大别—苏鲁及邻区上地幔的各向异性

Seismic anisotropy of upper mantle beneath the Dabie-Sulu and its adjacent areas

大别—苏鲁是扬子与华北的碰撞造山带,对该地区上地幔各向异性的研究有助于了解该区的地幔动力学机制.本文选用了中国数字化地震台网和区域数字地震台网(山东、安徽、江苏、河南、湖北)三分量宽频带的远震地震波形数据,分别采用最小能量法和旋转相关法,对大别—苏鲁及邻区进行剪切波偏振分析,计算了研究区台站下方介质的各向异性分裂参数:快波偏振方向(Φ)和快慢波延迟时间(δt).本文研究结果发现,研究区内快、慢波延迟时间0.5～1.63s,推测各向异性层深度为57.5～187.6km,由软流圈和岩石圈地幔的各向异性共同作用引起.快波偏振方向在4个不同构造区表现出不同的特点:华北板块快波偏振方向为近E-W向,根据地质资料,我们分析认为华北板块的各向异性受地幔软流圈流动的影响明显;大别造山带各向异性平行于大别主构造,反映造山过程中岩石圈物质沿大别造山轴部NW-SE向迁移的特点;在大别南侧和东侧的扬子板块快波偏振方向分别表现为近垂直于造山带走向和NEE-SWW,苏鲁造山带各向异性结果为NEE-SWW,与地表构造有一定的夹角,同时与板块运动方向相差较大,分析认为扬子板块和苏鲁造山带各向异性是由地幔软流圈流动和印支—燕山期构造运动残留在岩石圈地幔的"化石各向异性"共同作用的结果.

The Dabie-Sulu orogenic belt is a collision zone of the Yangtze block and North China block,and the analysis on upper-mantle anisotropy beneath the study region will help us to understand the mantle dynamics process.In this paper,we have analyzed observations of low frequency SKS waves recorded by Shandong,Jiangsu,Anhui,Henan and Hubei digital seismograph networks and China Broadband Digital Seismograph Network（2006.05~2011.05）.With rotation correlation and transverse component minimization methods,we have obtained splitting parameters（Φ,δt） beneath Dabie-Sulu and its adjacent areas.The results show that the delay times between fast and slow wave range from 0.5 s to 1.63 s in the study,which indicate that the anisotropic structures are at depths about 57.5~187.45 km,including the effects of lithosphere and asthenosphere.The fast wave polarization directions beneath this area are divided into four groups.The fast wave polarizations beneath North China are nearly E-W,and according to the geology,we infer that the upper mantle anisotropy beneath North China is caused by the convection of the asthenosphere.The anisotropy results beneath Dabie orogenic belt show a good relation with the strike direction of the belt,which reflect the north-south-trending compression and collision of Yangtze block and North China block.In Yangtze,the results to the south of Dabie are nearly perpendicular to the Dabie orogenic belt,and the results to the east of Dabie show NEE-SWW trends.The anisotropy beneath Sulu orogenic belt is NEE-SWW,which are different from both the strike direction of the surface structures and the absolute plate motion.The upper mantle anisotropy can be explained by the combined effects of asthenosphere convection and the so-called ＂fossil anisotropy＂ frozen in the lithosphere during the Indosinian-Yanshanian period.