Electrical mantle anisotropy and crustal conductor: a 3-D conductivity model of the Rwenzori Region in western Uganda
Electrical mantle anisotropy and crustal conductor: a 3-D conductivity model of the Rwenzori Region in western Uganda
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DOI：
10.1111/j.1365-246x.2011.05006.x
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发表时间：
2011-06
影响因子：
2.8
通讯作者：
Michael Häuserer;A. Junge
中科院分区：
文献类型：
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作者：
Michael Häuserer;A. Junge
大地电磁测量在乌干达鲁文佐里山脉以东进行。在23个测点估算了10 ~ 10 000 s周期内地磁场和相张量元之间的传递函数。传递函数表明，在鲁文佐里山脉和东北裂谷肩的接触带，地壳内部存在复杂的三维导电性结构。介绍了相位张量椭圆以棒状形式的另一种表示形式。在100 s以上的周期内，各观测点的主相张量条均向SSW-NNE方向移动，相张量不变量φmin和φmax相差至少20°。这种明显的行为和在同一周期范围内较小的垂直磁场可以用30-50 km的各向异性电导率来解释，其导电方向垂直于裂谷轴。这种各向异性可能来自于大部分上地幔的橄榄石晶体取向，因此首选的取向可能是由于Rwenzoris下地壳的分层引起的运动。在Rwenzoris东南部15公里深度处发现了一个高导电层，与地震低速带相匹配。这可能表明下地壳内部有一个部分融化的区域，与附近的火山场相连。一般来说，相位张量元素φmin在45°以上增加的周期长于2000 s，表明电导率随深度的增加而增加。
SUMMARY Magnetotelluric measurements were performed east of the Rwenzori Mountains, Uganda. At 23 sites transfer functions between magnetic and telluric fields and phase tensor elements were estimated in the period range 10–10 000 s. The transfer functions indicate a complex 3-D conductivity structure within the crust predominantly at the contact zone of the Rwenzori Mountains and the rift shoulder to the northeast. An alternative representation of the phase tensor ellipses in form of bars is introduced. For periods above 100 s, the major phase tensor bars of all sites strike SSW–NNE and the phase tensor invariants φmin and φmax exhibit a difference of a least 20°. This conspicuous behaviour and the small vertical magnetic field in the same period range can be explained by an anisotropic conductivity in the depth range 30–50 km with the conductive direction perpendicular to the Rift axis. The anisotropy might originate from orientated Olivine crystals in the most upper mantle, whereby the preferred orientation might be motion induced due to delamination of the lower crust beneath the Rwenzoris. A high conducting layer is found to the southeast of the Rwenzoris at 15-km depth matching a seismic low-velocity zone. Possibly it indicates a zone of partial melt within the lower crust connected to volcanic fields nearby. Generally, the phase tensor elements φmin increase above 45° for periods longer than 2000 s denoting an increase of conductivity towards greater depth.