Evidence from Earthquake Data for a Partially Molten Crustal Layer in Southern Tibet

Abstract

Earthquake data collected by the INDEPTH-II Passive-Source Experiment show that there is a substantial south to north variation in the velocity structure of the crust beneath southern Tibet. North of the Zangbo suture, beneath the southern Lhasa block, a midcrustal low-velocity zone is revealed by inversion of receiver functions, Rayleigh-wave phase velocities, and modeling of the radial component of teleseismic P-waveforms. Conversely, to the south beneath the Tethyan Himalaya, no low-velocity zone was observed. The presence of the midcrustal low-velocity zone in the north implies that a partially molten layer is in the middle crust beneath the northern Yadong-Gulu rift and possibly much of southern Tibet.

Finite frequency tomography in southeastern Tibet: Evidence for the causal relationship between mantle lithosphere delamination and the north–south trending rifts

“…Several recent Himalayan-Tibetan tectonic investigations have proposed a weak middle-lower crust, which allows crustal flow, to explain the building of the southeastern margin of the Tibetan Plateau [Clark and Royden, 2000;Beaumont et al, 2001;Schoenbohm et al, 2006]. This idea is supported by results from geophysical studies that show evidence for localized partial melt within the middle crust in Tibet [Kind et al, 1996;Wei et al, 2001] and a radial anisotropy that can be caused by channel flow within the mid-to-lower Tibetan crust [Shapiro et al, 2004]. However, contrary to the notion that the north -south rift zones in the Tibetan Plateau are shallow features, formed by the eastward motion of the shallow crust that are decoupled from the mantle lithosphere by a low-viscosity lower crust, our results suggest an upper mantle origin of the rift zones, at least in southeastern Tibet, where mantle lithosphere delamination plays a key role in the process of the rise of the plateau.…”

Complex images of Moho and variation of Vp/Vs across the Himalaya and South Tibet, from a joint receiver‐function and wide‐angle‐reflection approach

“…North of the ITS, RF exhibit unusually strong negative peaks, e.g. station G in Figure 3a, from which presence of partial melt within the crust is in agreement with Kind et al [1996]. Station E (Figure 3b) shows a less extreme low-velocity part in the deeper pile of layers of alternating high and low velocities, but high-velocity layers need to reach mantle velocities, consistently with the independent WARR analysis of reflection amplitude that suggested superposed Moho segments.…”

“…This Moho marker has not been sampled since then over a great length, by several surveys that have brought a great amount of insight into other aspects [Zhao et al, 1993;Brown et al, 1996;Makovsky et al, 1999;Hauck et al, 1998]. Kind et al [1996] did not resolve Moho depth variations in the Lhasa block but their receiver-functions were widely spaced. Zhao et al [2001] constructed a model with an essentially flat Moho from the northern Lhasa block into the Qang Tang area by using a recent cross-strike WARR experiment.…”

Lithospheric and upper mantle structure of southern Tibet from a seismological passive source experiment

“…In contrast, in the seismic reflection section the Moho was only observed at the southern end of the INDEPTH I profile . A second important observation obtained by Kind et al [1996] using receiver functions was the crustal low-velocity zone north of the Zangbo suture. This feature was correlated with other observations associated with the low-velocity zone, namely, "bright spots" seen in the seismic reflection profile , strong traces containing only (for flat layers) P-to-S converted energy at discontinuities underneath the station.…”

“…Besides the improved method we also use more data than Kind et al [1996]. We have added teleseismic recordings of the dense wide-angle German Depth Profiling of Tibet and the Himalayas (GEDEPTH) deployment, which proved to be very successful because of the close spacing of these stations.…”