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

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.