T. K. Wang

Rifting and magmatism in the northeastern South China Sea from wide‐angle tomography and seismic reflection imaging

We present a new travel time tomography velocity model and seismic reflection images that delineate the rift architecture and magmatic features of the rifted margin in the northeastern South China Sea. These data reveal moderately stretched crust ~25 km thick along the continental shelf and thin but laterally variable crustal thickness in the distal margin. Along the continental slope, crust rapidly thins to ~4 km in a basin characterized by tilted fault blocks that sole into a low-angle detachment. Strain was localized to a degree within the highly stretched basin but failed to progress to breakup and seafloor spreading. Crust in the distal margin is ~12–15 km thick. Few extensional structures are apparent in the distal margin, but seismic velocities are suggestive of highly thinned and magmatically intruded continental crust. The magmatic features we interpret include volcanic zones at the top of the basement that deform or disrupt overlying postrift strata, sills intruded into the postrift sedimentary section, and a high-velocity (~6.9–7.5 km/s) lower crustal layer that we take to be magmatic underplating or pervasive lower crustal intrusions. These features primarily occur in the distal margin and may have been emplaced during postrift seafloor spreading. The postrift magmatism may have been induced by convective removal of continental lithosphere following breakup and the onset of seafloor spreading in the South China Sea.

Velocity Models and Pre-stack Depth Migration of Gas Hydrates on Taiwan Accretionary Prism

Reflection images of the accretionary prism along seismic profiles collected from R/V Maurice Ewing in 1995 and from R/V Marcus G. Langseth in 2009 off SW Taiwan are presented for charactering gas hydrates in this study. Pre-stack depth migrations of the seismic profiles indicate the thickness of sediments about 2-3 km and the depth of the bottom simulated reflector (BSR) about 300 m below the sea floor. Layers of gas hydrate and free gas are about 100 m thick and 80 m thick, respectively. Horizontal velocity analysis of the seismic profiles show a gas-hydrate layer above BSR with a P-wave velocity of about 2100 m/s and a free-gas layer below BSR with a P-wave velocity of about 1800 m/s. Most BSRs are non-continuous in the accretionary prism that may result from the gas dissipation from thrust faults, fractures and severe deformation due to subduction. Gas migrated from thrust faults and accumulated beneath the lower slope of the accretionary prism may result in more clear and continuous BSRs in the lower slope than those in the upper slope of the accretionary prism. The difference of BSRs may be associated with the organic sediments came from the river of Taiwan.

Imaging of Gas Hydrates in the Northernmost South China Sea from Multi-component OBS Data

More than 100 Ocean-bottom seismometers (OBS), with a relatively dense spacing of about 500-1000 m, were deployed and recovered for exploring gas hydrates in the northernmost South China Sea. P-wave velocity models have been determined from travel-time inversion of hydrophone and vertical components of OBS data whereas the Poisson’s ratio models are determined from shear waves, converted at the sea floor and the bottom-simulating reflector (BSR), selected from two horizontal components of OBS data. A layer-stripping and Monte-Carlo inversion of the blocky model of the Poisson’s ratio is applied. Furthermore, based on P-wave velocity, Poisson’s ratio and rock physics of the three-phase effective medium, saturation of gas hydrates is estimated. The results show that hydrates are imaged by a relatively low Poisson’s ratio (0.44-0.46) below anticlines and free gas is characterized by a relatively high Poisson’s ratio (0.485-0.5) beneath most of the BSR. We also observe that saturation of hydrates (30%) in the passive continental margin of the South China Sea is greater than that (15-25%) in the active accretionary wedge off SW Taiwan.

Origin of Gas Hydrates Imaged from the Acoustic Basement in the Northernmost South China Sea by Using OBS

During the survey of TAiwan Integrated GEodynamics Research (TAIGER) in 2009, we recovered 36 ocean-bottom seismometers (OBS) along 3 multi-channel seismic (MCS) profiles for investigating the origin of gas hydrates in the northernmost South China Sea off SW Taiwan. In the accretionary prism of the Manila subduction zone, we observed gas hydrates accumulated in the closed areas where were formed by folding imaged from a strong variation of the lateral velocity (3-4.5 km/s) in the acoustic basement. In the continental slope, gas was migrated from the rifting basement to the anticline. The rifting basement with a relatively low velocity of 3-4.5 km/s in the extended continent of the northernmost South China Sea resulted from magma intrusion along normal faults since the paleo-seafloor spreading. We suggest that gas generated in the deeper structures may be migrated along faults to accumulate hydrates in the sedimentary layers.