Cooling of the lithosphere beneath the Nansha Block, South China Sea

Abstract

Throughout the Late Cretaceous and Cenozoic, the Nansha Block experienced stretching and extension and moved southwards away from the South China continent. As an exotic micro-continent, the Nansha Block has markedly different lithospheric rheological properties from its surrounding regions. The temperature represents one of the key parameters controlling the dynamics and rheological behaviors of the lithosphere and mantle. Therefore, we calculated the 3D temperature field of the crust and upper mantle beneath the Nansha Block using two methods, namely, solving the heat conduction equation and inverting the S-wave velocity structure. The results show that the ratio of the crustal heat flow to the seafloor heat flow is greater than 50% in the central Nansha Block. The crustal temperatures in the Nansha Block are low. The temperature at the Moho discontinuity is approximately equal to the Curie temperature (523 °C). The temperatures of the shallow mantle (50 km) beneath the Nansha Block (800 °C) are lower than those of the surrounding areas. However, the cool mantle below the Nansha Block gradually transitions into a warm mantle with increasing depth. At a depth of 150 km, the mantle temperatures beneath the Nansha Block (1600 °C) are higher than those of the surrounding areas. The bottom boundary of the “thermal” lithosphere beneath the Nansha Block is at approximately 100 km, which is deeper than that in the surrounding areas. This result is different from the bottom boundary of the “seismic” lithosphere at a depth of 80 km under the Nansha Block, which is similar to that in the surrounding areas. The low lithospheric temperatures beneath the Nansha Block imply that the lithosphere of the Nansha Block experienced a rapid cooling process.