Abstract:

Fold scarps, a type of geomorphic scarp developed near the active hinge of active folds due to the local compressive stress, are formed by folding mechanisms of hinge migration or limb rotation. At present, there are several proven methods, which are only based on the fold scarp geometry combined with the occurrences of underlying beds and do not use the subsurface geometry of thrust fault and fold to obtain the folding history. The use of these methods is of great significance to illuminate the seismic hazards and tectonic processes associated with blind thrust systems.
The Sansuchang fold-thrust belt is a fault-propagation anticline controlled by the Sansuchang blind thrust fault located in the southern Longmen Shan foreland area. Previous study used the area-depth method to calculate the shortening history of the Sansuchang anticline since the late Pleistocene(73~93ka)based on the terrace deformation of Qingyijiang River. However, due to the serious erosion damage to the terrace after its formation, the shortening history obtained by incomplete terrace deformation needs to be further verified.
A~9km long scarp was found on the Dansi paleo-alluvial fan on the eastern limb of the Sansuchang fold-thrust belt. According to the detailed field investigation and the fold geometry built by the seismic profile, we found the scarp is near the synclinal hinge, which separates beds dipping 10°~17° and 43°~57° east and parallels with the Sansuchang fold hinge. Therefore, we determined the scarp is a fold scarp formed by the forelimb hinge migration of the fault-propagation fold.
The maximum height of the scarp, extracted by the swath topographic profile across the scarp, is about 28~35m. According to the parameters of the fold scarp height, the underlying beds dip angle near the fold scarp, and the quantitative geometric relationship between shortening and the blind Sansuchang thrust fault, it can be estimated that, after the deposition of the Dansi paleo-pluvial fan((185±19)ka), the anticline forelimb horizontal shortening rate is~0.1mm/a, the fault tip propagation rate of the Sansuchang blind fault is(0.5+0.3/-0.1)mm/a, and the total shortening rate of the Sansuchang anticline is(0.3+0.2/-0.1)mm/a.
The folding rates of the Sansuchang fold-thrust belt since the late middle Pleistocene has been obtained by the local deformation characteristics of the fold scarp in this study. The result is basically consistent with the shortening rate since late Pleistocene obtained by complete terrace deformation across the anticline, which proves that the shortening rate of the Sansuchang anticline is relatively stable at~0.3mm/a. It provides a new idea for studying the activity characteristics of fold-thrust belts in the southern Longmen Shan foreland thrust belt area with a fast denudation rate and discontinuous geomorphic surface.

Key words: fold scarp, fault-propagation fold, Longmen Shan, Sansuchang anticline, Late Quaternary shortening rate

摘要：

褶皱陡坎是发育在褶皱活动轴面变形应变集中区的地貌陡坎。通过褶皱陡坎的几何形态及下伏基岩地层的产状可以约束活动褶皱的变形历史。文中在龙门山前陆冲断带南段三苏场背斜北段东翼向斜轴面附近的中更新世丹思冲积扇面上发现了与褶皱轴线平行、 坡向E的线性地貌陡坎。通过对该陡坎的详细野外调查, 结合利用地震反射剖面建立的几何学模型, 确定该陡坎是由于断展褶皱前翼向斜枢纽的迁移而形成的褶皱陡坎。利用横跨陡坎的条带地形剖面获取该段陡坎的最大高度为28~35m。根据褶皱陡坎的高度与下伏活动枢纽两侧地层的倾角(分别为10°~17°、 43°~57°)及深部隐伏三苏场逆断层缩短量间的定量几何关系, 估算丹思冲积扇形成以来背斜东翼的水平缩短速率约为0.1mm/a, 三苏场断层的扩展速率为(0.5+0.3/-0.1)mm/a, 背斜的总缩短速率为(0.3+0.2/-0.1)mm/a。利用褶皱陡坎获取活动褶皱晚第四纪变形量的方法, 为研究高剥蚀速率、 变形地貌面不连续的龙门山前陆冲断带的活动性提供了一种新的思路。

关键词: 褶皱陡坎, 断展褶皱, 龙门山, 三苏场背斜, 晚第四纪缩短速率