胶东半岛地壳速度结构及其构造意义

胶东半岛地处华北平原东北部沿海地区,北东向断裂带异常发育,该区不仅是华北震区中、强地震活动的频发区,又是中-新生代地壳构造活动的大陆边缘带.中国东部最大规模的郯庐断裂带纵贯胶东半岛西部,苏鲁超高压变质带位于该半岛南侧,彰显其地理位置重要而特殊.为深入认识我国近海与海陆过渡带地壳结构特征及其深部动力学背景等基础科学问题,2013年在胶东半岛东部实施了海陆联合探测项目.本文对布设的北西向宽角反射/折射探测剖面进行了计算处理,获取了该测线的地壳精细结构及其构造特征.结果表明:胶东半岛地壳结构复杂,该区基底埋深较浅,地表速度偏高,地壳速度结构呈现东西迥异的非均匀性特征,具有横向分层、纵向分块的典型特性.作为郯庐断裂带重要组成部分的牟平—即墨断裂带是胶东半岛上非常重要的断裂带,该断裂带东西两侧的地壳结构特征差异非常明显,断裂带东侧速度偏低,而西侧速度偏高,地壳各分层界面形态在该断裂带两侧起伏变化明显,地壳结构整体上呈现南东浅北西较深的结构特征.总体来看,胶东半岛不同区段呈现出的结构差异与该区大地构造单元的划分基本相符,在界面起伏变化明显的区域与地表穿过的断裂带遥相呼应.作为华北板块和扬子块体相互碰撞的边缘地带,胶东半岛复杂的深部结构特征与西太平洋板块对其的俯冲挤压有着密切的关联,该研究对了解本区地壳构造变化的深部动力学背景有着重要的研究意义.

Crustal velocity structure beneath Jiaodong Peninsula and its tectonic implications

In order to understand better oil and gas resources and earthquake hazards in China offshore and marine-continental transitional zones and interior structure and evolution process of the earth, the First Institute of Oceanography (FIO), State Oceanic Administration (SOA), Institute of Geology and Geophysics, China Academy of Sciences (IGGCAS) and Geophysical Exploration Center of CEA jointly implemented the research on key technology of onshore-offshore deep geophysical exploration. During the mid-late 2013, we deployed a NW-trending onshore-offshore profile in eastern Jiaodong Peninsula to reveal shallow-deep structural relationship, seismogenic environment and crustal structure in active faults of the Jiaodong Peninsula and its adjacent regions. The onshore-offshore seismic survey simultaneously received structural information from the upper mantle by terrestrial artificial source explosion, submarine detectors and the terrestrial seismic instruments. The artificial seismic exploration is one of creditable methods to obtain the relatively accurate deep crustal structure,which has the higher data resolution, the identified source location and reliable seismic phase. In order to suppress the interference and improve the signal-to-noise ratio of instruments, observation instruments are placed away from main traffic routes as far as possible. To obtain high-quality seismic data, we apply the seismic ray tracing method to calculation. First of all, we identify and read the wave group phase of shot points, then repeatedly modify the given initial velocity model to make the most effective fitting between the theoretical arrival time and the measured arrival time, and finally get the 2D deep velocity structure of seismic along the profile with the active source. From the obtained 2D velocity structure, we get following results: (1) Five sets of seismic phase, i.e.PG, P1, P2, P3, PM, are identified, of which PG, PM are the dominant wave groups with strong energy and continuous contrast tracking while other wave groups are relatively weak with the loss of some phase in some sections. (2) The study area can be divided into the upper crust and lower crust with the C2 interface as the boundary between the upper and lower crust and M interface being the boundary between crust and mantle. (3) The crustal structure in Jiaodong Peninsula is complex. The buried depth of the basement is shallow and the surface velocity is relatively high. The crustal velocity structure presents non-uniform characters with horizontal stratification and longitudinal blocks. (4) Interface trending features of C1 and C2 are similar, showing gradually shallower from NW to SE with evident undulation in some sections. C3 interface only appears in the NW section and has not been found yet in the SE section. The M interface is deeper in the middle and shallow subsurface at both ends, and at the same time the thickness of the lower crust is thicker in the west and thin in the east. (5) As the important part of the Tanlu fault zone, the Muping-Jimo fault zone is a major fault zone in Jiaodong Peninsula. Crustal structures are obviously different on either side of this fault zone. The speed on the east is lower while higher on the west side, and layered interface of the crust also varies obviously on both sides. The crustal structure is shallower in SE and deeper in NW on the whole. Overall, structural differences in different sections of Jiaodong Peninsula are in line with the tectonic unit division. The regions with significant undulation at the interface coordinate with the fault zone through the surface from afar. As the collision edge zone between North China block and the Yangtze block, located on the onshore-offshore structure edge which is related with the subduction of Pacific plate to Eurasian plate, Jiaodong Peninsula is featured by complicated crustal structure due to the combined effects of two kinds of dynamic backgrounds. In the light of the distribution of the NNE-trending fault zone, the Pacific plate plays a more important role in the subduction of the Jiaodong peninsula.