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首页> 外文期刊>Pure and Applied Geophysics >Upper Mantle Seismic Structure Beneath Southern Africa: Constraints on the Buoyancy Supporting the African Superswell
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Upper Mantle Seismic Structure Beneath Southern Africa: Constraints on the Buoyancy Supporting the African Superswell

机译：南部非洲下方的上地幔地震构造：对支撑非洲超涌的浮力的限制

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摘要

We present new one-dimensional SH-wave velocity models of the upper mantle beneath the Kalahari craton in southern Africa obtained from waveform inversion of regional seismograms from an Mw = 5.9 earthquake located near Lake Tanganyika recorded on broadband seismic stations deployed during the 1997–1999 Southern African Seismic Experiment. The velocity in the lithosphere beneath the Kalahari craton is similar to that of other shields, and there is little evidence for a significant low velocity zone beneath the lithosphere. The lower part of the lithosphere, from 110 to 220 km depth, is slightly slower than beneath other shields, possibly due to higher temperatures or a decrease in Mg number (Mg#). If the slower velocities are caused by a thermal anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by shallow buoyancy from a hot lithosphere. However, a decrease in the Mg# of the lower lithosphere would increase the density and counteract the buoyancy effect of the higher temperatures. We obtain a thickness of 250 ± 30 km for the mantle transition zone, which is similar to the global average, but the velocity gradient between the 410 and 660 km discontinuities is less steep than in global models, such as PREM and IASP91. We also obtain velocity jumps of between 0.16 ± 0.1 and 0.21 ± 0.1 km/s across the 410 km discontinuity. Our results suggest that there may be a thermal or chemical anomaly in the mantle transition zone, or alternatively that the shear wave velocity structure of the transition zone in global reference models needs to be refined. Overall, our seismic models provide little support for an upper mantle source of buoyancy for the unusually high elevation of the Kalahari craton, and hence the southern African portion of the African Superswell.
机译：我们提供了非洲南部卡拉哈里克拉通下方上地幔的一维SH波速度模型，该模型是根据1997-1999年在宽带地震台站上记录的坦Tang尼喀湖附近Mw = 5.9地震的区域地震图的波形反演获得的南部非洲地震实验。卡拉哈里克拉通下方岩石圈的速度类似于其他盾构的速度，并且几乎没有证据表明岩石圈下方存在明显的低速带。岩石圈的下部（从110到220 km的深度）比其他屏蔽层的底部稍慢，这可能是由于温度升高或Mg值（Mg＃）降低所致。如果较慢的速度是由热异常引起的，那么喀拉哈里克拉通异常高的高度的略小于一半可以用热岩石圈的浅浮力来解释。但是，下部岩石圈Mg＃的降低将增加密度并抵消较高温度的浮力作用。对于地幔过渡带，我们获得了250±30 km的厚度，这与全球平均值相似，但是在410 km和660 km间断点之间的速度梯度不如诸如PREM和IASP91这样的全球模型陡峭。我们还获得了在410 km不连续处的0.16±0.1到0.21±0.1 km / s的速度跳跃。我们的结果表明，地幔过渡带可能存在热异常或化学异常，或者整体参考模型中过渡带的横波速度结构需要完善。总体而言，我们的地震模型几乎没有为卡拉哈里克拉通异常高高的上地幔浮力提供支持，因此也为非洲超涌浪的南部非洲部分提供了支撑。

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