THREE-DIMENSIONAL VELOCITY MODEL OF THE TECTONOSPHERE UNDER THE TIAN SHAN AND PAMIR REGIONS

Abstract

The orogenic process in Central Asia is difficult to understand without an understanding of the structure of the lithosphere and asthenosphere in this region. A considerable amount of information on structure at depth is provided by analysis of anomalies (discrepancies) in the travel time of P waves from distant earthquakes, recorded by local networks of seismic stations. In deriving velocity models of the Tian Shan and Pamir regions it was found that the calculational procedures were difficult to implement on a computer. Furthermore, deviation of three-dimensional models could not be undertaken owing to the lack of a suitable algorithm and a lack of data on discrepancies for earthquakes in the western azimuthal sector (if the azimuthal distribution of sources is the model without correction for the inhomogeneity of the crust; the corrections produce no significant effect. The formal characteristics of the solutions are: α = 10-3, number of parameters 40, M = 21, q = 57 percent standard error δV (percent of V0) about 0.3 percent at the center and about 0.6 percent at the periphery of the region. Overall, the results indicate that in a large part of the region of present-day orogeny the crust is underlain by a layer of high-velocity 'cold' mantle. The low-velocity zone beneath the northern Pamirs is apparently too deep to be a cause of tectonic activity. The upper mantle under the eastern Tian Shan differs sharply from the rest of the region in that it has a 'rift' structure. This structure may have begun to develop long before the recent uplifting.