琼东南盆地新生代构造东西分异的动力学机制研究

Qiongdongnan Basin is located at the transition region of the Indo-China extrusion system and the extension system of South China Sea continental margine, and is departed by the central tectonic transform belt which is NWW direction, of which sides the structural evolution is obviously different: (1) From early Oligocene(T70, about 30Ma), the basement began to subside strongly, and the direction of structures and sedimentary center turned from NE to EW in eastern basin. While in western basin, the direction of structures is EW, and the extension and subsidence increased rapidly westward, especially in the northern depression zone. In the meantime, there developed inversion structure by compressive stress in the southwest part of the basin; (2) Aftr early Miocene(T60, since 21Ma), the center of the post-rift sediment migrated on east-west direction; (3) During the period of 10-5Ma, there commonly occured accelerate subsidence in the basin. The subsidence increased westward to the transition zone of Yinggehai Basin and Qiongdongnan Basin. On the east, quickly open of Xisha trough resulted in the development of deep water canyon. Lacking of deep water area data, preliminary studies understood inadequately for the performance, starting and finishing time, formation mechanism of eastern-weatern difference of Qiongdongnan Basin in each period, so there are still many controversy. Based on the new added deep water drilling and high resolution seismic data, this application will use numerical simulation method to caculate tectonic stress field of the basin, combining whith analogue modeling to research the effect of Red River fault and expansion of northwest sub-basin of South China Sea to eastern-weatern difference, and to find out the the movement rule of the centeral structure transform belt and its role in the development of the basin.

琼东南盆地发育在印支挤出体系和南海陆缘伸展体系的过渡区上，以NWW向中央构造转换带为界，两侧构造演化存在明显的东西分异：(1)自早渐新世(T70,约30Ma)，盆地东侧强烈沉降，断裂和沉积中心从NE向转为近EW向，盆地西侧整体走向NE，向西张裂和沉降快速增强，北部坳陷带活动尤为强烈，断裂呈EW走向，西南部局部挤压反转；(2)早中新世后(T60,即21Ma以来)，沉降中心存在东西向迁移；(3)10-5Ma期间，盆地普遍存在加速沉降，西侧向莺琼过渡区增加，东侧西沙海槽快速打开，导致深水峡谷开始发育。由于缺乏深水区资料，前期的研究对于琼东南盆地各时期东西分异的表现、起止时间、形成机制等认识不足，存在较多争议。本申请拟利用新补充的深水区钻井、地震数据，运用构造应力场数值模拟方法，结合物理模拟，分析红河断裂和西北次海盆扩张等因素对盆地东西分异的影响，探讨中央构造转换带的活动规律及其对盆地发育的作用。

基于反射地震数据和井资料，利用玫瑰花图、断层活动速率、平衡剖面、构造沉降等一系列构造地质学方法，分析了断裂的活动规律、地壳的运动特征，并结合岩浆活动、地壳结构特征、沉积反射特征及周边地质事件，研究了琼东南盆地新生代构造特征和陆架坡折的演化。取得以下主要认识：.（1）琼东南盆地中央坳陷带由陵水-松南低凸起分为东、西两个部分。盆地中央坳陷的形态向西、向东逐渐变宽，形成一对喇叭状形态组合。断层活动速率显示，断层发育分别由东、西两侧逐步向盆地中央扩展。因此，盆地东部的拉伸应力似乎来自于南海扩张，而盆地西部则受红河断裂活动的影响。.（2）盆地东部可能存在三期岩浆事件，地壳强烈减薄可能与其岩浆背景有关。在南部坳陷带东部，地层的变形特征揭示在T60(21Ma)前后发生岩浆沿断裂的侵入事件；第二期岩浆事件发生在T40(10.5Ma)以前，之后的热衰减导致了10.5Ma 以来的加速沉降；最后一期岩浆事件发生在10Ma之后，并形成现今的高热流特征。在盆地西部，地壳向南增厚，根据其较为宽缓的地壳拉伸系数曲线形态及莫霍面埋深特征，推测陆壳破裂期间发育东南流向的下地壳流，并叠加晚期由于巨厚上覆沉积地层均衡作用产生的下地壳流。.（3）琼东南盆地新生代经历了裂陷、热沉降、加速沉降三个构造期，相应地其陆架坡折也经历了不同的演化过程，并且在时间和空间上具有东西差异性。琼东南盆地古近纪裂陷期发育构造型坡折带；新近纪热沉降阶段，在浅海环境下发育了坡度较缓的过渡型浅海相沉积地层，无明显坡折带发育；加速沉降阶段发育沉积型陆架坡折及无陆架坡折的缓坡型沉积层序，前者可进一步划分为前积型和加积型陆架坡折。.（4）陆架坡折前积/加积起始点在空间和时间上具有迁移性：空间上，盆地西部的乐东、陵水凹陷前积起始点在现今深盆的北部，并远离深凹边缘，过渡到松南、宝岛凹陷，加积层序基本上是受下伏盆缘断裂的控制，紧邻凹陷边缘；时间序列上，陆架坡折的形成具有“东早西晚，分段发育，最终统一”的特征。.（5）断裂活动的差异性控制了古近纪裂陷期构造型坡折带的类型和截止时间，构造沉降控制了新近纪加速沉降期前积型陆架坡折开始发育的位置和时间，沉积物供应量则控制了陆架坡折沉积层序的叠加类型。.（6）坡折带的演化受南海洋脊跃迁、菲律宾板块的碰撞和西沙海槽的打开、红河断裂右旋走滑的影响。