Syn-Rift Subsidence Deficit of the Qiongdongnan Basin and the Synchronous Rapid Uplift of South Hainan Island: Some Clues on Surface Response to the Hainan Plume, NW South China Sea

Abstract:

Based on tomography, geochemical and petrological evidences, a mantle plume called Hainan Plume has been suggested lying under Hainan Island. Our recent studies on basin subsidence and surface topography indicated that there existed surface vertical motion response to the suggested mantle plume. Post-rift rapid subsidence in the Qiongdongnan basin, which lies south of Hainan Island, is well known and accepted phenomenon. A particular feature is that the rapid subsidence did not just follow the end of syn-rift phase at ~ 21 Ma, but began after a ~10 my relatively tectonic quiescence. Different tectonic processes have been proposed to account for this rapid subsidence event. However, all these proposed mechanisms just focus on the rapid subsidence episode since Late Miocene. Our details subsidence analyses show that its tectonic subsidence is of a two-stage anomalous character: the deficit subsidence at the end of the syn-rift phase and the well known post-rift rapid subsidence after 10.5 Ma. Numerical analyses show that the stretching factor β_s based on syn-rift sequences is much less than the observed crustal stretching factor β_c, and the present basement depth can be almost predictable if assuming that the lithosphere was thinned with stretching factor β_c; The crustal thinning happened during the syn-rift phase (before 21 Ma), and a new rifting and lower crustal flow after 21 Ma were not supported, proposing further that the syn-rift subsidence is deficit. Our further analyses proposed that the syn-rift subsidence deficit might be caused by the dynamic support of the secondary plume, and the post-rift rapid subsidence might result from the losing support of the thermal source decaying, therefore occurred to compensate the subsidence deficit of the syn-rift phase. The cooling history of the southern Hainan deduced by low temperature chronometry data, also indicated there exist an uplift event in the Late Oligocene (Shi et al., 2011). All the samples show that there is a Late Oligocene rapid uplift and cooling episode in the Southern Hainan. It is hard to explain the rapid cooling (uplift) episode in the eastern rim of the Hainan since there are no big faults close to the eastern Hainan. The deficit subsidence and uplift event might be the surface response to the deep thermal anomaly.