【作者】 王未来；

【导师】 吴建平；

【作者基本信息】 中国地震局地球物理研究所 ， 固体地球物理， 2014， 博士

【摘要】 中国西南地区是由不同地质历史时期的许多微小板块拼贴而成,是特提斯构造发育的重要区段；同时,该地区在西藏高原物质向东挤出、地壳增厚和下地壳流模型中,都扮演了极为重要的角色。因此,研究中国西南地区壳幔特征对于深入理解特提斯构造特征和青藏高原构造活动机制具有重要意义。区内分布有大量深大断裂带,自20世纪70年代以来,该区域相继发生了一系列M>7.0强震,强震活动给当地造成了巨大经济损失和人员伤亡。开展中国西南地区深部构造特征的详细研究,对于认识该区大地构造、地壳演化和地震成因乃至强震活动趋势都具有非常重要的意义。近年来,中国地震局利用“地震科学探测台阵系统”的观测设备在中国西南地区开展了多个大规模流动地震台阵观测,研究区流动和固定宽频带地震台站数量超过900个,平均台间距小于40km。本文将充分利用流动地震台站和固定台站的观测资料,采用P波接收函数H-κ叠加技术获得地壳厚度和泊松比的分布图像；采用P波接收函数三维偏移成像获得上地幔过渡带速度间断面的空间分布图像；采用S接收函数偏移成像研究岩石圈底界面的横向变化；利用面波层析成像获得周期达100s以上瑞利波相速度分布图像；采用接收函数和面波联合反演技术,揭示横跨重要构造边界200km深度范围的S波速度结构。总体而言,中国西南地区地壳厚度变化与构造背景具有一致性。地壳厚度从西北部的约60km向东北、东南、西南向呈弧形状减薄,其中东南部和西南部最薄约30km。松潘甘孜地块的平均地壳厚度为60km,向东剧烈减薄到四川盆地的45km,向东南方向减薄相对缓慢,显示出青藏高原东向物质逃逸被四川盆地阻止后,进一步向东南逃逸的特征。中、高泊松比(σ≥0.26)主要分布在松潘甘孜地块、攀枝花-峨眉山一带、四川盆地、腾冲地块。这些区域断裂带分布密集交错,呈现出较强的地壳活动特征。四川盆地西南部和攀枝花-峨眉山一带存在高密度、较高的P波速度。高密度、高波速与高泊松比说明地壳存在较多的镁铁质成分,这可能与峨眉山玄武岩分布有关。而鲜水河南段-小江断裂穿过攀枝花-峨眉山地区北部,下地壳局部熔融引起的高泊松比与中上地壳镁铁质引起的高泊松比叠加,在此区域出现了研究区内最高值泊松比的分布。腾冲地块分布有活动火山,TNC.YN高达0.32的高泊松比表示地下有熔融或半熔融的岩浆囊存在,这与该区域下方的低波速相一致。低泊松比(σ≤0.26)主要分布在研究区的东南部：扬子克拉通、华夏地块、滇缅泰地块。在排除沉积层影响的情况下,位于红河断裂附近台站相对周围台站具有相对较高的泊松比(部分台站泊松比达0.28-0.30),而上地幔顶部速度偏低并且速度结构具有壳幔过渡带特征,暗示着红河断裂为超壳断裂,高泊松比可能与下地壳底部局部熔融有关。四川盆地下方西南部岩石圈厚度达250km,逐渐减薄到中部和南部岩石圈的200km厚度,进一步减薄到东部边缘的约100km厚度,向北进一步减薄到北部边缘的约130km厚度。滇缅泰地块上地幔顶部100km范围内存在双层低速界面,同时也足以确认这一区域岩石圈厚度小于100km。华夏地块下方上地幔顶部存在双层低速界面,而且与扬子克拉通岩石圈接触关系在北部和西部有所不同,西部可能有相互融合的现象,而北部可能还存在深浅部断离的情况。秦岭大别构造带西段和渭河平原地区在100km深度处存在明显的大范围低速界面。中国西南地区的上地幔过渡带厚度在扬子克拉通(主要包括四川盆地地区和峨眉山-攀枝花等地区)地区基本大于250km,在华夏地块基本都小于250km,在松潘甘孜地区接近扬子克拉通的地区厚度较厚,再往西过渡带厚度则小于250km。上地幔过渡带较厚区域与500km深度的P波速度结构中高速体对应,可能与俯冲物质有关。典型地区台站下方S波速度结构：腾冲地区的上地幔顶部从45km到90km速度相对较低。华夏地块的呈现双低速层模型,上层低速层接近60km处但厚度相对较薄,下层低速层在100-140km相对较厚。松潘甘孜地块下方存在较浅埋深的上地幔低速层,深度范围为80-120km。四川盆地内部上地幔顶部50-170km都具有较高速度分布和较大的岩石圈厚度。更多还原

【Abstract】 Southwest China is a collage of many small plates from different geological history, and it is an important segment during the Tethyan tectonic development. Meanwhile, this region plays an important role in the extrusion model of the Tibet plateau materials towards east, the crustal thickening model and the lower crust flow model. Therefore, it is significant to study the features of the crust and upper mantle beneath southwest China, to obtain a deep understanding of the Tethyan tectonic features and the tectonic activity mechanisms of the Qinghai-Tibet plateau. A large number of deep and large faults are distributed in this region. Since the1970s, a series of large earthquakes (M>7.0) happened here. The occurrence of large earthquakes has caused tremendous casualties and great damage to local society and economy. It is significant to carry out detailed studies on the deep structural features of southwest China, to know the regional geo-tectonics, crustal evolution, the cause of earthquakes and even the trend of strong earthquakes.In recent years, a number of large-scale mobile seismic array observations are carried out in southwest China by the China Earthquake Administration and the observation equipment is from the "ChinArray". More than900mobile and fixed broadband seismic stations are deployed in the study region, with an average station spacing of less than40km. In this dissertation, we will make full use of the observational data from the mobile and fixed seismic stations, including using the H-κ stacking technique of P-wave receiver functions to obtain the distribution of crustal thicknesses and Poisson’s ratios; using the3-D seismic migration image of P-wave receiver functions to obtain the spatial distribution of the velocity discontinuity of the upper mantle transition zone; using the seismic migration image of S-wave receiver functions to study the lateral variation in the lithospheric bottom interface; using surface wave tomography to obtain the Rayleigh wave phase velocity maps at periods up to100s, and using the joint inversion of receiver functions and surface waves to reveal the S-wave velocity structure stretching over important tectonic boundaries within200km in depth.Overall, the variation in the crustal thicknesses of southwest China is consistent with the tectonic background. The crustal thicknesses become thinner arc-shaped from-60km in the northwest towards the northeast, southeast and southwest, and the thinnest parts in the southeast and southwest are-30km. The average crustal thickness of the Songpan-Ganzi terrane is-60km. It becomes thinner severely to45km eastwards in Sichuan basin and becomes thinner more slowly towards southeast, indicating the Qinghai-Tibet plateau materials further escape towards southeast after they are prevented by the Sichuan basin.Intermediate and high Poisson’s ratios (σ≥0.26) are mainly distributed in the Songpan-Ganzi terrane, Panzhihua-Emeishan region, Sichuan basin and Tengchong terrane. In these regions, the faults are very dense and criss-cross, showing strong crustal activity features. Because high density, high velocity and high Poisson’s ratio correspond to more mafic compositions in the crust, the high density and higher P-wave velocity in southwestern Sichuan basin and Panzhihua-Emeishan region may be related to the Emeishan basalt distribution. The south part of the Xianshuihe-Xiaojiang fault penetrates northern Panzhihua-Emeishan region, and high Poisson’s ratio caused by partial melting in the lower crust is strengthened by the mafic in the intermediate and upper crust, causing the highest Poisson’s ratio to appear in this region. There are volcanoes in the Tengchong terrane; the Poisson’s ratios are up to0.32at station TNC.YN, implying melting or partial melting magma chambers exist there, which is consistent with the low velocities beneath this region. Low Poisson’s ratios (σ≤0.26) are mainly shown in the southeast part of the study region:Yangtze craton, Cathaysian block and Yunnan-Burma-Thailand block. After we exclude the impact of the sediment, higher Poisson’s ratios are shown beneath the stations near the Red River fault (up to0.28-0.30for part of the stations) compared to the surrounding stations. The velocities in the uppermost mantle are low and the velocity structure is similar to those at the crust-mantle transition zone, indicating the Red River fault is a deeply penetrating fault, and the higher Poisson’s ratios may be related to the partial melting at the bottom of the lower crust.The lithospheric thickness is up to250km in southwestern Sichuan basin, and gradually becomes thinner to200km in the central and southern parts, and further becomes thinner to-100km on the eastern edge and-130km on the northern edge. Double-layer low velocity interfaces exist within100km at the top of the upper mantle beneath the Yunnan-Burma-Thailand block and it is also enough to confirm that the lithospheric thickness in this region is less than100km. Double-layer low velocity interfaces also exist at the top of the upper mantle beneath the Cathaysian block, and its contact styles to the lithosphere of the Yangtze craton are different in the north and west:mutual integration may exist in the west while partition between shallow and deep part may exist in the north. Significant and large-scale low velocity interface exists at100km depth beneath the west of Qinling-Dabie tectonic belt and Weihe plain.The thickness of the upper mantle transition zone is basically greater than250km in the Yangtze craton (mainly including regions like the Sichuan basin and Panzhihua-Emeishan region), and less than250km in the Cathaysian block; it is close to the thickness of the Yangtze craton in thick area and is less than250km westwards. The thick area of the upper mantle transition zone corresponds to the high velocity bodies of P-wave velocity structure at500km depth, which may be related to the subduction materials.S-wave velocity structure beneath stations in typical regions:the S-wave velocities are lower at the top of the upper mantle from45km to90km beneath the Tengchong region. Double-layer low velocity model is shown beneath the Cathaysian block; the upper low velocity layer is thinner and is-60km at depth, while the lower low velocity layer is thicker and is-100-140km at depth. There is a low velocity layer in the upper mantle ranging80-120km at depth beneath the Songpan-Ganzi terrane. A higher velocity layer at the top of the upper mantle ranging50-170km at depth with a large lithosphere thickness is located beneath the Sichuan basin.更多还原