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_Journal of Structural Geology 32 (2010) 746-754_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Polyphase rifting of greater Pearl River Delta region (South China): Evidence for possible rapid changes in regional stress configuration Lung Sang Chan a,*, Wenlue Shen a, Manuel Pubellier b a Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong b CNRS-UMR 8538, Laboratoire de Géologie, École Normale Supérieure, 24, rue Lhomond, F-75231 Paris Cedex 05, France Article history: Received 25 July 2008 Received in revised form 22 April 2010 Accepted 29 April 2010 Available online 12 May 2010 Keywords: Pearl River Delta tectonics Polyphase rifting South China Cenozoic tectonics Active-passive margin transition Abstract The greater Pearl River Delta (PRD) Region, consisting of several late Mesozoic-Cenozoic basins, has preserved information on the tectonic history of the coastal region of South China. An integrated morphological and structural study of the basins has revealed several phases of extension subsequent to the collapse of the Mesozoic arc magmatism. A N-S extension associated with the exhumation of the magmatic arc during the late Mesozoic-early Cenozoic was followed by an ENE-WSW extension producing NW-striking normal faults in the region. Paleostress analysis of fault slips measured at seven localities reveals stress configurations consistent with field observations. The basin development during the Cenozoic was strongly controlled by relative motion of neighboring plates as well as preexisting structures. The results suggest that stress configuration can change relatively rapidly in a continental margin undergoing a transition from an active-margin to passive-margin. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction The coastal province of South China represents a continental margin that underwent a transition from an active-margin to a passive-margin during the Late Cretaceous-early Cenozoic time. The region is characterized by an extensive Mesozoic magmatic belt and prominent NE-striking fault zones developed during the northward subduction of a paleo-Kula-Pacific plate beneath South China (Mark and Geoffrey, 1982). Extensive exhumation of plutons occurred during the active to passive continental margin transition (Hsu et al., 1990; Lapierre et al., 1997). The rifting probably commenced during as early as the Late Cretaceous, producing faulted basins both onshore and in the continental shelf (Faure et al., 1996; Zhou and Li, 2000; Zhu et al., 2004). The offshore basins, including the Zhujiang and Qiongdongnan Basins, are elongated in a generally E-W direction and were formed mainly during the late Eocene-Oligocene (Li and Rao, 1994; Zhou et al., 1995; Ludmann and Wong, 1999; Ren et al., 2002). The tectonic origin of the terrestrial basins in the onshore region is not well understood. Zhu et al. (2004) regarded them as pull-apart basins while Zhou et al. (1995) suggested that they were formed by simple rifting along two adjoining faults. A better understanding of the * Corresponding author. Fax: +852 2517 6912. E-mail address: chanls@hku.hk (L.S. Chan). 0191-8141 $ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.04.015 nature of the basins is therefore critically important for the reconstruction of the tectonic history of South China during the continental margin transition. In the study described in this paper, we carried out detailed structural measurements at selected localities in the greater Pearl River Delta area in an attempt to unravel the tectonic deformation history of South China during the continental margin transition. Topographic features and drainage characteristics were delineated using satellite images and Shuttle Radar Topography Mission (SRTM) data. Field measurements of geological structures and a paleo-stress analysis were carried out to delineate the post-orogenic extension history of the study area. 2. Morphotectonic and structural setting of the greater Pearl River Delta region The study area, located between longitudes 111°30'E and 115°E and latitudes 21°40'N and 24°N, is part of the greater Pearl River Delta region in South China (Fig. 1). The region consists of a series of NE-SW striking fault zones intersected by less prominent NW-SE striking faults (Fig. 1). The two sets of faults dissect the entire region into a checkerboard-like pattern. The cross-cutting relationship between the two sets of faults is obscure, but present-day seismicity shows that both sets of faults are apparently active. Geophysical data compiled by LDAC (1989), Zeng et al. (1997), Wei L.S. Chan et al. Journal of Structural Geology 32 (2010) 746-754 Fig. 1. Simplified geology of the greater Pearl River Delta (PRD) region, based on Bureau of Geology and Mineral Resources of Guangdong Province (1988). (2001) and Pubellier and Chan (2006) reveal a gradual thinning of the crust towards the southeasterly direction and an E-W directed principal stress in the present day (Fig. 2). The major NE-striking fault zones, including the Wuchuan-Sihui Fault (WSF), the Conghua-Yangjiang Fault (CYF), the Heyuan Fault (HYF) and the Lianhuashan Fault (LHSF), developed along older ductile shear zones and were probably reactivated during the exhumation of the basement (Sewell et al., 2000; Fletcher et al., 2004). Fig. 2. Estimated crustal thickness, focal plane mechanism and present-day principal stress directions of Guangdong area based on Zeng et al. (1997), Wei (2001) and Pubellier and Chan (2006). 748 L.S. Chan et al. Journal of Structural Geology 32 (2010) 746-754 Fig. 3. Drainage anomalies in the PRD region and surrounding areas. (a) Drainage network in the coastal region of South China; (b) Xijiang river which follows an east-west striking fault and makes an abrupt turn towards the SSE; (c) Antecedent drainage of the Beijiang river indicates control of the drainage by E-W structures. The area contains some of the largest and Cenozoic-age terrestrial basins in South China, including the late Mesozoic-age Kaiping Basin and the Cenozoic-age Dongguan, Sanshui and Pearl River Delta (PRD) Basins. The sediments within the Mesozoic basins are generally tilted and occasionally folded, while the sediments in the Cenozoic basins are mostly sub-horizontal. The PRD Basin marks a rectangle-shaped depression extending from the coast northwestwards for more than 100 km into the magmatic arc. Its age is controversial; Zhou et al. (1995) suggested that a proto-PRD basin was formed during the late Mesozoic-early Cenozoic either as pull-apart or a back-arc basin, while Huang et al. (1982) and Wei (2001) regarded it as a Quaternary graben. Three major drainage systems, namely the Xijiang (Western River), Beijiang (Northern River) and Dongjiang (Eastern River), are present within the study area (Fig. 3). A strong structural control on the development of the drainage systems is evident. Both the Xijiang and Dongjiang rivers follow an E-W course along latitude 23.2°N for a combined distance of over 500 km. West of the PRD Basin, the eastward flowing Xijiang river cuts through over 1 km of Devonian formations in an anticline and takes an abrupt turn towards the SSE into the PRD Basin near Sanshui. The Dongjiang River follows a westerly course and turns similarly towards the SSE near Guangzhou. The three rivers join to form the Pearl River Delta System, which is characterized by a series of subparallel and linear distributaries trending in a NNW-SSE direction. The ability of the distributaries to maintain very linear courses despite the lack of relief is indicative of the strong control of the river development by neotectonic motions involving basement structures. 3. Structural measurements and paleo-stress inversion The presence of multiple post-orogenic extensional episodes is evident in the greater PRD region. Detailed structural measurements were conducted at ten localities within area; the localities are grouped into five areas based on their geographic locations. Bedding attitude and kinematic indicators such as drag fold, tension gashes, schistosity, C-S fabrics, Riedel shears, fault-plane orientation, and fault-slip were studied and measured. The multiple inverse method developed by Yamaji (2000) and subsequently modified by Yamaji (2003) was applied on the fault-slip data to delineate phases of kinematic movement and the associated stress tensors for the localities. The inversion necessarily assumes a linear and isotropic plasticity of the rock mass, and the slip vectors occur in the direction of the maximum shear stress on the slip surface. Fault-slip data are grouped into subsets using a recursive procedure, and the optimum stress tensor is calculated for each subset of fault slips by minimizing the angular misfits between the predicted and the measured fault-slip values. A good clustering of stress solutions thus represents a persistent stress field, and the number of solution clusters indicates the number of tectonic episodes experienced by the rock mass. L.S. Chan et al. Journal of Structural Geology 32 (2010) 746-754 Fig. 4. Fault-slip data and paleo-stress inversion in the greater Pearl River Delta region using the multi-inversion method. Two phases of extension are evident in the stress tensors obtained from the inversion. In the field, the fault-slip vectors were either determined from slickensides on the fault planes or inferred from secondary structures such as tension gashes, Riedel shears and P-shears. For the Riedel shears and P-shears, the direction on the fault surface normal to the line of intersection between the fault plane and the shears was used as the slip vector. For tilted layers, the fault-slip data were corrected for the bedding attitude before the inversion was applied. A sub-group size of 8 was used in the inversion to_ Ключевые слова: e, r, o