高黎贡山脉南部的晚新生代构造-重力垮塌及其成因

Extensional Collapse of the Southern Part of the Gaoligong Range in the Western Yunnan, China and Its Tectonic Origin

云南西部高黎贡山脉构成青藏高原东南边界的一部分，平均海拔3000m以上，山脉南部高度突然下降到海拔2000m左右，线性延伸的山体变为平缓起伏的面状高地。野外工作发现，这一地貌突变是由于伸展变形和重力垮塌造成的。晚新生代以来高黎贡山脉中段和南段东西两侧均发育伸展构造，其中分隔盆地与山脉地貌单元的主要正断层被称为高黎贡山脉东坡和西坡主边界正断层，它们控制了山脉两侧盆地沉积及腾冲地块内部的火山喷发作用，造成山脉与盆地的地势差距、山体的构造剥蚀以及水系的下切。高黎贡山脉及其西侧的腾冲地块构成青藏高原周边唯一的伸展边界。这些伸展构造是畹町、瑞丽、梁河等左行走滑断裂的尾端转换构造。畹町断裂的左行走滑运动将上部地壳物质向东、向西拉开，使得高黎贡山脉南部发生构造重力垮塌变形，形成三角形的构造窗，暴露出由变质结晶岩系构成的保山地块基底。磷灰石裂变径迹年龄表明，大约距今14Ma开始的构造-重力垮塌去顶作用首先发生在山脉南部，并逐渐向北发展，高黎贡山脉中部在距今8～5Ma进入伸展构造的强烈活动期。

The Gaoligong range, with an average elevation of 3000 m, lies within the westernmost Yunnan, China. It marks the west edge of the southeastern margin of the Tibetan plateau, bounding the Nu River extending on the east in N-S direction. From Longling southward, the altitude of the south part of the range drops abruptly to about 2000m and the range loses its linear features accordingly. Our fieldwork found that the southern part of the range is bounded by two normal fault systems along its eastern and western margins. The one on the west dips to the west, dividing the Tengchong graben in the hanging wall, the latter is filled with the lacustrine deposits and locus of frequent earthquake and volcanic activities and the oldest sediments are filled with the Pliocene. The normal fault system bounded the range on the east consists of two main branches, the East Main Boundary Normal fault on the west and the Lameng fault on the east. The East Main Boundary Normal fault cuts between the metamorphic rocks of Proterozoic age and the elastic rocks of early Paleozoic age, whereas the Lameng fault cuts between the early Paleozoic and Mesozoic rocks, along which a series of grabens and horsts were developed. The core of the southern part of the Gaoligong range is occupied by a large batholith of Triassic age, intruded into the Proterozoic and Paleozoic rocks. The results of apatite fission-track age dating reveal that the southern part of the Gaoligong range has experienced a significant cooling event, initiated at 14 Ma in the south and 8~5 Ma in the north. We interpret this quick cooling event was associated with the EW extension across the southern part of the Gaoligong range, accommodated by its eastern and western boundary normal faults. These two normal fault systems join with the NE-trending Wanding fault to the south, and the latter defines the southern boundary of the southern part of the Gaoligong range. This fault is active due to its leftlateral movement, accommodating the clockwise rotation of the Tengchong and Baoshan blocks, trigged by the N-S right-lateral movement along the Sagaing fault to the west. Based on the spatial and temporal correlations between the eastern and western boundary faults of the southern part of the Gaoligong range and the Wanding fault, we propose that the eastern and western boundary normal faults formed as the transfer fault transferring the NE--SW left-lateral movement along the Wanding fault, resulted in EW extensional collapse of the southern part of the Gaoligong range. As a consequence, the basement rocks of the Gaoligong range, characterized by Proterozoic metamorphic rocks and Triassic granite, were exposed to the surface, occurred in the late Cenozoic time.