In this paper, we use a robust inversion algorithm, which we have tested in many regional studies, to obtain the first global model of Curie-point depth (GCDM) from magnetic anomaly inversion based on fractal magnetization. Statistically, the oceanic Curie depth mean is smaller than the continental one, but continental Curie depths are almost bimodal, showing shallow Curie points in some old cratons. Oceanic Curie depths show modifications by hydrothermal circulations in young oceanic lithosphere and thermal perturbations in old oceanic lithosphere. Oceanic Curie depths also show strong dependence on the spreading rate along active spreading centers. Curie depths and heat flow are correlated, following optimal theoretical curves of average thermal conductivities K = ~2.0 W(m°C)⁻¹ for the ocean and K = ~2.5 W(m°C)⁻¹ for the continent. The calculated heat flow from Curie depths and large-interval gridding of measured heat flow all indicate that the global heat flow average is about 70.0 mW/m², leading to a global heat loss ranging from ~34.6 to 36.6 TW.

In 2016, we carried out a coincidental multichannel reflection seismic and wide‐angle reflection/refraction seismic experiment along a 320‐km‐long profile across the northeastern margin of the South China Sea. Based on these new data, we studied velocity structure of the transitional continental crust using both forward modeling and travel‐time tomography. The velocity model shows that Mesozoic sediments have velocities of 4.3–5.3 km/s and are about 3–6 km thick in the magnetically quiet Chaoshan Depression and the attenuated transitional continental crust. Using forward magnetic modeling, we confirm that the presence of this thick Mesozoic layer with low magnetic susceptibility induces the magnetic quiet zone and show that the lower crust high‐velocity materials have limited effect on magnetic anomalies. This Mesozoic layer pinches out seaward near the continent‐ocean boundary (COB). Two apparently isolated high‐velocity anomalies are revealed in the lower crust of the continental slope. The first has velocities of 7.0–7.5 km/s and a maximum thickness of 8 km, and the second has velocities of 7.0–7.3 km/s and a maximum thickness of 3 km. We find that the discontinuity of the high‐velocity zones is not due to the lack of some seismic phases or lose of a seismometer. We suggest that postspreading magmatism caused these high‐velocity zones. A third high‐velocity zone, showing a distinctly large velocity gradient, is revealed near the COB. This indicates upper mantle upwelling and/or exhumation right at the COB, despite recent unsuccess in directly sampling these deep materials in the South China Sea.

The post-spreading magmatic activities in the northeastern South China Sea (SCS) margin are very strong, evidenced by widely distributed high-velocity lower crust (HVLC) and numerous volcanoes. However, there are large contrasts in magmatic activities and crustal structure between the Southern Depression (TSD) of the Tainan Basin and the volcanic continental slope area further south. We analyse their crustal P-wave velocity structures based on a newly acquired wide-angle ocean bottom seismic data set. The Cenozoic strata below the TSD, a Cenozoic failed rift, are relatively thick (∼3–4.5 km) with velocities from 1.6 to 3.6–3.9 km s^–1, whereas the Mesozoic strata are relatively thin (∼1–2.5 km) with velocities from 4.3 to 4.6–5.2 km s^–1. In the TSD, magmatic activities are relatively weak and the crust is severely thinned (∼4 km). The crust is 9–15 km thick below the volcanic continental slope area, which shows extensive volcanism. We identified HVLC below the failed rift of the TSD (Zone 1) and attributed it to mantle serpentinization, whereas the imaged HVLC below the volcanic continental slope (Zone 3) and HVLC adjacent to the failed rift of the TSD (Zone 2) are due to post-spreading magmatic underplating/intrusions. At the model distance ∼90 km, lateral transition from magmatic underplating/intrusions to mantle serpentinization occurred abruptly. We concur that post-spreading cooling and thermal contraction in the nearby SCS oceanic lithosphere can trigger decompressive melting and deformation in the thinned continental slope zone. Our study shows that, in addition to mantle serpentinization in the continent–ocean transition (COT) zone, mantle can also be serpentinized below the rift during early-stage rifting. Weak syn-rifting magmatism and mantle serpentinization below the failed rift support that the northeastern SCS has a magma-poor margin.