摘要
郯庐断裂带是贯穿我国东部北北东走向的一条深大断裂,其中南段及其邻区(115°E—121°E,29.5°N—35°N)穿过了大别造山带、苏鲁造山带、长江中下游成矿带及合肥盆地.为了研究该区域地壳速度结构及变形特征,我们使用安徽省和江苏省及其周边地区105个台站(固定台站98个,流动台站7个)的垂向连续波形数据,时间范围从2014年5月到2015年7月,共计14个月.利用背景噪声互相关方法,从垂直分量互相关函数中最终提取了2590条瑞利面波相速度频散曲线,反演得到周期范围为5～30s的瑞利波方位各向异性相速度分布图,再反演每个网格点瑞利面波相速度频散得到一维层状横波速度模型,然后拼合起来组成三维横波速度模型.根据本文反演结果并综合已有资料,我们得出如下结论:(1)在北大别、蚌埠隆起、长江中下游成矿带、合肥盆地北部大桥凹陷区域存在中地壳横波高速体,可能与岩石圈和下地壳拆沉以及中生代中国东部大范围岩浆活动有直接关系,更深层原因可能与古太平洋俯冲相关;(2)苏鲁造山带南缘,垂直于嘉山响水断裂,从南向北中上地壳低速体深度变浅,这个低速体可能是高压/超高压变质岩与扬子板块接触处的破碎带,是扬子板块与华北板块接触的边界;(3)郯庐断裂合肥—嘉山段两侧以及大别造山带东缘短周期瑞利面波相速度快轴方向与郯庐断裂带走向基本一致,可能是三叠纪碰撞期与白垩纪时期的大规模左旋走滑活动的结果;(4)合肥盆地南部15～20s周期的瑞利波相速度快轴方向为北西-南东向,反应该区域中下地壳快波方向为北西-南东向,推测是大别造山带折返的痕迹;(5)郯庐断裂带的结构和地震活动性存在明显的分段性,嘉山—郯城段郯庐断裂带现今地震活动性弱,但发生过较强的古地震,推断现今郯庐断裂带宿迁段可能处于闭锁状态,从长远来看要注意该地区发生大震的可能.
Tanlu fault is a deep penetrating fault with an NNE strike in eastern China.Its central-southern segment and the adjacent area traverse the Dabie orogenic belt,the Sulu orogenic belt,the Middle-Lower Yangtze metallogenic belt,and the Heifei Basin.In order to study the crustal velocity structure and deformation characteristics in this region,we used continuous waveform data from May 2014 to July 2015,a total of 14 months,of 105 stations(98 permanent stations and 7 temporary stations)in Anhui and Jiangsu Provinces and its surrounding area.Using the ambient noise cross-correlation method,we finally extracted 2590 surface Rayleigh wave phase velocity dispersion curves from the vertical component cross-correlation function,and then obtained an azimuthally anisotropic Rayleigh wave phase velocity maps in the period band 5～30 s.Then we inverted the Rayleigh wave phase velocity dispersion at each grid point for a onedimensional layered shear velocity model,which was then combined to form a three-dimensional shear velocity model.Based on the inversion results and the existed data,we draw the conclusions as follows.(1)The northern Dabie orogenic belt,Bengbu uplift,Middle-Lower Yangtze metallogenic belt,and Daqiao-Dingyuan depression in the northern Hefei Basin exhibit higher shear velocity anomalies in the middle crust,which may be directly related to the lithosphere and lower crust delamination and Mesozoic volcanic magmatism in eastern China,and the deeper causes may be related to the Paleo-Pacific subduction.(2)The low velocity body shallows from south to north in the upper-middle crust of the southern margin of the Sulu orogenic belt,which is perpendicular to the Jiashui-Xiangshui fault.The low velocity body may be the fracture zone where the ultra-high/high pressure metamorphic rocks come into contact with the Yangtze plate.It is a contact boundary between the Yangtze plate and the North China plate.(3)The fast-axis direction of the short period Rayleigh wave phase velocity on both sides of the Hefei-Jiashan section of the Tanlu fault and the eastern boundary of the Dabie orogenic belt is basically consistent with strike of the Tanlu fault,which may be the result of the large-scale sinistral strike-slip activity of the Tanlu fault during the Triassic collision period and the Cretaceous;(4)The fast-axis direction of Rayleigh wave in the period band 15～20 sis northwest-southeast in the south of the Hefei Basin,indicating a northwest-southeast fast wave direction in the middle and lower crust in this area,presumably reflecting the trace of the exhumation of the Dabie orogenic belt.(5)The structure and seismicity of the Tanlu fault zone are clearly segmented.The seismicity of the Tanlu fault from Jiashan to Tancheng is weak at present,but strong paleo-earthquakes occurred.It is inferred that the Suqian section of the Tanlu fault may be in a locking state.In the long run,attention should be paid to the possibility of large earthquakes occurring in this area.