Study On The Main Fault Activities And Tectonic Deformation Tectonics In The Eastern Margin Of The Qinghai-Tibet Plateau

The eastern margin of the Qinghai-Tibet Plateau (eastern Chuanqing Block) is atypical region for studying on the kinetics of the Qinghai-Tibet Plateau and ambitusbasins.In this unique region, the tectonic deformation mechanism from the affectiondue to the collision between the Indian Plate and the Eurosian Plate; that is, the upliftis produced by thrust faulting and crustal shortening or by inflation of the ductilelower crust. The Wenchuan Ms8.0earthquake of May12th,2008located at theLongmen Shan fault of the eastern margin of Qinghai-Tibet Plateau attracted moreresearchers to discuss the recent tectonic deformation in this region.With the basic goal of studying the active faults in this region, this paperdiscusses in detail the active features on the faults and gains some data, such as faultsegmentation, slips rate. In the meantime, by other methods of landform, the crustmovement observation, earthquake activities and fault-combination, this paperpresents the reasonable explanation for the crust deformation in the eastern margin ofQinghai-Tibet Plateau.(1)By the studies on the geological, geomorphology and seismic data, the mainboundary faults in the Qinghai-Tibet Plateau (eastern Chuanqing Block) shows thedifferent activities since late Quaternary. Among them, the southeastern section(Tazang fault) of NW trending eastern Kunlun fault shows the main horizontalmovement with a maxima sinistral slip rate of3mm/a. The NS trending Min Jiangfault and Hu Ya fault show the vertical movement with a slip rate of1mm/a or so. TheNE trending Longmen Shan faults presents a thrust movement with an amount ofdextral horizontal movement. A slip rate of1mm/a or so can be seen in the eachsingle fault in the fault zone.(2)The lateral branch faults near the boundary fault show some active evidenceson geology, landform since late-Quaternary. The NW trending Diebu-Bailongjiang fault zone is composed of several parallel branch faults, which show the active featureof sinistral slip with some vertical amount. On the each section, the slip rate is lessthan the amount on the boundary fault-eastern Kunlun fault. The NE trendingLongriba fault is divided by two active branch faults, on which, the dextral slip rate isbigger than the amount on the east side main faults—Min Jiang fault and Hu Ya fault.(3)These main faults in this region show obvious segmentation feature. Amongthem, the newest active section of the central fault-Beichuan-Yingxiu fault,Longmenshan faults, produced the Wenchuan Ms8.0earthquake of May12th,2008.Some important evidences of ground fault and earthquake rupture investigation andtrenching, seismic activity and geophysical field in depth explain that its northeasternend should extend to Qiaozhuang Town, Qingchuan County, and approach to the backrange faults of the Longmen Shan faults. The total section length is about290km.(4)In the eastern margin of the Qinghai-Tibet Plateau, some branch faults nearthe main faults in the north part of the margin, take an important role in the process ofregional tectonic deformation. Combining with the parallel branch faults,Diebu-Bailongjiang faults, the south eastern section (Tazang fault) of NW trendingeastern Kunlun fault accept and regulate the regional crust deformation with SEEdirection form the crust material escape due to the rapid uplift of the Qinghai-TibetPlateau. In the east part of the margin, the NE trending Longriba faults also regulatesand decomposes the regional tectonic deformation, with the main border fault-MinShan block and Longmen Shan faults, a typical thrust-faulting zone from west to east.So Longriba faults possibly become a kind of trailing edge reverse slip and can absorband regulate a part of crust deformation obviously. Through the studies, the total slipvalues along the eastern margin of the Qinghai-Tibet Plateau are matched. It indicatesthat the slip and movement along the brittle faults in the crust matches with the forcefrom the west.(5)On the basis of research on the geomorphology, the main faults in this regionhave a common characteristic of succession since the Quaternary. The tectonicaffection should play a controlling role on the process of shaping topography in theregion.(6)The earthquakes occurred at this region all belong to the shallow focus type,Recent tectonic stress field shows a feature of near horizontal maxima press forcewith a NWW-NW direction. The strong earthquake almost toke place in the border ofactive blocks, joints of active faults and special section of strong new tectonicmovement. The result of recent crust deformation observation (GPS) shows that the main faults obviously control and regulate the crust deformation.(7)The crust shortening due to the thrust-slip movement in the brittle faults ismain source for the orogenic movement in this region. The regional crust deformationwith SEE direction form the crust material escape due to the rapid uplift of theQinghai-Tibet Plateau drives huge brittle faults in the upper crust to slip or thrustalong the low-velocity layer as a detachment-slip zone in the depth of20km.Through the thrust movement of the Longmen Shan faults, with tectonicdeformation transition of horizontal slip in the end of the north part in this area, thecross section crust has been shortened and thicken so as to produce such distincthigh land.