贡嘎山快速隆升的磷灰石裂变径迹证据及其隆升机制讨论

Apatite fission track evidence for rapid uplift of the Gongga Mountain and discussion of its mechanism

鲜水河断裂是青藏高原东南缘的一条北西向大型左旋走滑断裂,其南东段逐渐向南偏转,并与近南北向的安宁河断裂相接,在两个断裂相接处西侧耸立着海拔7556 m高的贡嘎山.磷灰石裂变径迹(AFT)测试可知,贡嘎山及其邻区12个样品的年龄分布在0.2±0.1 Ma～2.7±0.7 Ma之间,平均径迹长度在13.64～15.19μm之间,表明贡嘎山及其邻区第四纪时期一直处于快速剥蚀状态.结合前人在此地区的低温热年代研究成果,揭示出两个现象:(1)贡嘎山岩体及鲜水河断裂与龙门山断裂所夹的三角区域为快速隆升区域,而其西侧、北侧的高原腹地的隆升速率远低于这两个区域;(2)贡嘎山岩体从北向南隆升速率逐渐变大,其最南端1 Ma以来的隆升速率超过3.3±0.8 mm/a.这些现象表明青藏高原在整体横向挤出、缓慢隆升的基础上,还存在着一些特殊的局部快速隆升区域.通过对川滇地块水平运动的矢量分解,我们认为贡嘎山花岗岩体是鲜水河断裂至安宁河断裂间挤压弯曲段吸收、转换川滇地块南东向水平运动导致局部快速隆升的产物,在这一过程中,由于垂直于断裂的挤压分量从北到南逐渐增大,导致了岩体从北往南的隆升速率逐渐增大.

The near-NW trending Xianshuihe fault is a huge and active sinistral strike-slip fault located in the southeast margin of the Tibetan plateau. Its southeast section links with Anninghe fault which extends from north to south, and the 7556 m high Gongga mountain is located on the west of the junction of the two faults. In order to reveal the uplifting history of the Gongga mountain and its adjacent area, we took 12 samples for apatite fission track （AFT） dating. The AFT ages between 0.2±0.1 Ma and 2.7±0.7 Ma and the mean length （ML） between 13.64μm and 15. 19 μm indicate that the Gongga mountain and its adjacent area are denuded rapidly in Quaternary. Combining other researchersr samples with our samples, we find two phenomena： firstly, the Gonggashan granite which is mostly located to the west of the Xianshuihe fault and the triangle area between the Xianshuihe fault and the Longmenshan fault uplift quickly, when other areas uplift with a much slower rate; secondly, the uplift rate of Gonggashan granite progressively turns higher from north to south, and the highest uplift rate from circa 1 Ma to now may equal or exceed 3.3 ± 0.8 mm/a. The phenomenon indicates that,-in the Tibetan plateau, there are some special areas which uplift quickly when most of the plateau uplifts at a much slower rate. According to the vector resolution of the Sichuan-Yunnan block＇s horizontal movement, we think that the rapid uplift of Gonggashan granite is caused by the change of Xianshuihe fault＇s strike, and it absorbs and converts the southeastward horizontal movement of the Sichuan-Yunnan block. In this process, Gonggashan granite uplifts with a higher and higher rate from north to south, because the vector component perpendicular to the fault turns bigger and bigger.