Crustal structure of northern and southern Tibet from surface wave dispersion analysis

Abstract

[1] Group and phase velocities of fundamental mode Rayleigh waves, in the period range of 10 to 70 s, are obtained for southern and northern Tibet. Significant variations in crustal velocity structure are found. The group velocity minimum for Tibet occurs at $33 s and the minimum is $0.12 km/s lower for southern Tibet than for northern Tibet. At periods greater than 50 s, however, group velocities are up to 0.2 km/s faster in southern Tibet. The group and phase velocities are inverted for layered S wave models. The dispersion observations in southern Tibet can only be fit with a low-velocity layer in the middle crust. In contrast, the velocity models for northern Tibet do not require any lowvelocity zone in the crust. The S wave velocity of the lower crust of southern Tibet is $0.2 km/s faster than the lower crust of northern Tibet. In southern Tibet the sub-Moho velocity increases with a positive gradient that is similar to a shield, while there is no velocity gradient beneath northern Tibet. The high-velocity lower crust of southern Tibet is consistent with the underthrusting of Indian continental lithosphere. The most plausible explanation of the mid-crustal low velocity zone is the presence of crustal melt resulting from H 2 O-saturated melting of the interplate shear zone between the underthrusting Indian crust and overflowing Asian crust. The lack of a pronounced crustal low-velocity zone in northern Tibet is an indication of a relatively dry crust. The low S wave velocity in the lower crust of northern Tibet is interpreted to be due to a combination of compositional differences, high temperatures, presumably caused by a high mantle heat flux, and possibly small amounts of partial melt. Combined with all available observations in Tibet, the new surface wave results are consistent with a hot and weak upper mantle beneath northern Tibet.INDEX TERMS: 7205 Seismology: Continental crust (1242); 7218 Seismology: Lithosphere and upper mantle; 7255 Seismology: Surface waves and free oscillations; 8102 Tectonophysics: Continental contractional orogenic belts; KEYWORDS: Tibet, crustal velocity structure, surface wave, Rayleigh waves, continental collision Citation: Rapine, R., F. Tilmann, M. West, J. Ni, and A. Rodgers, Crustal structure of northern and southern Tibet from surface wave dispersion analysis,

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

“…That means that the low P n -velocities in the Changtang Block are true and not effected by a dipping Moho. There was also no rise of the Moho found by the dispersion studies of Rapine et al (2003), but their data would possibly not be able to resolve a small step ( < 5 km) in the Moho.…”

“…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.…”

“…Also, in the paper of Kind et al (2002), a 300 jC hotter lithosphere in the north is estimated from receiver function studies. Rapine et al (2003) find a low lower-crustal S-wave velocities and the absence of an S velocity gradient in the mantle beneath northern Tibet from dispersion studies. McNamara et al (1995), studying S n regionally, map a large elliptically formed attenuation area in central Tibet, mainly in the Changtang Block.…”

Constraining P-wave velocity variations in the upper mantle beneath Southeast Asia

“…1). The major large-scale features produced by the collision and post-collisional convergence of the Indian and Eurasian plates include the Tibetan Plateau, where the continental crust has approximately doubled in thickness (Molnar and Tapponnier, 1975;Tapponnier et al, 2001;Rapine et al, 2003). Offshore, oceanic lithosphere is predominantly of early Cenozoic age, and there is ongoing subduction of the Pacific, Philippine Sea, and Indo Australian plates beneath Eurasia (van der Hilst et al, 1991;Fukao et al, 1992;Widiyantoro and van der Hilst, 1996) with eastward slab roll back since Oligocene times (van der Hilst and Seno, 1993;Northrup et al, 1995).…”

Collaborative Research: High-Resolution Seismic Velocity and Attenuation Models of Western China

“…Li et al, 2008;Tilman et al, 2003;Mitchell, 1997;Rodgers et al, 1997;Rapine et al 2003b;McNamara et al, 1997]. Here we summarize some of the current state-of-the-art knowledge of the seismic structure of the Tibetan plateau and its relationship to tectonic hypotheses.…”