珠江口盆地惠州运动的断裂响应研究:以阳江东凹为例

惠州运动是最近确定的发生于珠江口盆地早、晚文昌过渡期的构造运动,以珠一坳陷对其响应最为典型。然而,这一运动是否波及珠江口盆地其他坳陷,以及它们的响应特征如何并不清楚。本文以珠三坳陷阳江东凹断裂体系为突破口,首先对其几何学和运动学特征进行了研究,然后从断裂体系的优势方位、组合样式、活动和迁移规律等角度对珠江口盆地对惠州运动的响应特征进行了研究。研究发现:阳江东凹古近系主要发育NE-NEE和E-W-NWW两组不同级别的断裂,这些断裂的发育演化主要受先存构造和区域构造运动两种因素控制。其中,先存构造主要影响主断裂带的优势发育部位、排列方式和整体走向等要素,可为这些断裂沿线后期构造的形成提供有利条件,而区域构造运动则导致该区优势断裂体系发生转换。对于阳江东凹而言,正是阳江—一统断裂前新生界左阶分支的左旋复活,才导致惠州运动后其内部形成了一系列小型走滑拉分盆地;虽然阳江东凹古近系断裂体系仍保留其前新生代先存构造的基本特征,如方位和组合样式等,但与珠一坳陷一致,惠州运动前后,其优势断裂体系也由早文昌沉积期的NE-NEE向张性断裂向晚文昌沉积期的近E-W-NWW向张扭性断裂转换,断裂活动强度也由南往北、由西向东发生了迁移和转换。珠江口盆地古近系断裂体系对惠州运动的统一响应说明惠州运动为南海北部影响广泛的构造运动,珠江口盆地不同坳陷对惠州运动的响应特征研究可为其内部各坳陷孤立发育的文昌组的跨盆对比提供构造依据,同时对于破解阳江东凹油气勘探困局也具有重要意义。

Fault response to the Huizhou Movement in the Pearl River Mouth Basin: Insights from a case study of the Eastern Yangjiang Sag

The Huizhou Movement is a newly-defined tectonic movement occurring to the Pearl River Mouth Basin during the transition period between the early and late Wenchang rifting stages. This movement is well-responded in the Zhu Ⅰ Depression of the basin, however, its impact and response characteristics in other depressions of the basin are not known. Taking the fault system of the Eastern Yangjiang Sag, Zhu Ⅲ Depression as an example, this paper first studies the geometry and kinematic characteristics of the fault system, then summarizes its response characteristics to the Huizhou Movement in terms of its preferential orientation, assemblage style, and activity and activity migration patterns. It was found that mainly NE-NEE- and EW-NWW-trending faults of different scales develop in the Paleogene strata of the Eastern Yangjiang Sag, and their formation and evolution are mainly dominated by the pre-existing fabrics/weaknesses and regional tectonic movements. The pre-existing fabrics/weaknesses determine the preferential position, array style and general orientation of the main fault belts, imparting a favorable structural condition for the formation of various later structures along the fault belts. In the Eastern Yangjiang Sag, it is the reactivation of the pre-Cenozoic left-stepping echelon array branches of the NW-trending Yangjiang-Yitong'ansha Fault Zone that leads to the formation of a series of small strike-slip pull-apart basins along the Yangjiang-Yitong'ansha Fault Zone during its sinistral motion after the Huizhou Movement. Regional tectonic movements result in structural transformation of basin's dominant fault system including both main and secondary faults. For instance, although the Paleogene fault system of the Eastern Yangjiang Sag retains the general characteristics of the pre-existing pre-Cenozoic fabrics, such as fault position, array style and orientation, its dominant fault system, like in the Zhu Ⅰ Depression, transforms from the NE-NEE-trending extensional faults in the early Wencheng deposition stage to the sub-parallel E-W-NWW-trending transtentional faults in the late Wenchang deposition stage in response to the Huizhou Movement, and fault activities also migrate from south to north and from east to west, accordingly. The consistent response of the fault system to the Huizhou Movement in the Pearl River Mouth Basin indicates the Huizhou Movement is a wide-spread tectonic movement occurring to the north margin of the South China Sea. Therefore parallel studies of fault response characteristics in different depressions of the Pearl River Mouth Basin can provide a tectonic basis for inter-depression/inter-sag comparison of the Wenchang Formation developed in isolation across the whole basin, and facilitate oil and gas exploration in the Eastern Yangjiang Sag.