and unravel this relationship. We infer that lateral advection in the mantle is small and that the sinking rate was about 2 cm/yr in the lower mantle and about 4 cm/yr above the transition zone.

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