Crustal structure of central Tibet as derived from project INDEPTH wide-angle seismic data

Abstract

Summary In the summer of 1998, project INDEPTH recorded a 400 km long NNW–SSE wide‐angle seismic profile in central Tibet, from the Lhasa terrane across the Banggong‐Nujiang suture (BNS) at about 89.5°E and into the Qiangtang terrane. Analysis of the P‐wave data reveals that (1) the crustal thickness is 65 ± 5 km beneath the line; (2) there is no 20 km step in the Moho in the vicinity of the BNS, as has been suggested to exist along‐strike to the east based on prior fan profiling; (3) a thick high‐velocity lower crustal layer is evident along the length of the profile (20–35 km thick, 6.5–7.3 km s−1); and (4) in contrast to the southern Lhasa terrane, there is no obvious evidence of a mid‐crustal low‐velocity layer in the P‐wave data, although the data do not negate the possibility of such a layer of modest proportions. Combining the results from the INDEPTH III wide‐angle profile with other seismic results allows a cross‐section of Moho depths to be constructed across Tibet. This cross‐section shows that crustal thickness tends to decrease from south to north, with values of 70–80 km south of the middle of the Lhasa terrane, 60–70 km in the northern part of the Lhasa terrane and the Qiangtang terrane, and less than 60 km in the Qaidam basin. The overall northward thinning of the crust evident in the combined seismic observations, coupled with the essentially uniform surface elevation of the plateau south of the Qaidam basin, is supportive of the inference that northern Tibet until the Qaidam basin is underlain by somewhat thinner crust, which is isostatically supported by relatively low‐density, hot upper mantle with respect to southern Tibet.

About the lithospheric structure of central Tibet, based on seismic data from the INDEPTH III profile

“…This co-incidence in location means that around the BNS, at least in the upper and middle crust, there is a near vertical LVZ, most probably a fault zone. Zhao et al (2001) and Haines et al (2003) also have mapped a zone of very low velocities around the BNS, but did not postulate a fault zone in their interpretations. About 20 -30 km south of the BNS, in the Lumpola valley, along the Karakorum-Jiali Fault (KJF), deep-reaching shear deformation is suspected (Huang et al, 2000).…”

“…1). Five record sections of the vertical components of the geophones with some correlation are contained in Zhao et al (2001). All record sections have been band-passed filtered (2 -12 Hz) and normalized.…”

“…As mentioned in the Introduction section and described in detail in Zhao et al (2001), 11 big shots were fired and their signals were recorded by the 62 seismometers along the 400-km long line of INDEPTH III (see Fig. 1).…”

“…During the active shooting phase, 11 large shots delivered seismic signals to the 62 seismometers, creating a dense network of overlapping travel time branches. Based on the seismic data of INDEPTH III, a number of papers were recently published: Huang et al (2000) report on the seismic anisotropy in central Tibet and its correlation to tectonics, based on teleseismic studies, and Zhao et al (2001) report on modeling the crustal structure in central Tibet from analyzing travel times from the 11 big shots along INDEPTH III. Rapine et al (2003) constrain the regional scale crustal structure in Tibet based on surface wave dispersion analysis; Haines et al (2003) concentrate on the shot-point data of INDEPTH III and deep crustal processes; and Kind et al (2002) present discontinuity images of the mantle, based on the migration of receiver functions and show that the Poisson ratio is almost normal in north central Tibet.…”

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

“…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.…”