Flexural subsidence by 29 Ma on the NE edge of Tibet from the magnetostratigraphy of Linxia Basin, China   总被引：43，自引：0，他引：43  

Xiaomin Fang  Carmala Garzione  Rob Van der Voo  Jijun Li  Majie Fan 《Earth and Planetary Science Letters》2003,210(3-4):545-560

This study provides a detailed magnetostratigraphic record of subsidence in the Linxia Basin, documenting a 27 Myr long sedimentary record from the northeastern edge of the Tibetan Plateau. Deposition in the Linxia Basin began at 29 Ma and continued nearly uninterruptedly until 1.7 Ma. Increasing rates of subsidence between 29 and 6 Ma in the Linxia Basin suggest deposition in the foredeep portion of a flexural basin and constrain the timing of shortening in the northeastern margin of the plateau to Late Oligocene–Late Miocene time. By Late Miocene–Early Pliocene time, a decrease in subsidence rates in the Linxia Basin associated with thrust faulting and a 10° clockwise rotation in the basin indicates that the deformation front of the Tibetan plateau had propagated into the currently deforming region northeast of the plateau.  相似文献   

Enhanced red fluorescence emission in the oxygen minimum zone of the Arabian Sea     

Wiebke Breves  Rüdiger Heuermann  Rainer Reuter 《Ocean Dynamics》2003,53(2):86-97

 Depth profiles of fluorescence at several excitation and emission wavelengths were measured along with CTD data during the cruise So119 of RV Sonne in the Arabian Sea from 12 May to 10 June 1997. In addition to chlorophyll fluorescence from phytoplankton in the near-surface layer, the profiles in the oxygen minimum region well below the euphotic zone show enhanced red fluorescence. Red fluorescence intensity is inversely related to the oxygen concentration in intermediate and deep waters. A relationship to characteristic water masses of the region cannot be found in the data, and this holds also with chemical data such as DOC. Absorbance spectra of water samples taken in the oxygen minimum zone show an absorption band at 420 nm wavelength with about 50 nm bandwidth, much weaker than gelbstoff absorbance in the same wavelength range. The absorption band remains stable after sample filtration with 0.45 μm glass fibre filters. Hence, the size of the absorbing matter is in the range of dissolved molecules or particles much smaller than 1 μm. Fluorescence spectra of unfiltered samples with 420 nm excitation show a weak emission band at 600 nm and a more pronounced one at 660 nm wavelength. The trailing edge of the 660 nm band falls into the range of chlorophyll emission, thus giving rise to the observed depth profiles of red fluorescence in the oxygen minimum zone. Red fluorescence measured on samples remain stable during a few hours after sampling even in the presence of oxygen. It is not detectable after several weeks of sample storage in the dark and cannot be reproduced even after depletion of dissolved oxygen. Received: 22 May 2002 / Accepted: 18 February 2003 Responsible Editor: Andreas Oschlies Acknowledgements. This work was supported by a grant from the Federal Minister of Education and Technology, Bonn, within the frame work of the JGOFS Arabian Sea program. We are grateful to the captain and the crew of RV Sonne for their support. We are indebted to Mrs. Kirsten Neumann from the Institute of Marine Chemistry and Biogeochemistry of the University of Hamburg for providing the oxygen data, and to Mr. Nikolai Delling from the same institute for making the DOC and chlorophyll data available to us.  相似文献   

Smithsonian Institution's Global Volcanism Network     

Rick Wunderman 《Bulletin of Volcanology》2003,65(1):73-74

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Extracting flow structures from tracer data     

Judit Schneider  Tamás Tél 《Ocean Dynamics》2003,53(2):64-72

A method of visualizing structures in closed chaotic flows out of homogenous particle distributions is presented in the example of models of a meandering jet. To this end, the system will be leaked or opened up by defining a region of the flow, so that a particle is considered to be escaped if it leaves this region. By applying this method to an ensemble of nonescaped tracers, we are able to characterize mixing processes by visualizing the converging and stretching filamentations (stable and unstable manifolds) in the flow without using additional mathematical tools. The possibility of applying the algorithm to analyze buoy data, and a comparison with the finite time manifolds are discussed.  相似文献   

Contemporary crustal deformation of the Chinese continent and tectonic block model   总被引：1，自引：0，他引：1  

Min Wang  Zhengkang Shen  Zhijun Niu  Zusheng Zhang  Hanrong Sun  Weijun Gan  Qi Wang  Qun Ren 《中国科学D辑(英文版)》2003,46(2):25-40

We obtain the preliminary result of crustal deformation velocity field for the Chinese continent by analyzing GPS data from the Crustal Motion Observation Network of China (CMONOC), particularly the data from the regional networks of CMONOC observed in 1999 and 2001. We delineate 9 technically active blocks and 2 broadly distributed deformation zones out of a dense GPS velocity field, and derive block motion Euler poles for the blocks and their relative motion rates. Our result reveals that there are 3 categories of deformation patterns in the Chinese continent. The first category, associated with the interior of the Tibetan Plateau and the Tianshan orogenic belt, shows broadly distributed deformation within the regions. The third category, associated with the Tarim Basin and the region east of the north-south seismic belt of China, shows block-like motion, with deformation accommodated along the block boundaries only. The second category, mainly associated with the borderland of the Tibetan Plateau, such as the Qaidam, Qilian, Xining (in eastern Qinghai), and the Diamond-shaped (in western Sichuan and Yunnan) blocks, has the deformation pattern between the first and the third, i.e. these regions appear to deform block-like, but with smaller sizes and less strength for the blocks. Based on the analysis of the lithospheric structures and the deformation patterns of the regions above, we come to the inference that the deformation modes of the Chinese continental crust are mainly controlled by the crustal structure. The crust of the eastern China and the Tarim Basin is mechanically strong, and its deformation takes the form of relative motion between rigid blocks. On the other hand, the northward indentation of the Indian plate into the Asia continent has created the uplift of the Tibetan Plateau and the Tianshan Mountains, thickened their crust, and raised the temperature in the crust. The lower crust thus has become ductile, evidenced in low seismic velocity and high electric conductivity observed. The brittle part of the crust, driven by the visco-plastic flow of the lower crust, deforms extensively at all scales. The regions of the second category located at the borderland of the Tibetan Plateau are at the transition zone between the regions of the first and the third categories in terms of the crustal structure. Driven by the lateral boundary forces, their deformation style is also between the two, in the form of block motion and deformation with smaller blocks and less internal strength.  相似文献   

A numerical method of viscoelastic finite element coupled with spring-block model to simulate the interaction of faults and crustal blocks     

Yong Yu  Hanjing Hong  Peixun Liu  Wei Tao  Xiuzhen Zheng 《中国科学D辑(英文版)》2003,46(2):118-129

A numerical method of viscoelastic finite element coupled with spring-block model is developed to study temporal processes from the slow tectonic motion of large-scale crust to the rapid failure of small-scale faults. Our modeling demonstrates that the motion of crustal blocks is driven by forces from tectonic plate boundaries, and the deformation is distributed on faults for the stress accumulating. The coupling model generates earthquake sequences that display a magnitude-frequency scaling consistent with Gutenberg-Richter law. The frictional heterogeneities affeci earthquakes occurrence and stresses distribution of crustal blocks. Rupture of earthquakes starts at the nucleation node, and propagates bilaterally along faults with the stress triggering, release and redistribution. The failure of faults releases part of crustal stresses, the stress state of crustal blocks near fault is affected by the rupture of local segments on the fault, and the stress state of crustal blocks far away from the fault is controlled by the seismic activity of the whole fault.  相似文献   

Numerical simulations of deformation and movement of blocks within North China in response to 1976 Tangshan earthquake     

Wuming Bai  Banghui Lin  Zu’an’an Chen 《中国科学D辑(英文版)》2003,46(2):141-152

Using methods of discontinuous deformation analysis and finite element (DDA+FEM), this paper simulates dynamic processes of the Tangshan earthquake of 1976, which occurred in the northern North China where its internal blocks apparently interacted. Studies focus upon both the movement and deformation of the blocks, in particular, the Ordos block, and variations of stress states on the boundary faults. The Tangshan earthquake was composed of three events: slipping motions of NNE-striking major fault, NE-striking fault near the northeastern end of the NNE-striking fault, and NW-striking fault on the southeastern side of the NNE-striking fault. Compared with previous studies, our model yields a result that is more agreeable with the configuration of aftershock distributions. A number of data are presented, such as the principle stress field during the earthquake, contours of the maximum shear stress, the strike-slip deformation between blocks near the earthquake focus, time-dependent variations of slips of earthquake-triggered faulting, the maximum slip distance, and stress drops. These results are in accord with the earthquake source mechanism, basic parameters from earthquake wave study, macro-isoseismic line, observed horizontal displacement vectors, etc. The Tangshan earthquake exerted different influences on the adjacent blocks and boundary faults between them, thus resulting in differential movement and deformation. The Ordos block seems to have experienced the small-scale counterclockwise rotation and deformation, but its northeast part, bounded on the east by the Taihangshan and on the north by the Yanshan and Yinshan belts, underwent relatively stronger deformation. The Tangshan earthquake also changed the stress state of boundary faults of the North China, leading to an increase in shear stress and a decrease in normal stress in the NW-trending Zhangjiakou-Penglai fault through Tangshan City and the northern border faults of the Ordos block, and therefore raises the potential risk of earthquake occurrence. This result is supported by the facts that a series of Ms ≥ 6 earthquakes took place at the northern margin of the Ordos block after the Tangshan earthquake.  相似文献   

Active blocks and strong seismic activity in North China region     

Zhujun Han  Jie Xu  Yongkang Ran  Lichun Chen  Xiaoping Yang 《中国科学D辑(英文版)》2003,46(2):153-167

The active North China block consists of three second-order blocks: Ordos, North China Plain, and East Shandong-Huanghai Sea blocks. Two active tectonic zones, the Anyang-Heze-Linyi and Tangshan-Cixian zones, exist in the active North China Plain block and have separated the active block into 3 third-order active blocks, Taihangshan, Hebei-Shandong, and Henan-Huai blocks. The 3 third-order active blocks are characterized by their entire motion and are clearly different in their Cenozoic structures and deep structures. The active boundary tectonic zones between the third-order active blocks are less than those between the first- and second-order active blocks in their movement strength, extent, and seismic activity. The density of M· ·6 earthquakes in the boundary zones between active blocks is higher than that within the blocks by 9–22 times in the North China region, up to one order of magnitude on average. M· · 7 earthquakes occurred basically in the boundary zones between active blocks. The difference is not occasional, but reflects the nature of intraplate movement and the characteristics of strong seismic activity and is the powerful evidence for hypothesis of active blocks.  相似文献   

Fine structure of Pn velocity beneath Sichuan-Yunnan region     

Jinli Huang  Xiaodong Song  Suyun Wang 《中国科学D辑(英文版)》2003,46(2):201-209

We use 23298 Pn arrival-time data from Chinese national and provincial earthquake bulletins to invert fine structure of Pn velocity and anisotropy at the top of the mantle beneath the Sichuan-Yunnan and its adjacent region. The results suggest that the Pn velocity in this region shows significant lateral variation; the Pn velocity varies from 7.7 to 8.3 km/s. The Pn-velocity variation correlates well with the tectonic activity and heat flow of the region. Low Pn velocity is observed in southwest Yunnan, Tengchong volcano area, and the Panxi tectonic area. These areas have very active seismicity and tectonic activity with high surface heat flow. On the other hand, high Pn velocity is observed in some stable regions, such as the central region of the Yangtze Platform; the most pronounced high velocity area is located in the Sichuan Basin, south of Chengdu. Pn anisotropy shows a complex pattern of regional deformation. The Pn fast direction shows a prominent clockwise rotation pattern from east of the Tibetan block to the Sichuan-Yunnan diamond block to southwest Yunnan, which may be related to southeastward escape of the Tibetan Plateau material due to the collision of the Indian Plate to the Eurasia Plate. Thus there appears to be strong correlation between the crustal deformation and the upper mantle structure in the region. The delay times of events and stations show that the crust thickness decreases from the Tibetan Plateau to eastern China, which is consistent with the results from deep seismic sounding.  相似文献   

Optimum multiple tuned mass dampers for structures under the ground acceleration based on the uniform distribution of system parameters     

Chunxiang Li  Yanxia Liu 《地震工程与结构动力学》2003,32(5):671-690

The five MTMD models, with natural frequencies being uniformly distributed around their mean frequency, have been recently presented by the first author. They are shown to have the near‐zero optimum average damping ratio (more precisely, for a given mass ratio there is an upper limit on the total number, beyond which the near‐zero optimum average damping ratio occurs). In this paper, the eight new MTMD models (i.e. the UM‐MTMD1～UM‐MTMD3, US‐MTMD1～US‐MTMD3, UD‐MTMD1 and UD‐MTMD2), with the system parameters (mass, stiffness and damping coefficient) being, respectively, uniformly distributed around their average values, have been, for the first time here, proposed to seek for the MTMD models without the near‐zero optimum average damping ratio. The structure is represented by the mode‐generalized system corresponding to the specific vibration mode that needs to be controlled. Through minimization of the minimum values of the maximum dynamic magnification factors (DMF) of the structure with the eight MTMD models (i.e. through the implementation of Min.Min.Max.DMF), the optimum parameters and values of Min.Min.Max.DMF for these eight MTMD models are investigated to evaluate and compare their control performance. The optimum parameters include the optimum mass spacing, stiffness spacing, damping coefficient spacing, frequency spacing, average damping ratio and tuning frequency ratio. The six MTMD models without the near‐zero optimum average damping ratio (i.e. the UM‐MTMD1～UM‐MTMD3, US‐MTMD1, US‐MTMD2 and UD‐MTMD2) are found through extensive numerical analyses. Likewise, the optimum UM‐MTMD3 offers the higher effectiveness and robustness and requires the smaller damping with respect to the rest of the MTMD models in reducing the responses of structures subjected to earthquakes. Additionally, it is interesting to note, by comparing the optimum UM‐MTMD3 with the optimum MTMD‐1 recently investigated by the first author, that the effectiveness and robustness for the optimum UM‐MTMD3 is almost identical to that for the optimum MTMD‐1 (without inclusion of the optimum MTMD‐1 with the near‐zero optimum average damping ratio). Recognizing these performance benefits, it is preferable to employ the optimum UM‐MTMD3 or the optimum MTMD‐1 without the near‐zero optimum average damping ratio, when installing the MTMD for the suppression of undesirable oscillations of structures under earthquakes. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献