Hyperextended crustal structure of the Qiongd

image: The pink blocks show the distribution of late Cenozoic magmatism in the QDNB and its adjacent areas
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Credit: ©Science China Press

This study is led by Dr. Shaohong Xia (South China Sea Institute of Oceanology, Chinese Academy of Sciences). The Qiongdongnan Basin (QDNB), a typical rifted basin in the northwestern South China Sea (SCS), is located along the elongating direction of the oceanic ridge of the SCS Northwest Subbasin. It has become a natural laboratory for studying geodynamic processes such as lithospheric extension and thinning, magmatism, and basin evolution. Both global and regional seismic tomography results show a low-velocity conduit originating in the lower mantle and extending to the shallow lithosphere in the northern SCS; This conduit is called the Hainan mantle plume. Considering the QDNB is surrounded by these magmatic zones, the study team mainly focuses on two research issues. (1) What are the characteristics of the rifted crustal structure in the middle of the QDNB? (2) How has the Hainan mantle plume affected the crustal structure of the QDNB?

Shaohong Xia, together with his group, conducted an onshore-offshore seismic experiment across the QDNB. They adopted a combination of onshore seismic station and ocean bottom seismometer (OBS) to receive artificial seismic source signals. Four portable seismometers were on the seafloor with a spacing of approximately 12 km. They obtained the crustal structures of the QDNB.

The results show that the crustal stretching factors of the Northern and Southern Rises of the QDNB are estimated to be 1.3–2.0, indicating slight or moderate crustal thinning. Whereas, the crustal stretching factor in the Central Depression of the QDNB is estimated to be more than 3.0, which means that the crust has been hyperextended. Controlled by two detachment faults (F2 and F11), the upper and lower crust show brittle and ductile thinning, respectively. Three anomalous low-velocity conduits penetrating the crystalline crust are developed in the Songnan Uplift and the Southern Rise of the QDNB. Their P-wave velocities are 5.5–6.0 km/s, which are significantly lower than those of the surrounding crust (6.0–6.8 km/s). The Late Cenozoic magmatic intrusions (eg, sills and dikes) and hydrothermal activities can be observed in the sedimentary strata above these low-velocity conduits.

They interpret that the low-velocity conduits might be the crustal magmatic footprints of the Hainan mantle plume, to the correlated distributions of OIB-type magmatism, the influence range according to the Hainan mantle plume, and the high heat flow of 95 MW/m2.

So, the crustal structure of the QDNB is a product of tectonic interactions by crustal hyperextension and subsequent Hainan mantle plume magmatism. This study provides a good basis for understanding the impact of a mantle plume on a pre-existing rifted margin.

See the article:

Xia S, Fan C, Wang D, Cao J, Zhao F. 2022. Hyperextended crustal structure of the Qiongdongnan Basin and subsequent magmatic influence from the Hainan mantle plume. Science China Earth Sciences65(5): 845-862, https://doi.org/10.1007/s11430-021-9894-7

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