【作者】 宋文杰；

【导师】 程荃； 朱介寿；

【作者基本信息】 成都理工大学 ， 应用地球物理， 2011， 硕士

【摘要】 自2005年以来,我们跨龙门山断裂带布设了龙泉驿至道孚(剖面1)和资阳至玛曲(剖面2)两条被动源地震剖面。利用被动源剖面记录到的远震地震波数据通过接收函数方法对其进行接收函数反演成像,得到每个台站下方地壳及上地幔的速度界面。并结合研究区台站下的S波速度和人工地震剖面的地壳分层及速度分布,对地壳结构、活断层面及滑脱面进行研究,对于探讨青藏高原东缘龙门山与扬子克拉通的碰撞接触关系及动力学问题具有重要的意义。研究结果表明:1.扬子地台西缘地壳厚度在35~48km的横向变化;在龙门山推覆体莫霍面起伏变化不大,地壳厚度约45km;松潘甘孜地块在剖面1中莫霍面由跨过龙门山中央断裂带后形成一个16~20km的陡降带,地壳厚度在45~68km之间变化;而在剖面2中莫霍面在跨过龙门山后山断裂带西侧约30km形成12km左右的缓降带,地壳厚度在45~58km之间变化。这结果表明从扬子板块向西至松潘甘孜地块,地壳厚度逐渐增厚,松潘甘孜地块南面莫霍面比北面深约10km左右,剖面1在龙门山推覆体内形成的陡降带与龙门山汶川主震位置的中央断裂带走势一致,而剖面2在龙门山推覆体内形成的陡降带距离龙门山中央断裂带约70km左右,这与龙门山中央断裂带的走势不一致,也与地貌和地表构造特征不吻合。根据扬子板块和松潘甘孜地块地壳厚度的分区性,说明:松潘甘孜地块南部—龙门山地带是莫霍界面强烈变化转折部位,是扬子地块西缘和松潘甘孜地块的构造边界;松潘甘孜北部—莫霍面强烈变化部位在龙门山中央断裂西侧70km,是扬子地块西缘和松潘甘孜地块的构造边界,龙门山构造带不是扬子地块西缘和松潘甘孜地块的构造边界。2.松潘-甘孜地块及龙门山推覆体的中地壳内20~30km存在一个厚10km左右的朝北西缓倾并转为近水平延伸的低速层,该低速层的南东端结束于龙门山中央断裂与前山断裂之间的下方。龙门山前山断裂及以东的扬子地台西缘地壳壳内低速层不发育。松潘-甘孜地块及龙门山推覆体中的低速中地壳易形成滑脱层,使其上覆的上地壳与低速中地壳滑脱拆离,脆性上地壳向东仰冲推覆,使得比较软的松潘-甘孜地块对坚硬的四川陆块,在龙门山由北西向南东逆冲推覆滑动形成了走滑-逆冲推覆构造,并形成前山、中央和后山几条主要铲式断层。龙门山推覆体上覆上地壳和中下地壳滑脱拆离,致使剖面2得到的莫霍面在白溪乡东侧缓降带与地表断裂带不一致。3.松潘甘孜地块岩石圈厚度在130~180km之间,软流圈下界面深约280~290km;龙门山推覆体岩石圈厚度约为140km,软流圈下界面深约260~280km;扬子地块岩石圈厚度为130~140km,软流圈下界面深约270~290km。从扬子地块至松潘甘孜地块向西岩石圈厚度逐渐增厚,起伏不大;软流圈下界面深度无大的起伏,也无明显变化规律。扬子地块和松潘甘孜地块同时存在410km和660km间断面。更多还原

【Abstract】 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.更多还原