青藏高原东缘—扬子特提斯构造域深部结构与地壳形变研究

青藏高原东缘和扬子西缘的构造带是中国特提斯构造域的重要组成部分,该构造域受欧亚板块与印度板块陆—陆碰撞、高原隆升、块体裂解或拼接挤压等强烈构造活动的影响,记录和保存了多期次的特提斯构造演化历史痕迹。同时,该研究区域也是中国西部地区地壳形变最强烈的地区之一,其浅表形变特征与深部构造之间存在怎样的关联和制约机制是目前国际地球科学的一个研究热点。本研究依据作者十多年来持续在该区域开展的地质—地球物理研究,通过深部地球物理多参数结构成像、沉积盆地分析、地壳形变和强震孕育机制等综合对比分析,发现在青藏高原东缘的下地壳存在低速和高泊松比异常带,该异常体与来自青藏高原上涌的软流圈热物质汇聚,导致从扬子西缘到青藏高原的下地壳和上地幔的深部结构发生显著变化。沿着龙门山断裂带,中、下地壳存在交叠相间的低速(高泊松比)和高速(低泊松比)区域,这些深部结构分布特征与地表形变及前陆盆地隆坳格局具有较好的一致性。基于上述认识,提出了青藏高原东缘—扬子板块的深部接触模式及其相应的盆山耦合关系,阐明了板块碰撞—耦合的深部动力学过程对剧烈地壳形变、盆地隆坳格局和强震诱发的制约关系。本研究成果将为深入认识青藏高原东缘高原急剧隆升、盆地基底结构与隆拗格局,以及强烈地壳形变的深部动力学机制提供参考信息。

Deep structure and crustal deformation in the Tethys tectonic domain of the eastern Xizang(Tibetan)margin—Yangtze Platform

Objectives:The collisional orogenic belt composed of Songpan—Garze(Ganzi),Chuandian(Sichuan—Yunnan)and Yangtze blocks is an important part of the Tethys tectonic domain in China.The tectonic domain is affected by the strong tectonic activities such as the continent—continent collision between Eurasian plate and Indian plate,plateau uplifting,block cracking or splicing compression,and has recorded and preserved rich and comprehensive historical traces of Tethys tectonic activities.Meanwhile,it is one of the regions with the strongest crustal deformation in southwest China.What the relationship and constraint mechanism between the characteristics of superficial deformation and deep structure is a research hotspot in Earth science.Methods:To investigate the deep dynamic mechanism,crustal deformation and earthquake generation in the Tethys tectonic domain of China,we applied a joint tomographic inversion method for a large number of local and teleseismic data to have determined the 3-D seismic velocity and Poisson’s ratio models in the crust and upper mantle.Results:Based on the author’s continuous geology—geophysics studies for more than ten years,a lowvelocity and high Poisson’s ratio belt is observed along the Eastern margin of Xizang(Tibet).This anomalous belt meets together with the hot upwelling asthenospheric materials to form an upwelling channel upwards to the upper crust.At the same time,a sharp contrast anomaly of multiple parameters on two sides of the Longmenshan(Longmem Mountains)fault zone is revealed in the lower crust and uppermost mantle.A distinguish overlapping zone of low speed(high Poisson’s ratio)and high speed(low Poisson’s ratio)has been revealed in the middle—lower crust under the Longmenshan fault zone.These imaged deep structural anomalies have a good consistency with the spatial distributions of basin pattern of depression and uplifting,surface stress and crustal deformation.Based on these obtained deep structural characteristics,a deep contact model across the Eastern margin of Xizang(Tibet)together with corresponding basin—mountain coupling relationship is proposed by comprehensive comparative analyzing of multi-parameter structure imaging,crustal deformation and seismogenic mechanism of large earthquakes as well.The constraint mechanism of the deep dynamic process of plate colliding and coupling on severe crustal deformation and generation mechanism of large earthquakes is clarified.Conclusions:The results of our model could provide some reference information for further understanding of the deep dynamic mechanisms on the plateau sharp uplifting,basin basement structure,basin pattern of depression and uplifting,as well as strong crustal deformation.