青藏高原东南缘瑞利波群速度分布特征及其构造意义探讨

doi:10.6038/cjg20150509

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青藏高原东南缘是研究印度—欧亚板块碰撞过程、块体间相互作用和壳幔变形机制的重要地区.本文利用川滇地区流动地震台阵和固定地震台网共557个台站的连续波形数据,基于改进的背景噪声处理流程和分析方法得到了6023条瑞利波群速度频散曲线,反演获得了6~48 s的瑞利波群速度分布图像.结果显示在四川盆地内部短周期群速度分布较好地揭示了盆地内沉积层厚度的横向变化.在30~48 s周期,四川盆地西部群速度存在南低、北高的特征,推测是南部中下地壳和上地幔温度较高引起的.温度的增高降低了地壳的力学强度,在青藏高原东向挤压作用下盆地西南部地壳更易发生变形,并导致脆性上地壳在新生代产生地壳缩短和褶皱、断裂等地质活动.攀枝花及其周边地区从地壳浅部至上地幔深度的高速异常体,可能与基性和超基性岩的侵入有关.该高速体具有较大的介质强度,在一定程度上阻碍了青藏高原物质东南向的运移,这可能是造成丽江—小金河断裂两侧巨大高程差异的重要因素.自24 s开始,南盘江盆地表现为明显的高速异常,与华南块体西南部其他区域的深部结构存在明显差异.反演的S波速度结构揭示,自中上地壳至上地幔,南盘江盆地的速度一直高于北侧其他区域.结合此地区的地壳运动模式,推测介质S波速度较高、力学强度较大的南盘江盆地对青藏高原物质的东南向逃逸具有一定的阻挡作用.

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关键词 ：青藏高原东南缘, 噪声层析成像, 瑞利波, 群速度, 四川盆地, 南盘江盆地

Abstract：

The southeastern margin of the Tibetan Plateau is an important area for the study of continental collisions between India and Eurasia plates, block interactions, and crust-mantle deformation mechanism. The study on the 3-D velocity structure of crust and mantle in the area helps us understand the formation mechanism and geodynamic process of the plateau. Although many seismological researches have been conducted in this region, high-resolution S-wave velocity structures are still in shortage due to the sparse distribution of seismic stations. Using dense seismic stations set up recently, high-resolution group velocity images of the crust and mantle in this area are obtained with ambient noise tomography. The new results provide new insights for studying the mechanism of the crust-mantle coupling, deformations of the crust and mantle, and so on.
We collected continuous waveform data recorded by 557 temporal and permanent stations in Chuan-Dian fragment region. After removing instrument responses, bandpass filtering, frequency-time normalization, cross-correlation and stacking, more than 50000 empirical Green's functions were acquired. Furthermore, 13972 Rayleigh wave fundamental mode dispersion curves of high signal-to-noise ratio waveforms were measured with application of multiple filtering technique. To guarantee the validity of dispersion curves, we selected reliable dispersion curves according to the setting station-distance criteria and cluster analysis. At last, we determined group velocity structures of Rayleigh wave from periods of 6 to 48 s in the study area with traditional surface wave tomography technique.
From periods of 6 to 20 s, group velocity in west and north of Sichuan basin is characterized as low velocity, while the central and southeast Sichuan basin is imaged as high velocity. From periods of 30 to 48 s, the group velocity in Sichuan basin is obviously higher than that in the Tibetan Plateau and surrounding orogenic belts. In the meantime, the southwest part of the Sichuan basin is shown as relative low velocity. This phenomenon indicates the lateral variation of velocity structure in the lower crust and upper mantle. At 30 s, low group velocity is observed in Songpan-Ganzi fold belt, western Chuan-Dian fragment block and Xiaojiang fault zone. At Panzhihua and its surrounding area, a high-velocity anomaly exists at all periods. From 6 s to 24 s, this high-velocity anomaly is oriented in a nearly west-east direction, however between 30 s and 48 s in a northeast-southwest direction. Along with the growth of periods, the anomaly is concentrated to Panzhihua. At periods of 14 s and 20 s, high group velocity anomaly exists at the south of Lijiang-Xiaojinhe Fault and the anomaly is parallel with the fault. Since 24 s, high-velocity anomaly shows up at the southeast of the study region and becomes larger with periods turning longer. At 30 s, the range of the high velocity anomaly is consistent with Nanpanjiang Basin. The S-wave velocity cross section reveals that the S-wave velocities of crust and upper mantle in the Nanpanjiang Basin are higher than those in other surrounding regions.
High-resolution Rayleigh wave group velocity maps in the southeast of the Tibetan Plateau are obtained using ambient noise tomography based on intensive seismic observation data. Low velocities in Sichuan Basin at short periods reflect the sediments in the basin being younger than the surrounding blocks. In this basin, the different subsidence rate and sediments thickness since Jurassic result in the inhomegeneous distribution of low velocities. In the north and northwest of the basin, the sediments are thickest. For periods of 30 to 48 s, the low velocity in the northwestern margin of the Sichuan Basin may be related with high temperature in the lower crust and upper mantle. The high temperature decreases the mechanical strength of the crust in the southwestern basin, resulting in the crust deforming easier because of the pinching action of the Tibetan Plateau, and probably is the reason of Cenozoic folds, fault movements and crustal shortening in the brittle upper crust. High-velocity anomaly existing from the surface to upper mantle in Panzhihua and surrounding areas is probably caused by the intrusion of mafic-ultramafic magma in Emeishan magmatic activity. This high-velocity anomaly with strong medium strength impedes the material extrusion of Chuanxibei sub-block to some extent, bringing about the enormous difference of topography between the two sides of the Lijiang-Xiaojinhe Fault. The high group velocity anomaly in the southeast of the study area is consistent with the range of the Nanpanjiang Basin. Having a higher S-wave velocity, the Nanpanjiang Basin has different deep structure comparing with other regions to the north of Nanpanjiang. Meanwhile, this high velocity anomaly may offer an impact of blocking the southeast extrusion of the plateau material.

Key words： Southeastern margin of Tibetan Plateau Ambient noise tomography Rayleigh wave Group velocity Sichuan Basin Nanpanjiang Basin

收稿日期: 2014-08-18

PACS:

P315

基金资助:

国家自然科学基金(41304043,41374097)资助.

通讯作者: 吴建平,男,1964年生,研究员,博士生导师,主要从事地震学与地球内部物理研究.E-mail:wjpwu@cea-igp.ac.cn E-mail: wjpwu@cea-igp.ac.cn

作者简介: 范莉苹,女,1988年生,博士研究生,固体地球物理学专业.E-mail:fanlipingchampak@163.com

链接本文:

http://www.geophy.cn/CN/10.6038/cjg20150509 或 http://www.geophy.cn/CN/Y2015/V58/I5/1555

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