琼东南盆地深水区东区凹陷带结构构造及其演化特征

琼东南盆地深水区东区凹陷带,即松南—宝岛—长昌凹陷,位于琼东南盆地中央坳陷东端。在大量地震资料解释的基础上,对38条主要断层进行了详细分析。获得以下认识:(1)琼东南盆地深水区东区凹陷带平面上表现为近EW向展布的平行四边形,剖面结构表现为自西向东由半地堑—不对称的地堑—半地堑有规律变化。(2)琼东南盆地深水区东区凹陷带断裂系统可划分控制凹陷边界断层、控制洼陷沉积中心断层和调节性断层3类。(3)琼东南盆地深水区东区凹陷带古近纪时期受到太平洋板块俯冲和南海海盆扩张的双重影响,构造应力场发生NW—SE→SN转变。构造演化可划分为3个阶段:～32Ma,应力场以区域性NW—SE向伸展为主,断裂系统以NE—SW向为主,控制凹陷边界;32～26Ma,以南海海盆近SN向拉张应力场为主,断裂系统以NWW—SEE向为主,断层活动控制凹陷沉积中心;26～Ma,区域性伸展与南海海盆扩张应力均逐渐减弱,NE—SW向和NWW—SEE向断裂继承性发育。(4)琼东南盆地深水区东区凹陷带内部主要断层在渐新统崖城组和陵水组沉积时期活动速率快,地形高差大、沉积水体深、沉积厚度大,控制了崖城组和陵水组的大规模沉积,有利于烃源岩的发育。圈闭以受断层控制的断鼻和断块为主,长昌主洼凹中隆起带发育2个最为理想的构造圈闭。

STRUCTURAL PATTERN AND EVOLUTION OF EASTERN SAG BELT,IN DEEP-WATER AREA OF QIONGDONGNAN BASIN

The deep-water area of the northern continental margin in the South China Sea,apotential area with unique geological setting and excellent geological conditions for gas accumulation,has got great attention from scholars both at home and abroad.It is required to solve the basic geological problems in the early stage of the deep-water exploration,which has high risk and high economic threshold.The structural geology of the deep-water basins is no doubt one of most important geological problems to be solved.The Eastern Sag belt of the Qiongdongnan Basin,i.e.the Songnan-Baodao-Changchang Sag,is located in the east part of the Central Depression of the Qiongdongnan Basin,where complicated fault systems dominate.Our study suggests that：（1）the Eastern Sag belt of the Qiongdongnan Basin is in a shape of parallelogram in a plane view,with changes from a half-graben to an asymmetric graben to a half-graben in the section from west to east.（2）The fracture system can be subdivided into three types,i.e.the boundary fault in the border,the depositional fault in the center and the transfer fault in between.（3）These faults are developed and constrained by the joint action of the subduction of the Pacific Plate and the spreading of the South China Sea in Paleogene.As the tectonic stress field switched from NW-SE to NS,the structural evolution of the region could be divided into three phases.In the phase of pre-Eocene（～32Ma）,the boundary fault system with a NE-SW strike was controlled by the extensional stress in NW-SE direction;In the phase of Late-Eocene—early Oligocene（32-26Ma）,the growth fault system with a NWW-SEE strike was controlled by the extension in SN direction;In the phase after Late Oligocene（26～ Ma）,the fault systems in NE-SW and NWW-SEE direction inherited from the former as the decrease in the regional extension and the spreading of the South China Sea.The movement of faults was accelerated while the Yangcheng and Lingshui Formations were deposited,that controlled the mass depositio