Crustal Structure, Composition and Dynamics of the Northeastern Margin of Tibetan Plateau
|School||China Seismological Bureau, Institute of Geophysics|
|Course||Solid Earth Physics|
|Keywords||Northeastern margin of Lower crust Delamination Bayan Har Ordos block Poisson's ratio Crustal thickening Crustal thickness Wave velocity Lithosphere|
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.