关键词: 博罗—大亚湾断陷, 地热系统, 新生代, 地球物理

Abstract:

The Boluo-Dayawan fault depression (BDFD) is located in the Pearl River Mouth Basin in southwestern Guangdong Province, bordering in the north by Yangtze block and the west by Youjiang block of the Eurasian plate. At the beginning of the Cenozoic, intense tectonic activities, such as frequent magmatic intrusions, volcanic eruptions, strong earthquakes, and other neotectonics, occurred continuously in the studying area, owing to the present-day configuration of interactions between oceanic-continental systems and westwards directed oblique subduction of the Pacific plate beneath the Eurasia plate. The BDFD is in the active tectonic coast belt of southeastern China, one of the Pacific Fire Ring’s (PFR) most spectacular and productive areas of large scale of magmatic, young volcanic and geothermal manifestation. In particular, the depression hosts an exceptionally vigorous active geothermal province which coupled with its high surface heat flow and temperature gradient, is as big as all southern China continent. Structures of the geothermal anomalies have been characterized by the NW-trending normal faults surrounded by the BDFD due to extension; inside the structures developed thick continental clastic deposits, large area of outcropped granite intrusions with rich thermogenic elements, and numerous high temperature hot springs exposed along the NW-trending linear BDFD, inductive to a huge geothermal system. Abundant hot-water-dominated geothermal resources stored in shallow sedimentary layers have been exploited and utilized by previous hydrogeological and geothermal survey. However, deeper HDR geothermal energy exploration is limited by current technology and economic conditions. From the experience of geothermal energy exploration around the world, the relationship between geological condition and temperature distribution is crucial in HDR exploration. Here, we studied the depths of a variety of Moho’s surface and Curie point isotherm and the distribution of the Bouguer gravity anomalies patterns under the BDFD, by a comprehensive multidisciplinary (gravity, magnetic, electric, seismic characteristics, etc) analysis of geophysical data. Aa a result, we derived preliminarily the crustal geological structure and temperature conditions. Considering both HDR geothermal energy generation principles and basic regional tectonic background, we infer that the heat source for HDR is derived from mantle diapirism which caused the crust-mantle material non-uniform flow after producing partial melting. The melting position occurs at the bottom of continental crust region with huge upward heat transfer to the shallow layers. The ascending hot material and huge vertical stress leads to the Cenozoic new deep faults and subsidence of shallow terrain. Eventually, massive heat transfers to upper crustal rock mass through high thermal conductivity stratigraphy units and accumulates to form HDR thermal reservoir. The fracturing neotectonics superimpose on the previous Yanshanian tectonic system and reactivate the whole fault structures, facilitating heat exchange for the migrating shallow cold water. The tectonic mechanism and role of linear thermal uplift extension as shown in the conceptual model are in agreement with knowledge from other HDR geothermal studies.

Key words: Boluo-Dayawan fault depression (BDFD), geothermal system, Cenozoic, geophysics