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    华北地区大地电磁测深及岩石圈厚度讨论

    MT sounding and lithosphere thickness in North China

    • 摘要: 提要:近些年来,随着地球科学的进展,大陆岩石圈导电性结构的研究越来越引起人们的重视。这是因为固体地球物理学研究的结果告诉我们,有关大陆岩石圈导电性的研究有可能为当前地球科学各领域的进展提供重要的物理依据。大地电磁测深是从导电性的角度研究地壳和上地幔结构不可缺少的方法。长期以来,国内对此开展了大量工作;而当前,随着科学技术发展,无论是仪器、数据采集或数据处理和反演技术都有了长足的进步。中国的大地电磁测深已实现超宽频带、高精度测量技术;在数据处理和反演等方面也基本与国际先进技术接轨。因此,在2001年,沿着山西应县到山东商河,布置一条大地电磁测深剖面进行研究。采用现代先进的大地电磁数据处理技术和快速松弛二维反演方法获得该剖面二维电性结构模型,从而充分展示了华北地区岩石圈电性结构的特点。从电性特征上讲,华北岩石圈以太行山前断裂为界划分为东、西两区。东区为低阻区,上地壳电性结构基本与华北裂谷系的隆、坳构造格局相对应,岩石圈的电导最高达30 000 S,远远大于强烈活动的安第斯山岩浆弧区和西藏高原岩石圈的电导。西区为高阻区,太行山和恒山的岩石圈为高阻块体,表现出稳定大陆区岩石圈导电性结构的特点。但恒山高阻块体之下发现一组向西缓倾的高导层,其电导率为0.04~0.25 S/m,顶面在20 km深处,底面深约40 km。研究结果表明,华北裂谷盆地范围内地壳的电阻率很低,属于良导电性的地壳,这可能是由华北裂谷盆地的壳、幔热结构、热状态所决定的。根据对大陆电阻率模型的讨论,推测鄂尔多斯地块的上地幔也许不存在前人所界定的“软流圈”的性状;而华北裂谷盆地虽然是中新生代构造活动区,活动性较强,但与年轻的火山岛弧区或剧烈的构造活动区相比仍有一定差别,因而,上地幔“软流圈”的痕迹并不明显。这也许正是根据现今应县—商河剖面上大地电磁的探测结果看不出来存在通常所说的反映“软流圈”电性特征的“上地幔高导层”的原因。不过,根据这结果还是可以得出:在华北地区西部的地壳和岩石圈厚度大于东部。但更准确的结果还有待于更深入、更精细的地球物理探测。

       

      Abstract: Abstract: In recent years, with the development of geosciences, there has been a growing interest in the study of electrical conductivity structure of the continental lithosphere, which is because the results of solid geophysical research show that the study of electrical conductivity structure of the continental lithosphere can provide important physical grounds for research in other areas of geosciences. Magnetotelluric(MT) sounding is an indispensable geophysical method for the study of the structures of the crust and upper mantle in the context of electrical conductivity. For a long time, great efforts have been devoted to the study of MT technologies in China. At present, with the development of science and technology, major advances have been made in instrumentation, data acquisition or processing technology and inversion technology. In China, superwide-band and high-precision MT survey techniques have been applied and data processing and inversion technologies have been improved greatly and in the main are compatible with the world's advanced technologies. Therefore, an MT profile was run along the Yingxian, Shanxi, to Shanghe, Shandong, in 2001. A 2D conductivity structure model of the profile, obtained by using the advanced MT data processing method and rapid release inversion (RRI), shows the features of the conductivity structure of the lithosphere in North China. According to the electrical features, the North China lithosphere is divided into the eastern and western parts by the frontal fault of the Taihang Mountains. The eastern part is characterized by low resistivity and the western part by high resistivity. In the eastern part, the electrical structure of the upper curst corresponds roughly to a tectonic framework of alternating uplifts and depressions in the North China rift system and the lithosphere conductance reaches a maximum of 30 000 S, being far greater than that of the Andes magma arc area with strong volcanic activities and that of the Tibetan Plateau. In the western part, the lithosphere of the Taihang and Hengshan mountains, marked by high-resistance blocks, is characterized by the conductivity structure of the stable continental lithosphere. However, a group of gently west-inclined high-conductive layers, with a conductivity of 0.04—0.25 S/m and a top depth of 20 km and a bottom depth of 40 km, were discovered under the high-resistance block of Hengshan Mountain.Study indicates that the very low-resistivity feature of the crust below the North China rift basin is probably determined by the thermal structure and regime in the curst and mantle of the rift basin. According to a discussion on continental resistivity models, it can be inferred that the “asthenosphere” character defined by predecessors might not exist in the upper mantle below the Ordos block. Although the North China rift basin is a Meso-Cenozoic tectonically active area, its activity is weaker than that of younger volcanic arcs or zones with violent tectonic movement. Therefore, the asthenosphere character of the North China rift basin is not distinct. That is one of the possible reasons why there is no "high-conductive layer of the upper mantle" in the MT profile from Yingxian to Shanghe, which images the asthenosphere electrical characters. However, according to the survey results, it can be inferred that the crust and lithosphere of western North China are thicker than those of eastern North China. In order to obtain more accurate results, more intensive and precise geophysical surveys and studies remain to be conducted.

       

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