LARGE DEEP-LEVEL DECOLLEMENT STRUCTURE ON THE NORTHERN MARGIN OF THE YANGTZE PLATE AND ITS DYNAMIC ANALYSIS
Following the collision of the Sino-Korean plate and the Yangtze plate in the Early Devonian large-scale northward intracontinental subduction of the Yangtze plate relative to the Sino-Korean plate took place in Indosinian time. Through a study of the structural deformation of the basement (Pt_3) and cover (Z-T) on the southern margin of the Dabie-Qingling Mountains and in the lower Yangtze region. the author has demonstrated that the large deep-level ductile decollement-shear zone in the northern part of the Yangtze plate is precisely the product of this subduction. 1. From above downwards the structural deformation in the cover (Z-T) shows a change from upright folds→isoclinal folds→recumbent folds. The cleavages also transform from upright folds→inclined→horizontal ones and exhibit a style of listric structure. Recumbent folds of varying sizes are developed within the Sinian strata, accompanied by subhorizontal flow cleavages and strong stretching of gravels in gravel-bearing phyllite (Zhougang Formation Z_1~1) and moraine conglomerate (Sujiawan Formation Z_1~2), forming the NNE- or N-S- trending stretching lineation. 2. Strong subhorizontal flow cleavages and lineations, "A"-typpe recumbent folds, sheath folds and N-S- or NNE-trending stretching lineations are developed in the basement (Pt_3; the Yuelinghe, Liangyun, Hong'an, Zhangbaling and Haizhou Groups). The nonsymmetry of the crystal tail of feldspar and pressure shadow of pyrite along the xz plane, the s-c structure of mica and structural recrystallized quartz, the rotation of the inclusions of basic volcanic rocks and the gently inclined myllonite zone. all this indicates the existence of a high- strain shear zone. 3. From above downwards the metamorphism is progressive. and there appear high-pressure metamorphic minerals such as glaucophane, crossite, chloritoid, 3T-phengite and jadeite in the decollement layer of the basement. 4. The nonsymmetrical fabric analysis of quartz within the decollement layer (basement) implies the existence of south-directed shear, while the hom- ogenous fabrics in the gneiss of the Wudang Group below the Liangyun Group suggest that the deep-buried metamorphic rocks far from the decollement-shear zone has not been involved in the Indosinian decollement. 5. The finite strain analysis of Sinian deformed gravels (by use of Dunnet method) gave a Flinn index (K=0. 945. 1. 124) of nearly one. So a simple shear mechanism is considered. 6. Microstructure analysis shows that shear heating was produced in the late stage of the shear action and indicates the formation of structural recry- stallized quartz, high-temperature quartz fabric (1010), partial melting in strata and Indosinian granite. 7. The age of glaucophane and albite in the decollement layer is 210-230 Ma, representing the age of a large decollement. The author considers that the Indosinian fold belt of the Yangtze platform is an intracontinental mountain chain and did not result from simple compression. It is genetically related to the large-scale dutile decollement-shear zone between the cover and basement. This decollement structure is tho largest deep-level one ever discovered in China at present. The author also considers that the Tancheng-Lujiang fault was formed a bit later than the Indosinian decollement structure, as the result that the subsuction velocity of its eastern sector was greater than that of the western sector, It is a sinistral strike-slip, transform, ductile shear zone of "intracontinental subduction type".