Tectonic Geomorphology Of The Nantinghe Fault In Southwestern Yunnan

The ongoing convergence between India and Eurasia apparently is accommodatednot merely by crustal shortening in Tibet, instead also by motions along strike slipfaults which are usually boundaries between tectonic blocks, especially in the Tibetanplateau and adjacent Yunnan and Sichuan provinces. Many researchers have studiedthese large-scale faults from tectonics, geomorphology, chronology, and geophysics,and proposed various models of kinematics and dynamics for the Tibetan plateau.Among them, the slip-line field theory, block model, channel flow model andmicoplate model focus on the boundaries between tectonics blocks, while the faultsand movement within these blocks have captured a little attentionSoutheast of the Tibetan plateau lie the Chuandian block and southwestern (SW)Yunnan block bounded by the Red River fault. Two major sets of active faults arepresent in the SW Yunnan block: one is NE-trending arc-shaped left-lateral strike-slipfaults including the Longlin-Ruili fault, Nantinghe fault, Menglian fault and Daluofault, which run about the eastern Himalayan syntaxis with an interval150km; theother is NW-NNW directed right-lateral strike-slip faults, which comprise the Lincangfault and Hanmuba fault. Either of the two fault sets can generate major earthquakes.Base on the GPS, earthquake date and geomorphology, the overall movement mannerof the SW Yunnan block has been well described. But the limited data about the faultswithin the block makes it difficult to understand the kinematics and dynamics as wellas tectonics and seismicity of the block’s interior.This thesis focuses on the NE-trending Nantinghe fault and adjacent areas, whichis the largest one of the first fault set in the SW Yunnan block aforementioned, bearing most conspicuous characters. The approach is to perform quantitative study oftectonic geomorphology around this fault. From the results of such research, thisthesis further analyzes the kinematics and dynamics of the NE-trending arc-shapedfaults in the SW Yunnan block and discusses their implications for tectonics of theTibetan plateau.In the early time, the research of the Nantinghe fault was motivated by thegeological surveys for construction of the hydropower station. The YunnanSeismological Bureau suggested that this fault is the most active in the SW Yunnanblock, of which the NE section crosses the Lancangjiang fault obliquely. But ZhuYuxin disagreed this conclusion, and thought that the Nantinghe fault does not crossthrough the Lancangjiang fault, instead terminates in the Linchang-Menghai granitemasses west of the Nantinghe fault. No systematic work about geometry of theNantinghe fault has been made. The differences in activity between the northern andsouthern sections of this fault is also a controversial issue. Base on the thickness ofQuaternary basins along the fault and paleo-earthquake profiles, Zhu Yuxinconsidered that although the north segment’s activity is weaker than the southsegment, the former remains active in the Quaternary. While Wang Jinnan believedthat the late Quaternary activity of the north segment is weak. Moreover, there is noreport about the activity parameters such as slip rates and paleo-earthquake events onthe Nantinghe fault, which are fundamental to understanding the fault activity and thekinematics and dynamics of the block.Since the1970s, active tectonics has advanced from qualitative research toquantitative research. Many researchers focus on which qualitative parameters toobtain and how to obtain them. It is usually accepted that the following parameters arenecessary for quantitative descriptions of active faults: length of a fault or segment,displacements, slip rates, and paleo-earthquake events. Because of complex nature ofproblems concerned and limited capability of human recognition, there are still someerrors and uncertainties in these parameters. For the Nantinghe fault, such studies arealso far from sufficient because of the following reasons.(1) Abundant rain and strongsurface denudation may have destroyed offset landforms and caused missing of paleo-earthquake records.(2) Limited by the chronology in early time, there existbig uncertainties in slip rates and ages of paleo-earthquake events.(3) Theidentification of paleo-earthquake events and fault segmentation also bearuncertainties. So it is necessary to develop a set of methods to acquire theseaccurately parameters and to assess their quality.Tectonic geomorphology provides a useful tool to help solve the problems above.Tectonic movements have been exerting significant influence upon formation oftopography and landforms, and such processes are usually extremely slow over verylong time which cannot be documented by human history and any instruments.Observations, especially direct measurements of various feature of geomorphologycan reveal details of tectonic movements, including slip on active faults. In the earlytime, such studies were usually limited in one or two parameters of geomorphology tocharacterize active tectonics. With rapid development of computer and DEMtechnologies, it is possible to use multiple parameters of landforms to describeregional tectonic activity. Previous work in this aspect focuses on large scales, while alittle on small scale faults or individual faults. And existing studies are mostlyconcerned with normal or thrust faults dominated by vertical motion.This thesis combines tectonic and geomorphic parameters to study the Nantinghefault, using multiple methods such as field investigations, remote sensinginterpretation, paleo-earthquake data, AMS14C dating, DGPS survey, and DEManalysis. The primary contents and conclusions of are presented below.(1) Relationship between fractal parameters and tectonic activity in SW YunnanUsing the90m SRTM DEM and fractal theory, this work obtained2D fractalparameters of SW Yunnan and discussed the relationship between these parametersand tectonics.1) The relationship between fractal factors and lithology or climate is weak. There isalmost no influence of lithology or climate on fractal dimensions.2) The relationship between tectonic activity and fractal dimensions is strong. Thefractal dimensions are low in the areas with intense tectonic activity, and arerelatively high in the regions which are stable in tectonics. 3) The ordinate-intercepts are not sensitive to tectonic activity. The distributions ofordinate-intercepts are consistent in the areas with or without intense tectonicactivity.4) The distribution maps of fractal dimensions show the different characteristics of theparallel NE-trending faults, which implies possible differences in their movementparameters.(2) Geometric characteristics of the Nantinghe faultBased on comprehensive remote sensing interpretation on high-resolutionsatellite images such as ALOS data and aerial photographs as well as fieldinvestigations, this work suggested that the NE end of the Nantinghe fault is locatedon the west side of Lancangjiang fault. The Nantinghe fault extends in N40-60°Edirection.The Nantinghe fault can be divided into two segments bounded by nearbyDaxueshan Town. According to geometric characteristics, each segment can befurther divided into three segments, totaling six (hereafter called segment A to F).Segment A is featured by sinistral strike-slip displacement of rivers and linear gorges.In the south of segment A, there is a2km-long reverse scarp. In northern segment B,the sinistral strike-slip displacement of terraces of the Nanqiao river and verticaldisplacement of the same terraces on two sides of river are obvious. In the middle ofsegment B, the fault stretches along with the Nanqiao river. The south section ofsegment B exhibits triangular facets and deep gorges. Segment C is also characterizedby sinistral strike-slip of rivers and linear gorges. The characteristics of segment D aredifferent in north section and south section. In the northern segment D, the fault tracesare clearly evidenced by sinistral strike-slip of rivers, but in the south section the faulttraces are not clear. On images, segment E is recognized by large gullies and lineargorges. Sinistral strike-slip on terraces and alluvial fans are also seen on segment E.Except the middle, the south and north parts of segment F are not notable.(3) Late Quaternary activity of the Nantinghe fault.Displacement measurement of terraces and AMS14C dating estimate the sinistral-slip rate to be3.6±0.4mm/a and the throw rate as1.1±0.3mm/a along thefault since8ka. Trenching at there locations suggests that the active fault isdominated by strike-slip and it has generated two earthquakes in last1000a.(4) Relationship between tectonic geomorphology parameters and tectonics.Based on high-resolution DEM extracted from ALOS data, this work obtainedthree tectonic geomorphology parameters, which are deflection angle of catchment,steepness index and hypsometric integral in180catchments along the Nantinghe fault.The steepness index and hypsometric integral, which are used by many researchersimplies the rock uplift rates. The deflection angle of catchment is for the first timeapplied to reflect the sinistral strike-slip displacements. Using the distribution patternsof these factors this work examined the geometry and activity of the Nantinghe fault,which were derived from comprehensive remote sensing interpretation and fieldinvestigations.(5) Kinematics and dynamics of SW Yunnan blockBased on previous studies and comparison with the new results presented above,this work suggested that the sinistral-slip of a series of NE-trending faults in the SWYunnan block are resulted from clockwise rotation when the material of the Tibetanplateau is transferred toward southeast.