墨脱走滑剪切带的构造演化及其对南迦巴瓦构造结形成的制约

The Namche Barwa Syntaxis (NBS), formed by the northward subduction of east India corner beneath the Eurasian plate, is separated from the Lhasa terrane in the north by the sharply curved Indus-Yarlung suture zone. The NBS is a very special terrane in the Himalayan orogenic belt, which is characterized by 1) the strongest tectonic stress, 2) the fastest rates of exhumation and erosion, 3) the strongest deformation, anatexis, metamorphism and magmatism. Strike-slip faulting is one of the most important factors in the formation of the NBS. Therefore, we will focus on the tectonic evolution of the Medog shear zone (the eastern flank of the NBS) by integrating the geometry, fabric kinematics and thermochronological data. These new data, which will be acquired during this project, would potentially provide new constraints on the forming processes and dynamics of the NBS. This study is of great scientific significance to not only the formation and evolution of the NBS, but also the formation and extension of the Himalayan orogenic belt.

南迦巴瓦构造结以雅鲁藏布江大拐弯缝合带为北界与拉萨地体相邻，是印度板块东犄角楔入欧亚大陆的特殊地质体，是喜马拉雅造山带中构造应力最强、隆升和剥蚀最快、新生代岩浆活动、深熔作用和变质作用最强的地区，而走滑作用是南迦巴瓦构造结形成的重要因素之一。本项目拟以“南迦巴瓦构造结”东侧走滑剪切带（墨脱右行剪切带）为切入点，通过其几何学、组构运动学、以及热年代学研究，探讨墨脱走滑剪切带的演化，以及对南迦巴瓦构造结形成过程和动力学机制的制约。该研究不仅对南迦巴瓦构造结的形成和演化，而且对喜马拉雅造山带的形成和扩展过程的认识具有重要的科学意义。

本研究以南迦巴瓦构造结东界墨脱走滑剪切带为主要研究对象，综合野外填图、岩相学观察、岩石组构学及构造-热年代学等多学科研究，认为：(1) 墨脱剪切带具有各段差异性，从南至北分别是：南段北东走向具右行兼上盘下滑性质，中段近南北向具右行走滑性质、北段北西走向具右行兼逆冲性质；(2) 墨脱剪切带经历了由高温（柱面[c]滑移，>650℃）至低温（底面<a>滑移，300-400℃）的演化过程；(3) 墨脱剪切带活动时间始于29.4 Ma~28.6 Ma，并经历了多期构造热事件：~23.4 Ma，16.9–12.6 Ma及~5.3 Ma。同时，我们将墨脱剪切带与南迦巴瓦构造结西界东久-米林走滑剪切带进行对比，认为：(4) 两条剪切带具有变形的连续性，南迦巴瓦变质体受制于二者而相对拉萨地体向北推移，并以高角度插入拉萨地体之下；(5) 南迦巴瓦构造结边界断裂主变形活动时限为32.7-23.2Ma，与STDS、MCT及构造结周缘大型走滑断裂的活动时限相吻合。