龙门山构造带中段新近系砾岩层砾组特征及其构造意义

Gravel fabric characteristics of Neogene conglomerate layers in the middle sector of the Longmenshan structural belt and its tectonic significance

通过对龙门山构造带中段玉堂镇新近系砾岩层的砾组分析,阐述了砾岩层的成因、来源、搬运方式和构造背景等。研究表明,玉堂镇新近系砾岩层为近—中源河流相冲积物,物源为来自北西方向龙门山构造带中的基岩地层,砾石成分复杂,底部砾岩层的砾石成分以盖层沉积岩和浅变质岩为主,中部砾岩层中基底花岗岩砾石成分则大幅度增加,水动力条件不稳定,砾石原始最大扁平面倾向具有向NW转变的趋势。反映龙门山地区新近纪地壳运动渐趋强烈,且古构造应力场方向发生过由NWW向NW向的转变。

Based on an analysis of gravel fabric features of Neogene conglomerate layers at Yutang Town in the middle sector of the Longmenshan structural belt,this paper describes the origin,source,hydrodynamic condition and tectonic setting of the Neogene conglomerate layers.Studies show that the conglomerate layers are proximal alluvial deposits and their source rocks are bedrocks of the Longmenshan structural belt,and the composition of gravels is complex.The gravel components from the lower conglomerate layers are mainly composed of sedimentary rocks and epimetamorphic rocks from the caprocks.Gravels of basement granite largely increase in the middle conglomerate layers.The hydrodynamic condition is not stable.The original maximum flat surface of the gravels tends to turn northwestward,suggesting the increasingly intense crustal movement and a NWW to NW change of paleotectonic stress field during Neogene in the Longmenshan area.