Seismic signatures of detached lithospheric fragments in the mantle beneath eastern Himalaya and southern Tibet

In this study, we investigate the mantle transition zone (MTZ) structure beneath eastern Himalaya and southern Tibet using ~ 9000 high quality receiver functions from 96 broadband stations spanning the entire region. The pervasive early arrival times(~ 1.7 s compared to IASP91) of the P-to-s conversions from the 410 km discontinuity are attributed to the high shear wave velocities associated with the subducted Indian (and Asian) lithosphere(s). Global and regional shear wave velocity models obtained from seismic tomography evince such high velocity anomalies both in the uppermost mantle and within the MTZ, in this complex collision environment. In contrast, the conversions from the 660 km discontinuity are either normal or delayed (up to 1 s) providing evidences for a thickened MTZ beneath most of the study region. Although presence of water in the MTZ can result in such a thickening, issues like 1) low amplitudes of the P410s and P660s conversions, 2) small dependence of their amplitudes with frequency, 3) absence of a detectable low velocity layer atop 410 and 4) lack of evidence for a 520 km discontinuity, preclude such an interpretation. Instead, we attribute this thickening to lowered temperatures affected by the possible presence of detached cold and dense lithospheric slabs within the MTZ. Such a detachment might have been facilitated either by convection or gravity removal of the lithosphere thickened due to continued subduction of the Indian plate since the Mesozoic.