Seismological Investigations of the Continental Upper Mantle

我们理解大陆变形和动力学的一个关键因素是我们对大陆上地幔详细结构和性质的了解。地球的地幔很少能被直接取样，而地球物理技术提供了最全面的方法来理解横跨大陆的岩石圈结构。在大陆之下，在古老的克拉通区域和年轻的地体之间观测到的地震波速度的巨大差异主要可以用温度的变化来解释。然而，在一些区域和局部实验中看到的地震速度的急剧横向变化不能用热结构的差异来解释。最近的表面波层析成像结果表明，除了快速的横向速度变化外，切变波速的垂直变化与简单的成分模型和克拉通地热不相容。在一些古老的大陆区域之下，莫霍下的地震速度比预期的要慢，在最上层的地幔上似乎有一个正的速度梯度。为了研究这一问题，有必要发展使用不同技术进行综合解释的方法。异常结构在表面波层析成像中是突出的，然而这些研究具有有限的垂直分辨率，不足以成像物理结构的任何非常快速的转变。这种限制可以通过结合表面波层析成像和接收函数来解决。接收函数利用以速度对比转换的地震波，因此将突出物理性质的不连续。在地震台站间隔紧密的地区，不仅可以调查垂直变化，还可以调查物理性质的横向不连续性。该提案将通过联合解释非洲和澳大利亚永久地震台站下的表面波频散和接收函数来研究大陆上地幔的结构和性质。比较不同构造背景下的速度模型对于更好地理解大陆岩石圈的物理演化具有重要意义。是在所有环境中都观察到慢速度和正梯度，还是构造过程的一个特殊特征？在非洲南部，这些方法还可以在临时地震阵列的数据上进行测试。阵列数据应该提供更详细的大陆构造信息，更好的分辨率允许描述结构的横向变化。然而，评估1-2年临时部署的地震仪是否能提供与多年永久台站数据获得的模型一样可靠的模型也很重要。在南部非洲，一项额外的地球物理技术将被纳入解释。与地震学技术相比，使用大地电磁数据的方法主要是岩石中小相性质连通性的函数（例如，低阶部分熔体、流体、石墨形式的碳），从而提供岩石圈结构的补充信息。因此，大地电磁研究的结果可以与地震学模型相结合，进一步增强我们对大陆岩石圈的认识。这一建议的目的是发展一种方法，使地震模型能够根据最上层地幔的物理性质进行定量解释。是否有可能准确地观察到异常剪切波速的深度范围，它们是否与特定的矿物学有关？研究的总体结果将大大提高我们对大陆岩石圈现状的认识。

A key element in our understanding of continental deformation and dynamics is our knowledge of the detailed structure and properties of the continental upper mantle. Very little of the Earth's mantle can be directly sampled, and geophysical techniques provide the most comprehensive approach to understanding the structure of the lithosphere across the continents. Beneath the continents the large differences in seismic wavespeed observed between old cratonic regions and younger terranes can be predominately explained by variations in temperature. However, sharp lateral variations in seismic velocity, as seen in some regional and local experiments, cannot be explained by differences in thermal structure. Recent results from surface wave tomography suggest that as well as the rapid lateral changes in velocity, the vertical variations in shear wavespeed are not compatible with simple compositional models and cratonic geotherms. Below a number of old continental regions seismic velocities beneath the Moho are slower than expected and there appears to be a positive velocity gradient in the uppermost mantle. In order to investigate this problem it is necessary to develop methods to make a combined interpretation using different techniques. The anomalous structure is highlighted in surface wave tomography, however these studies have a limited vertical resolution insufficient to image any very rapid transition in physical structure. The limitation can be addressed by combining surface wave tomography with receiver functions. The receiver function makes use of seismic waves which are converted at velocity contrasts and will therefore highlight discontinuities in physical properties. In regions with closely spaced seismic stations it should be possible to investigate not only the vertical variations, but also any lateral discontinuities in physical properties. The proposal will investigate the structure and properties of the continental upper mantle through a joint interpretation of surface wave dispersion and receiver functions beneath permanent seismic stations in Africa and Australia. Comparison of velocity models beneath different tectonic settings is important to enable a better understanding of the physical evolution of the continental lithosphere. Are the slow velocity and positive gradient observed in all settings, or a particular feature of a tectonic process? In southern Africa the methods can also be tested on data from a temporary seismic array. Array data should yield more detailed information on continental structure, the better resolution allowing the characterisation of lateral changes in structure. However, it is also important to assess whether a 1-2 year temporary deployment of seismometers can provide as reliable a model as those obtained on many years of permanent station data. Within southern Africa an additional geophysical technique will be incorporated into the interpretation. In contrast to seismological techniques, methods using magnetotelluric data are predominately a function of the connectivity of minor phase properties in a rock (e.g., low order partial melt, fluids, carbon in graphite form) and thus give complementary information on the structure of the lithosphere. The results from magnetotelluric studies can therefore be combined with the seismological models and further enhance our knowledge of the continental lithosphere. The aim of this proposal is to develop methods that will allow a quantitative interpretation of seismological models in terms of the physical properties of the uppermost mantle. Is it possible to accurately observe the depth range of anomalous shear wavespeeds, and can they be associated with a particular mineralogy? The overall results of the study will greatly improve our knowledge of the present state of the continental lithosphere.