龙门山南段印支晚期构造活动的厘定-来自构造几何分析及碎屑锆石年代学的约束

A study on the late Indosinian tectonic activity of the south part of Longmenshan: constraints from structural analyses and detrital zircon chronology

关键词:  龙门山南段  地震反射剖面  构造正演模拟  锆石U-Pb定年

Key words:  southern section of the Longmenshan thrust belt  seismic section  forward modeling  U-Pb chronology analyses of detrital zircons

摘要:

印支运动在龙门山构造带的形成与演化中扮演着重要作用。为明确龙门山南段晚印支以来构造活动特点，本研究选取山前带某地震剖面进行构造解析和正演模拟，并采用碎屑锆石U-Pb年代进行定量约束。结果表明龙门山南段上三叠统物源与松潘-甘孜地块具有亲缘性，应来自北部的秦岭造山带以及松潘-甘孜褶皱带内中下三叠统的再旋回。研究区晚印支以来经历多次逆冲推覆，位移量约为1.7km，与中北段相比，其构造活动相对较弱。结合区域资料，认为龙门山印支期的构造活动整体呈由北向南传递之特征。

Abstract:

The Indosinian tectonic event played an important role in the formation and evolution of the Longmenshan thrust belt. In order to depict the structural characteristic of the southern section of the Longmenshan thrust belt during the late Indosinian epoch, integration of structural analysis and forward modeling of seismic section and detrital zircon U-Pb chronology analyses from the piedmont belt are used in this study. The results show that, the provenance of the syntectonic sediments from the southern section of the Longmenshan thrust belt was affinity to the Songpan-Ganzi block, thus most likely from the Qinling Orogenic Belt and the recycling of the folded Middle to Lower Triassic strata of Songpan-Ganzi block. The balanced restoration of the seismic section reveals a. 1.7 km shortening at the southern section of the Longmenshan thrust belt, which is weaker than that of middle and north part of Longmenshan thrust belt. Combined with the regional geological data, the tectonic activity of Longmenshan thrust belt during Indosinian was migrated from north to south, thus the piedmont zone of southern section of the Longmenshan thrust belt can receive material from the recycled Middle to Lower Triassic of Songpan-Ganzi block.