The dynamic mechanism of post-rift accelerated subsidence in Qiongdongnan Basin, northern South China Sea
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ORIGINAL RESEARCH PAPER
The dynamic mechanism of post-rift accelerated subsidence in Qiongdongnan Basin, northern South China Sea Zhongxian Zhao • Zhen Sun • Zhenfeng Wang • Zhipeng Sun • Jianbao Liu • Zhangwen Wang • Longtao Sun
Received: 10 April 2013 / Accepted: 9 August 2013 / Published online: 22 August 2013 Ó The Author(s) 2013. This article is published with open access at Springerlink.com
Abstract A 1-D unloaded tectonic subsidence (air-loaded tectonic subsidence) model is proposed and applied to the Qiongdongnan Basin. Results show that three episodes of subsidence exist in Cenozoic, that is, syn-rift rapid subsidence (Eocene–Oligocene) with subsidence rate at 20–100 m/m.y., post-rift slow thermal subsidence (early-middle Miocene) around 40 m/m.y., and post-rift accelerated subsidence (since late Miocene) 40–140 m/m.y., which is substantially deviated from the exponentially decayed thermal subsidence model. For exploring the mechanism of post-rift accelerated subsidence, the faulting analyses are conducted and results show that there is a dramatically decrease in the numbers of active faults and fault growth rate since 21 Ma, which indicates that no active brittle crust extension occurred during post-rift period. Furthermore, previous studies have demonstrated that the stretching of the upper crust is far less than that affecting the whole crust. Therefore, we infer that the lower crust thinned during the post-rift period and a new model of basin development and evolution is put forward to explain the post-rift accelerated subsidence and depth-dependent crust thinning in the Qiongdongnan Basin, which is supported by gravity data. Keywords Accelerated subsidence Unloaded tectonic subsidence Lower crust thinning Depthdependent crust thinning Qiongdongnan Basin
Z. Zhao (&) Z. Sun J. Liu Z. Wang L. Sun CAS Key Lab of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China e-mail: [email protected] Z. Wang Z. Sun Zhanjiang Branch of CNOOC Limited, Zhanjiang 524057, China
Introduction Qiongdongnan Basin, located at the western end of the northern continental margin of South China Sea, is mainly composed of a shelf and adjacent deepwater depressions, rich in oil and gas resources (Zhu et al. 2012; Zhang 2010). The Cenozoic tectonic setting of the Qiongdongnan Basin is complicated, and is impacted by the mutli-episodic, multi-directional stretching of South China Block (Ru and Pigott 1986; Zhou et al. 1995), as well as the sinistral and dextral strike-slip of the Red River Fault (Harrison et al. 1992; Sun et al. 2003; Xiang et al. 2004), and multi-phase spreading of the South China Sea (Briais et al. 1993; Li et al. 2011). In Qiongdongnan Basin, an episode of accelerated subsidence exists during the post-rift (Li et al. 1998), which is dramatically deviated from the exponential decayed thermal subsidence in the typical Atlantic continental margin (Steckler and Watts 1978). However, the specific time and location of the accelerated su
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