Teleseismic imaging of the mantle beneath southernmost China: New insights into the Hainan plume Shaohong Xia a,b, ⁎, Dapeng Zhao b , Jinlong Sun a , Haibo Huang a a CAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China b Department of Geophysics, Tohoku University, Sendai 980-8578, Japan abstract article info Article history: Received 26 September 2015 Received in revised form 24 May 2016 Accepted 24 May 2016 Available online 1 June 2016 Handling Editor: A.R.A. Aitken Intraplate volcanism during the Late Cenozoic in the Leiqiong area of southernmost China, with basaltic lava flows covering a total of more than 7000 km 2 , has been attributed to an underlying Hainan plume. To clarify de- tailed features of the Hainan plume, such as the morphology of its magmatic conduits, the depth of its magmatic pool in the upper mantle and the pattern of mantle upwelling, we determined tomographic images of the mantle down to a depth of 1100 km beneath southern China using 18,503 high-quality arrival-time data of 392 distant earthquakes recorded by a dense seismic array. Our results show a mushroom-like continuous low-velocity anomaly characterized by a columnar tail with a diameter of 200–300 km extending down to the lower mantle beneath north of the Hainan hotspot and a head spreading laterally in and around the mantle transition zone, in- dicating a magmatic pool in the upper mantle. Further upward, the plume head is decomposed into smaller patches, and when reaching the base of the lithosphere, a pancake-like anomaly has formed to feed the Hainan hotspot. This result challenges the classical model of a fixed thermal plume that rises vertically to the surface. Hence we propose a new layering-style model for the magmatic upwelling of the Hainan plume. Our results in- dicate spatial complexities and variations of mantle plumes probably due to heterogeneous compositions and thermochemical structures of the deep mantle. © 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: Hainan plume Teleseismic tomography Intraplate volcanism Thermochemical mantle Southern China 1. Introduction The Late Cenozoic intraplate basaltic magmatism, as a prominent feature in Southeast Asia, occurred extensively and voluminously in the Leiqiong area, the Indochina peninsula and the South China Sea basin after the cessation of seafloor spreading of the South China Sea. These basaltic provinces, dominated by tholeiites and alkali basalts (Wang et al., 2012), share the same isotopic and geochemical character- istics (Wang et al., 2013; Yan et al., 2014) and display light rare-earth- element enriched patterns and typical oceanic island basalt (OIB)-type incompatible element distributions (Tu et al., 1991; Zou and Fan, 2010). Field investigations and drilling have shown that at least nine volca- nic eruption episodes had taken place in the Leiqiong area during the Miocene and the Holocene, with a peak age of late Pliocene to middle Pleistocene (Flower et al., 1992). The basaltic plateau in the Hainan Is- land has elevations up to N 100 m, with a maximum thickness of about 1000 m, and the basaltic lava flows cover a total of more than 7000 km 2 in the Leiqiong area (Fig. 1). The time-averaged supply of magma was estimated to be 0.1–0.25 km 3 yr -1 in the Leiqiong area (Flower et al., 1992), which is significantly greater than that of the typ- ical OIB shields (e.g., Swanson, 1972) and close to the estimates for the major flood basalt episodes (Cox, 1980). One popular view is that intraplate volcanism is driven by a hot deep mantle plume (Morgan, 1971). Regional and global tomographic studies have revealed a continuous low-velocity (low-V) anomaly reflecting magmatic upwelling from the lower mantle beneath Leiqiong and its adjacent area, indicating the existence of the Hainan plume (e.g., Lebedev and Nolet, 2003; Zhao, 2004, 2007; Montelli et al., 2004, 2006; Huang and Zhao, 2006). Local seismic tomography has clearly im- aged the Hainan plume in the upper mantle (Lei et al., 2009; Huang, 2014). Additional evidence for the Hainan plume includes extensive synchronous OIB-type basalts (e.g., Flower et al., 1992; Zou and Fan, 2010), a thin mantle transition zone (Wang and Huang, 2012), a high mantle potential temperature (e.g., Hoang and Flower, 1998; Wang et al., 2012), and geochemical signatures of the basalts reflecting a lower-mantle plume origin (e.g., Zou and Fan, 2010; Wang et al., 2013). Recent studies on mantle plumes, however, have uncovered that the simple, vertical upwelling of the classical plume model is contradictory with geophysical observations (e.g., Wolfe et al., 2009; Villagomez et al., 2014), and the morphology of magmatic conduits and the pattern of plume upwelling are highly dependent on the mantle temperature, Gondwana Research 36 (2016) 46–56 ⁎ Corresponding author at: CAS Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China. Tel.: +86 20 8902 3155. E-mail address: shxia@scsio.ac.cn (S. Xia). http://dx.doi.org/10.1016/j.gr.2016.05.003 1342-937X/© 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr