Assessment of Late Quaternary folding and shortening of the Longmenshan fold-and-thrust system (SE China) using fluvial terraces

Determining the magnitude of late Quaternary foreland deformation along the Longmenshan is relevant to assessing its role in regional crustal deformation. The Longmenshan foreland fold-and-thrust system mainly consists of a blind frontal fault zone, the Pujiang-Xinjin anticline, and the Longquanshan anticline. Surface deformation occurs mainly by folding, as well as some surface rupture, linked to the main Longmenshan fault zone through the deep detachment fault. The 2008 Wenchuan Ms 8.0 earthquake occurred along the front and central faults of Longmenshan fault zone. Fluvial landforms are very sensitive to tectonic activity. Quantitative analysis of warped terraces is an effective method for reconstructing the Late Quaternary growth of folds. The Minjiang River crosses the entire foreland thrust-fold system after flowing out of the Longmenshan fault zone. Using remote sensing image and digital elevation model interpretation, we conducted geomorphological and Quaternary sedimentation analyses along the middle part of the Minjiang River. Accurate measurements of river terraces using GPS, along with quaternary geochronology (OSL, AMS-14C, et al), permit reconstructing Minjiang River terraces sequence and longitudinal profile of the terraces to assess Late Quaternary surface deformation. Results suggest that: (1) The Wenchuan-Maowen fault activity predominately in the horizontal direction, with a minute vertical component; (2) The main vertical displacement of the Longmenshan fault zone occurs in Yingxiu-Beichuan fault;(3) The vertical deformation of frontal fault and the frontal buried fault is weak;(4) The Pujiang-Xinjin fault is more active which affected the formation of the T1 terrace;(5) The activity Longquanshan fault is weak, it is relatively stable since T3 formed.