南海北部陆缘不均匀伸展及脉动式构造升降史

Heterogeneous extension and pulsed tectonic subsidence in the northern South China Sea margin

南海北部陆缘在区域板块构造和深部地幔流影响下,发育了复杂的构造、岩浆和基底升降过程。本文采用最新提出的伸展校正回剥技术和4条长地震剖面,对南海北部陆缘新生代伸展变形和从张裂到裂后的构造升降过程开展详细研究。结果表明南海北部陆缘伸展减薄差别大,强伸展与弱伸展不仅具有南北分带特点,还具有东西交替出现的分块特征。琼东南盆地、珠江口盆地中部、台西南盆地伸展减薄大,部分地壳被减薄至10km以下;而珠江口西部和东部伸展减薄小,地壳厚度大都在20km以上。强伸展与弱伸展陆缘总应变不同,但应变速率均具有张裂早期慢、晚期快的二阶段特征。强伸展陆缘最大应变速率在4×10^(-15)~7×10^(-15)s^(-1)之间,地壳最大伸展系数为4~10;弱伸展陆缘最大应变速率小于1×10^(-15)s^(-1),最大伸展系数小于1.9。南海北部陆缘构造升降具有脉动式、快慢交替的特征。张裂期构造沉降快,裂后构造升降表现为4幕先慢后快及东早西晚的特点。陆缘西部琼东南盆地脉动式升降主要与深部地幔上升流引起的动力地形有关。陆缘东部珠江口和台西南盆地脉动式构造升降则可能受南海打开过程及吕宋岛弧与南海陆缘碰撞的影响。

Under the influence of regional plate tectonics and deep mantle flow,the northern continental margin of the South China Sea has developed the complicated tectonic,magmatic and basement subsidence processes.In this study,the modified technique of back-stripping and four long seismic profiles are applied to conduct an in-depth investigation into the Cenozoic extensional deformation of the northern South China Sea margin and the syn-to post-rift tectonic subsidence history.Results show a significant difference in the crustal thinning in the northern South China Sea margin.From east to west,the strong and weak extensional zones occur alternatively.The Qiongdongnan Basin,the middle segment of the Pearl River Mouth Basin,and the Taixinan Basin experienced strong extension,and part of the crust is extremely thinned to less than 10 km;while the west and east segments of the Pearl River Mouth underwent minor extension,and the crustal thickness mostly remains above 20 km.The total strain between the strong and weak extensional margins are largely different,however,the strain rates both show a two-stage characteristics:slow in the early syn-rift period and fast in the post-rift period.In the strong extensional margin,the largest strain rate ranges between 4×10^(-15) and 7×10^(-15)·s^(-1),and the maximum extensional factor is 4~10;while in the weak extensional margin,the largest strain rate is less than 1×10^(-15)·s^(-1),and the maximum extension factor is less than 1.9.Moreover,the northern continental margin of the South China Sea is characterized by pulsed and alternative fast and slow tectonic subsidence.The tectonic subsidence was rapid during the syn-rift period.The post-rift period is characterized by four episodes of slow-fast-slow-slow tectonic subsidence,which happened early in the east then late in the west.The pulsed tectonic subsidence in the Qiongdongnan Basin is mainly related to deep mantle upwelling.The pulsed tectonic subsidence in the Pearl River Mouth and the Taixinan Basins may be affected by the diachronous opening of the South China Sea and the collision between the Luzon Island Arc and the South China Sea margin.