Based on the stacking velocity spectrum data of 14 near‐vertical deep reflection profiles in the deep waters of Zhujiangkou–Qiongdongnan basins along the northern continental margin of South China Sea, we use the Dix formula to transform the stacking velocities into corresponding crust interval velocities (in TWT domain) and construct the crustal velocity structure in depth domain by time‐depth transformation. Integrating all the velocity profiles we analyze the spatial variation of P wave velocity at different depths and the stratified geometry of crust in the Zhujiangkou and Qiongdongnan basins in the continental margin of northern South China Sea. The result indicates that the crust of Qiongdongnan basin can be divided into a 4~8 km thick sediment layer (Vp is 1.7~4.7 km/s), a 4~10 km thick upper crust layer (Vp is 5.2~6.3 km/s), a lower‐crust layer about 5 km thick (Vp is 6.4~7.0 km/s), and a high‐velocity lower‐crust bottom layer about 2~6 km thick (Vp > 7.0 km/s). The existence of the high‐velocity lower crust layer can be considered as the seismological signature of underplating at the lower crust bottom induced by lithosphere extension in South China and its continental margin, or the residual mafic layer of the original Cathaysia lower crust. Synthesizing the available geophysical sounding results we construct the images of Moho and the lithosphere bottom across the continent of South China and the continental slope of South China Sea, which reveals the lithosphere thinning in continental South China and the northern continental margin of South China Sea.
Compared with continents, there are less studies on lithospheric thermal structure in oceans or continent-ocean transition zones. Based on submarine heat flow data and related petrological thermal properties available in the Qiongdongnan Basin, we calculate the constitution of heat flow and deep temperature of different layers along four distinct seismic profiles. The crustal structure in this area is divided into four layers based on latest analysis of P-wave velocity variation, that is sediment cover. Upper crust, normal lower crust and anomalous lower crust with high velocity, respectively. The results demonstrate that mantle heat flow increases from shallow water to deep water, which predominantly results in the current distribution of submarine heat flow in the Qiongdongnan Basin. Besides, its ratio to submarine heat flow is 76.3±7.0% on average, suggesting a typical feature of the lithospheric thermal structure model consisting of "cold crust and hot mantle". In addition, Moho temperature beneath the Qiongdongnan Basin concentrates in the range 500∼700 • C with a lower temperature region and two higher temperature regions, which is primarily ascribed to the extent of the lithospheric thinning of the northern margin and well-developed faults of the South China Sea.
In this paper we deduce the analytic solutions of the fi rst-and second-order vertical derivative zero points for gravity anomalies in simple regular models with single, double, and multiple edges and analyze their spatial variation. For another simple regular models where it is diffi cult to obtain the analytic expression of the zero point, we try to use the profi le zero points to analyze the spatial variation. The test results show that the spatial variation laws of both first-and second-order vertical derivative zero points are almost the same but the second-order derivative zero point position is closer to the top surface edge of the geological bodies than the fi rst-order vertical derivative and has a relatively high resolution. Moreover, with an increase in buried depth, for a single boundary model, the vertical derivative zero point location tends to move from the top surface edge to the outside of the buried body but fi nally converges to a fi xed value. For a double boundary model, the vertical derivative zero point location tends to migrate from the top surface edge to the outside of the buried body. For multiple boundary models, the vertical derivative zero point location converges from the top surface edge to the outside of the buried body where some zero points coincide and finally vanish. Finally, the effectiveness and reliability of the proposed method is verified using real fi eld data.
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