The Gashydrate-related Active Tectonics In The Deep-water Of The Northern South China Sea And Its Genetic Mechanisms

The basins of the Northern South China Sea experienced a complex geologicevolution, which located in the overlapping zone of the Pacific Tectonic Domain andthe Tethys Tectonic Domain and enclosed by Eurasian Plate, India-Australian Plateand Pacific Plate. The study area is located in the epicontinental deep-water area(>300m), covering the most parts of the Qiongdongnan Basin, the Zhujiangkou Basinand the Taixinan Basin. The study area is also a highlighted zone of the study andexploration of the gashydrate. A series of important achievements has revealed therelationship between gashydrate and special geological bodies. But the availablematerials are limited to local areas and lack of macroscopic view. The abundantseismic data obtained from the project on gashydrate provides some favourable termsto the study of the relationship between gashydrate and gashydrate-related activetectonics.Based on the detailed interpretation of seismic data and predecessors’achievements, this paper plans to give a study on the genetic mechanisms of activefaults, submarine landslides, diapir structures and layer-bound faults and theirrelationship to the gashydrate. The problem of time-sequence in the field ofgashydrate study has been proposed for a long time, but no scholars investigated therelationship from this angle. As an important geophysical identification, BottomSimulating Reflector (BSR) plays an important role in the research and exploration ofgashydrate. Therefore, a series of key problems discussed in this paper is based on therecognition of BSRs. Based on the research of active faults, submarine landslides,diapir structures and layer-bound faults, some models on accumulation system ofgashydrates are finally gived for different regions. The following conlcusions are asfollows based on this study.（1）The heterogeneity of active faults since late Miocene is obvious in the study area. The NE-trending faults have a long-term activity. Influenced by thepre-Cenozoic NW-trending faults, the NW-NWW-trending sinistral and trantensionalfaults developing during the Dongsha Movement has not been at large scale. But theinitial strength of the Dongsha Movement may be large with a wider influence rangethan previously considered, then quickly retreated to the Dongsha Uplift and itsperipheral regions. The northern South China Sea basically remained the tensile shearstress field after late Miocene, and some of NE-, NW-NWW-and EW-trending faultswere still active after the Dongsha Movement. The local existence of compressionphenomenon such as reverse faults, tectonic inversion and folded strata is a result ofuneven horizonal compression induced by uplifting of the ancient Dongsha Uplift.The active faults play an important role in the vertical migration of deep-seated fluids,but the specific accumulation association is complex.（2）Two new submarine landslide-triggering models called the “oceanic corecomplex” pattern and “high-pressure sandbody” pattern are proposed and the typicalsubmarine landslides can be summarized to three morphological types. Thegas-hydrate-related diapir types can be divided into mud diapirs and magma diapirs.The clustering characteristic of Quaternary magma diapirs is obvious and theinfluence of the magma activity to the gashydrate stability zone (GHSZ) isdisappeared now. In the study area there is less BSR coexisted with magma diapirs.Controlled by different stress fields, the mechanisms to mud diapirs in different areaare different. From the time-sequence angle, the relationship between knowngashydrate and active faults, submarine landslides and diapir structures can be dividedinto a series of types respectively, to which different exploitative requirements isconsidered.（3）The mechanism of new-founded layer-bound faults in the study area isvarious in different areas. This formation of layer-bound faults underwent amulti-stage and multi-effecting-joint process, some of which are directly controlled orinfluenced by the structural activities and their superposition. For a long time,domestic and abroad scholars have paid more attention to the study of non-structural interpretation of layer-bound faults, while ignoring the coupling of tectonic stress andnon-structural factors. The layer-bound faults can effectively change the properties ofsediments, fine-grained sedimentary rocks can be appeared as good reservoirs ofgashydrate. But this effect has not been founded in the Northern South China Sea.（4）The gashydrate accumulation system consists of hydrocarbon generationsub-system, fluid migration sub-system and accumulation-seepage sub-system. Thedeep-water basins in the Northern South China Sea have a good hydrocarbon-sourcecondition and a variety of fluid migration pathways, but the model of gashydrateaccumulation system is also various in different regions being characterized bydifferent geological conditions. The great differences in the BSR-bearing stara in theQiongdongnan Basin and the Zhujingkou Basin can be interpreted by the couplingrelationship between the degree of the effectively accumulation and GHSZ.