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NEAR-SURFACE CHARACTER OF PERMANENT GEOLOGIC DEFORMATION ACROSS THE MEGA-STRIKE-SLIP FAULTS IN THE NORTHERN TIBETAN PLATEAU

XU Xi-wei1)YU Gui-hua1)CHEN Gui-hua1)LI Chen-xia1)ZHANG Lan-feng1)Yann Klinger2)Paul Tapponnier2)LIU Jing3)1)Institute of Geology,China Earthquake Administration,Beijing 100029,China2)Institut de Physique du Globe,75005 Paris,France3)Institute of Tibet Plateau Research,Chinese Academy of Sciences,Beijing 100085,China  
The Altyn Tagh,Kunlun and Haiyuan Faults are three major left-lateral strike-slip faults with high geologic and GPS-derived horizontal slip rates as well as frequent surface-rupturing earthquakes in the northern Tibetan Plateau.There exist local structures,such as pull-apart basins in stepovers and sag ponds,where fine-grained and/or organic interfaulting sediments have been continuously filled and co-seismic faulting traces have been well preserved in those sediments.Trenching across the strike-slip faults and those local structures,stratum-logging of the trench walls and structural-stratigraphic examination can uncover basic features of the permanent and cumulative geologic deformation zone of a strike-slip fault that has experienced several surface-rupturing earthquake cycles.The geologic section of the Banguoba trench 9km east of Old A'kesai Town across the recent traces of the Altyn Tagh Fault records 7 paleoearthquake events with a co-seismic left-lateral slip of 7±1m for the latest event and its cumulative geologic deformation zone is only 8m in width.The geologic section of the western Old A'kesai trench across a pull-apart basin of the Altyn Tagh Fault records at least 4 paleoearthquake events and its permanent and cumulative geologic deformation zone is only 13m in width.The geologic section of the Xidatan trench across the Kunlun Fault reveals 5 paleoearthquake events and their cumulative geologic deformation zones are 12~13m in width.The Maqin trench across a pull-apart of the Kunlun Fault also reveals 5 paleo-earthquake events and most of the structural deformation,about 15m wide,is concentrated in the pull-apart,while the widest structural deformation,including the associated distortion nearby the boundary fault of the pull-apart,is less than 35m.Two trenches excavated across the southern and northern boundary faults of the Songshan pull-apart basin along the Maomaoshan-Laohuashan segment of the Haiyuan Fault show up 6 paleoevents and their permanent geologic deformation zones are less than 10m wide for single boundary fault.Of course,as a extensional jog,the pull-apart basin over a hundred meters wide will experience severe tensional and transtensional surface ruptures during an earthquake,and the pull-apart basin itself may be taken as one part of the permanent geologic deformation zone.Thus,the repeatedly faulting of the Altyn Tagh,Kunlun,and Haiyuan Faults during the past several surface-rupturing earthquake cycles is localized along their strike and the width of their permanent geologic deformation zone for a single strike-slip fault is over 10 meters,but less than 30 meters in general.
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