摘要：
The structural style of the Pearl River Mouth Basin (PRMB) at the northern continental margin of the South China Sea (SCS) is quite complex, showing significant spatio-temporal differences influenced by the basement-involved faults striking in NNE, NE, NW and near E-W directions.Based on the structural style changes according to seismic data interpretation, we conclude that the middle section of the PRMB develops a ramp-flat detachment fault that slowly dips to the south;and the Paleogene basin structure is taken as an extensional system of the supra-detachment.The Xijiang Sag in the northern PRMB lies in the head region of the extensional system, from where the boundary normal fault extends to the deep detachment fault in listric geometry, restraining the Paleogene to form a half graben faulted structure.The Panyu Low Uplift in the middle is situated in the low-angle fault zone of the detachment fault, where extensional displacement of the supra-detachment leads to its overlying with the Enping Formation on both sides.The Baiyun Sag in the south overlays on the ramp of the detachment fauand the Paleogene filling in the sag is characterized by a synform-type"faulted-sag"structure.The Southern Uplift is located in the deeper flat of the detachment fault controlling the Liwan Sag by its branch faults, with a complex"faultedsag"structure of the Paleogene.Geometry and kinematics of the detachment fault and branch faults on the extensional system control the basin structural style and evolution in different tectonic units.The model demonstrates that both supra-and under-detachment are extensional deformations that could occur in different crustal layers.However, spatio-temporal differences should be expected for different deformation styles and stretch strain distributions.In general, the supra-detachment deformation features brittle extensional structure and the uneven stretch strain distribution is mainly concentrated in the listric normal faults linked to the detachment fault, but the footwall of the detachment is predominantly of ductile stretching deformation with relatively uniform distribution of stretch strain.In the head region of the detachment system (Xijiang Sag) , the supra-detachment extension is greater than the under-detachment counterpart;however, the opposite is true in the root region (Baiyun and Liwan sags).The master boundary faults on the northern edge of the Xijiang Sag are listric normal faults striking NE in the Wenchang period as the head of the detachment fault, but cut off by high angle normal faults striking near E-W in the Enping period so that the head of the detachment fault is abandoned or migrated to the Panyu Low Uplift from the northern edge of the Xijiang Sag.The spatio-temporal differences in basin structure style and deformation are influenced by many factors such as pre-existing basement structure, lithosphere rheology, and scale of extension and so on, but the main factor is the response to lithospheric stretching deformation and lithosphere thermal change due to asthenosphere flows.A dynamic model, i.e.the speed up flow of asthenosphere from northwest to southeast drags the overlying lithosphere to divergent motion and causes the continent rifting, can reasonably explain the structural formation and evolution in the middle section of the PRMB.That is, the asthenosphere one-way flow drags the overlying lithosphere to its simple shearing extensional deformation and forms the detachment fault, while the relatively uniform stretch strain of the under-detachment, with diminishing toughness from south to north, is passed onto the supradetachment with non-uniform brittle extensional deformation enhanced gradually from south to north.%位于南海北部大陆边缘上的珠江口盆地发育NNE向、NE向、NW向、近EW向等多组基底断裂, 盆地结构复杂, 并表现出明显的时空差异性.本文基于珠江口盆地中段地震资料解释的构造样式的变化推断地壳中存在一条向南缓倾斜、呈坡坪式形态的拆离断层, 古近系构造属于这条拆离断层上盘的伸展构造系统.北部的西江凹陷属于拆离断层伸展构造系统的头部, 凹陷边界正断层铲式断层面形态向深层延伸并收敛在拆离断层面上, 凹陷表现为半地堑"断陷"样式;中部的番禺低隆起对应于拆离断层的低角度断坪部位, 拆离断层上盘断块的伸展位移导致两侧的恩平组超覆在低隆起上;南部的白云凹陷位于拆离断层的深部断坡部位, 充填的文昌组和恩平组表现为"断坳"或"坳断"样式;南部隆起位于拆离断层深部断坪部位, 其上盘发育的分支断层控制着荔湾凹陷古近系、新近系的发育并使之表现为复杂的断陷-断坳构造样式.该模型强调拆离断层上盘与下盘、不同地壳结构层均发生不同程度的伸展变形, 且伸展变形方式、应变量等存在时空差异, 而拆离断层正是不同构造单元、不同地壳构造层之间的调节性构造面.总体上, 拆离断层上盘以脆性伸展构造变形为主, 分支断层控制不同构造单元古近纪的构造演化, 下盘则是以韧性伸展变形为主, 并拖曳上盘发生不均一的伸展应变;西江凹陷的伸展应变量大于拆离断层下盘的伸展应变量, 白云凹陷的伸展应变量则小于拆离断层下盘的伸展应变量.以西江凹陷北部边缘的NE向铲式正断层为头部的拆离断层控制了文昌组沉积, 但在恩平组沉积期被近EW向高角度正断层切割破坏而被遗弃, 拆离断层系统的头部由西江凹陷北部边缘迁移至番禺低隆起.盆地结构及断裂系统的时空差异性受盆地基底先存构造、地壳与岩石圈结构及伸展量等多方面因素的影响, 但主要是对软流圈流动及岩石圈热结构变化的响应.用软流圈由北西向南东流动拖曳上覆岩石圈发生伸展变形的动力学模型能合理地解释珠江口盆地中段古近系构造的形成和演化.
