Surface wave tomography: Imaging of the lithosphere-asthenosphere boundary beneath central and southern Africa?
Surface wave tomography: Imaging of the lithosphere-asthenosphere boundary beneath central and southern Africa?
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DOI：
10.1016/j.lithos.2010.05.011
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发表时间：
2010-11
期刊：
影响因子：
3.5
通讯作者：
S. Fishwick
中科院分区：
文献类型：
--
作者：
S. Fishwick
岩石圈-软流圈边界（LAB）在地球科学中仍然是一个有争议的课题，在地幔区域之下似乎是一个特别难以一致成像的边界。地震方法给出了不同的指标上地幔的速度结构：层析成像模型提供了各种横向尺度的速度变化的估计，但有有限的垂直分辨率;接收器函数技术提供了很好的指示深度地震不连续，但较少的信息上的绝对速度。这项研究评估了不同的方法是否对南部非洲LAB的深度给出了一致的估计。使用在非洲地区的近12，000路径的表面波数据集，计算中部和南部非洲的新的层析模型。为了显示断层扫描的非唯一性，结果是两个不同的参数化。这些模型显示了非洲中部和南部的东非地区不同的速度结构，从而估计了LAB的深度，从坦桑尼亚大约150公里的深度到卡拉哈里沙漠下大约200公里的深度，再到刚果部分地区下225 - 250公里的深度。在广泛的范围内，这些深度估计是兼容的金伯利岩捕虏体的地质测温。在坦桑尼亚等地区，金伯利岩岩浆活动被观察到发生在上地幔地震速度结构中的沿着强水平梯度-岩石圈结构中的潜在边缘特征。相比之下，在喀拉哈里沙漠之下的详细比较表明，在这个地区，并考虑到目前的层析成像分辨率，金伯利岩和速度梯度之间没有明确的联系。然而，一般来说，金伯利岩不采样最快的地震速度的区域。断层成像模型的LAB深度估计值与接收器函数的估计值之间的关系尚不清楚。接收器函数技术在南部非洲的结果往往会在比热定义的LAB估计更浅（100 - 150公里）或更深（300 - 350公里）的深度放置不连续性。因此，应特别注意自动关联从快到慢的地震速度作为一个热定义的岩石圈-软流圈边界相同的位置的不连续性。
The lithosphere–asthenosphere boundary (LAB) remains a controversial subject in Earth sciences, and beneath cratonic regions appears to be a particularly difficult boundary to consistently image. Seismic methods give different indicators on the velocity structure of the upper mantle: tomographic models provide estimates of the velocity variations at a variety of lateral scales, but have limited vertical resolution; receiver function techniques provide good indication of the depth to seismic discontinuities, but less information on the absolute velocities. This study assesses whether the different methods give consistent estimates for the depth of the LAB in southern Africa. Using a surface wave dataset with nearly 12,000 paths in the African region, new tomographic models of central and southern Africa are calculated. To show the non-unique nature of tomography, results are presented for two different parameterisations. The models indicate varying velocity structure beneath the cratonic regions of central and southern Africa, which yield estimates of the LAB depth from around 150km depth in Tanzania, to approximately 200km depth beneath the Kalahari Craton, down to depths of 225–250km beneath parts of the Congo. At a broad-scale these depth estimates are compatible with geothermometry from kimberlite xenoliths. In regions such as Tanzania, the kimberlite magmatism is observed to occur along strong horizontal gradients in upper mantle seismic velocity structure — potentially edge features in lithospheric structure. In contrast, a detailed comparison beneath the Kalahari Craton indicates that in this region, and given the present resolution of the tomography, there is no clear link between the kimberlites and velocity gradients. However, in general the kimberlites do not sample the regions of fastest seismic velocities. The relationship between LAB depth estimates from the tomographic modelling and those estimates from receiver functions is not clear. Results from receiver function techniques beneath southern Africa tend to place discontinuities at either shallower (100–150km), or deeper (300–350km), depths than the thermally defined LAB estimates. As such, particular care should be taken in automatically associating a discontinuity from fast to slow seismic velocities as the same location as a thermally defined lithosphere–asthenosphere boundary.