Detection of a collision zone in south Indian shield region from magnetotelluric studies

Magnetotelluric (MT) investigations were carried out along a profile in the greenschist–granulite transition zone within the south Indian shield region (SISR). The profile runs over a length of 110 km from Kuppam in the north to Bommidi in the south. It covers the transition zone with 12 MT stations using a wide-band (1 kHz–1 ks) data acquisition system. The Mettur shear zone (MTSZ) forms the NE extension of Moyar–Bhavani shear zone that traverses along the transition zone. The regional geoelectric strike direction of N40°E identified from the present study is consistent with the strike direction of the MTSZ in the center of the profile. The 2-D conductivity model derived from the data display distinct high electrical resistivity character (10,000 Ω m) below the Archaean Dharwar craton and less resistive (< 3000 Ω m) under the southern granulite terrain located south of the MTSZ. The MTSZ separating the two regions is characterized by steep anomalous high conductive feature at lower crustal depths. The deep seismic sounding (DSS) study carried out along the profile shows dipping signatures on either side of the shear zone. The variation of deep electrical resistivity together with the dipping signature of reflectors indicate two distinct terrains, namely, the Archaean Dharwar Craton in the north and the Proterozoic granulite terrain towards south. They got accreted along the MTSZ, which could represent a possible collision boundary.     

Three-dimensional finite-element modelling of magnetotelluric data with a divergence correction

A finite-element method for computing the electric field in a 3-D conductivity model of the Earth for plane wave sources, thus enabling magnetotelluric responses to be calculated, is presented. The method incorporates in the iterative solution of the electric-field system of equations the divergence correction technique introduced for finite-difference solutions by Smith (1996). The correction technique accelerates the development of the discontinuity of the normal component of the approximate electric field across conductivity discontinuities. The convergence rate of the iterative solution is improved significantly, especially for low frequencies. The correction technique involves computing the divergence of the current density for the approximate electric field, computing the static potential whose source is this divergence of the current density, and ‘correcting’ the approximate electric field by subtracting from it the gradient of the potential. This is repeated at regular intervals during the iterative solution of the electric-field system of equations. For the method presented here, the Earth model is discretised using a rectilinear mesh comprising uniform cells. Edge-element basis functions are used to approximate the electric field and nodal basis functions are used to approximate the correction potential. The Galerkin method is used to derive the systems of equations for the approximate electric field and correction potential from the respective differential equations. A bi-conjugate gradient solver was found to be adequate for the system of equations for the correction potential; a generalised minimum residual solver was found to be better for the electric-field system of equations. The method is illustrated using the COMMEMI 3D-1A and 3D-2A models.     

喜马拉雅东构造结岩石圈板片深俯冲的地球物理证据

2009～2010年在南迦巴瓦地区进行了宽频带地震和大地电磁探测,分别处理获得东构造结及其邻区的地下300km以上的P波速度图像和两条大地电磁电阻率剖面。通过资料的对比和综合解释,发现电阻率分布与地震波速有较好的对应关系。研究结果表明:南迦巴瓦变质体的上地壳部分呈现明显高速高阻特征,为两侧的雅鲁藏布江缝合带所夹持;中下地壳具有不均匀性,且普遍呈低速低阻特征;印度板块在藏东南向欧亚板块的俯冲前缘越过嘉黎断裂,抵达班公湖-怒江缝合带;在拉萨地体的高速俯冲板片以下100km至200km深度范围内存在大规模的低速异常带,其上盘中下地壳也广泛发育低速高导体,指示青藏高原东南缘可能存在韧性易流动的物质向东、东南逃逸的通道,为印度板块在南迦巴瓦的深俯冲动力学模式提供了地球物理证据。     

Magnetotellurics applied to the study of the crustal structures in Senegal (West Africa)

Abstract

In Senegal, magnetotellurie (MT) method has been used in an attempt to resolve the principal structural features by their electrical response. On the basis of numerical modelling of data, an unified model of possible crustal structure is presented for the West african margin. The results are in agreement with other independent geophysical and geological information.     

Application of the 3D magnetotelluric inversion code in a geologically complex area

The WSINV3DMT code makes the implementation of 3D inversion of magnetotelluric data feasible using a single PC. Audio‐magnetotelluric data were collected along two profiles in a Cu‐Ni mining area in Xinjiang, China, where the apparent resistivity and phase curves, the phase tensors and the magnetic induction vectors indicate a complex 3D conductivity structure. 3D inversions were carried out to reveal the electrical structure of the area. The final 3D model is selected from the inversion results using different initial Lagrange values and steps. The relatively low root‐mean‐square (rms) misfit and model norm indicate a reliable electrical model. The final model includes four types of low resistivity areas, the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone. The second ones are not controlled by magnetotelluric sites and embody little information of the observed data, they are considered as tedious structures. The third one is near to the regional Kangguer fault and should be treated carefully considering the effect of the fault. The last ones are isolated and existing at a limited level as the first ones, they should be paid more attention to.     

喜马拉雅西北部逆冲带的地壳电性结构

印度板块北部地形起伏较大的喜马拉雅山地区由几个构造互异的地质单元组成,依地形高、低把喜马拉雅碰撞带分成低喜马拉雅和高喜马拉雅.为了研究与主要逆冲带（含主缝合带MCT和主边界带MBT）有关的地壳电性结构,沿Rohtangpass (海拔4000 m) 到Mandi (海拔400 m)剖面进行了MT探测.通过对16个测点观测资料的分析和考虑地形的二维反演,获得了沿剖面的二维电性结构.电性结构显示,在Chail和主逆冲边界带下方,东西走向的缝合带突然转向北.在下喜马拉雅的Rampur 区段的元古代基底为范围较大的高阻体,而浅部地壳被逆冲带分成向北倾的电导性块体和电阻性块体.Chail 逆冲带东侧低喜马拉雅Rampur 区段的推挤和它西侧的基底脊柱体导致主边界带及相关的逆冲带（Kangra 拐角）向北转弯,Kangra拐角处的应力可能是由于西侧基底脊柱体进入到Kangra 区引起的.     

阿尔金走滑断裂带昌马段的电性结构样式及构造意义

阿尔金断裂带东段走滑速率沿断裂走向方向存在明显的流失现象,有关阿尔金断裂带的影响范围及走滑速率变化的机制需要有更多的深部结构证据来提供支撑.本文以阿尔金断裂带昌马段为窗口,获取了4条横穿阿尔金断裂带及相邻地区的大地电磁测深剖面.二维电性剖面显示在阿尔金断裂带北侧中上地壳以连续的高阻体为主,而南侧祁连山内部的深部电性结构在横向上有较为复杂的变化.这一点与区域构造背景相对应,即北侧的塔里木盆地东缘依然具有较好的整体性,南侧的祁连山是青藏高原北缘生长的最前端,变形强烈.在断裂带的结构特征上,阿尔金断裂带沿走向方向的切割深度在昌马盆地西侧发生了显著的降低,与阿尔金断裂带相对应的电性边界在这里向南偏移了约15 km,对应F18断裂,并与昌马盆地相接.祁连山北部的断裂带,包括昌马断裂、旱峡—大黄沟断裂总体呈现出低角度南倾的样式,切过高阻异常体的顶部.虽然昌马盆地可以起到连接断裂带的阶区的作用,将部分阿尔金断裂的走滑分量转移到盆地南侧的昌马断裂上,但是昌马断裂的走滑速率从西向东是增加的,东侧的走滑速率甚至大于阿尔金断裂沿走向方向的流失分量.我们认为在青藏高原北部主要断裂带的活动还是受印度—欧亚板块碰撞引起的远程挤压效应的影响,包括阿尔金断裂以及祁连山内部系列断层都处于斜向挤压应力环境.在这种基本构造模式下,阿尔金断裂、断裂F18、昌马盆地、昌马断裂构成了一个局部的走滑速率分解-转换-吸收体系,对局部应力状态产生影响.

     

基于空气地球分离策略的大地电磁三维正演

大地电磁正演常采用电场双旋度方程,由于工作频率低、空气电导率为零,电场双旋度方程含有空解空间,导致离散化的线性方程组严重病态、具有高的条件数.为降低大地电磁场双旋度正演方程的病态性,本文提出了一种基于空气与地球分离策略的大地电磁混合正演公式.首先,在空气层中将双旋度算子转换为Laplace算子,在地下利用电导率大于零的特征保留双旋度算子,建立空气与地下分离的混合公式系统,以便实现混合公式系统对应的离散化线性方程组条件数的大幅度降低.然后,在空气与地下分界面上,施加电磁场的切向与法向连续性条件实现空气与地下区域的耦合与信息交换,以保证电磁场分布的唯一性.再则,利用非结构化四面体网格离散技术来高精度模拟任意起伏地形和复杂地下三维地电结构,并结合节点有限元与棱边有限元法,高精度求解该混合公式系统.最后,利用国际标准大地电磁测试模型来验证该空气与地球分离算法的正确性,并展示其有效降低原有电场双旋度方程的病态性的能力.本文提出的空气与地球分离思想为寻求大地电磁最优化控制方程开启了新的求解思路,预计会产生更多、性能更佳的大地电磁混合公式,以便服务于大地电磁正反演技术的快速发展.

     

Space and time spectra of stationary stochastic waves,with special reference to microtremors

It is of crucial importance to investigate the spatial structures of ancient landslides in the eastern Tibetan Plateau's alpine canyons as they could provide valuable insights into the evolutionary history of the landslides and indicate the potential for future reactivation. This study examines the Deda ancient landslide, situated in the Chalong-ranbu fault zone, where creep deformation suggests a complex underground structure. By integrating remote sensing, field surveys, Audio-frequency Magnetotellurics (AMT), and Microtremor Survey Method (MSM) techniques, along with engineering geological drilling for validation, to uncover the landslide’s spatial features. The research indicates that a fault is developed in the upper part of the Deda ancient landslide, and the gully divides it into Deda landslide accumulation zone I and Deda landslide accumulation zone II in space. The distinctive geological characteristics detectable by MSM in the shallow subsurface and by AMT in deeper layers. Our findings include the identification of two sliding zones in the Deda I landslide, the shallow sliding zone (DD-I-S1) depth is approximately 20 m, and the deep sliding zone (DD-I-S2) depth is 36.2‒49.9 m. The sliding zone (DD-II-S1) depth of the Deda II landslide is 37.6‒43.1 m. A novel MSM-based method for sliding zone identification is proposed, achieving less than 5% discrepancy in depth determination when compared with drilling data. These results provide a valuable reference for the spatial structural analysis of large-deep-seated landslides in geologically complex regions like the eastern Tibetan Plateau.