Zhiming Bai

Crustal structure across the Dabie?Sulu orogenic belt revealed by seismic velocity profiles

The Tan-Lu fault (TLF) separates the Dabie and Sulu orogenic belts, well known for their ultra high pressure (UHP) metamorphic rocks in eastern China. We reinterpret one of the wide-angle seismic profiles traversing the TLF using traveltime tomography methods, and compare the results with the interpretation of three other seismic profiles across the TLF, to enable us to study the relationship of the five tectonic units comprising the North China plate (NCP), the Yangtze plate (YTZP), the TLF, the Dabie–Sulu orogenic belt (DSOB), and the ultra-high pressure metamorphic belt (UHPMB) that is exposed within the DSOB. The results demonstrate that there is strong lateral heterogeneity within the studied area. The TLFs penetrating depth deepens along a S–N direction. In the central section of the fault, the TLF can be traced to the middle crust but in the northern section it penetrates to the Moho. The average P-wave velocity in the UHPMB and DSOB is 0.1–0.4 km s−1 faster than that of the YTZP, NCP and TLF for upper crusts with depths 13 km. The bottom borders of the middle and lower crusts of the UHPMB and DSOB are apparently deeper than the other three tectonic units, and the Moho beneath UHPMB around Dabieshan may be deeper than 40 km. The general similarities of the crustal velocity structures between the Dabie and Sulu UHPMB may suggest a similar exhuming mechanism of UHP metamorphic rocks, before the large-scale TLF strike slip, driven by the subduction of the Yangtze block. The velocity gradient of the crust–mantle transition beneath the Sulu UHPMB implies the intrusion of basaltic melts from the upper mantle.

Lithological model of the South China crust based on integrated geophysical data

The structure and petrology of the earth’s crust is critical to understand the growth and evolution of the continents. In this paper, we present the crustal lithological model along the 400-km-long seismic profile between Lianxian, near Hunan Province, and Gangkou Island, near Guangzhou City, South China. This model is based on an integrated geophysical data set including P-wave velocity (VP), S-wave velocity (VS), VP/VS ratio, mass density (ρ) and Lam´ e impedances (ρλ, ρμ), which are compared to those determined by laboratory measurements on a variety of crustal rock samples. The Bouguer gravity anomaly together with the seismic velocity enables us to constrain density. The heat flow and thermal field allow us to carry out corrections for temperature. Pressure correction is based on depth. We directly compare the property parameters determined from the South China seismic data with laboratory measurements made at the same conditions of pressure and temperature. Inversion of the available data for rock lithology indicates that there are substantial differences in the crustal lithology of the Yangtze and Cathaysia blocks. While the average lithology of the upper crust in both blocks is mainly characterized by granite–granodiorite and biotite gneiss, the granite–granodiorite layer is much thicker beneath the Cathaysia block. The middle crust in these two domains is not entirely similar, being granite–granodiorite and granite gneiss as the best fit for the Yangtze block, and granite gneiss and biotite gneiss for the Cathaysia block. The lower crust is composed of biotite gneiss, paragranulite and amphibolite for the Yangtze block, whereas biotite gneiss, paragranulite, diorite, mica quartz schist, amphibolite, green schist facies basalt and hornblende provide the best fit for the Cathaysia block. The results demonstrate that to the east of the Chenzhou–Linwu fault (CLF) that is the southern segment of the Jiangshan–Shaoxing fault, the lithology displays relatively abrupt lateral variations, in contrast to the west of the fault. This suggests that the deformation is well developed in the whole crust beneath the Cathaysia block, in agreement with seismic evidence on the eastward migration of the orogeny and the development of a vast magmatic province. The CLF clearly marks the change in the property parameters of both tectonic blocks, so it appears a natural boundary between the Yangtze and Cathaysia blocks, and it is a crust-scale fault.

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.

Joint tomographic inversion of first‐arrival and reflection traveltimes for recovering 2‐D seismic velocity structure with an irregular free surface

Irregular surface flattening, which is based on a boundary conforming grid and the transformation between curvilinear and Cartesian coordinate systems, is a mathematical method that can elegantly handle irregular surfaces, but has been limited to obtaining first arrivals only. By combining a multistage scheme with the fast‐sweeping method (FSM, the method to obtain first‐arrival traveltime in curvilinear coordinates), the reflected waves from a crustal interface can be traced in a topographic model, in which the reflected wave‐front is obtained by reinitializing traveltimes in the interface for upwind branches. A local triangulation is applied to make a connection between velocity and interface nodes. Then a joint inversion of first‐arrival and reflection traveltimes for imaging seismic velocity structures in complex terrains is presented. Numerical examples all perform well with different seismic velocity models. The increasing topographic complexity and even use of a high curvature reflector in these models demonstrate the reliability, accuracy and robustness of the new working scheme; checkerboard testing illustrates the methods high resolution. Noise tolerance testing indicates the methods ability to yield practical traveltime tomography. Further development of the multistage scheme will allow other later arrivals to be traced and used in the traveltime inversion.

Phase shift approximation for the post-critical seismic wave

Post-critical seismic waves are widely used in crustal exploration of the seismic velocity structure, and are gaining interest in the oil/gas seismic community to image the deeper structure beneath the high velocity basalt layer. They are featured with their phase shifts and strength changes, which should be taken into account in seismic data processing, such as velocity analysis and true amplitude migration, etc. In order to simplify the exact but complicated formula of reflection and transmission coefficients, numerous approximate expressions for reflection and transmission coefficients for pre-critical incidence are obtained. In the post-critical case, there is Downtons approximation with acceptable accuracy approximation when the velocity changes smoothly. However if the velocity model changes rapidly, the error will be relatively very large, limiting the use of the approach. In order to improve the post-critical approximation, we utilize Taylor expansion of ray parameters with angle increment (compared to critical angle) in wide-angle seismic reflection and transmission coefficients. The explicit expressions for amplitude and phase shift (time shift) for the post-critical incident angle are obtained. Our results confirm that the wide-angle seismic reflection/transmission phase shifts are strongly frequency dependent; phase shifts of low frequency wide-angle seismic waves are more predominant and their correction should be considered in seismic processing and imaging. Numerical examples demonstrate that (1) the accuracies of these approximations are high compared to the classic Akis formula and Downtons approximation, and (2) the wide-angle effect can be effectively reduced with phase-shift correction by utilizing our time-shift approximation to the seismic traveltimes.