华南陆缘高热流区的壳幔温度结构与动力学背景

华南陆缘是我国重要的矿产、地热资源区.晚中生代以来，在太平洋板块西向俯冲，地幔热对流活动共同作用下，该区出现多期岩浆-热事件和大规模爆发式成矿作用.在前人研究基础上，本文利用地表热流观测资料、地震剪切波资料、重力位球谐系数，计算了壳-幔温度结构，分析了动力学背景.计算结果表明：华南陆缘东南沿海地带，地壳10 km以浅温度达200℃以上，居里点温度475℃，莫霍面平均温度550℃.地壳浅层较热，花岗岩中放射性元素衰变放热是地壳浅层地下水热活动的重要热源，但地壳总体温度不高，为"冷壳热幔"型热结构.地幔中，90 km深度，温度950~1250℃；120 km深度，温度1050~1400℃；150 km深度，温度1200~1450℃；220 km深度，温度1500~1700℃."热"岩石圈底界深度在110~150 km之间，西深东浅.岩石圈内，地幔应力场为挤压-伸展相间格局；岩石圈之下，地幔应力场为一个以南昌为中心、长轴NE-SW向的椭圆.分析认为，晚中生代以来，太平洋板块的西向俯冲，导致华南陆缘在区域性SE向地幔对流背景上叠加局域性不稳定热扰动，在175~85Ma期间，上地幔物质向上流动，形成不同的岩浆活动高峰期.同时，岩石圈地幔受俯冲洋壳流体的影响，含水量高，黏度小，在地幔流切向应力场作用下，岩石圈底界由西向东"波浪"状减薄.现今岩石圈之下仍具备地幔小尺度热对流温度条件，但除地表浅层外，地壳整体温度不高，岩石圈构造稳定.

Temperature structure and dynamic background of crust and mantle beneath the high heat flow area of the South China continental margin

The South China continental margin is an important region of mineral and geothermal resources of our country. In this area, the Pacific plate's westward subduction and the small-scale unstable mantle thermal convection associated with the subduction have led to complex tectonic deformation, multi-period magmatic activities, and extensive mineralization since late Mesozoic. Based on surface heat flow data and shear wave velocity data, as well as harmonic coefficients of gravity, this paper calculates the crust and mantle temperature structure and analyzes the geodynamic background. The results show that the thermal structure is of the "cold-crust/hot-mantle" type in the South China continental margin. Over 10 km of the shallow crust, the temperature is above 200℃.The Curie temperature is 475℃ and the average temperature of Moho is 550℃. Although there is a high temperature layer in the shallow crust caused by the high decay heat of the radioactive elements in the granite, the overall crustal temperature is low and the tectonics is stable. In the mantle, at 90 km, the temperature is 950~1250℃ at depth 90 km, 1050~1400℃ at 120 km, 1200~1450℃ at 150 km, 1500~1700℃ at 220 km, respectively. The "hot" lithospheric bottom boundary determined by the 1300℃ adiabatic isothermal surface is between 110~150 km which becomes thinner from west to east. The mantle stress field shows a pattern of alternating compression and extension in the lithosphere and an elliptical with a NE-SW directed long-axis centered at Nanchang city. We suggest that the subduction of the Pacific plate is the main factor that controlled the deep structure and hot-tectonic process for the continental margin of South China. The westward subduction of the Pacific plate has aroused local thermal activity of the deep mantle and localized unstable mantle convective disturbance superimposed on the regional southeast mantle convective background since late Mesozoic. During the period of 175~85 Ma, the upper mantle material flowed upwards and formed different magma activity peak periods. Meanwhile, the lithospheric mantle was affected by the subducted oceanic crust fluid with high water content and small viscosity. Under the action of the tangential stress field of the mantle, the thickness of the lithosphere was thinned from west to east in a "water wave" form. The magmatic activity of the continental margin in southern China had largely completed in the Early Cretaceous. Although there are still small-scale thermal convective temperature conditions in the current deep mantle, in addition to the surface shallow higher temperature, the overall crustal temperature is relatively low and the lithosphere is stable.