新生代华南及邻区上地幔各向异性深部动力学机制的数值模拟

Upper mantle anisotropy and dynamics beneath Cenozoic South China and its surroundings:insights from numerical simulation

晚中生代以来,华南地区同时受到印度—欧亚板块碰撞和太平洋—菲律宾板块俯冲及后撤作用的影响,壳幔结构复杂.深入了解华南地区深部地幔流模式和地幔各向异性特征是认识华南复杂的深部构造演化过程与动力学机制的基础.本文采用三维全球地幔对流模型,从软流圈剪切变形的角度计算了软流圈的各向异性,尝试探讨了华南地区各向异性的起源和深部地幔流特征.华南地块东部,软流圈各向异性呈NW-SE向,各向异性主要来源于软流圈,壳幔具有垂直连贯的变形特征;南北构造带的中段,软流圈各向异性大致为N-S向,这一区域的造山作用虽然对岩石圈造成了巨大变形,但是并未显著影响软流圈变形,并且各向异性的主要来源可能是岩石圈地幔;在南北构造带中,30°N可能是地幔各向异性的过渡带,30°N以南的川滇地区,软流圈各向异性的方向出现了环形特征;菲律宾板块向欧亚板块下的俯冲到达地幔转换带,这种俯冲可能带动了西太平洋地幔向华南块体下的流动;华南地区的软流圈流场自西向东显示出顺时针旋转的特征,并在扬子板块东部与来自菲律宾板块下的西南向的地幔流相遇.

Since the Late Mesozoic,South China has a complicated crust-mantle deformation due to the India-Eurasia convergence and the subduction of the Philippine Sea plate.The understanding of mantle anisotropy and flow pattern is the basis to reveal the deep tectonic evolution and mantle dynamics in South China.Using 3-D global mantle convection models,we analyze the asthenospheric anisotropy from the view of asthenospheric shear deformation and further explore the origin of anisotropy and mantle flow pattern.The asthenosphere may contribute to the seismic anisotropy in east part of South China block and the predicted asthenospheric anisotropy is dominant SE-trending.A vertically coherent lithospheric and asthenospheric deformation is inferred in the east part of South China.In the middle segment of the north-south tectonic belt,predicted anisotropy is approximately in N-S trends.Although orogeny in this region has caused great deformation of the lithosphere,it has not significantly affected the deformation of asthenosphere.Seismic anisotropy in this region may be significantly contributed to the lithospheric mantle.30°N may mark the transition zone of mantle anisotropy in the north-south tectonic belt.In Chuandian region,a circular pattern of anisotropy is revealed.The subduction of Philippine Sea plate to Eurasia plate may extend to the mantle transition zone and induce the flow of mantle.Asthenospheric flow beneath the South China shows a pattern of large-scale clockwise rotation from west to east and encounters the southwest mantle flow from Philippine Sea plate in east part of Yangtze block.