Upper Mantle Flow Beneath the Hangay Dome, Central Mongolia

In order to investigate the upper mantle deformation beneath central Mongolia and its relation with the major structures such as the Hangay dome, the Gobi-Altai range, the Siberian platform and the Baikal rift, a NS-trending profile of broad band seismic stations has been deployed for 5.5 months in 2003 from the southern Siberian craton to the Gobi-Altai range and crossing the whole Hangay dome. Mantle flow is deduced from the splitting of teleseismic shear waves such as SKS phases and reveals a homogeneous NW-SE trending pattern. Because the observed delay times of 1.5 to more than 2.0 s suggest a coherent mantle flow over large mantle thicknesses and since the observed fast directions are parallel to the trend of the lithospheric structures and close to the trend of the plate motion, we propose that both the lithosphere and the asthenosphere may add their anisotropic effects beneath central Mongolia. Seismic anisotropy beneath the permanent station ULN in Ulaanbatar in eastern Mongolia is consistent with such interpretation since it reveals the presence of two anisotropic layers, the lower one oriented NW-SE, close to the trend of the plate motion and the upper one NE-SW, close to the trend of the lithospheric structures in this area. In order to interpret the slight clockwise rotation of the fast SKS directions with respect to the HS3-Nuvel1A plate motion vector, we propose that the sub-lithospheric flow beneath Mongolia is deflected by the thick root of the Siberian platform. In the Eurasian reference frame, the SKS splitting beneath central Mongolia show a rather good parallelism with the GPS vectors (Calais et al., 2003) suggesting that the crust escapes toward the east coherently with the lithospheric and asthenospheric mantle flow. A different behavior is observed in western Mongolia, where the GPS vectors trend NS whereas the fast SKS directions trend EW, suggesting a complete decoupling between the upper crust moving northwards and the asthenospheric mantle flowing eastwards.