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

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