Abstract:

In central Asia, there exists a notable NE trending boundary (TianshanBaikal boundary) along the western end of the Tianshan Mountains, northwestern end of the Altai Mountains and the Baikal area. On either side of this boundary, the features of topography, tectonics, seismicity and other geophysical fields are quite different. There is no major fault aligned with this boundary, while a series of NW striking faults intersect it at nearly right angles, of which the northwestern sections terminate nearby the TianshanBaikal boundary. Overall, the stable Kazakh platform and Siberia platform are situated in the northwestern side of this boundary, and tectonically active Tianshan, Altai and western Mongolia characterize its southeastern side. Therefore this TianshanBaikal boundary marks the termination of crustal deformation within the continent. Processes of terrane amalgamation in central Asia finished at the end of the Paleozoic when the Tianshan, Altai and other orogens were welded with the Kazakh and Siberia platforms, followed by consolidation during the Mesozoic. Since the Cenozoic, impacted by the IndiaAsia collision and continued postcollisional plate convergence, the Tianshan, Altai and other orogens were rejuvenated, resulting in extensive crustal deformation under NS compression, which concentrates along the EW trending thrust and NW striking strikeslip faults, as well as diffused earthquakes. The spread of these deformations toward north was terminated or cut off by the rigid and intact platforms of Kazakh and Siberia, creating the NE striking TianshanBaikal boundary. Crustal strength and deformation behaviors of blocks in the Asian continent have a conspicuous inheritance. Since the Cenozoic, the original platforms (cratons) remain intact and stable, almost without interior deformation, while those old orogens underwent intensive deformations again, which were associated with IndiaAsia collision as well as deep structures of crust and upper mantle. Besides, it also played an important role in strain partitioning that the platforms of Kazakh and Siberia, northwest of the TianshanBaikal boundary, hampered the diffusion of deformation.

Key words: central Asia, TianshanBaikal, deformation termination, crust and upper mantle