Using Passive Seismological Profile Experiment In Deep Crustal Structure Of Longmenshan Fault Zone And Adjacent Areas

We have deployed two passive seismological profiles across Longmenshan fault zone since 2005,namely, profile 1 extending from Longquanyi to Daofu and profile 2 extending from Ziyang to Maqu. The seismic data from distant earthquakes, acquired from both profiles, have been analyzed to obtain velocity interfaces within lithosphere and upper mantle for each station via teleseismic receive function method. Combining the S-waves velocity data, crustal layering and velocity distribution in the seismic profiles, the crustal structure, active fault asperity and detachment surface. This thesis is to illustrate several significant implications for the kinematical mechanism of collision between Eastern Tibetan plateau and Yangtze craton.Our research has made three main discoveries, as follows,1. The crust thickness of Western margin of the Yangtze craton ranges from 35 to 48 km. The Moho boundary under Longmenshan nappe is of little ups and downs around 45 km. The Moho boundary forms a rapidly decreasing zone of 16~20 km, starting from the East across Longmenshan central fault zone towards Songpan-Ganzi block, with the crustal thickness decreasing from 45 to68 km. In profile 2, a 12 km decrease of Moho depth from 30 kmacross the Longmenshan fault zone, is a gentles decrease compared to the previous one. The crustal thickness ranges from 45~58 km. These results both demonstrate the thickness of the crust increases from Yangtze craton to Songpan-Ganzi block.The crustal thickness of southern Songpan-Ganzi block is 10 km thicker than its northern counterpart. In profile 1, the direction of the decrease zone located in the Longmenshan nappe is consistent with the direction of the Longmengshan central fracture zone.However, in profile 2, the decrease zone within the Longmenshan nappe is 70 km apart from the Longmenshan central fracture zone, showing inconsistence with both Longmenshan central fault zone and topographic features. Based on the crustal thickness partition property of Yangtze and Songpan-Ganzi block, we propose that Songpan-Ganzi block south-Longmenshan is the tipping point of the Moho depth and also the tectonic boundary between western Yangtze craton and Songpan-Ganzi block ; Songpan-Ganzi block north- the decrease zone within the Longmenshan nappe is 70 km apart from the Longmenshan central fracture zone and also the tectonic boundary between western Yangtze craton and Songpan-Ganzi block.2. A low-velocity zone with the thickness around 10 km is located in the middle crust (20~30 km depth) of Songpan-Ganzi block and Longmenshan nappe region. This low-velocity zone slightly dips northwestward then and turns to be lateral with its southeast end between the Longmenshan central fault zone and the front hill fault zone. There is no sign showing this same low-velocity zone existing in the crust of Yangtze craton and Songpan-Ganzi block. The existence of this low-velocity middle crust makes it easy to form detachments between the upper crust and the middle crust in these regions.Furthermore, the force from the Tibetan plateau push the detached upper crust to the East, resulting in the northwestern dipping thrust nappe structure as well as three main fault zones in the Longmenshan region. The same detachment in the between the upper crust and middle-lower crust in Longmenshan causes the inconsistence observed along profile 2.3. The thickness of the lithosphere of Songpan-Ganzi block is between 130~180 km, with the depth of the bottom of asthenosphere at about 280~290 km. The thickness of the lithosphere in Longmenshan nappe belt region is around 140 km with the depth of the bottom of asthenosphere at about 260~280 km. For the Yangtze craton, the lithosphere thickness is 130~140 km and the depth of asthenosphere locates at 270~290 km. From these data, we can conclude that the lithosphere thickness from Yangtze craton to Songpan-Ganzi block has a slight but steady increase and the depth of the asthenosphere barely changes across these two tectonic bodies. There exist 410-km-discontinuity and 660-km-discontinuity under both the Yangtze craton and the Songpan-Ganzi block.