Gravity Inversion and Thermal Modeling for the Crust‐Mantle Structure of the Southwest Subbasin in the South China Sea

Abstract

The crust‐mantle structure and magmatic activity are one of the key problems in the study on formation and evolution of the southwest subbasin in the South China Sea. According to gravity data from the NH973‐1 profile, we construct a density model of crust‐mantle structure with the gravity inversion method. And then, we make modeling of the temperature structure and thermal evolution in the basin after sea‐floor spreading. Gravity inversion indicates that there is a deeper sunken sedimentation area and a depressed Moho surface region under the southwest subbasin center. Thermal modeling shows that temperature variation is not consistency degradation between crust and mantle, but in a way of crust warming with mantle cooling. After the termination of floor spreading, the crust warming lasted for about 7 Ma, followed by slow cooling, also with the mantle. Then, crust thermal evolution mode was changed into a new way that its temperature declined as same as mantle. In this way, crust and mantle entered the stage of thermal subsiding synchronously. Along profile NH973‐1, the characteristics of crustal warming are different among the south, middle and north parts. At the depth of 7~9 km under the place, about 70 km to the north of the oceanic basin spreading center, the rate of temperature increase sustained above 200°C between 9.6 Ma and 4.6 Ma. This appending temperature would drive up the local temperature value to 1100°C which was of advantage to crust magmatic activity in the end time of seabed spreading, supported by the P‐T diagram. We infer that magmatic partial melting would be likely to happen at this time if there was water‐bearing and decompressing conditions in crust produced by local faults.

Focused Mantle Upwelling Beneath the Southeastern Asian Basalt Province Revealed by Seismic Surface Wave Tomography

“…The Vs model at a shallow 30 km depth (Figure 2a) correlates well with known tectonic units and is consistent with previous regional Vs tomographic findings. For example, significant low velocities are imaged beneath the southeastern margin of the Tibetan Plateau where a relatively thick crust is present, whereas high velocities are observed across the SCS and Celebes Sea that could be explained by their relatively thin crust (e.g., Chen et al., 2021; Tang & Zheng, 2013; Zhao et al., 2019). In particular, the Celebes Sea is imaged with a more prominent high velocity in our model compared to the Vs model of Huang and Xu (2011) (Figure S12 in Supporting Information ), which indicates that our Vs model achieves higher imaging resolution, especially in resolving relatively small‐scale anomalies thanks to the dense regional ray‐path coverage (Figure S5 in Supporting Information ).…”

Moho Inversion by Gravity Anomalies in the South China Sea: Updates and Improved Iteration of the Parker‐Oldenburg Algorithm

“…Owing to the lack of accurate density information, we must choose a safer and more widely recognized indicator to estimate the lateral variation in the SCS Moho density contrast. With contemporaneous stratigraphic densities varying considerably from region to region, obvious lateral density zoning features occur in the SCS (J. Chen et al., 2012). As shown in Figure 9, the study area can be roughly divided into three crustal type’s crusts according to the submarine topography in geology: oceanic crusts, continental‐oceanic transitional crusts, and thinning continental crusts.…”

The Moho Depth of the South China Sea Basin From Three‐dimensional Gravity Inversion With Constraint Points and Its Characteristics

“…The East Basin and Southwest Basin exhibit an obvious NE-trending residual spreading center along the Changlong Seamount, Zhongnan Seamount, Zhenbei Seamount and Huangyan Island that extends to 112 • E in the southwest; the Moho depth is greater than 12 km and the crustal thickness is over 6 km. The thickened crust along the spreading ridge is possibly the result of thermal subsidence (Zhang and Li, 2011) and it stretches continuously, indicating the two basins stopped at the same time. The Zhongnan Seamount, Zhenbei Seamount and Huangyan Island at the residual spreading center show that crust thickened significantly and the Moho depth is above 14 km (>11 km in crustal thickness), which is consistent with the 3D crustal structure from OBS (Wang et al, 2016).…”