Teleseismic anistotropy and its features in the upper mantle beneath the Tibet Plateau and neighboring areas. Metallogenic implications

Abstract

The extensive seismic studies carried out in Tibet and its neighboring areas in China provided teleseismic data revealing upper mantle structure and indicating structural features at varying levels down through the upper mantle. An example is included to contribute to the study of deep-rooted pathways guiding the ascent of magmatic and hydrothermal fluids potential to concentrate metals in the upper crust. Systematic seismic studies in Tibet and its neighboring areas started in June 1992 under the Tibetan Plateau Seismic Experiment - a joint Sino-French project by the Ministry of Geology and Mineral Resources of China (MGMR) and Institut National des Sciences d'Univers (CNRS), France (Jiang et al., 1995; Hirn et al., 1995). A seismic profile across Tibet, extending essentially NNE fromTingri in southern Tibet to Golmud in Qinghai Province on the north, included 108 digital seismic stations and revealed shear wave anisotropy beneath the Tibetan Plateau and the Qinghai Province (Shi et al., 1999). Our present study analyzed the characteristics of shear wave anisotropy achieved from three-component broadband teleseismic data collected in Tibetan Plateau and its neighboring areas. The time lag between fast and slow wave was usually found bigger than 1.0 s which can not be attributed to crust. The values bigger than 2.0 s may indicate anisotropic materials deep in the upper mantle. The strongest anisotropy appears along the margins of high velocity terrains, often associated with the partial molten material deep in the mantle. In these areas, the formation of anisotropy should have subtle relations with hydrothermal process, earthquake activity and uplifting of Tibetan Plateau. Near the strike-slip faults on the edge of these terrains, the anisotropy direction is consistent with the strikes of the fault systems. This consistency implies that the faults in surface geology may be effected by anisotropy features of upper mantle at the depths of at least 200 km. Figure 1a (after Jiang & Su, 1999) gives a seismic velocity image reaching the depths of 400 km, produced by means of tomography with a rich set of data acquired in western China, Tibet, Qinghai, and Xinjiang. Low-velocity bodies located in the active tectonic zones can be found all along the profile of thousands of kilometers, which are supposed to have close ties with mantle-derived materials, suggesting potential correlation with giant ore deposits. The interpreted mantle plume at the Kunlun Pass (Figure 1b) occupies a special structural position at the boundary between theTibet block and Qaidam Basin. Other preferred locations can be identified especially along the AltynTagh Fault at the boundary between theTibetan Plateau and Tarim Basin. The morphology of the broad region hosting the Tarim Basin (Fig. 1 in the paper by Wittlinger et al., 1998) indicates that some major NW-trending faults may intersect the Altyn Tagh Fault. Also the Magnetic Lineament Map of China (Jiang & Ma, 1991) shows major lineaments (especially the northwest ones) crossing the boundary between the Tibetan Plateau and Tarim Basin and can assist in defining structural intersections as possible targets for mineral exploration.