Abstract:

In eastern Asia, there exists a huge triangleshaped tectonic region with extensive major earthquakes. Roughly, the Himalayan arc, PamirsAltaiBaikal, and 105°E are the three boundaries of this region which covers plateaus, mountains and intermountain basins. Within this triangle region, the fragmented crust is characterized by intensive tectonic motion and extensive deformation that occurred along boundaries between blocks as well as in blocks. Outside this region, the rigid massifs moved as a whole with little quakes and weak deformation. On a large tectonic scale, such a broad intraplate deformation region was originated from dynamic interactions between the India, Philippine SeaWest Pacific and Eurasia plates, as well as the impact of mantle flow at depth. The Indian subcontinent that was moving toward north at about 40 mm/a had plunged beneath Tibet, resulting in various motions and deformation along the Himalayan arc, which diffused over a long distance in the hinterland of Asia. Along 95°100°E the originally northward crustal motion turned to east and southeast, impeding the northward motion around the eastern Himalayan syntaxis. At the western Himalayan syntaxis, the Pamirs continued to move into central Asia leading to crustal deformation and earthquakes which were largely accommodated by those old EW or NW trending faults in the bordering areas between China, Mongolia and Russia, and restricted by the stable land northwest of Tian ShanAltaiBaikal. The subduction of the Philippine and Pacific plates to the Eurasian continent had generated a very long and narrow seismic zone along trenches and islandarcs in the marginal seas, while posed a little horizontal compression on the Asian continent, thus unable to impede the eastward motion of eastern Asian continent. The partial reason for this is likely the impact of southeastward mantle flow beneath Eurasia on the subducting slabs; the southeastward mantle flow converged with the northward mantle flow from below India in central Tibet where they turned toward east and southeast, which is consistent with the pattern of surface movements observed by GPS.

Key words: eastern Asia, huge triangleangle seismic region, India plate, Philippine SeaPacific, Eurasian continent, mantle flow