The Pearl River Mouth basin(PRMB) is a marginal sedimentary basin of the South China Sea.It trends NE and is divided into three segments from west to east by two NW-trending faults.Changing dramatically in structures along and across strike,the PRMB is a good example to analyze main factors that might control the process of a continental rift basin's extension.Through five series of analogue experiments,we investigate the role of different factors,such as pre-existing discontinuities of crust,rheological profiles of lithosphere,kinematics of extension and presence of magmatic bodies and strong crustal portions(rigid massifs) on the development of basin's structures.After being compared with the architecture of the natural prototype,the results of the analogue models were compared with the architecture of the natural prototype and used to infer the role of the different factors controlling the formation and evolution of the PRMB.The main conclusions are as follows.(1) Affected by pre-Cenozoic structures,the PRMB was controlled by crosscut NE-and NW-trending initial faults,and the NW-trending Yitong'ansha(一统暗沙) fault may be a through-going fault along dip and offset the NE-trending rift and faults,while the Enpingdong(恩平东) fault might exist only in the middle and south.(2) The NW-trending faults may orient WNW to be sinistrally transtensional under SE to nearly NS extension.(3) The thickness ratio of brittle over ductile crust in Baiyun(白云) sag is less than normal,suggesting an initially hot and weak lithosphere.(4) The magma must have taken part in the rifting process from early stage,it may occur initially upon or slightly south of the divergent boundary in the middle segment.The flow of magma toward rift boundary faults caused extra vertical subsidence above the initial magma reservoir without creating a large extensional fault.(5) The rigid massif contributed to the strain partition along and across basin strike.
Affected by thermal perturbation due to mantle uprising, the rheological structure of the lithosphere could be modified, which could lead to different rifting patterns from shelf to slope in a pas-sive continental margin. From the observed deformation style on the northern South China Sea and analogue modeling experiments, we find that the rift zone located on the shelf is characterized by half grabens or simple grabens controlled mainly by long faults with large vertical offset, supposed to be formed with normal lithosphere extension. On the slope, where the lithosphere is very hot due to mantle upwelling and heating, composite grabens composed of symmetric grabens developed. The boundary and inner faults are all short with small vertical offset. Between the zones with very hot and normal lithosphere, composite half grabens composed of half grabens or asymmetric grabens formed, whose boundary faults are long with large vertical offset, while the inner faults are relatively short. Along with the thickness decrease of the brittle upper crust due to high temperature, the deformation becomesmore sensitive to the shape of a pre-existing weakness zone and shows orientation variation along strike. When there was a bend in the pre-existing weakness zone, and the basal plate was pulled by a clockwise rotating stress, the strongest deformation always occurs along the middle segment and at the transition area from the middle to the eastern segments, which con-tributes to a hotter lithosphere in the middle segment, where the Baiyun (白云) sag formed.
To investigate the thinning of the whole crust, and the contribution of the upper versus lower crust to the stretching since Cenozoic, we calculated the stretching factor of the northern margin of South China Sea with data such as whole crustal thickness, depth of Cenozoic sedimentary basement and the horizontal displacement of faults. An isometric line drawing on whole crustal stretching factor is then obtained. Along the seismic Line 1530 in Baiyun sag, we also calculated the stretching factors of the upper and lower crust. The results suggest that the whole crustal thickness decreases seaward while the whole crustal stretching factor increases from shelf to slope. The Moho upwells highest beneath where the crust is thinned most. The value of the whole crustal stretching factor ranges from 1.5 to 6. Two areas were thinned intensely: the center of Yinggehai Basin, and the Baiyun sag in the Pearl River Mouth Basin. The calculation of the upper and lower crustal stretching factors from DSP1530 in Baiyun sag shows that the original crust of Baiyun sag should be thinned before deformation. Its pre-Cenozoic evolution as well as tectonic position during Cenozoic might be responsible for that.
