Arc tectonic elements and the upper mantle structure of the Central and Southeastern Asia: seismic tomography and seismicity data

Abstract

Analysis of the upper mantle plumes spatial distribution in the inner part of the Sunda arc shows a number of plume bodies interrupting the stagnating slab framed from the south by the sinking slab of the Sunda Arc. Possible mechanisms providing this structure of the mantle are (i) sublatitudinal toroidal mantle flow through a gap in a flat slab and (ii) roll-back capable of forming a gap in a flat slab and launching upper mantle plumes in it without deep (1000 km) roots. The space above the slab top surface consists of local hot mantle bodies, which are secondary plumes and often form local rift segments. The three-dimensional mapping of δVp in the Tibet and Central Asia region contains structural styles similar to the Sunda Arc region. There is a region of subhorizontal fragments of slabs and a gap in which plume anomalies of deep and secondary origin are established. The vectors of the movements of rock masses along the shape of the Sunda Arc detachment planes, detected from seismic events, are directed outward from the center of the curvature of the arc in which secondary upper mantle plumes are concentrated. This indicates the presence of thrust processes at the arc front that are not associated with the subducting plate. Thrusting at the arc is accompanied by less number of events along antithetical thrusts. The fan-shaped orientation of azimuth movements along the Himalayas is directed to Hindustan. This shows that the main indicator of tectonic activity — seismic events — has a direction of rock mass displacement to the south from the back-arc stretching region within Tibet with the formation of thrust deformations during movements along the detachment planes. In the Himalayan arc, as well as the Sunda Arc, two directions of seismic movements are distinguished. The first direction corresponds to the model of the Indian Plate subduction. The second direction combines the displacement of the thrusts on the Indian Plate.