小江断裂带巧家段晚第四纪走滑速率研究

The late Quaternary strike-slip rate of the Qiaojia segment of the Xiaojiang fault zone

鲜水河-小江左旋走滑断裂系是调节青藏高原东南部物质向东南挤出的大型边界断裂。云南巧家断裂作为小江断裂带北段,其晚第四纪走滑速率是认识川滇地块东部边界应变调节方式的关键。本文利用无人机航摄和地面激光扫描技术,获取了该断裂段穿过金沙江河谷区红路和蒙姑两处的高分辨率地形数据,恢复出断层错动T2和T3两期阶地陡坎上缘的左旋位错量分别为120±5~128±1 m和193±1~202±1 m。根据T3中次生碳酸盐的AMS-14C法测年结果,结合已有的类似阶地年龄数据,并经气候曲线校正后认为,区域上T2和T3被废弃应分别发生在冰后期和末次盛冰期末期,时间为8.5~11.2 ka BP和18.6~21.4 ka BP。据此估算,小江断裂带巧家段的晚第四纪平均走滑速率为10~13 mm/a。进一步统计分析小江断裂带的晚第四纪走滑速率,发现巧家至宜良以北的段落,总体保持着10~15 mm/a的高走滑速率。但从宜良向南,断裂走滑速率出现了分段递减的特征,至建水以南快速减小到中-北段的近十分之一。小江断裂带中-北段的高走滑速率以及向南的分段式递减现象,反映在宜良以北,小江断裂带的走滑剪切作用是调节川滇地块向东南旋转-挤出运动的主要方式,但向南伴随变形分解作用,调节方式转变为了伸展、旋转和逆冲等多种方式共存的复杂形式。因此,进一步精细化定量限定川滇地块东部边界断裂的应变分解作用,是深入认识青藏高原物质挤出方式及其机制的关键。

The sinistral strike-slip Xianshuihe-Xiaojiang fault system is one of the largest boundary fault zones that accommodate the southeast extrusion of crustal masses from the southeastern Tibetan Plateau.The late Quaternary strike-slip rate of the Qiaojia fault,the northern segment of the Xiaojiang fault zone,is the key to understand the strain accommodation mode of the eastern boundary of the Sichuan-Yunnan Block.In this paper,high-resolution topographic data of the fault section at Honglu and Menggu along the Jinsha River Valley area are obtained,using Structure from Motion(SfM)and Terrestrial Laser Scanner(TLS)techniques.It is shown that the left-lateral displacements cutting across the upper parts of the T2 and T3 terraces are 120±5~128±1 m and 193±1~202±1 m,respectively.AMS-14C dating of secondary carbonates from the T3,in combination with the previous dating results of similar terraces and correction by climate curves,reveals that the abandonment of T2 and T3 in the region occurred at ca.8.5~11.2 ka BP and 18.6~21.4 ka BP in the post glacial period and at the end of the last maximum glaciation period,respectively.Accordingly,the average late Quaternary strike-slip rate of the Qiaojia fault is estimated at ca.10~13 mm/a.The late Quaternary stike-slip rates of the other two segments of the Xiaojiang fault zone are further summarized in this study.It is shown that the fault zone generally maintained a high strike-slip rate of 10~15 mm/a from Qiaojia to the north of Yiliang.However,the strike-slip rate decreases step-by-step from Yiliang southward and rapidly decreases to nearly one tenth of the rates of the middle-north segment to the south of Jianshui.The step-by-step decrease along the south part of the fault zone suggests that the extrusion movement of Sichuan-Yunnan Block has changed from sinistral strike-slip shearing to concurring extension,rotation and thrusting in this region.Therefore,further quantitative constraining the strain partitioning along the boundary faults of the Sichuan-Yunnan block may provide deep insights into the accommodation mode of mass extrusion in the southeastern Tibetan Plateau.