T13E-01 INVITED 13:45h
The making and remaking of the North China Block
The North China Block (NCB) consists of two major Archaean crustal nuclei that collided together by ca. 1800 Ma (e.g., Zhao, 2001), although some argue that they came together as early as ca. 2500 Ma (e.g., Zhai and Liu, 2003; Kusky and Li, 2004). No orogenic event is known within the NCB from ca. 1800 Ma until Permian. The shape and lithospheric architecture of this seemingly stable continental block were significantly reworked by Mesozoic-Cenozoic events. Continental collision between the NCB and South China Block (SCB) started from their eastern ends as early as Permian, leading to the initial continental subduction and subsequent exhumation of 240-235 Ma UHP metamorphic rocks (e.g., Hacker et al., 2000; Ayers et al., 2002) to the crustal level by ca. 220 Ma (Hacker et al., 2000). This was followed by the development of flake tectonics in the region east of the Tanlu Fault between Late Triassic and mid-Jurassic (Li, 1994, 1998), where the lower crust and lithospheric mantle of the SCB continued to subduct beneath the NCB, whereas the upper crust of the SCB, along with the UHP rocks at the upper crustal level along the former plate margin, were obducted over the lower crust of the NCB for up to 500 km. We thus have a composite crust in the Sulu-Yellow Sea-central Korea region. Extensive thin- to thick-skinned thrusting also developed in both the Jiao-Liao region east of the Tanlu Fault and the Xu-Huai and Luxi regions west of the Tanlu Fault during this process, all within the upper crust of the NCB. This complex crustal architecture may still influence distribution of epicentres in the region. Late Jurassic-Cretaceous thin-skinned thrusts were well developed along northern NCB (e.g., Davis et al., 1998) possibly due to the closure of the Mongolo-Okhotsk sea to the north. For reasons still being debated, eastern NCB underwent dramatic lithospheric thinning (locally total erosion of lithospheric mantle?) during the Cretaceous and early Tertiary (e.g., O'Reilly et al., 2001). Although new (still very thin) mantle lithosphere was subsequently regenerated in this region, lithospheric thinning continues today in central NCB (Xu et al., 2004). This significantly thinned (80-60 km) lithosphere may be partly responsible for the abnormally high intra-plate seismicity in the region.
T13E-02 INVITED 14:05h
Seismic imaging of North China: insight into intraplate volcanism and seismotectonics
We used seismic tomography to study the detailed three-dimensional (3-D) seismic velocity structure of the crust and mantle beneath North China for understanding the intraplate volcanism and seismotectonics of the Asian continent. Two active volcanoes, Changbai and Wudalianchi, exist in Northeast China and they have erupted several times in the last 1000 years. The origin of the active intraplate volcanoes is still unclear. Global tomography shows that the subducting Pacific slab becomes stagnant under NE Asia and strong low-velocity (low-V) anomalies exist in the upper mantle under the two volcanoes (Zhao, 2004). Recently we determined a 3-D P-wave velocity structure under the Changbai volcano using teleseismic data recorded by 19 portable seismic stations in NE China (Zhao et al., 2004). Our result shows a columnar low-V anomaly extending to 400 km depth and high-velocity anomalies in the mantle transition zone with deep-focus earthquakes of about 600 km depth. These results indicatie that the Changbai and Wudalianchi volcanoes are not hotspot like Hawaii but a kind of back-arc volcano related to the deep subduction and stagnancy of the Pacific slab under NE Asia. A detailed 3-D P-wave tomography of the crust and uppermost mantle under the Beijing region is determined by using local earthquake arrival times recorded by the newly installed Chinese Capital Seismic Network with 101 short-period seismic stations coving the region densely and uniformly (Huang and Zhao, 2004). The results show that large crustal earthquakes, such as the 1679 Sanhe earthquake (M 8.0) and the 1976 Tangshan earthquake (M 7.8), generally occurred in high-velocity areas in the upper to middle crust. In the lower crust to the uppermost mantle under the source zones of the large earthquakes, however, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids. The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes. Similar features are also found in the source areas of the 1995 Kobe earthquake (M 7.2) in Japan (Zhao et al., 1996) and the 2001 Bhuj earthquake (M 7.8) in India (Mishra and Zhao, 2003). Zhao, D. (2004) Global tomographic images of mantle plumes and subducting slabs: insight into deep Earth dynamics. Phys. Earth Planet. Inter. 146, 3-34. Zhao, D., J. Lei, R. Tang (2004) Origin of the intraplate Changbai volcano in Northeast China: Evidence from seismic tomography. Chinese Science Bulletin 49(13), 1401-1408. Huang, J., D. Zhao (2004) Crustal heterogeneity and seismotectonics of the region around Beijing, China. Tectonophysics 385, 159-180.
http://www.ehime-u.ac.jp/~grc/
T13E-03 14:25h
Tomgraphic Structure of East Asia: Data and Results
P-wave arrival times within the study area of 20°N-55°N and 100°E-145°E, were selected from three sources, including the ISC Bulletin of 1964-2001, the Annual Bulletin of Chinese Earthquakes of 1985-1998, and the Temporary Report of Chinese Earthquakes of 1999 to 2001. We also collected several thousands of P-wave arrival times recorded by Chinese regional networks in northeast China to improve the ray coverage there. Selection of earthquakes was based on the criteria that an individual teleseismic event has to be recorded not only by at least 10 stations but also by at least one station of the Chinese Seismic Networks. In total, we had selected 400,000 P-wave arrival times from 8002 regional earthquakes and 370,000 P-wave arrival times from 9000 teleseismic events. The combination of travel time residuals of regional earthquakes and relative travel time residuals of teleseismic events were used simultaneously in the tomographic inversion to determine the 3-D P-wave velocity structure above 1000 km depth. Moho depth variations in the region obtained from deep seismic reflection studies were incorporated in the tomographic inversion. Most important findings of this regional tomography are as follows. (1) No fast P-wave velocity anomalies can be related to subducted oceanic slabs beneath the 660-km discontinuity; instead the subducted oceanic slabs become flattened and stagnant in the transition zone. (2) The western end of the flat stagnant slabs is located ~ 1500 km west of the active trench in the western Pacific, which is correlated with the prominent surface topographic break in eastern China along the NNE-trending Taihang Mountain Range (~105°E). (3) Slow P-wave velocity anomalies are present at depths of 100-250 km below the active volcanic arc and the East Asia.
T13E-04 14:40h
Tomgraphic Structure of East Asia: Geodynamic implications
P-wave arrival times of both regional and teleseismic earthquakes were inverted to obtain mantle structures of East Asia. Most important findings of this regional tomography are as follows. (1) No fast P-wave velocity anomalies can be related to subducted oceanic slabs beneath the 660-km discontinuity; instead the subducted oceanic slabs become flattened and stagnant in the transition zone. (2) The western end of the flat stagnant slabs is located ~ 1500 km west of the active trench in the western Pacific, which is correlated with the prominent surface topographic break in eastern China, between the Erdos Plateau to the west and the North China plain in the east, along the NNE-trending Taihang Mountain Range (~105 E). Based on these observations, we suggested that vigorous mantle convection is operating within this horizontally expanded mantle wedge above both the active subducting slabs in the western Pacific and the stagnant flat slabs beneath much of the North China plain. This horizontally expanded convection was probably resulted from both rapid eastward migration of the western Pacific trench system and the sinking of the Mesozoic and Cenozoic slabs now trapped at the 660-km transition zone. Both the widespread Cenozoic volcanism and associated extensional basins in East Asia could have been the manifestation of this vigorous upper mantle convection. Finally negative thermal anomaly associated with the stagnant slabs above the 660-km discontinuity has not only caused a broad depression of the boundary due to its negative Clapeyron slope but also effectively shielded the asthenosphere and continental lithosphere above from any possible influence of mantle plumes in the lower mantle.
T13E-05 14:55h
Tomographic Inversion of Pn Travel Times in China: Implications for lithosphere thinning and deformation of North China
Seismic Pn waves sample the top of the upper mantle right beneath the Moho, providing an important tool to probe the temperature, composition, and deformation of the mantle lithosphere. We have recently conducted an inversion of Pn travel times in China from a large collection of Chinese and international earthquake bulletins (Liang et al., 2004, in press). The lithosphere mantle Pn velocities of China are characterized by a mosaic of very fast and very slow anomalies, mirroring the heterogeneity of geology at surface. The major basins show distinct Pn anomalies with high Pn velocity for the basins in the west and low Pn velocity for the basins in North China. The Ordos basin is unique: the Pn velocity in the eastern part of the basin is lower than the average, but the Pn velocity for the western part of the basin is higher than the average. A large area of North China shows prominent low Pn velocities beneath the Archean basement. The thinnest crust of the whole China continent is located in the North China basin. The shape of the crustal thickness contours, trending NNE, correlates well with the shape of the NNE trending North China basin and the Songliao basin, with the crust thickening beneath the Daxin'anling and Taihang mountain ranges to the west. The boundary along the Daxin'anling and Taihang mountain ranges between the thicker crust to the west and thinner one to the east roughly follows the major lineament of the Bouguer gravity anomaly. Our observations are consistent with rifting, lithospheric thinning, and mantle upwelling in the region. The Pn anisotropy is consistent with a dextral simple shear in the NNE direction in the lithosphere mantle during the last (and ongoing) major deformation period. The locations of gold ore deposits in North China and oil deposits in North China and Songliao basins correlate remarkably well with low Pn velocities of the region, suggesting the metallogenesis and oil formation of the region may closely related to magma and thermal activity in lithospheric mantle and crust-mantle interaction since Mesozoic and Cenozoic.
T13E-06 15:10h
Asian Active Tectonics: A Finite Element Study
Asia is the largest region of intense intraplate deformation and seismicity. We have developed a 3D finite element model to investigate the driving forces, rheological structures, and boundary conditions that control the global pattern of strain rate and stress field in Asia. The model lithosphere is a 3D spherical shell of power law rheology, supported by a viscous foundation. Lateral variations of topography and crustal thickness are included explicitly in the model. At the Indian-Eurasian boundary the relative velocities from the Nuvel-1A model are imposed. A free slip boundary condition is assigned to the west ($60\deg$E) and north ($70\deg$N) boundaries of the model domain. At other boundaries, both a lithostatic stress condition and a free slip boundary condition are tested. Our results indicate that most of the Indian-Asian collisional force is absorbed within the Tibetan Plateau and surrounding regions, with little impact on active deformation in the region east of $100\deg$E. On the other hand, gravitational buoyancy forces from the high topography of the Tibetan Plateau and other part of central Asia have broad impact on active tectonics in all parts of Asia. The predicted strain rate pattern is highly sensitive to the rheology structure. We will show that geodetic measurements incorporated with proper geodynamic modeling could provide useful constraints on lithospherical rheology, thus improving our understanding of the dynamics of intraplate deformation in Asia.
T13E-07 15:25h
Stress Interactions Between the 1976 Magnitude 7.8 Tangshan Earthquake and Adjacent Fault Systems in Northern China
The 28 July 1976 ML = 7.8 Tangshan earthquake struck a highly populated metropolitan center in northern China and was one of the most devastating earthquakes in modern history. Its occurrence has significantly changed the Coulomb stresses on a complex network of strike-slip, normal, and thrust faults in the region, potentially heightened the odds of future earthquakes on some of these fault segments. We have conducted a detailed analysis of the 3D stress effects of the Tangshan earthquake on its neighboring faults, the relationship between stress transfer and aftershock locations, and the implications for future seismic hazard in the region. Available seismic and geodetic data, although limited, indicate that the Tangshan main shock sequence is composed of complex rupture on 2-3 fault segments. The dominant rupture mode is right-lateral strike-slip on two adjoining sub-segments that strike N5-aE and N35-aE, respectively. We calculated that the Tangshan main shock sequence has increased the Coulomb failure stress by more than 1 bar in the vicinity of the Lunanxian district to the east, where the largest aftershock (ML = 7.1) occurred 15 hours after the Tangshan main event. The second largest aftershock (ML = 6.8) occurred on the Ninghe fault to the southwest of the main rupture, in a transitional region between the calculated Coulomb stress increase and decrease. The majority of the ML > 5.0 aftershocks also occurred in areas of calculated Coulomb stress increase. Our analyses further indicate that the Coulomb stress on portions of other fault segments, including the Leting and Lulong fault to the east and Yejito fault to the north, may also have been increased. Thus it is critical to obtain estimates of earthquake repeat times on these and other tectonic faults and to acquire continuous GPS and space geodetic measurements. Investigation of stress interaction and earthquake triggering in northern China is not only highly societal relevant but also important for advancing our understanding of the fundamental characteristics of earthquakes in regions of diffuse continental deformation.