Elevation of the 410 km discontinuity beneath the centra Tien Shan: Evidence for a detached lithospheric root

Abstract

We analyzed P-SV converted phases recorded by the Kyrgyzstan Broadband Network (KNET) to investigate the nature of phase transitions in the upper mantle beneath the central Tien Shan and the Kazakh shield. We find that P-SV phases from the 410 km discontinuity recorded by several stations located along the range front are about 2 s earlier from beneath the Tien Shan than they are from beneath the Kazach shield. Because the delay times at stations located on the Kazakh shield are normal and seismic velocities in the upper mantle beneath this part of the Tien Shan apparently are low (about 4% less than the Kazakh shield to the north), the early arrival of the conversion from 410 km implies that this discontinuity is elevated by about 20 km. Because the 410 km discontinuity can be elevated by the introduction of cold material, our results are consistent with a model in which the lithosphere in this area developed a root during collision that later detached and is now residing near 410 km depth. Copyright 1997 by the American Geophysical Union.