东喜马拉雅缺口的地质与地貌成因

东喜马拉雅缺口位于西藏东南部米林地区,平均海拔高度只有4500m,远远低于喜马拉雅山其它地段。我们的研究揭示,它的形成是由一条规模很大的,称之为米林韧性正断层的活动造成的。断层带的宽度至少有20km,大体倾向西,主要由眼球状糜棱岩组成,岩石中的拉伸线理以及眼球旋转的方式表明位于其东西两侧的高喜马拉雅深变质岩系和特提斯喜马拉雅中浅变质岩系之间发生过大规模的拆离运动,导致了东喜马拉雅构造结的最高峰——南迦巴瓦(7756m)的早期抬升以及特提斯喜马拉雅的重力垮塌。该断裂的南西端和藏南拆离系(STDS)相交,因此,它很可能是藏南拆离系的东翼断裂,同样形成于中新世。拆离构造的发生表明喜马拉雅山在中新世发生南北向构造缩短的同时还伴随着近东西向的拉伸。米林断裂的北东端和派区断裂相接。后者在中新世呈左旋剪切,构成东喜马拉雅挤入构造的西边界。米林断裂和上述两个断裂的衔接关系表明该断裂是一个协调高喜马拉雅和特提斯喜马拉雅之间斜向拆离运动的转换断层。

GEOLOGIC AND GEOMORPHIC ORIGINS OF THE EAST HIMALAYAN GAP

Two transects, mapped across the eastern boundary between the Tethys Himalayan sedimentary sequence and the High Himalayan metamorphic sequence in Milin area of southeastern Tibet, demonstrate that a roughly W dipping normal fault, namely Milin, separates these two sequences. Triassic rocks in the hanging wall consist of schist and quartzite, dipping to SW with an angle of ~60 degree. They contain mylonitic fabrics and show evidence for normal slip. Those rocks in the footwall consist of augen gneiss, with dipping direction varying from NW to SW. The sense of the rotation of plagioclase augens indicates the hanging wall going downward. The contrasting difference between their metamorphic degree indicates that amount of normal displacement along the Milin fault is very large. In this region the average elevation of the Himalayas is only about 4500 m, much lower than that of any other part of the Himalayas, namely here the East Himalayan gap. Its formation can be interpreted to have resulted from the normal faulting along the Milin fault. To the south, the Milin fault joins with the eastern end of the South Tibet detachment system (STDS), suggesting that it may have constituted the eastern flank fault of STDS. With the highest peak of 7756 m (Namche Barwa), the core of the eastern Himalayan syntaxis is bounded by the Milin fault on the west, indicating that its rapid uplift may have also accommodated by the normal faulting, which occurred in the Miocene. Occurrence of the normal faulting across the eastern Himalayan gap indicates that the Himalayas also underwent E W elongation while the N S shortening occurred in the Miocene.