摘要
长期以来,华南地区的壳/幔结构、壳/幔结构与大地构造的关系、花岗岩的分布特征与形成原因等问题一直是研究的热点,至今仍无定论。本文依托国家“深部探测技术与实验研究专项”SinoProbe02-04-02课题,在华南东南部进行了面积性三维大地电磁观测研究,获得了一些有意义的成果,希望能对以上问题有所帮助。课题的研究区位于东经113°-121°、北纬23°-29°之间,剖面穿过华夏地块和东南沿海地块,走向为NWW,垂直于研究区内主要构造方向NNE向。利用宽频大地电磁仪(V5-2000)和长周期大地电磁仪(LEMI-417)共计观测获得了11条大地电磁剖面,413个宽频大地电磁测深点、98个长周期大地电磁测深点。本文主要对研究区内南侧5条剖面的190个宽频大地电磁测深点、46个长周期大地电磁测深点进行了处理并分析研究。在野外数据观测时我们通过对测点的合理偏移、灵活的参数选择、严谨的布站操作和设置远参考站等措施,获得了一批高质量的大地电磁数据。对野外数据进行处理时,作者分别利用了Robust技术、远参考技术、人机交互和静位移校正,使实测数据的噪声得到了合理的压制,并尝试对宽频大地电磁数据与长周期大地电磁数据进行了拼接。利用处理后的数据可以获得研究区内的纵向电导、感应矢量实部、极化图和二维偏离度等信息,经分析表明:华南东南部的地下结构整体呈现出二维特征,局部地区三维特征比较明显;最佳电性主轴方向均为NE向;构造走向方向主要为NE向,局部地区为NW向、近SN向或近EW向;结合感应矢量实部和二维电性结构剖面可以很容易的判断出断裂带的位置、规模和走向,研究表明该地区的断裂带主要走向为NNE向。对每个测点的视电阻率曲线进行了极化模式的判断,分别进行了TE、TM和联合模式的反演,获得了华南东南部的5条大地电磁二维电性结构剖面。结合研究区前人所做的地质学、岩石学、地球物理学、地球化学、地热学等资料,对华南东南部的断裂特征、构造单元划分、壳/幔电性结构特征及动力学背景、花岗岩的分布特征与形成原因、深部热状态分别进行了分析和研究。主要研究成果如下:(1)根据感应矢量实部和5条大地电磁二维电性结构剖面在研究区内确定出8条NNE走向的断裂带,分别为吴川-吉安断裂带(F1)、赣江断裂带(F2)、阳江-金华断裂带(F3)、河源-邵武断裂带(F4)、澳门-建阳断裂带(F5)、丽水-海丰断裂带(F6)、福安-海丰断裂带(F7)、长乐-诏安断裂带(F8),并对它们的特征进行了相应的分析。(2)本文根据5条剖面的二维电性结构特征和确定出的断裂带,并结合前人的研究成果,把研究区划分为华夏地块和东南沿海地块,其中华夏地块内又分为武功-诸广褶皱带,武夷山隆起区和粤东北凹陷区。(3)研究区内壳/幔厚度起伏很大,在沿海地区和远离沿海的内陆地区壳/幔厚度较大,其中岩石圈厚度均超过100km,沿海地区的厚壳/幔形成与太平洋板块对欧亚板块的俯冲有关,而内陆地区的厚壳/幔与碰撞汇聚作用和陆内造山作用有关;在剖面的中间位置壳/幔厚度较小,并存在有明显的软流圈隆起,隆起带呈NE向展布,软流圈的隆起与华南东南部的壳/幔拆沉减薄密切相关。(4)华南东南部各时期的花岗岩分布十分广泛,存在有两个明显的花岗岩带,分别为NNE向中新生代花岗岩带、近EW向花岗岩带。本文分析了花岗岩平面和纵向的分布特征,根据花岗岩的分布特征对其进行了新的分区,并尝试分析了各区花岗岩的形成背景。(5)分析了壳/幔中电导率和温度的关系,发现在壳/幔的深部电导率与温度有直接的关系,高电导率对应介质的高温,于是作者根据深部的电导率特征分析了壳/幔深部的热状态,结果表明由电性特征可以获得准确、直观的壳/幔深部热状态,并其它方法获得的壳/幔深部热状态有很好的一致性。
For a long period of time, the crust/mantle structure of south China, the connection betweencrust/mantle structure and tectonics, the distribution characteristics of granite and its formation arealways the hot researches; there have been no definite answers yet. In this paper, we carried outthe area three-dimensional magneto telluric observational studies supported by“SinoProbe-DeepExploration in China”SinoProbe02-04-02subject in southeast of south China and got somemeaningful results, wishing to have the mentioned problems settled to some extent.The location of research area is between east longitude113°-121°, north latitude23°-29°, theprofiles go across the cathaysia block and the southeast coast block, their directions are NWW,perpendicular to the direction of main structure, the NNE direction. Using broadbandmagnetotelluric instrument (V5-2000) and long-period magnetotelluric instrument (LEMI-417),we have got11Magneto telluric profiles,413points of broadband magnetotelluric points and98long-period points. This paper mainly investigates the190points of broadband magnetotelluricand46long-period points in the5south profiles.In field observation, we got a lot of high quality field data through proper migration for points,flexible parameter selection, rigorous operations and installation of reference station. We used theRobust technology, remote reference technology, human-computer interaction and staticdisplacement correction respectively to suppress the noise and attempted to join the broadbandmagnetotelluric data and long-period magnetotelluric data together. We got the information aboutlongitudinal conduct, polar plots and2D irrelevance of the research area, the analysis suggest that:southeast part of south China presents2D features, in some areas the3D features are moreobvious; the direction of main axis of optimal electric property is NE; the main direction ofstructure is NE and some are NW, near SN or near EW; combining the real part of vector with2Dstructure profile of electrical property, the position, size and direction of the fault can be easilypredicted. The studies suggest that the main direction of fault zone is NNE.We evaluated the polarization mode for apparent resistivity curve of each point and done theTE, TM and joint inversion. Then we got five magnetotelluric2D electronic structure profiles.Based on the previous study of geology, petrology, geophysics, geochemistry and geothermic, weinvestigated features of fault in southeast of south China, the division of tectonic units,crust/mantle electronic structure features and its dynamic, distribution characteristics of graniteand its formation and deep thermal condition respectively. The research results are follows:(1) According to the real part of induction vector and five magnetotelluric2D electronicstructure profiles, we determine8NNE fault zones, that are Wuquan-Jian fault zone(F1),Ganjiangfault zone (F2), Yangjian-Jinhua fault zone(F3), Heyuan-Shaowu fault zone (F4), Aomen-Jianyuanfoult zone(F5), Lishui-Haifei fault zone(F6), Fuan-Haifeng fault zone(F7), Changle-Zhaoan faultzone, then we analyze the characteristics of each fault zone.(2) According to2D electronic structure features of5profiles and the fault zones wedetermined, based on the previous research results, we divide the research area into Huaxia block and southeast coast block. The Huaxia block consists of Wugong-Zhuguang fold zone, Wuyishanuplift area and northeast Guangdong depressed area.(3) The crust/mantle thicknesses are fluctuant, the crust/mantle in the coast area and inlandare pretty thick, the average thickness of lithosphere is more than100km, formation ofcrust/mantle in coast area is related to the interaction between the Pacific plate and Eurasian plate.The formation of crust/mantle inland is related to Collision convergence effect and Orogeny effect,so the thickness of crust/mantle is small in the profile. And there is obvious asthenosphere uplift.The direction of uplift zone is NE, asthenosphere uplift closely relate to the delamination thinningdense of southeast region of south China.(4) The granite in southeast of south China during different time period is widespread, thereare two obvious granite zones, the NNE Mesozoic and cenozoic granite zone, EW-trending granitebelt. In this paper, we analyze the horizontal and vertical features of granite distribution, based onthe features of granite distribution, redistrict the granite distribution, and try to analyze thebackground information about the formation of granite in each area.(5) We analyze the relationship between the electrical conductivity of crust/mantle andtemperature, and discover that the electrical conductive in the deep crust/mantle is related totemperature directly, high electrical conductivity corresponds to high temperature. In this case, weanalyze the thermal state of deep crust/mantle according to the features of electrical conductivity.The results suggest that the thermal state of deep crust/mantle obtained from the electricalconductivity is accurately and intuitively, consistent with that obtained by other methods.