南海西北部莺歌海盆地低速层的特征及其成因探讨

CHARACTERISTICS AND GENESIS OF LOW VELOCITYLAYER IN YINGGEHAI BASIN，NW SOUTH CHINA SEA

对垂直于莺歌海盆地走向的两条广角反射地震剖面数据的模拟结果表明，莺歌海盆地在3．5—9km深度范围存在低速层。对多道反射地震数据及钻孔测井数据的进一步分析表明，该低速层的上界面埋深从盆地中心的1．5km往周边变为5km，表现出明显的穿时性，包含的地层在盆地中心地带有部分第四系和第三系，往北西和南东只有部分第三系，没有第四系。低速层形成的原因在盆地不同部位有所差异：盆地的西北和东南部主要由沉积物快速堆积及其引起的欠压实造成，而盆地中心地带则除沉积物快速堆积外，多期的活动热流体及其底辟作用可能是造成低速层上界面抬升的主要原因。

Two wide-angle seismic profiles across the Yinggehai Basin indicated a low ve-locity layer (LVL) in sediments at depth between 3. 5-9 km. Velocity analyses ofmulti-channel seismic reflection data and well logging data confirmed the existence ofLVL. The upper boundary of the LVL is diachronous and varies in depth between 1. 5km and 5 km, mostly shallower towards the center of the basin. Involved strata includeparts of Quaternary to Tertiary in the center of the basin, but parts of Tertiary only tothe north and southeast of the basin. Several factors have contributed to the genesis ofLVL. The high accumulation rate and subsequent under-compaction of sediments weremain factors for the northern and southeastern portions of the basin, while the multi-ep-isodic fluid activity and related diapir process played important roles in the center of thebasin and raised the upper boundary of LVL over there.