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摘要
Accretionary prism consists of ophiolitic slices and ocean plate stratigraphy of trench flysch, pelagic and hemipelagic sediments, and oceanic islands/seamounts, which is developed by tectonism and sedimentation that take place within subduction zone in front of the convergent plate. It is charactered by large-scale deformation of imbricated thrust faults, duplexes, and tight and overturned folds and small-scale deformation including schistosity, cataclastic fabric, small fold, and kink. Accretionary prism comprises continuous unit and mélange that are controlled by location of decollement, which is one typical tectonic facies of accretionary orogen. Spatial-temporal relationship between the accretionary prism, forearc basin, and island arc/continental arc can directly point to subduction polarity and indicate ocean evolution and orogeny. Together with accurate recognition and detailed anatomy of texture and composition of accretionary prism, systemic discriminant and reconstruction ocean plate stratigraphy(OPS) provides evidences for defining the location of plate boundary and vanished ocean basin, fine division of orogenic texture, and reconstruction of orogenic processes. Geological mapping with large scale is an effective method to study texture and composition of accretionary prism and reconstruction ocean plate stratigraphy in the orogenic belts. Geological mapping results demonstrate that the Shihuiyao accretionary prism in the Lajishan area of the South Qilian belt consists of Middle Cambrian to Early Ordovician oceanic materials including trench, ocean basin, and seamount OPS that show as several thrust sheets separated by southward-inclined thrust faults from north to south. Trench and ocean basin OPS sheets show intact OPS sequence, but seamount sheets comprise intact and mélange portions. The Dingmaoshan accretionary prism is dominated by seamount assemblage with well-preserved intact sequence, which is the largest seamount in the South Qilian belt. The assemblages of different type of OPS portions and associated internal deformation texture indicate southward-subduction of the Proto-Tethyan ocean during Cambrian to Early Ordovician. This inference is consistent with the spatial-temporal relationship between the accretionary prism and ophiolite and island arc. These results are vital to understand tectonic evolution of Early Paleozoic volcanism, magmatism and sedimentation of the South Qilian belt and reconstruct evolution of the ocean basin. ? 2021, Science Press. All right reserved.
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