三江造山带后碰撞断裂构造带的结构与演化:以新生代剑川—兰坪盆地为例

Structure and Evolution of the Post-Collisional Fault Structures in the Three River Orogenic Belt, Exemplified by the Cenozoic Jianchuan-Lanping Basin

三江造山带作为调节印度 -欧亚板块间的碰撞及后碰撞过程的重要构造带 ,具有长期多阶段复杂演化过程。在剑川—兰坪新生代盆地发育的澜沧江断裂带、中轴断裂带和乔后断裂带是三江造山带不同时期演化的具体体现。它们在产状、组成和特点等方面有着显著的差异。构造分析表明 ,澜沧江断裂带与印度 -欧亚岩石圈板块间的早期碰撞过程密切相关 ,它制约着剑川—兰坪盆地的发育 ;乔后断裂带和中轴断裂带对于剑川—兰坪盆地具有强烈的改造作用 ,是后碰撞调解过程的结果。三江造山带对于印度 -欧亚板块间碰撞的调解过程 ,包括了水平侧向逃逸与垂向物质蠕散。在剑川—兰坪地区 ,前者表现为乔后断裂早期阶段右行走滑作用产生了大约 90km的走向错移 ,而后者形成了广布于中轴断裂内遍布的小型逆冲构造和乔后断裂西部的大量逆冲和推覆体构造。

The Jianchuan-Lanping Cenozoic basin is located in the Three River orogenic belt, an important tectonic belt generated during collisional or post-collisional processes due to India-Asia interaction. There are several fault zones in the basin and adjacent areas, including the Lancangjiang fault zone, the Central axis fault zone and the Qiaohou fault zone. The N-S extending Lancangjiang fault zone is characterized by occurrence of mylonites and strongly foliated granitic rocks and meta-sedimentary rocks. Foliations dip steeply either to the east or to the west with stretching lineations in the direction along their strikes. Shear sense indicators in tectonites suggest a left-lateral shear during mylonitizartion along the fault zone. The Central axis fault zone is a zone of brittle fractures in N-S direction. The fracture surfaces mainly dip to the west with dip angles varying from 0° to 90°. The shallowly dipping surfaces often cut across those with high angles form thrusts and nappe structures at different scales. Qiaohou fault zone dipping to the NNE with dip angles mainly of 60°-70° to the east is the northern extension of the Ailaoshan-Red River fault zone. Evidences for both strike-slip faulting and thrusting are obvious along the fault zone. From the absence or occurrence of vein materials along fault zones or on fault surfaces the fault zones are subdivided into two different types, wet and dry, which gives clues to the involvement of fluid phases during faulting. Veins of irregular forms indicate a high fluid pressure during deformation and striae of new mineral grains like calcite on fault surfaces are resulted from low pressure effect during faulting. Structural analysis of the three fault zones and consideration of their relationships in the regional context suggest that the Lancangjiang fault zone is related to the collision between Indian and Asian plates and faulting along the Central Axis fault zone and Qiaohou fault zone is attributed to post-collisional effects. On the other hand, the fault zones have important contributions to post-collisional crustal adjustment, including lateral (e. g. large scale horizontal displacement along the Qiaohou fault zone) and vertical (E-W shortening due to thrusting) extrusions.