A fission-track and (U–Th)/He thermochronometric study of the northern margin of the South China Sea: An example of a complex passive margin Yan Yi a, ⁎, Andy Carter b , Bin Xia a , Lin Ge a , Stephanie Brichau b , Hu Xiaoqiong a a Key Laboratory of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, PR China b School of Earth Sciences, Birkbeck College, Malet Street, London WC1E 7HX, UK abstract article info Article history: Received 8 December 2008 Received in revised form 23 April 2009 Accepted 28 April 2009 Available online 5 May 2009 Keywords: South China Sea Passive margin Fission track Thermochronology Extension Zircon fission track (ZFT), apatite fission track (AFT) and (U–Th)/He thermochronometric data are used to reconstruct the Cenozoic exhumation history of the South China continental margin. A south to north sample transect from coast to continental interior yielded ZFT ages between 116.6±4.7 Ma and 87.3±4.0, indicating that by the Late Cretaceous samples were at depths of 5–6 km in the upper crust. Apatite FT ages range between 60.9 ±3.6 and 37.3±2.3 Ma with mean track lengths between 13.26±0.16 μm and 13.95±0.19 μm whilst AHe ages are marginally younger 47.5±1.9–15.3 ± 0.5 Ma. These results show the sampled rocks resided in the top 1–1.5 km of the crust for most of the Cenozoic. Thermal history modeling of the combined FT and (U–Th)/He datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin. 1) Initial phase of rapid cooling that youngs to the north, 2) a period of relative (but not perfect) thermal stasis at ~70–60 °C which increases in duration from the south to the north; 3) final-stage cooling to surface temperatures that initiated in all samples between 15 and 10 Ma. The timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin. The history of South China margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the South China Sea. This rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the South China block and site of active rifting to the south. Consequently whilst the South China margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The continental margin of South China (Fig. 1) has a long and complex history that extends back to the Mesozoic when it formed part of an active “Andean type” margin associated with north directed subduction of the Paleo-Pacific Plate (Lapierre et al., 1997; Sewell and Campbell, 1997; Zhou and Li, 2000; Li and Li, 2007). In the Paleocene the margin experienced significant extension, and a number of mechanisms have been proposed to explain this including rollback of the subduction zone (Ren et al., 2002), southward subduction of a proto South China Sea along a north Borneo trench (Hall, 2002) and extrusion of Indochina to the southeast relative to a stationary China block (Briais et al., 1993; Replumaz and Tapponnier, 2003) although the latter can now be discounted on the basis of new seismic reflection profile evidence from across the boundary between Sundaland and the southern-rifted margin (Clift et al., 2008a,b). Whilst the mechan- isms for extension continues to be debated it is known that rifting and then opening of the South China Sea established a passive margin along the southern edge of the South China block in the Eocene (Lapierre et al., 1997; Zhou and Li, 2000). The age of seafloor spreading in the South China Sea along a WSW–ENE axis is generally considered to date to Chron 11, ~ 30 Ma (Taylor and Hayes, 1980; Briais et al., 1993; Barckhausen and Roeser, 2004) but this mainly applies to opening in the central area. Further to the NE, towards Taiwan, more recent studies point to initiation at Chron 16 at ~37 Ma (Hsu et al., 2004). Seismic tomographic studies of the crust and lithosphere of the South China margin are inconclusive as to whether there is magmatic underplating and/or abnormally hot mantle upwelling (Nissen et al., 1995; Yan et al., 2001; Hayes and Nissen, 2005; Xu et al., 2008) and thus whether the margin is a volcanic or non-volcanic passive margin. Tomography of the South China crust shows the continental margin of the South China Sea has distinct NE–SW trending regions of high velocity material in the lower crust (Xu et al., 2008) which Nissen et al. (1995) interpreted as reflecting pre-rift events that included intrusion of mantle material into the lower crust. On the basis of modeling pre-seafloor spreading extension coupled with ODP 184 heat flow measurements in the South China Sea Clift et al. (2001) Tectonophysics 474 (2009) 584–594 ⁎ Corresponding author. Tel.: +86 20 85290212. E-mail addresses: yanyi@gig.ac.cn, yycarl@sina.com (Y. Yi). 0040-1951/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2009.04.030 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto