西藏班公湖—怒江构造带尼玛盆地构造性质

雅鲁藏布江洋俯冲及印度-欧亚陆陆碰撞导致了强烈的大陆岩石圈挤压变形与青藏高原的隆升。研究青藏高原内部破碎带构造-沉积演化,对理解相关变形如何向欧亚大陆腹地扩展传递至关重要。班公湖—怒江缝合带内发育一系列白垩纪—新生代陆相沉积盆地,保存了关于该时期高原内部构造-沉积演化的丰富信息。针对该类盆地的构造性质和形成机制有走滑拉分盆地、断陷盆地、前陆盆地3类不同观点。若要检定上述观点,需要开展如下工作:(1)查明盆地基底与充填建造变形特征;(2)结合构造背景探究其演化机制。鉴于此,本文对该带内尼玛盆地开展大比例尺地质填图与构造分析,结合前人成果,对盆地构造背景、构造性质和构造演化进行了探讨。主要取得了如下认识:(1)尼玛盆地基底为班公湖—怒江洋闭合形成的软碰撞缝合带内的变质岩与海相沉积岩。基底断裂为近东西走向,倾向或南或北的逆冲断裂。(2)盆地充填建造为上白垩统—新近系多旋回河湖相沉积。其变形样式主要为轴向近东西延伸的非对称褶皱,局部卷入基底断裂变形。多幕次变形自边缘向盆地中心前展式递进发展。(3)盆地可以划分为盆北掀斜隆起、南部推覆扇状隆起两处主要剥蚀物源区、中部基底断片掀斜隆起一处次要剥蚀物源区,以及北部叠瓦状压陷区与南部对冲压陷区两处主要构造沉积单元,其构造格架可以概括为“三隆夹两坳”。(4)尼玛盆地肇始于班公湖—怒江洋闭合导致的南北向地壳缩短。其后,雅鲁藏布江洋北向俯冲与印度-欧亚碰撞所致南北向挤压,导致盆地基底断裂发生周期性活动,伴有多旋回磨拉石建造与递进变形。简言之,尼玛盆地为软碰撞缝合带之上发育的山间压陷盆地。

Structural characteristics of the Nima Basin in the Bangong-Nujiang tectonic belt,central Tibet

The Neo-Tethyan subduction and subsequent India-Eurasia collision resulted in significant contractional deformation of the continental lithosphere and the uplifting of the Tibetan Plateau. Revealing the formation mechanism and process of giant fracture zone in Tibet is crucial for understanding the propagation of the deformation toward the hinterland of the Eurasian continent. Several continental basins of Cretaceous-Cenozoic age developed along the Bangong-Nujiang suture zone, providing a splendid record of the tectonic-sedimentary evolution of the central Tibet. So far, competing geodynamic models are proposed to explain the structural characteristics and the formation of these basins, including strike-slip faulting, extensional rifting and foreland flexural depression. The key to testing these models includes (1) fully documenting the structural characteristics of the basin basement and sedimentary infills and (2) clarifying the structural evolution of the Nima Basin in a regional tectonic context. With these goals in mind, we conducted large-scale geological mapping and structural analyses on the Nima Basin in the Bangong-Nujiang suture. Together with previous research findings, we discussed the tectonic settings, structural characteristics and structural evolution of the Nima Basin and reached the following conclusions: (1) The basement of the Nima Basin mainly consists of metamorphic rocks and marine sedimentary sequences within the Bangong-Nujiang suture zone formed through “soft” collision, and is fractured, with several E-W striking northerly/southerly dipping reverse faults. (2) The Nima Basin is infilled with upper Cretaceous-Neogene multi-cycle fluvial and lacustrine deposits; the terrigenous sedimentary infills later deformed into asymmetric folds with E-W trending axial planes and locally involved in the reactivated basement faults, while multi-phase contractional deformation propagated from the southern edge toward the basin center. (3) The surface structural pattern of the Nima Basin features “two depressions between three uplifts”, where the terrigenous sediments are mostly sourced from the northern imbricate thrust system and the southern fan-shaped extrusion structure, and, along with clasts derived from the central thrust nappes, accumulate in the two depressions—a northern one at the footwall of the imbricate thrusts and a southern one sandwiched between two reverse faults with opposing polarities. (4) The demise of the Bangong-Nujiang Ocean resulted in regional N-S crustal shortening and syncontractional development of the Nima basin in the suture zone, after which the Neo-Tethyan subduction and subsequent India-Asia continental collision triggered repeated reactivation of the basement faults within the Nima region, which invoked both multi-cycle deposition of molasse sequences and propagative deformation. In short, the Nima Basin is a contractional depression basin superimposed over a “soft-collision” type suture zone.