RAYLEIGH-WAVE GROUP-VELOCITY TOMOGRAPHY OF SIBERIA, CHINA AND THE VICINITY

Rayleigh waves are used in a tomographic inversion to obtain group vel ocity maps of East Asia (40 degrees E-160 degrees E and 20 degrees N-7 0 degrees N). The period range studied is 30 to 70 seconds. Seismogram s used for this study were recorded at CDSN stations; at a temporary b roadband seismic array in Tibet, at several SRO stations, and Kirnos-e quipped stations established in Asia by the former Soviet Union, in Si beria, in the Sakhalin and in Mongolia. Altogether more than 1200 path s were available in the tomographic inversion. The study area includes the Angara craton, the geologically ancient core of Asia, and the sub sequently accreted units, the Altaids (a Paleozoic collision complex), the Sino-Korean platform (a chain of Archaen terranes separated by be lts of active structures), the south China platform (a collage of Prec ambrian, Paleozoic and Mesozoic metamorphic and igneous terranes), as well as the Tibetan plateau (an active tectonic feature created in lat e Cenozoic through collision of the Indian subcontinent and the Asian continent). Many of these main units are recognizable in the tomograph ic images as distinctive units; Tibet appears as a prominent low veloc ity (about -15% From the average) structure, with western and central Tibet often appearing as the areas with the lowest velocities, the Cen tral Asian fold-belt, and the Angara craton are consistently high grou p velocity areas. Some lesser tectonic features are also recognizable. For example, Lake Baikal is seen as a high velocity feature at period s greater than 40 seconds. However, the high group velocity feature do es not stop near the southern end of Lake Baikal, it extends south-sou thwestward across Mongolia. The North China Plain, a part of the platf orm where extensional tectonics dominate, is an area of high velocitie s as a result of relatively thin crust. The south China block, the lea st tectonically active region of China, is generally an area of high v elocity. For periods longer than 40 seconds, a NNE trending high group velocity gradient clearly exists in eastern China, the velocities are noticeably higher in the east. From the group velocity maps, average dispersion curves at twelve locations were determined and inverted to obtain velocity structures. Main results of group velocity inversion i nclude: (1) a Tibetan crust of around 60 km thick; with low crustal an d upper mantle shear velocities, at 3.3 kms and 4.2 km/s, respectively ; (2) with the Moho constrained at 40-43 km, the Angara craton and the Central Asian foldbelt have a V-s in excess of 4.6 km:s, (3) relative ly low shear velocities are obtained for tectonically active areas. In many parts of the study area, where Precambrian basement is exposed, the process in the crust and upper mantis due to recent tectonic activ ities have modified the crust and upper mantle velocity structures und er the Precambrian terranes, they are no longer underlain by high velo city crust and mantle.