造山带中增生楔识别与地质意义

增生楔主要由海沟复理石、远洋—半远洋沉积和洋岛/海山等大洋板块地层岩石及蛇绿岩共同构成，是汇聚板块边缘古俯冲带构造—沉积作用的综合产物，以发育叠瓦状逆冲断层、双冲断层和紧闭—倒转褶皱，以及片理、碎裂构造、小型褶皱、膝折等不同尺度的构造为特征。增生楔组成包括连续单元和混杂岩两部分，严格受滑脱面位置控制。增生楔是增生型造山带最基本大地构造相，它与弧前盆地、岛弧/大陆边缘弧的时空配置关系可直接指示大洋板块俯冲极性、揭示洋盆演化与造山作用方式。准确识别增生楔，详细解剖其结构特征与组成特征，综合判别并恢复大洋板块地层，可为古板块边界与古大洋盆地消亡位置确定、造山带结构精细划分及造山作用过程重建提供基本依据。大比例尺地质填图是研究增生楔结构和组成、以及大洋板块地层重建的有效手段。以南祁连拉脊山石灰窑增生楔为例，地质填图结果表明它是由中寒武世—早奥陶世洋壳物质构成，自北而南依次可分为海沟—大洋盆地—海山OPS和六道沟SSZ型蛇绿岩，被南倾逆冲断层分割成多个岩片。海沟—大洋盆地OPS岩片基本保留完整地层序列，海山OPS岩片包括连续单元和混杂岩两部分。顶帽山增生楔主要由海山OPS构成，内部结构层序相对完好，是南祁连构造带中保留最为完整、规模最大的海山OPS岩石组合序列。上述不同类型OPS片段和六道沟SSZ型蛇绿岩组成的增生楔与其南侧岛弧带同时代。增生楔—岩浆弧的时空配置暗示寒武纪—早奥陶世时期原特提斯洋自北向南俯冲极性。这些认识为南祁连早古生代火山—岩浆—沉积构造演化以及是否存在古洋盆与古洋盆演化重建提供基本佐证。

Recognition and significance of accretionary prism in orogenic belts

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.