Melt infiltration of the lower lithosphere beneath the Tanzania craton and the Albertine rift inferred from S receiver functions
Melt infiltration of the lower lithosphere beneath the Tanzania craton and the Albertine rift inferred from S receiver functions
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DOI：
10.1029/2012gc004167
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发表时间：
2012-08
期刊：
影响因子：
3.7
通讯作者：
I. Wölbern;G. Rümpker;K. Link;F. Sodoudi
中科院分区：
文献类型：
--
作者：
I. Wölbern;G. Rümpker;K. Link;F. Sodoudi
岩石圈和软流圈之间的过渡受到许多当代研究的影响，因为它的性质仍然知之甚少。最厚的岩石圈与古老的克拉通和地台有关，已经表明，地震调查可能无法成像这些地区的岩石圈-软流圈边界。相反，最近的几项研究提出，在几个克拉通之下存在一个来源不明的岩石圈中部不连续面。在这项研究中，我们调查了东非的坦桑尼亚克拉通，它被东非裂谷系统的东西两个分支所包围。我们给出了来自S接收函数的证据，即在50-100公里和140-200公里深度连续两个不连续面，对应于坦桑尼亚克拉通以及阿尔伯特和爱德华裂谷段下显著的S波速降低。通过与合成波形的比较，我们发现较低的不连续性与实验室显示的速度降低6-9%相吻合。较浅的界面显示了从克拉通下方12%到阿尔伯特-爱德华裂谷下方24%的速度下降。它被解释为一个渗透前锋，标志着软流圈熔融上升引起的蚀变岩石圈的上边界。基于包体样品的S速度变化的计算证实了这一点，包体样品显示出致密的结晶矿脉系统，作为渗入熔体的通道。这些矿脉中的矿物组合富含金云母和辉石岩，这可以解释剪切波速降低的原因。熔体渗透是在克拉通岩石圈内形成岩石圈中部不连续面的一种合适机制，克拉通岩石圈被异常热的地幔覆盖。
The transition between the lithosphere and the asthenosphere is subject to numerous contemporary studies as its nature is still poorly understood. The thickest lithosphere is associated with old cratons and platforms and it has been shown that seismic investigations may fail to image the lithosphere‐asthenosphere boundary in these areas. Instead, several recent studies have proposed a mid‐lithospheric discontinuity of unknown origin existing under several cratons. In this study we investigate the Tanzania craton in East Africa which is enclosed by the eastern and western branches of the East African Rift System. We present evidence from S receiver functions for two consecutive discontinuities at depths of 50–100 km and 140–200 km, which correspond to significant S wave velocity reductions under the Tanzania craton and the Albert and Edward rift segments. By comparison with synthetic waveforms we show that the lower discontinuity coincides with the LAB exhibiting velocity reductions of 6–9%. The shallower interface reveals a velocity drop that varies from 12% beneath the craton to 24% below the Albert‐Edward rift. It is interpreted as an infiltration front marking the upper boundary of altered lithosphere due to ascending asthenospheric melts. This is corroborated by computing S velocity variations based on xenolith samples which exhibit a dense system of crystallized veins acting as pathways of the infiltrating melt. Mineral assemblages in these veins are rich in phlogopite and pyroxenite which can explain the reduced shear wave velocities. Melt infiltration represents a suitable mechanism to form a mid‐lithospheric discontinuity within cratonic lithosphere that is underlain by anomalously hot mantle.