A Study On Structural Relief In The Bengco Area Of Tibet Based On Spaceborne InSAR

Since several decades, researches on the Tibet Plateau have been a hotspot ofgeosciences. So far, the uplift history and dynamic mechanisms of the Tibet Plateauhave been debated. There are two reasons. One is the complicated geological structure,the other is lack of the high resolution basic data because of the tough topographiccondition .So is hard to make thorough research.Tectonic connection is complicated in the Bengco area of Tibet, where a lot oflakes and swamp basins scatter. These basins have many similarities ingeomorphologies and movement characteristics nowadays. At the same time, theBengco fault zone intersects near 90° with the Gulu basin and Pengco basin. Althoughtheir kinetic features are different, certain relation exists between them. Therefore,thorough research in this region will help understand recent movement relations ofsouthern tectonics in the Tibet Plateau, and has significance to the dynamic study ofthe whole Tibet Plateau. However, in Bengco, because of complex topographicconditions, research degree is lower. Traditional scientific research means are difficultto meet study demands in this region. At present, developed space vs earthobservational techniques sufferer from lower environmental restriction, specially theInterferometric Synthetic Aperture Radar can work day and night,and all weatherconditions. It can carry out high-precision topological mapping, and providepossibility to further study in this region.This thesis makes use of Spaceborne Synthetic Aperture Radar Interferometric togenerate high-precision digital elevation models of the Bengco zone in Tibet, and toevaluate its applications;On the basis of abstracted high-precision digital elevationmodels, I integrate previous research findings to analyze regional geology and majortectonic geometry at the Bengco zone in Tibet, and to discuss preliminarily theformation relation between South-North tectonics and East-West tectonics.The major work and results arepresented below.(4) Adapt ERS-1/2 TANDEM data, and make use of InSAR to extract digitalelevation model of Bengco in Tibet. The Bengco region is located in eastlongitude 90~92°, north latitude 30.5~32.5°.Four pairs of ERS-1/2 TANDEMdata is used which were imaged in April 1996 have four pairs, the ERS-1/2data were processed by EarthView InSAR to generated Geotiff format DEM,for which the coordinate system of processed DEM is UTM-WGS-84,andpixel size is 20m×20m.(5) Analyze the theoretical precise of ERS-1/2 DEM, and compare applicablywith SRTM DEM published by America in 2004 and Chinese 1:250,000 DEM.From the gained coherence images, I find the images in plains and hills arewhite, displaying better coherence. While in high mountains, because steepvallies and ravines form radar shadow, and display grey black. In lake districts,radar wave produces total reflection, where there is no echo signal. Coherencecoefficiencies are naturalized to 0~1 and counted, except for water areas andimage edges with 0, the other more than a half have > 0.7 coherencecoefficiencies. Thus these data have very good coherence. Compared withChinese 1:250,000 DEM, SRTM DEM and ERS-1/2 DEM, I find that in thestructural relief analyse aspect, ERS-1/2 DEM data have higher resolution thanother two data. From reflected geomorphic information, in this area, 1:250,000DEM resolution is worse and unable to fit the morphotectonic study. SRTMDEM and ERS-1/2 DEM data both clearly reflect geomorphologicalfluctuation characteristics such as mountain lands, linear tectonics, and displaycoherent large-size geomorphological outlines. It is observed that SRTM DEMhas already fulfilled the precision demand for large-size morphotectonicresearches, At microscale geomorphology, ERS-1/2 DEM is superior to SRTMDEM. ERS-1/2 DEM geomorphological zoom degree obviously is higher thanthat of SRTM DEM. Geomorphological characteristic of microreliefs is easyto the distinguished. Linear structures in topography are also discerned inflatter areas. Although SRTM DEM could reflect some microtopography, it isnot as elaborate as ERS-1/2 DEM in details.(6) Based on the digital elevation model, I mainly analyze tectonicgeomorphology of the Bengco fault zone, and discuss preliminarily regionalmovement and tectonic relations in this area. At the southeastern edge of theBengco basin, fluctuated mountain topography causes an inaccurate digitalelevation model. The southwestern side of the Dazi-Dasha fault in this areahas flatter landform. From fault scarps and plain terrain's relation, the fault hasslip movement here. Relative height differences also indicate that normalfaults exist in this area. In the middle of the Bengco fault zone, the Bengcuobasin is of strip-shape. The statistics of geomorphological profilemeasurements demonstrate that except for that the basin's middle breadth isslightly larger, two long sides have approximately equal distances at otherlocations.The nearing east-western trending profile, through the Bengcuobasin, displays that this basin has an inclination plane from southeast tonorthwest. The southeastern and northwestern borders have soaringescarpments. At the eastsouthern edge of the basin, which is close to twoboundary slip faults, the hillslope is approximately perpendicular to swampland, it is inferred that this is boundary normal fault location. Normal faults ofboth sides formed ladder. In addition, movement signs of normal fault werefound in the southeastern marsh. So I preliminarily infered that southeasternbasin border formed three terraces because it sinked northwestwards.Mountains in northwestern end of the basin have a gradual fault trend.Distances between fault blocks are approximately comparable. On the vicinityof the Dazi-Dasha fault, a nearly northsouth trending normal fault seems tooccur, while there is not normal fault evidence near the Naka-Naduiduo fault.The northwestern part of Bengco lake reaches as far as the hill bottom. TheNaka-Naduiduo and Dazi-Dasha faults form evident fault valleys here, andextend northwestward parallelly, The widths of two valleys are almost equal,about 2 km. The Naka-Naduiduo fault is cut by gullies at 91°7′17″E,31°19′35″N, and loses tectonic and geomorphic features. Nearby theDazi-Dasha fault vally interior, a fault should plausibly connetcted with thePengco fault. This physiognomy which cuts through the Dazi-Dasha fault islocated in between 90°59′46″E,31°18′51″N and 91°31′31″E,31°20′8″N. Interms of geomorphic appearance of the Bengcuo fault zone and the same gullynear the Pengco lake, two secondary faults didn't form in the same time. TheNaka-Naduiduo fault has higher elevation and located at the upper reach of thegully. At this point, gully severs the Naka-Naduiduo fault. As a result, theNaka-Naduiduo fault formed earlier than the gully.While at the Dazi-Dashafault, this fault cuts through the gully, showing that the Dazi-Dasha faultformed later than the gully . The Dazi-Dasha fault has moved in recent peroids.So I infer that the Dazi-Dasha fault should form in later age than theNaka-Naduiduo fault does. In the Bengco area, another large tectonic belt-theGulu fault zone is the northern segment of the Gulu-Dangxiong-Yadong faultzone which controls the formation of the Gulu basin. The Gulu basin is flatter,where cliffs in both sides are higher in west and lower in east. The westerncliffs of the basin approximately erect, while eastern ones lightly lean. Nearthe Bengco fault zone, breadth of this basin has variations, the northernbreadth becomes larger than the southern breath.This basin spreads northwardin dustpan-shape, and a gap appears intersecting with the Bengco fault zone,which reflects reconstruction of the Bengco fault zone to the Gulu basin. Onvicinity of the Bengco fault zone, there are some smallapproximatelynorthsouth faulted basins, among the others, Shenco, Bamuco,Pengco, Dongco and Cona basins cross-split to pinch and swell form. Theyare all located in northwestern end of the Bengcuo fault zone, extending in anapproximately identical direction, and exhibit many similarities in topographicform. At the same time, earthquakes in this area scatter as apparent zonaldistribution. Based on comprehensive analyses to the above results, this thesissummed up regional tectonic connections: in the Tibet Plateau, nearly eastwestdirected pullapart movement continuously proceeds. The Bengco fault zonereconstructed the Gulu basin, which suggest some larger-sized approximatelysouthnorth trending grabens formed in the early stage,, along withpolyaddition of nearly northsouth squeezing, nearly eastwest shear slideoccurs in the interior of the Tibet Plateau, and functions conversion. In recentperiods of movement, secondary nearly northsouth structures appearedbetween large nearly north south structures. These nearly northsouth structuresformed later than nearly east-west ones. Some structures infiltrate nearlyeast-west structures. For example, the Pengcuo fault zone traverses theBengcuo fault zone.