Receive Function Experiment In Lithosphere Structure Of Longmen Shan Fault

We have deployed two passive seismological profiles across Longmenshan faultzone since2005,namely, profile1extending from Longquanyi to Daofu and profile2extending from Ziyang to Maqu. The seismic data from distant earthquakes, acquiredfrom both profiles, have been analyzed to obtain velocity interfaces within lithosphereand upper mantle for each station via teleseismic receive function method. Combiningthe S-waves velocity data, crustal layering and velocity distribution in theseismic profiles, the crustal structure, active fault asperity and detachment surface.This thesis is to illustrate several significant implications for the kinematicalmechanism of collision between Eastern Tibetan plateau and Yangtze craton.We use the teleseismic receiver function method to invert the image of crustaland lithospheric structure, The crust thickness of Western margin of the Yangtzecraton ranges from35to48km. The Moho boundary under Longmenshan nappe is oflittle ups and downs around45km. The Moho boundary forms a rapidly decreasingzone of16~20km, starting from the East across Longmenshan central fault zonetowards Songpan-Ganzi block, with the crustal thickness decreasing from45to68km.In profile2, a12km decrease of Moho depth from30kmacross the Longmenshanfault zone, is a gentles decrease compared to the previous one. The crustal thicknessranges from45~58km. In profile3, a8km decrease of Moho depth near theJiuzhaigou, The crustal thickness ranges from46~54km These results bothdemonstrate the thickness of the crust increases from Yangtze craton toSongpan-Ganzi block.The crustal thickness of southern Songpan-Ganzi block is10km thicker than its northern counterpart. In profile1, the direction of the decreasezone located in the Longmenshan nappe is consistent with the direction of theLongmengshan central fracture zone.However, in profile2, the decrease zone withinthe Longmenshan nappe is70km apart from the Longmenshan central fracture zone, showing inconsistence with both Longmenshan central fault zone and topographicfeatures. Based on the crustal thickness partition property of Yangtze andSongpan-Ganzi block, we propose that Songpan-Ganzi block south-Longmenshan isthe tipping point of the Moho depth and also the tectonic boundary between westernYangtze craton and Songpan-Ganzi block; Songpan-Ganzi block north-thedecrease zone within the Longmenshan nappe is70km apart from the Longmenshancentral fracture zone and also the tectonic boundary between western Yangtze cratonand Songpan-Ganzi block.According to the three section, the overall trend of Moho,The junction between Songpan-Ganzi block and the Sichuan Basin, The crustalthickness,southern deep than norther,western light than eastern.The paper finds that, the low velocity layer thickness is about30-40km, appearedin the Longmen Shan nappe of Ganzi-Songpan within the earth’s crust, low-velocitlayer extends to the front section of Longmen Mountain Fault Zone, Vs between2.75~3.45km/s, H-kappa and magnetotelluric method to support this viewpoint.According to the changes of Moho, low velocity layer position, we think, to theEast the Tibetan Plateau, plots to mount Longmen mobile, hard the Yangtze block stopthis behavior, therefore, part of the earth’s crust substance in the upward migration,increased Longmen mountain, another part of the subduction, gathered a large numberof stress, finally, stress suddenly release, Wenchuan earthquake.