四川盆地东南部早三叠世地震事件沉积及其地质意义

四川盆地南部及东南部下三叠统飞仙关组和嘉陵江组中常发育有一系列宏观及微观的软沉积物变形、震积角砾岩块、板状砾屑、层内微断层等特殊的地震成因沉积构造。飞仙关组震积岩在重庆北碚白庙子剖面飞仙关组第一、二段中产出，嘉陵江组震积岩产于四川叙永龙洞湾嘉陵江组第二段及四川兴文、贵州习水等地的相同层位中，其沉积序列明显，反映了古地震从开始至平静的全过程。作者认为，早三叠世时，上扬子板块西缘以川西义敦弧的演化为代表的三江构造带活动强烈，可能与四川盆地早三叠世震积岩的形成有联系，随着三江构造带板块碰撞、洋壳俯冲消减，能在较大区域内引发地震活动、火山喷发。

Seismites and Their Geological Significance in Early Triassic of Southeast Sichuan Basin

The seismites emerge in the first and sencond members of Lower Triassic Feixianguan Fm.in Baimiaozi section,Beibei,Chongqing,China and in the second member of Lower Triassic Jialingjiang Fm.in Longdongwan,Xuyong county,the southern Sichuan province,China,as well as in the same stratigraphic sequences in Xingwen county,Sichuan province and Xishui county,Guizhou province. There are some macroscopic and microscopic sedimentary structures of seismic events in the outcrops,drilling cores and thin sections including the following basic types could be observed: Soft-sediment liquefied deformation structures include curled deformation(sliding deformation structure,"sausage structure") and microscopic folds(laminated plastic deformation).In the first member of Feixianguan Fm.at Beibei,Chongqing.Microscopic folds(lamina plastic deformation) are found in the second member of Feixianguan Fm.at Longdongwan,and microscopic folds emerge in laminated thin-layer limestone,presenting slight folds in laminated thin-layer limestone and their low radian with undulation.In addition to study areas,some soft-sediment deformation structures such as convolute beddings,microscopic folds and so on are developed in the first member of Feixianguan Fm.at Jichang,Xuanhan county,Sichuan province,China,and may be related to paleoearthquake activities. The tabular-pebbles with different-size diameters are developed in the lower part of the second member of Feixianguan Fm.,interbedded with laminated thin-layer micritic limestone of normal sequences.The sedimentary characteristics of tabular gravel layers are also comparable with those in the second member of Feixianguan Fm.at Beibei,Chongqing. The gravels are developed with different size,shape and psephicitie in the first member of Feixianguan Fm.in Baimiaozi,Beibei Chongqing,and the second member of Jialingjiang Fm.in Longdongwan,Xuyong,Sichuan province,the shape being irregular mass,nearly spherical or tabular,the size ranging from 0.5 to 25cm,the lithology being grey or grey white thick-bedding massive micrite,some of which have certain psephicities. The intraformational microscopic faults are developed in the first and second member of Feixianguan Fm.at Beibei,Chongqing,they are limited to develop intraformationally,and don’t cut through the upper and lower beds with short slip of fault and slow angle of fault plane. Some well-developed microscopic seismites can also be discovered in some thin sections of the second member of Jialingjiang Fm.at Longdongwan,Xuyong county,and Jianshe,Dali,Chengdong,Wenxin county,Sichuan province,Huangmuping,Xishui county,Guizhou province,including the following basic types:microscopic liquefied curled deformation structure;microscopic tabular-pebbles layer;microscopic slip and microscopic flame structure. Seismites developed in the lower Triassic in Sichuan Basin have not only wide transverse distribution,but also are distributed longitudinally.Some seismites with volcanic debris sediments of Early Triassic have been found in other places of Sichuan basin besides research area.The author argues that the Sanjiang tectonic belt represented by the evolution of Yidun arc in the western Sichuan had strong activities at the west margin of the upper Yangtze plate,which may be related to the formation of seismites in the early Triassic.At the west margin of the upper Yangtze plate,the strongest tectonic deformation and metamorphism occurred in the early Triassic emerge between YiDui arc and Qiangtang mass,representing the subduction of the Paleo-Tethys.Magmatic and volcanic activities occur when the paleo-Tethyan oceanic crust subducts eastward along Litangbatang volcanic arc and westward along Jinsha River suture zone.In the early Triassic,seismic activities and volcanic eruption can be triggered in large areas with plate collision and ocean crust subduction.The author speculates that Yidui arc is the major cause of formation of seismites in Sichuan Basin that emerge in the early Triassic.