中沙海域的广角与多道地震探测

Wide-angle and multi-channel seismic surveys in Zhongsha waters

由于浅滩和暗礁广布,中沙海域(环礁区)长期以来处在南海地震探测的空白区域.为了揭示该区域的地壳结构及地层-构造特征,进而全面刻画南海陆缘新生代伸展过程,我们首次在中沙及其邻近海域完成了1条广角地震测线OBS2017-2和4条多道地震测线的数据采集.对得到的海底地震仪(OBS)数据进行了格式转换、位置校正等初步处理,识别了来自地壳深部的震相并对中沙环礁地壳结构进行了正演模拟;对穿过中沙环礁的多道地震剖面进行了地层-构造解释,共划分出6套地层.OBS2017-2测线处理结果显示,其数据质量良好,深部震相清晰,且位于环礁内的台站震相延伸较长,可达100km以上.中沙环礁地壳性质为轻微减薄的坚硬大陆块体,厚度约为25km左右.多道地震剖面显示,环礁之上沉积层厚度较薄,除环礁西南部存在一条基底断裂--"排洪-美溪断裂"外,基本不存在其他构造-岩浆活动.而在环礁坡脚周缘,有广泛的岩浆侵入和喷出体的存在.中沙环礁上较薄的上地壳厚度和沉积特征指示了该区域可能在古新世时期经历的强烈暴露剥蚀作用.中沙海域最新开展的广角和多道地震探测填补了南海地质地球物理研究的空白,为解读该区的构造-沉积演化提供了数据基础.

Due to the wide-spread shoals and reefs,Zhongsha waters(atoll)has long been a blank area for seismic exploration in the South China Sea.To reveal the crustal structures and the stratigraphic-tectonic characteristics of this area,and to comprehensively depict the Cenozoic extension process of the continental margins in the South China Sea,we carried out one wideangle seismic profile OBS2017-2 and four multi-channel seismic profiles in the Zhongsha Atoll and its adjacent areas for the first time.The Ocean Bottom Seismometer(OBS)data was processed by format conversion,position relocation,etc.The seismic phases from deep crust were identified and used to simulate the crustal structures by forward modeling approach.Six sets of sedimentary strata were established based on the stratigraphic and tectonic interpretation of the multi-channel seismic profiles.The data processing of the OBS2017-2 shows that the data quality is very high,the deep seismic phases are clear and extend longer on the atoll,reaching over 100 km.The Zhongsha Atoll is characterized by slightly-thinned continental crust,with thickness of about 25 km.Sedimentary layers in the atoll are relatively thin as shown in the multi-channel seismic data.Basically,there is few tectonic-magmatic activities exist within the atoll except for a basement-cutting fault that named Paihong-Meixi Fault in the southwest of the atoll.Extensive magmatic intrusions and volcanoes exist only around the slope toe of the Zhongsha Atoll.The thinned upper crust and the sedimentary characteristics in the Zhongsha Atoll indicate that this area may have experienced strong processes of exposure and denudation.The recent wide-angle and multi-channel seismic surveys in the Zhongsha waters have filled in the blank of geological/geophysical research in the South China Sea,providing important data for understanding the tectonicsedimentary evolution in this area.