龙门山中段中央断裂和前山断裂的晚新生代垂向活动性差异及其构造意义

2008汶川地震之后,多个研究组对龙门山的新生代剥蚀历史进行了研究,但是在龙门山推覆构造带中段,剥蚀历史研究主要集中在彭灌杂岩,而彭灌杂岩东侧(即中央断裂下盘)的热年代学资料相对缺乏,其剥蚀历史还比较模糊.对于彭灌杂岩东侧岩体的新生代剥蚀历史研究,不仅可以了解龙门山推覆构造带的新生代断层活动历史,而且对于青藏高原东缘的新生代隆升机制具有重要约束作用.在前人热年代学研究基础上,在龙门山推覆构造带中段中央断裂和前山断裂附近补充了一些裂变径迹样品.采用外探测器法(external detector method)对样品进行裂变径迹分析,实验测试在台湾中正大学裂变径迹实验室完成.实验获得了6个锆石裂变径迹和6个磷灰石裂变径迹年龄.前山断裂上盘,AFT(磷灰石裂变径迹)年龄以小鱼洞断裂为界存在明显的差异,其中小鱼洞断裂以南的样品AFT年龄为39Ma,小鱼洞断裂以北的4个AFT年龄介于6—8 Ma之间.研究揭示出中央断裂和前山断裂的新生代活动性以NW向小鱼洞断裂为界存在较大差异:距今8Ma以来,小鱼洞断裂以北,中央断裂和前山断裂的平均垂向滑动速率分别为约0.1mm·a-1和约0.55mm·a-1;小鱼洞断裂以南,平均垂向滑动速率则分别为约0.55mm·a-1和约0.1mm·a-1.低温热年代学方法获得的断层新生代垂向滑动速率与汶川地震断层垂向同震位移分布基本一致.前山断裂(小鱼洞断裂以北)距今8 Ma以来北西-南东向水平缩短量达到8~12km,表明地壳缩短是造成龙门山抬升和剥蚀的重要因素之一.本研究结论不支持下地壳增厚模型对于龙门山隆升的解释.

Differential Late-Cenozoic vertical motions of the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault in the central Longmenshan range and their tectonic implications

After the 2008 Wenchuan earthquake, several groups did some researches on the Cenozoic exhumation history of the Longmenshan range. However, to the east of the Pengguan massif, the thermochronology data are still scarce and the exhumation history is unknown, as the previous researches mainly focus on the Pengguan massif. The research on the exhumation history of the rocks to the east of the Pengguan massif can not only help to understand the Cenozoic faulting histories of the faults at the Longmenshan Thrust Belt, but also make significant constraints on the uplift mechanism of the eastern margin of the Tibetan plateau.We add some fission track samples along the Beichuan-Yingxiu fault and the Jiangyou-Guanxian fault, on the basis of previous studies. The samples are dated at the Fission Track Lab of the National Chung Cheng University, with the external detector method.We get 6 Zircon Fission Track (ZFT) data and 6 Apatite Fission Track (AFT) data. At the hanging wall of the Jiangyou-Guanxian fault, the AFT data show significant difference divided by the Xiaoyudong fault. To the south of the Xiaoyudong fault, the AFT age is 39 Ma, while the ages are between 6—8 Ma to the north.The research reveals that the vertical activities of the Beichuan-Yingxiu fault and the Jiangyou-Guanxian fault since 8 Ma are much different separated by the Xiaoyudong fault: north to the Xiaoyudong fault, the average vertical slip rate along the Beichuan-Yingxiu fault and the Jiangyou-Guanxian fault is about 0.1 mm·a^-1 and about 0.55 mm·a^-1, respectively; south to the Xiaoyudong fault, the average vertical slip rate along the Beichuan-Yingxiu fault and Jiangyou-Guanxian fault is about 0.55 mm·a^-1 and about 0.1 mm·a^-1, respectively. The late-Cenozoic vertical slip rate of the faults revealed by the thermochronology data is coincided with the co-seismic vertical displacement of the Wenchuan earthquake. The horizontal shortening across the Jiangyou-Guanxian fault (north to the Xiaoyudong fault) reaches 8~12 km since 8 Ma, which indicates that the crustal shortening performing as the thrust movement along the main faults is the one of the main causes for the uplift and exhumation of the Longmenshan range. Our results don't support the lower crust thickening model for the uplift of the Longmenshan range.