Loading...

Archive

    2012, Volume 19 Issue 4
    10 July 2012
    Previous Issue    Next Issue

    Article
    For Selected: Toggle Thumbnails
    Article
    The recent tectonic stress field of offshore of China mainland and adjacent areas.
    2012, 19(4): 1-7. 
    Abstract ( 1658 )   PDF (833KB) ( 5550 )  

    The offshore of China mainland includes the Bohai Sea, Yellow Sea and East China Sea. Using data of focal mechanism solutions, borehole breakouts and hydraulic fractures, there have been various degree studies  on the recent tectonic stress field of this region and adjacent areas. Based on these previous work, in combination with analyses of neotectonic movements of marine area and land and dynamic conditions, we attempt to make a further research on this subject. Our result shows that the recent tectonic stress field of this region is characterized by nearly horizontal compression, which is in NE direction in the north and gradually turns to NEE, EW, SEE and SE in the south, exhibiting a fan shape of diverging toward east. The intensity of tectonic stress is not uniform, relatively strong in the north and weak in the south. The marine and continental areas share a common recent tectonic stress field, which can be divided into Northeast China, North China and South China stress provinces. The generation of such a stress field is associated with the extrusion toward northeast and southeast at the eastern margin of the Tibetan plateau due to the continued northward indentation of the India plate, as well as the effect of the westward subduction of the Pacific and Philippine Sea plates in the east. And the eastward extrusion by the Tibetan plateau seems to be the dominant driving force.

    Related Articles | Metrics
    Relocation of the Yushu Ms 7.1 earthquake sequence and investigation on its seismotectonics.
    2012, 19(4): 8-14. 
    Abstract ( 1681 )   PDF (1037KB) ( 3077 )  

    We investigated the seismotectonics of the Yushu Ms7.1 earthquake by relocating the 1670 earthquakes in its sequence from April 11to September 15, 2010 using the double difference algorithm. Most of the relocated hypocentral depths are between 0 and 15 km. The relocated Ms7.1 mainshock occurred at the segment without surface rupture, while the aftershocks distribute along the surface rupture zone to its two sides in a linear pattern. At the northwestern end of the seismic belt without surface rupture, the aftershocks formed a wide fault nearly perpendicular to the main seismic belt. The seismic source rupture process delineated by the aftershocks is to be the bilateral rupture propagation contrast to the result of seismic rupture inversion as the unilateral rupture. The focal mechanism solutions of the mainshock and some large aftershocks suggest that the seismic faulting along the segment with surface rupture to be pure sinistral strike slip, whereas to the northwestern end, the rupture mechanism might be different from that of the mainshock, the wide seismic belt there might be formed by a conjugate rupture. It can be concluded that the aftershocks occurred after a time period were triggered in the process of stress adjustment, and not constrained inside the rupture zone created by the mainshock.

    Related Articles | Metrics
    A review of the mechanism of deep earthquakes:Current situation and problems.
    2012, 19(4): 15-29. 
    Abstract ( 2263 )   PDF (993KB) ( 5621 )  

    Deep earthquakes(including intermediatefocus and deepfocus earthquakes)play an important role in the study of structure of Earth deep interior, plate motion and dynamical mechanisms. Since the first discovery of deep earthquakes by Wadati,they have attracted extensive attention worldwide and had many high quality papers during this eighty years. However,the mechanism of deep earthquakes is still an unsolved problem. Now, it is commonly accepted that dehydration embrittlement is the mechanism of intermediatefocus earthquakes while transformational faulting is responsible for the deepfocus earthquakes. Although some researchers thought that the adiabatic shear instability is also suitable for intermediatefocus and deepfocus earthquakes, it needs more experimental evidence. Northeast China, located in the border of the Pacific Ocean Seismic Area, is also the only deepfocus earthquake zone in China. Combing the study of the structural geology of Earth surface, mineral rock physics, deep exploration, high temperature and high pressure experiments and numerical simulation analysis is an effective way to solve the problem of the mechanism of deep earthquakes. To strengthen the research of the mechanism of deepfocus earthquakes in the northeast area of China and western Pacific plate subduction is of a great significance for further studying the evolution of the tectonic patterns.

    Related Articles | Metrics
    Permeability of earthquake faults.
    Chen Jianye,Yang Xiaosong
    2012, 19(4): 30-40. 
    Abstract ( 1577 )   PDF (1001KB) ( 5593 )  

    With relation to the strength of earthquake fault and its stress release process, the pore pressure within the shear zone is always attracting high attention. The permeability structure of fault zone is of great importance to the understanding of the activating state of fluid, faults strength and stability as well as the coseismic process. The paper introduces the basic principle and methodology of fault permeability research, and summarizes the newest study results in this area. The brittle faults usually have a typical permeability structure, with the lowest value in the core and high values in the damage zone. The fault gouge is the most impermeable and shows high permeability isotropy. In a seismic cycle, the permeability of earthquake fault takes on a characteristic process of abrupt jumping and gradual decreasing. Cementation, fluidrock interaction and solutionprecipitation are the main mechanisms of fault healing. Thermal pressurization is an important and effective dynamic weakening mechanism. When permeability of fault gouge is less than 10-18 m2, earthquake will accompany with a strong thermal pressurization process.

    Related Articles | Metrics
    Geodynamic setting and further exploration of magmatismrelated mineralization concentrated in the Late Paleozoic in the northern Xinjiang Autonomous Region.
    2012, 19(4): 41-50. 
    Abstract ( 2148 )   PDF (1211KB) ( 1254 )  

    We report here our new perspectives on magmatismrelated mineralization temporally concentrated in the CarboniferousPermian periods in the vast region of the northern Xinjiang Autonomous Region. Our broad aim is to provide a conceptual basis for discussion towards an improved understanding of the mineralization and further exploration. We confirm the idea that the DevonianCarboniferous magmatism and related mineralization in the region is a consequence of plate tectonics, and such mineralization may have well continued until the Triassic. The onset of the mantle plume activities in the region is poorly constrained, which may date back to the Late Carboniferous and prevailed in the Permian. The coexistence and coevality of the two tectonomagmatic processes resulted in the abundant mineralization and its variability in time and space. The major types of mineralization include (1) magnetite deposits hosted in the strata of Late Devonian/Early Carboniferous age (perhaps formed from volcanic flows as interpreted, but more like skarn/hydrothermal deposits in character), (2) porphyry/skarn Cu (and Mo) deposits of Late Devonian through Early Triassic age, and (3) CuNi sulfide and TiVmagnetite deposits of Permian age associated with maficultramafic intrusive complexes. The latter deposits that are widespread along orogenic belts are coeval (280±5 Ma) with the hidden flood basalt in the Tarim Basin in southern Xinjiang (interpreted as a large igneous province or LIP). This observation suggests the possibility that the LIP may in fact be much greater in its spatial distribution. If proved, this will have profound implications for the Permian TiV magnetite and CuNi sulfide mineralization in the region, i.e., the association of such known deposits with the orogenic belts may reflect the tectonic exposure and more and larger deposits may in fact yet be discovered in more sites and over a much greater area away from the orogenic belts.

    Related Articles | Metrics
    Seismic evidence of thrustfold in Carlintype gold deposit, central southwestern Guizhou and its significance.
    2012, 19(4): 63-71. 
    Abstract ( 1493 )   PDF (1235KB) ( 1711 )  

    The seismic reflection data collected across central southwestern Guizhou clearly revealed the coal measures of upper Permian Longtan Formation and Dongwu and Guangxi unconformities in Carlintype gold deposit. Most Carlintype gold deposits are controlled by thrustfold in this area. Based on seismic profile, the gold deposits could be divided into two types, i.e., thrustcontrolled and thrustaccompanied deposit according to the position of deposit in the thrustfold belt. In thrustcontrolled deposits, the thrust with steep dip in superficial part and shallow dip in deep level was detached from coal measures, tuffs and Dongwu unconformable contact; faultrelated fold always occurred on the hanging wall block; gold ore bodies occurred in thrust with vein shape in shallow level and low angle stratadissecting shape in deep level. The thrustaccompanied gold ores occurred in interlayergliding structures caused by highangle faults with stratiformlike ore bodies. Northwestern and northeastern trending thrustfold zone were generated by successional closure of PaleoTethys Ocean in South China during the late Indosinian epoch. The seismic data also show that there are some thrustfolds similar to the structure of the structure zones where Carlintype gold deposits were discovered, suggesting an ore prospective area.

    Related Articles | Metrics