Undulating Moho beneath a near-uniform surface of central Tibet

Abstract

We present 805 reliable estimates of crustal thickness in central Tibet, revealing an undulating Moho beneath a near-uniform surface of the highest and the largest plateau in the world. Our results are derived from wide-angle reflections of P-waves from virtual seismic sources near seismographs (Virtual Deep Seismic Sounding, VDSS). The virtual sources are produced by large earthquakes well-distributed in back-azimuth; and we also utilize obvious changes of measured arrival times as a function of ray-parameter to constrain independently bulk thickness and P-wave speed of the crust (about 6.3 km/s), so there is little trade-off between the two parameters. Additionally, we consider effects of known heterogeneities in the upper mantle. In regions surrounding the Hi-CLIMB linear array, there is an eastward trend, in addition to the previously proposed northward trend, of crustal thinning. Respectively, these trends are approximately parallel and normal to the Indian mantle front (IMF), or the northern, leading edge of underthrust lithospheric mantle of India, deduced from a collection of additional geophysical evidence. The Moho near the IMF also shows large, 3-D variations in depths, prompting us to propose that the leading edge of intact Indian lower crust, or the Indian crustal front (ICF) that accompanied the underthrusting Indian lithospheric mantle beneath Tibet, lies along the zone of disturbed Moho. Based on results from about a dozen permanent broadband seismographs of the China National Seismic Network, more-or-less flat-lying, and thus presumably stable, cratonic Indian Moho south of the ICF apparently extends eastward for another 500 km to about 93°E.