The velocity structure of the upper mantle beneath Central Asia

Data on the deep structure of the Earth's crust and upper mantle are necessary for understanding and explaining the tectonic processes occurring in differ� ent regions of the world. The obtained information can be used for constructing geodynamical models that enable both explanation and prediction of further evolution of these processes. Therefore, of special interest are regions where riftogenesis and intensive orogenesis are currently observed. It was shown earlier (1-3) that the upper mantle beneath Central Asia is of an inhomogeneous struc� ture, and in both the lateral and vertical directions, at that. Such an inhomogeneous structure indicates the complex geodynamical regime of this region. The aim of the present communication is to investigate the deep structure of the discussed region in more detail and in a greater depth interval. The investigation of the Earth's crust and upper mantle velocity structure was carried out by the sur� facewave tomography for the case of a spherical sur� face (4). The materials for our study were records of surface waves from distant earthquakes of M > 5.5 (1991-2009), made in the "DRYa" channels of digital broadband seismic stations of the IRIS network. The range of epicentral distances was 1500-16 000 km. The dispersion curves of the group velocity for the main mode of the Rayleigh wave were calculated uti� lizing the spectralandtemporal analysis procedure (5) for more than 3200 seismic paths crossing the study region in different directions. In calculations, wave periods of 10 to 250 s were used, which, according to (6), corresponds to a depth extent of 650-700 km. Use of the representative sampling of dispersion curves has allowed us to calculate maps of distributions of the group velocities of the Rayleigh wave for the periods mentioned and to estimate the resolution of the map� ping results by calculation of the effective smoothing radius (4), which was 250-350 km in the lateral direc� tion. Based on these maps, local dispersion curves in particular points of the study region were calculated, and then these curves were turned into Swave velocity sections. The main tendencies of the mantle structure are illustrated by two 2D Swave velocity sections (Fig. 1). The A-B profile crosses the structures of the Mon� gol-Okhotsk Fold Belt in the territories of NE China and Transbaikalia, the Baikal Rift Zone, and the adja� cent areas of the Siberian Craton. The C-D profile starts in the Sayan Mountains, crosses the Khangai Arch and part of the Gobi desert, ending in China, near the Chinese-Mongolian boundary. The characteristic peculiarity of the discussed velocity sections is the presence of a clearly expressed asthenospheric layer possessing Swave velocities of 4.3-4.4 km/s. Beneath the Baikal Rift Zone, the top of this layer lies at about 120 km depth, while beneath other tectonically active structures it is located in the vicinity of the crustal base. The maximal thickness (150 km) of the asthenosphere is reached beneath the Khangai Arch, while in other parts of the profiles it is about 70 km thick.