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Paleo-Heat Flow Evolution of the Baiyun Sag, the Pearl River Mouth Basin

Abstract

The Baiyun Sag (BYS) is located in the continental margin of the northern South China Sea (SCS) , has been one of the key areas to characterize geothermal field and thermal evolution of passive continental margin. The geological evolution of the BYS was driven by the plate tectonic interactions between the Philippine Sea plate, the Eurasian plate and the Indo-Australian plate , and was influenced by seafloor spreading of the South China Sea (SCS) in the late Early Oligocene. Heat flow measurements show that the BYS is characterized by a high background heat flow, ranging from 72 mW/m2 to 94 mW/m2. The present-day heat flow of the northern South China Sea increases from the northern shelf with thick crust to the southern slope with thinned crust.

This study employs forward and inverse modelling to simulate the rift and post-rift processes exampled by three wells covering shelf to slope. Two new thermal evolution models of the BYS are established for continental shelf and continental slope. The new heat flow model of continental shelf is generally in agreement with the model of McKenzie. While heat flow of the new continental slope model continuously increases rather than decreases in previous models, which is resulted from lithospheric thinning and mantle upwelling during the Neogene passive continental margin stage. Heat flow gradually reduced after the cessation of the sea floor spreading (10 Ma).

Igneous rocks are generally formed after the late Miocene in the north margin of the SCS, which covered dozens of kilometers (Yan et al., 2006). The intrusion of magma into the brittle crust and mantle drastically heated the lithosphere, and led the BYS to deform strongly through lithosphere thinning. The local multi-staged magmatic activities mainly contribute to the high level of maturity in the Liwan Sag located in the lower slope.

In addition, thermal state and thermo-rheological structure of the crust and mantle during the period of rifting and drifting are key factors controlling the thermal evolution of the BYS. The the Panyu low uplift on the shelf is born from a normal lithosphere. For areas located in the continental slope, the lithosphere of BYS has thinned stepwisely and became hotter during the passive continental margin stage.