Folding Layers of Nanduan Formation, Xiding, Menghai, Western Yunnan

The Upper Paleozoic Nanduan Formation, a set of epimetamorphic clastic rocks, occurs in the accretionary complex of the Shuangjiang-Lancang subduction belt in Xiding area of Menghai, western Yunnan.Previously, the Nanduan Formation is thought to be a suite of normall stratigraphic sequences. In this paper, based on detailed geological investigation and research, it is believed that the Nanduan Formation is actually a tectonic-stratigraphic unit which is composed of folding layers. A lot of typical structures of folding layers, such as bedding schistosities, recumbent folds, viscous sausages and bedding ductile shear zones, have been identified in the Nanduan Formation. The folding layer structures in Nanduan Formation are with obvious characteristics such as stratified shear deformation, transverse structural replacement, and solid-state rheology. Considering regional tectonic background, it is suggested that the folding layer structures of the Nanduan Formation are formed by the extensional shear mechanism under the background of orogenic collisional contraction and compression after the closure of Changning-Menglian Paleo Tethys Ocean. At the same time, it is pointed out that the Nanduan Formation is a structural stratigraphic unit formed by structural reconstruction and reconstruction, and it should be called the Nanduan Rock Formation. The lithologic assemblage and thickness of the Nanduan Formation seen in present outcrops can only represent the reconstructed lithologic assemblage and thickness of folding layers, which are not the rhythmic assemblage of the original sedimentary strata and the thickness of the sedimentary strata. The Nanduan Formation should be divided, mapped and studied according to structural stratigraphic units.