Xiaobo Tian

Crustal structure across Longmenshan fault belt from passive source seismic profiling

[1] We analyse receiver functions from 29 broad-band seismographs along a 380-km profile across the Longmenshan (LMS) fault belt to determine crustal structure beneath the east Tibetan margin and Sichuan basin. The Moho deepens from about 50 km under Songpan-Ganzi in east Tibet to about 60 km beneath the LMS and then shallows to about 35 km under the western Sichuan basin. The average crustal Vp/Vs ratios vary in the range 1.75-1.88 under Songpan-Ganzi in east Tibet, 1.8-2.0 under the LMS, and decrease systematically across the NW part of the Sichuan basin to less than 1.70. A negative phase arrival above the Moho under Songpan-Ganzi and Sichuan basin is interpreted as a PS conversion from the top of a low-velocity layer in the lower crust. The very high crustal Vp/Vs ratio and negative polarity PS conversion at the top of lower crust in east Tibet are inferred to be seismic signatures of a low-viscosity channel in the eastern margin of the Tibetan plateau. The lateral variation of Moho topography, crustal Vp/Vs ratio and negative polarity PS conversion at the top of the lower crust along the profile seem consistent with a model of lower crust flow or tectonic escape.

Crustal structure beneath the Indochina peninsula from teleseismic receiver functions

[1] We analyze teleseismic receiver functions to determine the crustal structure beneath the Indochina peninsula which is located immediately south‐east of the eastern Himalayan syntaxis basin. We found that the Indochina peninsula is characterized by a thin (∼31 km) crust with a low Vp/Vs ratio (∼1.68). The intra‐lower crustal low‐velocity zone (LVZ) is observed beneath the northwestern part of our study region. We hypothesize that it is an extension of the lower crustal LVZ observed beneath the southeastern Tibet and the South China block and that it terminates at the Dien Bien Phu Fault (DBPF). A LVZ observed in the upper crust beneath southeast of the DBPF indicates that the crust is ductile and earthquakes are rare. Citation: Bai, L., X. Tian, and J. Ritsema (2010), Crustal structure beneath the Indochina peninsula from teleseismic receiver functions, Geophys. Res. Lett., 37, L24308, doi:10.1029/2010GL044874.

An improvement in the absorbing boundary technique for numerical simulation of elastic wave propagation

The computational domain is artificially designed in the numerical simulation of elastic waves propagating in unbounded media. As a result, boundary reflections arise from the edges of such truncated models. Although many complex schemes have been developed to solve this problem, the absorbing boundary technique of Cerjan et al (1985 Geophysics 50 705–8), as the simplest, is still widely used, especially for the pseudospectral method. To improve the efficiency of suppressing such boundary reflections in the simple technique, we apply damping to the movement velocity of nodes in a narrow strip along the edges. Several modelling results demonstrate its greater effectiveness than previous treatments which apply the damping to the displacement or both displacement and movement velocity of nodes.

Crustal velocity structure in the Emeishan large igneous province and evidence of the Permian mantle plume activity

The Emeishan large igneous province (ELIP) in SW China is interpreted to be associated with an ancient mantle plume. Most of the constraints on the role of mantle plume in the generation of the Emeishan flood basalts were provided by geological and geochemical methods, but the geophysical investigation is very limited. In order to better understand the deep structure and features of ELIP, we have studied the crustal velocity structure using the data acquired from the Lijiang-Panzhihua-Qingzhen wide-angle seismic profile. This profile crosses the three sub-zones of the ELIP (the inner, intermediate, and outer zones), divided based on the differential erosion and uplift of the Maokou limestone. The results provided by the active source seismic experiment demonstrate: (1) The average depth of the crystalline basement along the profile is about 2 km. (2) The middle crust in the Inner Zone is characterized by high-velocity anomalies, with the average velocity of 6.2–6.6 km/s, which is about 0.1–0.2 km/s higher than the normal one. The velocity of the lower crust in the inner zone is 6.9–7.2 km/s, higher than those observed in the intermediate and outer zones (6.7–7.0 km/s). Relatively low velocity anomalies appear in the upper, middle and lower crusts near the junction of the inner zone and intermediate zone, probably due to the effect of the Xiaojiang fault (XJF). (3) The average velocity of the crust is comparatively low on both sides of XJF, especially on the east side, and the average velocity of the consolidated continental crust is also low there. This may suggest that the XJF extends at least down to 40 km deep, even beyond through the crust. (4) The depth to the Moho discontinuity decrease gradually from 47–53 km in the inner zone, via 42–50 km in the intermediate zone to 38–42 km in the outer zone. In the inner zone, the Moho uplifts locally and the (consolidated) crust is characterized by high-velocity anomalies, which are likely related to intensive magma intrusion and underplating associated with melting of plume head. Overall the crustal velocity structure in the study area recorded the imprint left by the Permian Emeishan mantle plume.

Fossilized lithospheric deformation revealed by teleseismic shear wave splitting in eastern China

Global mantle convection significantly impacts the processes at Earth’s surface and has been used to gain insights on plate driving forces, lithospheric deformation, and the thermal and compositional structure of the mantle. Upper-mantle seismic anisotropy has been widely employed to study both present and past deformation processes at lithospheric and asthenospheric depths. The eastern China region was affected by extreme mantle perturbation and crust-mantle interaction during the Mesozoic, leading to large-scale destruction of the cratonic lithosphere, accompanied by widespread magmatism and metallogeny. Here we use teleseismic shear wave splitting measurements to evaluate the lithosphere and upper mantle deformation beneath this region. Our results from some of the individual and station averages show WNW-ESEto NW-SE–trending fast polarization direction, similar to those observed in eastern Asia in some previous studies, consistent with the direction of Pacific plate subduction during the Cenozoic. This feature suggests that the asthenospheric flow beneath the eastern China region is influenced by the subduction of the western Pacific or Philippine plate. However, most of our data show E-Wor ENE-WSW–trending fast polarization direction, which is inconsistent with subduction from the east. The seismic stations in this study are located near the Qinling-DabieSulu orogenic belt, which formed through the collision between the North and South China blocks during the Late Paleozoic– Triassic, and the anisotropy with an E-Wor ENE-WSW–trending fast polarization direction parallel to the southern edge of the North China block suggests lithospheric compressional deformation due to the collision between the North and South China blocks. Although the deep root of the craton was largely destroyed by cratonic reactivation in the late Mesozoic, our results suggest that the “fossilized” anisotropic signature is still preserved in the remnant lithosphere beneath eastern China.

Prevalent thickening and local thinning of the mantle transition zone beneath the Baikal rift zone and its dynamic implications

The Baikal rift is the most seismically active continental rift in the world and is significant for studying the dynamics of continental rifts, although its precise dynamic mechanisms remain controversial. We calculated receiver functions (1748) from Global Seismographic Network seismic stations TLY and ULN and stacked receiver functions in different bins. Here we present discontinuities at depths of 410 km and 660 km and thickness of the mantle transition zone (MTZ) beneath the study area. The MTZ structure shows an obvious thickening (292 km) in the Baikal rift zone except for an area of limited thinning (230 km), whereas it is basically normal (250 km) beneath the Mongolian area, to the southeast of the Baikal rift. Combining these results with previous findings, we propose that the large-scale thickening beneath the Baikal rift zone is likely to be caused by the Mesozoic collision between the Siberian Platform and the Mongolia-North China Block or magmatic intrusion into the lower crust, which would result in crust and lithosphere thickening. Thus, the lower crust becomes eclogitized and consequently detached into the deep mantle because of negative buoyancy. The detachment not only induces asthenosphere upwelling but also accelerates mantle convection of water detached from the subducted slab, which would increase mantle melting, while both processes promote the development of the rift. Our preliminary results indicate that the detachment and the consequent hot upwelling have an important influence on the development of the Baikal rift, and a small-scale mantle upwelling indicated by the located thinning may have destroyed the lithosphere and promoted this development.

Vertical crustal motions across Eastern Tibet revealed by topography-dependent seismic tomography

Using a topography-dependent tomographic scheme, the seismic velocity structure of the Eastern Tibetan Plateau, including the uplifted Longmenshan (LMS) orogenic belt, is accurately imaged in spite of the extreme topographic relief in the LMS region and thick sedimentary covers in the neighbouring Sichuan Basin. The obtained image shows a high-resolution upper crustal structure on a 500 km-long profile that is perpendicular to the LMS. The image clearly shows that the crystalline basement was uplifted within the LMS orogenic belt, and that the neighbouring Songpan-Ganzi Terrane was covered by a thick flysch belt, with evidence of near-surface thrust faults caused by convergence between Eastern Tibet and the Sichuan Basin. The indication that the lower crust beneath the LMS was folded and pushed upwards and the upper crust was removed by exhumation, supports the concept of a lower crustal channel flow beneath Eastern Tibet. The image also reveals that the destructive Wenchuan earthquake of year 2008 occurred in the upper crust, directly at the structural discontinuity between Eastern Tibet Plateau and the Sichuan Basin.

SANDWICH: A 2D Broadband Seismic Array in Central Tibet

ABSTRACT The tectonic processes that formed the Tibetan plateau have been a significant topic in earth science, but images of the subducting Indian continental lithosphere (ICL) are still not clear enough to reveal detailed continental collision processes. Seismological methods are the primary ways to obtain images of deep crust and upper‐mantle structures. However, previous temporary seismic stations have been unevenly distributed over central Tibet. The Institute of Geology and Geophysics, Chinese Academy of Sciences, has initiated a 2D broadband seismic network in central Tibet across the Bangong–Nujiang suture to fill in gaps among earlier north–south linear profiles for the purpose of detecting the lateral variation of the northern end of the subducting ICL. The health status for each station has been checked at each scheduled service trip. The noise level analysis shows a quiet background in central Tibet, with low cultural noise. Preliminary earthquake locations indicate that they are crustal and broadly distributed rather than only occurring along major faults, suggesting a diffused deformation in the conjugated strike‐slip fault zone. Preliminary receiver function analysis shows a complicated crust with significant east–west lateral variations.