东亚原特提斯洋(Ⅳ):北界西段早古生代构造变形

Proto-Tethys Ocean in East Asia(Ⅳ):Deformation and evolution of microcontinents in the west segment of the northern Proto-Tethys Tectonic Domain

北祁连造山带是原特提斯洋北支西段——古祁连洋闭合的地质记录,其经历了早古生代复杂的造山过程,但其俯冲极性、闭合时间、拼合方式还存在争议。通过详细的野外构造解析,并对变质年代学资料进行统计,在研究区识别出三幕早古生代褶皱变形。其中,第一幕变形发生在489~442Ma,形成于古祁连洋壳俯冲-碰撞阶段,主要表现为区域性的片理、片麻理或糜棱叶理;第二幕变形发生在422~406Ma,形成于俯冲板片的折返阶段,主要表现为轴面南倾的紧闭褶皱;第三幕变形则主要为轴面近于直立的宽缓褶皱。前两幕变形被第三幕变形叠加改造。祁连地区广泛分布着奥陶系-志留系与上覆泥盆系的角度不整合,不整合面上、下的地层对比指示西段不整合时间早于东段不整合时间,可能代表了古祁连洋西段拼合较早、东段拼合较晚的斜向"剪刀式"拼合。此外,多条穿过整个研究区的1∶20万地质剖面上的运动学解析,揭示了古祁连洋壳自北向南的俯冲极性。综合以上研究结果,认为古祁连洋壳最早的俯冲时间为544Ma,中祁连和阿拉善微陆块自462Ma开始碰撞拼合,古祁连洋于442Ma最终闭合。早古生代原特提斯洋北界西段俯冲方式为自南向北"后退式"俯冲,可能发生过俯冲带跃迁事件。

The North Qilian Orogen is a geological record of the Paleo-Qilian Ocean which is the west segment of the northern branch of the Proto-Tethys Ocean. The North Qilian Orogen underwent a complex orogenic process in the Early Paleozoic. However, the subduction polarity, closing time and assembly pattern are still on debate. Based on detailed structural analysis and compilation of the published metamorphic ages, three episodes of folding occurred during the Early Paleozoic are distinguished. The first-stage deformation is characterized by regional schistosity, gueissosity and mylonitic foliations, and it was formed due to the subduction- collision of the ancient Qilian oceanic crust during 489 - 442Ma. The second-stage deformation is marked by tight folds with South- dipping axial planes, and it occurred with the exhumation of the subduction plate during 422 - 406Ma. The third-stage deformation is characterized by upright open folds. The first and second stages of deformation are overprinted by the third stage of deformation. The angular unconformity between the Ordovician-Silurian and the overlying Devonian strata widely distributes in the Qilian area. The comparison between the overlying and the underlying strata indicates that the deformation in the western part occurred earlier than that of the eastern part, implying a scissors-type closure of the Paleo-Qilian Ocean. In addition, the motion senses revealed by many 1 ： 200000 geological sections across the entire study area indicate the southward subduetion of the ancient Qilian oceanic crust. The subduetion of the ancient Qilian Ocean started at 544Ma, and the collision between the Central Qilian and the Alax mierocontinents began at 462Ma. The ancient Qilian Ocean closed at 442Ma. The west segment of the northern boundary of the Proto-Tethys Ocean retreated northward during the Early Paleozoic, possibly generating the jumping of subduction zones.