Characteristics of Superposed Folds and Its Tectonic Significance in Saishitang Ore District, Qinghai Province—Tectonic Study on a Superposed Fold Zone

Recent geological studies have revealed that Qinghai Saishitang copper mining area is a deformation zone with several superimposed folds. To provide basic geological support for mine exploration programs, integrated tectonic analysis method drawn on outcrop structural analysis is adopted in this paper. Middle-Early Triassic rocks in mining area of poor outcrop condition are classified into four phases of fold association styles. All phases of folds are involved to produce various superimposed structures in light of outcrop and regional scale. In most cases, the presence of the Ramsy's Type 3 superposition, which is formed by the overlay of F2 folds on earlier F1 ones, suggesting that the two may be result of two successive deformation phases in one tectonic cycle and representing a crustal deformation mechanism transition from delamination shear to horizontal shortening. The intrusion and emplacement of Indosinian magmatic complex also took place between two deformation phases, which is probably a response to the far field transition from subduction to collision of the west Qinling and Qaidam blocks in the Indosinian tectonic period. The fold structure plays an important role on the formation, reconstruction and preservation of the polymetallic deposit in Saishitang mining area.