A Study On Active Tectonics In The Yuexi Basin

Active tectonics refers to all structures which have been active since the late Pleistocene, 100-120 ka, are still active recently, and will be active in a certain time period in the future. It includes active faults, active folds, active basins, active volcanoes, and the crustal-lithospheric blocks confined by them.A prominent scientific problem in the Sichuan-Yunnan Region is: How to allocate the deformation caused by the sinistral strike-slip motion of the NW trending Xianshuihe fault among several NS trending faults southward of Shimian. Understanding this problem is not only important to recognize the tectonic deformation of the Sichuan-Yunnan region, but also important to estimate the seismic risk of the Sichuan-Yunnan region. Finely quantitative research on the landforms in the Yuexi basin is important to recognize the evolution of the Yuexi basin and the kinematic features of the active tectonics in the southeastern Tibetan Plateau. The Yuexi basin, having an ellipse shape, lying in the western of Sichuan Province between Xiaoxiangling and Daliangshan, is a basin of tectonic origin, and is controlled by the Yuexi reverse fault which belonging to the the Daliangshan fault. The Yuexi river is running through the basin from south to north.This thesis tries to deep analysis the tectonic evolution of the Yuexi basin by quantitative research the tectonic landforms in the Yuexi basin, in order to further understand the activity of the Xianshui-Xiaojiang faults, the crust motion and tectonic deformation of the Tibetan Plateau, then we can provide proof to understand the deformation caused by the sinistral strike-slip motion of the Xianshuihe fault and how to allocated the deformation among several NS trending faults southward of Shimian.Based on the ideas for topic selection, the study contents and the used approach of this work are summarized as follows:(1)By aerial image interpretation and analysis of the topographic map and geological map, I compile the map of the geomorphologic division of regions.(2)Four balanced geological sections are drwan, i.e. two profiles in the Yuexi basin and two in the southern of the basin, then the crustal shortening amount and shortening rate in the basin and the Daliangshan zone are obtained.(3) I get the vertical displacement rate by making measurements on typical tectonic deformation, comparing the height of terrace (T2) on both side of the Yuexi river and combining the results of the luminescence dating.(4) By calculation the movement rate of the Yuexi fault and Puxiong fault, I analyze the important part of the study of active tectonics in Yuexi Basin in resolving the question about the missing of slip rate in the middle part of the Xianshuihe-Xiaojiang fault system.(5) The thesis makes a comprehensive analysis of evolution history of the Yuexi basin and its important implication in recognition of the kinematic features of active tectonic of the Tibet Plateau.The main conclusions of the work are as follows:(1) The development of landform in the basin show obviously asymmetric, which is mainly distributed in the eastern part of the basin. The Yuexi thrust strikes along the east boundary of the Yuexi basin. Field survey and aerial image interpretation, suggested that there are at least five geomorphic units in the east part of the basin: floodplain, alluvial fan and three terraces. In western basin, there are only three geomorphic units: floodplain and two terraces. TheYuexi basin is a fault-bounded basin, which is controlled by the Yuexi reverse fault which lies in the east boundary of the Yuexi basin.(2) Based on surveying of geomorphology ,combined with analyzing and contrasting of sample ages, the thrusting rate of the Yuexi reverse fault is estimated to be 1.30mm/a.(3) It is estimated that the average crust shortening in the Daliangshan zone is 10.9±1.6 km and the crust shortening rate is 17.8±2.2 % by using balanced cross-sections. The crustal shortening by folding occurred mainly in the Miocene and the Pliocene, lasting no more than 8.6 Ma. Then, the crustal shortening velocity of 1.3±0.2 mm/a can be estimated. Comparing with the left offset along the Daliangshan fault zone, it is recognized that the crustal shortening by folding plays an important part in transferring crust deformation southeastward along the Xianshuihe-Xiaojiang fault system.(4)The slip rate of Daliangshan fault zone in the Yuexi Basin is 3.1 mm/a, which is important to resolve the miss of slip rate in the middle part of the Xianshuihe-Xiaojiang fault system.