【作者】 刘明军；

【导师】 李松林；

【作者基本信息】 中国地震局地球物理研究所 ， 固体地球物理学， 2007， 博士

【摘要】 本文主要依据穿过巴颜喀拉地块的北部、秦岭地块、祁连地块、海原弧形构造区和鄂尔多斯地块的长约1000km的玛沁-兰州-靖边人工地震剖面资料，对青藏高原东北缘的地壳结构、组成和动力学进行研究。首先，重新对该剖面进行P波和S波的联合处理，获取P波、S波的速度结构和泊松比结构。然后，对影响地震波速度的温度和压力进行分析研究，推导消除温压影响将野外观测的原位P波速度校正到实验室温压条件下的校正公式。将校正后的速度与实验室岩石测量结果进行对比，确定研究区的岩性组成。最后，根据青藏高原东北缘地壳结构和组成的研究成果，探讨地壳增厚、拆沉作用和高原隆升等方面的地球动力学问题。P波、S波结构和泊松比结构研究揭示了青藏高原东北缘地壳的许多重要特征。①地壳厚度从北东向南西逐渐增加。在北东的鄂尔多斯地块的地壳平均厚度为42km，而在南西的巴颜喀拉地块厚63km，共增厚21km。其间，海原断裂以南和泽库以南地壳明显变厚。②地壳速度沿剖面有较大变化。鄂尔多斯地块、海原弧形构造区、祁连地块、秦岭地块和巴颜喀拉地块的平均P波(S波)速度依次为：6.30km(3.48km／s)、6.22km(3.40km／s)、6.25km／s(3.54km／s)、6.20km／s(3.51km／s)和6.10 km／s(3.46km／s)。③剖面地壳P波平均速度低于全球地壳P波平均速度。整条剖面地壳P波平均速度为6.22km／s；若除地壳顶部10km厚的地层，平均速度为6.27km／s，与全球平均速度(6.45km／s)相比低0.18km／s。④秦岭地块和海原弧形构造区存在低速带和叠层界面。在这两个地区，低速带有低S波速度和高泊松比；康氏界面和Moho界面为叠层，表明有强烈的构造活动。⑤青藏高原东北缘地壳增厚主要发生在下地壳。研究区中地壳的强反射被认为是康氏不连续面，地壳被分成两层，即上地壳和下地壳。鄂尔多斯地块下地壳厚21km，而巴颜喀拉地块下地壳厚36km，下地壳的增厚占整个地壳增厚的71.4％，可见地壳增厚主要发生在下地壳。本文系统地归纳总结出一套将野外观测的P波速度，校正到实验室温压条件下波速的具体可行的方法，推导出波速校正公式。用地震波速度推测大陆深部地壳组成必须首先消除深部温度和压力对地震波速度的影响。除了物质组成影响地震速度外，温度和压力也影响地震波速度，但温度和压力对大部分岩石泊松比的影响非常小，可以忽略不计。实验室岩石波速测量显示，在低压条件下，岩石裂隙发育，此时得到的波速并不能代表岩石的本征特征，只有当压力使裂隙基本闭合后，测量到的岩石波速才会与压力的增加呈线增加的关系，此状态下即可得波速随压力变化的压力系数。温度对波速的影响与压力相反，随着温度升高，波速会降低，据此可求出温度系数。利用压力系数和温度系数，可推导出将地壳原位地震波速校正到特定温压条件下速度的校正公式。校正公式中需要使用地壳深部的温度和压力值。确定深部地壳压力状态相对简单，而深部温度状态的确定则较复杂。为简单实用，将岩石静载荷压力即围压作为地壳压力值，压力值是深度和密度的函数：p=gρz，其中密度ρ采用大陆平均值2.83g／cm~3，g为重力加速度取9.81m／s~2，z为深度(km)。利用各构造单元的大地热流值，通过由热传导计算得出的地温曲线，测算出研究区地壳深部的温度值。利用波速校正公式，本文将研究区各构造单元不同深度界面的观测波速校正到室温和600MPa压力条件下，并与实验室岩石测量波速进行对比，得出岩性剖面。结果表明，青藏高原东北缘地壳平均P波校正波速为6.43km／s，低于Rudnick和Fountain(1995)的全球大陆地壳平均值6.67km／s，与Gao et al(1998b)得出的中国大陆平均值6.44km／s几乎一样。研究区地壳整体呈长英质。巴颜喀拉地块和秦岭地块南部的下地壳底部缺失P波校正速度V_p＞6.9km／s的基性岩，下地壳中酸性互层，下地壳整体呈酸性。其它地块下地壳底部有2-10km厚的V_p＞6.9km／s的基性岩，下地壳整体呈中性。中央造山带基底缝合带很可能就从研究区这两个不同岩性段中间泽库附近穿过，其北为硬基底，对应的下地壳底部有基性岩层；其南为软基底，对应的下地壳底部缺失基性岩。泊松比研究表明，巴颜喀拉地块和秦岭地块南部的低波速和高泊松比并不是部分熔融的结果，很可能是由于自由水而产生的。根据地壳结构和组成的研究成果，本文提出青藏高原地壳增厚主要发生在下地壳；推断地质历史时期巴颜喀拉地块和秦岭南部地块，在最近的地质历史时期可能发生过下地壳拆沉的事件，地壳整体组成向长英质方向演化；推测随着青藏高原的向东北生长和地壳持续增厚，秦岭北部地块和祁连地块下地壳基性麻粒岩将来有可能相变为榴辉岩，进而拆沉。拆沉作用在青藏高原不均一、阶段性隆升过程中起着重要作用。青藏高原岩石圈在经历水平向挤压缩短、垂向拉伸的过程中，地表缓慢隆升。拆沉作用使软流圈顶部的荷载减轻，导致高原加速隆升，因而青藏高原的隆升历史有快慢不均的现象。本研究有如下创新之处：①首次系统地归纳总结出一套将地震测深得到的原位P波速度，校正到实验室温压条件下波速的具体可行的方法。②首次根据地震测深资料，研究得出青藏高原东北缘地壳组成。③根据本文研究得到的地壳结构和组成结果，首次提出青藏高原东北缘存在下地壳拆沉作用。更多还原

【Abstract】 Based on the data of the 1000-km-long Maqin-Lanzhou-Jingbian seismicrefraction profile, which crosses the northern Baryan Har block, the Qinling block,Qilian block, the Haiyuan arcuate tectonic region, and the stable Ordos block, thecrustal structure, composition and dynamics of the northeastern margin of Tibetanplateau are studied. At first, the structures of P-wave velocity, S-wave velocity andPoisson’s ratio are got after the data of P-wave and S-wave are together reprocessedalong the profile. Then, temperature and pressure that influence velocity are analyzed,and the correction formula that minimizes the influence of temperature and pressureon V_p is deduced. The corrected velocities are compared with laboratorymeasurements of ultrasonic velocities and global models, and the crustal petrologiccomposition is determined in the study area. At last, according to crustal structure andcomposition in the northeastern margin of the Tibetan plateau, dynamic questionssuch as crustal thickening, delamination and uplift of the plateau are discussed.The P-wave and S-wave velocity structures and Poisson’s ratios reveal manysignificant characteristics in the profile.①The crustal thickness increases graduallyfrom northeast to southwest. The average crustal thickness increases from 42km inthe Ordos block to 63km in the Baryan Hat block, and becomes obviously thick southof the Haiyuan fault and south of Zeku.②Crustal velocities have big variationsalong the profile. The average crustal P-wave (S-wave) velocities are 6.30km/s(3.48kin/s) in the Ordos block, 6.22 km/s (3.40 km/s) in the Haiyuan arcuate tectonicregion, 6.25km/s(3.54km/s) in the Qilian block, 6.20 km/s (3.51 km/s) in the Qinlingblock, and 6.10 km/s (3.46 km/s) in the Baryan Har block.③The average crustalvelocity in the profile is lower than the average global velocity. The average crustalvelocity is 6.22 km/s along the profile. If the 10-km-thick layer in the top of the crustis removed, the average crustal velocity is 6.27km/s, which is 0.18km/s lower thanthe global average velocity of 6.45km/s.④There are low velocity zones andlaminated interfaces in the Qinling block and the Haiyuan arcuate tectonic region.There are low S-wave velocities and high Poisson’s ratios in the low velocity zones.Both the Conrad discontinuity and Moho in the Qinling block and in the Haiyuanarcuate tectonic region are laminated interfaces, implying intense tectonic activity.⑤Thickness increases of the lower crust is the main reason for the crustal thickening inthe NE margin of Tibetan plateau. A strong reflection in the mid-crust is thought tobe the Conrad discontinuity, so the crust is divided into two layers, the upper and thelower crust. The thickness of the lower crust increases from 21 km to 36 km as thecrustal thickness increases from 42km in the Ordos block to 63km in the Baryan Harblock south of the Kunlun fault. Therefore, the thickness increase of the lower crustaccounts for 71.4% of the crustal thickening.A set of feasible methods and correction formula are got, with which in situP-wave velocities observed in the field are corrected to velocities under the specialcondition of temperature and pressure in the laboratory. It is necessary to remove theinfluence of temperature and pressure on the velocity to infer the deep crustalcomposition using seismic velocity. Besides composition, the temperature and pressure affect velocity, but they have little affluence on Poisson’ ratios. Laboratorymeasurements of ultrasonic velocities show that under the condition of low pressurethere are many fractures and the velocities can not represent the intrinsiccharacteristics. Only after the pressure closes basically the fractures and the measuredvelocity increases linearly with pressure, can the pressure derivative be obtained. Theinfluence of temperature on the velocity is contrary to the influence of the pressure.The velocity deceases with temperature and temperature derivative can be got. Theformula that corrects in situ velocity to the velocity under special temperature andpressure is deduced using pressure derivative and temperature derivative. Thecorrection formula needs to use the temperature and pressure values.It is simple to determine the pressure state of the deep crust, but it is morecomplex to determine the deep temperature. The confined pressure is practicablyconsidered to be crustal pressure and the value is the function of the depth and density:p=gρz, where densityρ=2.83g/cm~3 and g=9.81m/s~2, and z is depth(km). Surfaceheat flow data and a family of conductive geotherms in different tectonic units areused to determine the temperature of the deep crust.Using velocity correction formula, the observed velocities at different interfaceof all tectonic units are corrected to a standard pressure of 600 MPa and roomtemperature, the corrected values are compared with the laboratory measurements ofaltrasonic velocities and the petrology profile is got along the Maqin-Lanzhou-Jinbianseismic profile. Results show that average collected velocity in the northeasternmargin of Tibetan plateau is 6.43 km/s, lower than the global average velocity of 6.67from Rudnick and Fountain(1995) and almost equal to the average velocity of 6.44km/s in the China mainland from Gao et al(1998b). The crustal bulk composition isfelsic. In Baryan Har block and southern part of the Qinling block, there is lack ofV_p＞6.9 km/s mafic rock layer in the lowest crust, felsic layer and intermediate layerare alternated in the lower crust, and the bulk composition in the lower crust is felsic.In other blocks, there is a 2-10 km thick mafic layer of V_p＞6.9km/s in the lowestcrust and the bulk composition is intermediate in the lower crust. It is possible that thebasement suture zone of the Central Orogenic Belt passes between these two differentpetrologic segments at Zeku. The basement north of the basement suture zone is thehard basement, corresponding with mafic rock layer in the lowest crust and thebasement south of the basement suture zone is the soft basement, corresponding withthe lack of mafic rock layer in the lowest crust. The study of Poisson’s ratios indicatethat low velocities and high Poisson’s do not necessarily represent the part melting,and they possibly result from free water.On the basis of the crustal structure and composition, the dissertation proposesthat the crustal thickening occurred mainly in the lower crust, that delaminationpossibly occurred in the lower crust of Baryan Har block and southern Qinling blockin the recent geologic time and the crustal bulk composition evolved to felsiccomposition, and that with the northeastern growth of Tibetan plateau and continuouscrustal thickening the lowest crust will delaminate aider the mafic granulatetransforms to eclogite in the northern Qinling block and Qilian block. Delamination plays an important role in the course of uneven and staggered upliftof the Tibetan plateau. The surface rose slowly when the Tibetan lithosphereunderwent horizontal compression and vertical extension. However, delaminationaccelerated the uplift of the plateau because delamination reduced the load on thetopmost of asthenosphere. Therefore, there was a phenomenon of different speeds inthe uplift history of the plateau.The project has such innovations as follows:①For the first time, a set of feasible methods are systemically summed up, withwhich in situ P-wave velocities observed by deep seismic sounding can be correctedto the velocity under the special condition of temperature and pressure in thelaboratory.②For the first time, the crustal composition is got in the NE margin of theTibetan plateau based on the data of deep seismic sounding.③For the first time, the delaminnation of lower crust is proposed in the NEmargin of the Tibetan plateau on the basis of the crustal structure and composition gotby the dissertation.更多还原