Non‐uniform hyper‐extension in advance of seafloor spreading on the vietnam continental margin and the SW South China Sea

Abstract

The SW South China Sea preserves a propagating oceanic spreading centre and associated continent-ocean transition (COT) that characterizes the continental margin offshore SE Vietnam. We investigated the nature of strain accommodation in the region immediately in front of the propagating rift using a combination of 1-and 2-D backstripping subsidence reconstructions, coupled with forward modelling based on the measured upper crustal extensional faulting applied to a flexural cantilever model. Major normal faulting ceases after an inversion event at ca. 16 Ma, although moderate extension was also noted at 22-23 Ma, representing the end of an extensional phase that initiated around 28 Ma. 1-D subsidence models indicate rapid 'syn-rift' subsidence, possibly lasting until 10 Ma, despite the lack of observed extensional faulting. We show that depth-dependent extension is required to explain the great depth of the basins despite the modest observed upper crustal faulting. Brittle faulting could not have extended much deeper than 10 km, suggestive of weak crust in the presence of high heatflow. The regional topographic slope on the basement suggests very low mid-crustal viscosities of 10 19 -10 20 Pa.s., consistent with the idea that flow in the ductile mid and lower crust was responsible for much of the subsidence prior to, and possibly after, seafloor spreading, which extended ca. 300 km from the tip of the mid ocean ridge. Flow is inferred to be dominant towards the spreading centre prior to 16 Ma. Extension in the COT postdates seafloor spreading and further supports the idea of this crust being very weak, albeit with more coherent, less extended crustal fragments, now forming banks offshore the Sunda continental shelf and surrounded by hyperextended crust of the COT.

Rifted continental margins: The case for depth-dependent extension

“…For these reasons we believe that some Type II margins will be shown to contain core complexes, like those in extended backarc regions, e.g. the Aegean Sea (Lister et al, 1984;Jolivet et al, 2010) and possibly in the South China Sea (Savva et al, 2013;Li et al, 2014).…”

Seismic structure of the northwestern margin of the South China Sea: implication for asymmetric continental extension

“…Wu et al (2005) considered the NE margin as the upper-plate in the simple-shear rifting model (Lister et al 1986), and suggested that the HVL originated from asthenospheric underplating. As analog and numerical models show (Huismans et al 2014), continental extension is in general depth-related and strongly depends on interstratified decoupling, which is also proven by detailed geophysical data of the SCS Li et al 2014b). To complicate matters, deep structures detected along several margins are also dependent on initial settings of rifting, such Figure 1.…”

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

“…Seismic and deep borehole data acquired along and near the fossil spreading axis in the NE, Central and SW sub-basins of East Vietnam Sea revealed basalt and pyroclastic deposits interbedded with early Miocene (>16 Ma) sediments (Li et al, 2014(Li et al, , 2015. However, the rate and extent of the synspreading volcanism were modest, corresponding to slower to intermediate spreading rate (20 km to 35 km/Myr); although the spreading rate sometimes reached 70-80 km/Myr (Li et al, 2014;IODP 349 Scientific report, 2014).…”

“…Plots of MgO (wt.%) against major oxides of Phu Quy basalts; except for SiO 2 which shows two broadly negative correlation trends with MgO for early and later eruptive lavas, the other oxides such as K 2 O, TiO 2 and P 2 O 5 define higher and lower fields, respectively, for late (circle) and early (diamond) lavas. Plotted for comparison are synopening East Vietnam Sea (cross, after Li et al, 2014) showing even lower in MgO, TiO 2 and K 2 O than Phu Quy basalts…”

“…Figure 8. Negative and positive correlation between SiO 2 and, respectively, Ba (a) and Ba/Nb (b) for early (diamond) and late (circle) Phu Quy basalts; also plotted are syn-opening East Vietnam Sea (after Li et al, 2014). Higher Ba accompanying lower SiO 2 and lower Ba/Nb (e.g.…”