汇聚背景下的多幕裂陷作用及其迁移机制:以南海北部珠江口盆地为例

南海的形成演化受控于印-澳、欧亚以及太平洋板块的相互运动,为研究汇聚背景下板块碰撞及其远程效应提供重要窗口。为了揭示该汇聚背景下的多幕裂陷过程,本文选取地质信息丰富的整个珠江口盆地为典型区,利用三条高精度地震剖面,对盆地各地质单元进行断层活动速率和构造沉降速率的定量计算及综合分析。结果表明盆地裂陷期东部、中部和西部主要控凹断层的平均活动速率分别为96 m/Ma、223 m/Ma和124 m/Ma,且其平均沉降速率依次为8.5 m/Ma、34 m/Ma和12.7 m/Ma,盆地整体呈现中部裂陷作用最强,其后向西部和东部逐渐减弱的特征。本文认为这与先存断裂以及初始地壳厚度有关:盆地东部和中部存在NE向先存断裂,并且东部先存断裂更加活跃,因此在新生代拉伸应力下东部更易表现为裂陷作用最强的区域,其次为中部和西部;而受前新生代时期俯冲作用的影响,岩浆的底垫作用引起盆地东部地壳增厚,东部裂陷作用强度急剧降低,造成裂陷作用强度的东西差异。此外,盆地南段凹陷裂陷期的断层活动和沉降速率发生激增,裂陷作用存在向南迁移的现象。本文推测在深度相关的伸展模式的影响下,南段凹陷地壳温度升高,强度减弱,因而在伸展应力下发生快速的拉伸减薄,导致裂陷中心向南迁移及岩浆物质上涌。同时,侵入的岩浆物质导致高角度正断层转换成低角度正断层,进一步促进裂陷中心向南迁移。

Multiphase rift and migration mechanism in the Pearl River Mouth Basin

The formation and evolution of the South China Sea is controlled by the mutual movements of the Indo-Australian, Eurasian and Pacific plates, which provides an important window for the study of plate collisions and their long-range effects in the context of plate convergence. To reveal the multi-episodic rifting process under this context, we select the entire Pearl River Mouth Basin rich in geologic information as the typical area of South China, and used three high-precision seismic sections to calculate the fault activity and tectonic subsidence rates and perform comprehensive analysis for each geological unit in the basin. According to our calculation, the average fault activity rates for the main sag-controlling faults of the eastern, central and western parts of the basin during the rifting period were 85, 203 and 93 m/Ma, respectively, and the average subsidence rates were 8.5, 34 and 12.7 m/Ma, respectively. Basinwide, rifting was strongest in the center and gradually weakened to the west and east, which, we believe, is related to the pre-existing faults and initial crustal thickness. The pre-existing NE-trending faults are known to be more active in the east than in the center. Therefore, under Cenozoic tensile stress, the east is more likely to experience the strongest rifting, followed by the center then the west. Whilst under the influence of pre-Cenozoic subduction, magmatic underplating caused the crust to thicken thus rifting intensity to drop sharply in the east, resulting in the difference in rifting intensity between west and east. In addition, the fault activity and subsidence rates in southern basin had surged during the rifting period, causing rifting to migrate to the south. We speculated that under the influence of depth-dependent extension, the southern sag witnessed increased crustal temperature and weakened crustal strength. As a result, rapid tensile thinning occurred under extensional stress, resulting in a southward migration of the rift center accompanied by mantle upwelling. At the same time, intrusive magmatic material caused the high-angle normal fault to convert into a low-angle normal fault, further promoting the southward migration of the rift center.