基于背景噪声面波成像法研究土耳其中部岩石圈速度结构

Lithospheric velocity structure of central Turkey based on ambient noise surface tomography

土耳其东部是由碰撞引起的挤压变形区域,而西部为俯冲引起的爱琴海伸展变形区域,土耳其中部作为两者之间的过渡拥有很复杂的地质情况,尤其是具有火山活动和长远的俯冲、大陆碰撞等构造历史,因此获得此区域岩石圈可靠的速度结构对认识俯冲末期的板块状态、岩浆活动等现象意义重大。为了更好地了解此区域的岩石圈速度结构,本研究使用了(31°E—38.8°E,34.5°N—42.0°N)范围内172个台站的背景噪声数据,通过互相关方法获得经验格林函数;之后利用频率-贝塞尔变换法得到了5—80 s周期范围内的基阶瑞雷波频散曲线,并在个别区域获得了高阶模式频散曲线;最后利用基于Broyden-Fletcher-Goldfarb-Shanno算法校正的拟牛顿迭代反演方法得到地表至124 km深的三维剪切波速度结构。结果表明:土耳其中部的速度横向变化剧烈(最大变化可达400 m/s),且与地质边界和缝合带区域密切相关,中安纳托利亚火山区及部分金牛座山脉东部的速度在0—100 km深度均小于4.3 km/s,因此推测此区域不存在岩石圈地幔;从塞浦路斯海沟开始俯冲的非洲大洋岩石圈,以近垂直俯冲在金牛座山脉中部下方,表现为明显的高速特征;土耳其中部在70—100 km深度广泛存在与上升软流圈物质相关的低速带,因此大部分研究区域的岩石圈波速小于全球平均剪切波速度,并且此区域岩石圈整体较薄、厚度多变。此外,本研究还发现在金牛座山脉中部和东部约13—23 km深度存在一个明显的低速带,推测可能与地块破裂导致的地层部分熔融有关。

The eastern part of Turkey is the compression deformation area caused by collision,while the western part is the extension deformation area of the Aegean Sea caused by subduction.As the transition between the two,the central part of Turkey has a very complex geological situation,volcanism,long-term subduction,continental collision and other tectonic history.Therefore,obtaining the reliable velocity structure of the lithosphere in this region is very important for understanding the plate state at the end of subduction,magmatism and other phenomena.In order to better understand the lithospheric velocity structure of this area,we study this region using ambient noise tomography based on the continuous noise data of 172 stations in the area(31°E−38.8°E,34.5°N−42.0°N).After obtaining the empirical Green’s function through the cross-correlation,the fundamental Rayleigh wave dispersion curves in the period range of 5−80 s is obtained by using the frequency-Bessel transform method,and the dispersion curve of overtones is obtained in some subarray.Subsequently,3-D shear velocity structure from surface to 124 km is obtained by quasi-Newton iterative version based on Broyden-Fletcher-Goldfarb-Shanno correction.These results show that the lateral velocity in central Turkey changes sharply(the maximum variation can reach 400 m/s),which is closely related to the geological boundary and suture.The velocity in the Central Anatolian volcanic province(CAVP)and some Eastern Taurus mountains(ETM)is less than 4.3 km/s at 0−110 km,so we speculate that there are no lithospheric mantle in this area.The African oceanic lithosphere,which began to subduct from the Cyprus trench,subducted beneath the Central Taurus mountains(CTM)at a near vertical angle,showing obvious high-velocity characteristics.For low velocity zones related to upwelling asthenosphere materials widely exist in this area from 70 km to 100 km,the lithospheric wave velocity in most of the study areas is less than the global average shear wave velocity,and the lithosphere is thin and variable in thickness.In particular,we also find that there is an obvious low velocity zone at a depth of 13−23 km in the CTM and ETM,which may be related to the partial melting of strata caused by block fracture.