Seismological Investigations of the Continental Upper Mantle

大陆上地幔的地震学研究

基本信息

批准号：
NE/G000859/1
负责人：
Stewart Fishwick
金额：
$ 7.81万
依托单位：
University of Leicester
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2009
资助国家：
英国
起止时间：
2009 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FG000859%2F1
关键词：
Seismological Investigations Continental Upper Mantle

项目摘要

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.

我们理解大陆变形和动力学的一个关键要素是我们对大陆上地幔的详细结构和性质的了解。地幔的部分很少可以直接采样，而地球物理技术提供了了解各大洲岩石圈结构的最全面的方法。在大陆之下，在古老的克拉通地区和较年轻的地体之间观察到的地震波速的巨大差异主要可以用温度的变化来解释。然而，正如在一些区域和局部实验中所看到的，地震速度的急剧横向变化不能用热结构的差异来解释。表面波层析成像的最新结果表明，除了速度的快速横向变化之外，剪切波速的垂直变化与简单的成分模型和克拉通地温不相容。在一些古老的大陆区域下方，莫霍面下方的地震速度比预期的要慢，并且最上地幔中似乎存在正速度梯度。为了研究这个问题，有必要开发使用不同技术进行组合解释的方法。表面波断层扫描突出显示了异常结构，但这些研究的垂直分辨率有限，不足以对物理结构中任何非常快速的转变进行成像。该限制可以通过将表面波断层扫描与接收器功能相结合来解决。接收器功能利用以速度对比转换的地震波，因此将突出物理特性的不连续性。在地震台间距很近的地区，不仅可以研究垂直变化，还可以研究物理特性的任何横向不连续性。该提案将通过对非洲和澳大利亚永久地震台下方的表面波色散和接收器功能的联合解释来研究大陆上地幔的结构和特性。比较不同构造背景下的速度模型对于更好地了解大陆岩石圈的物理演化非常重要。是在所有环境中都观察到慢速和正梯度，还是构造过程的特定特征？在南部非洲，这些方法还可以根据临时地震阵列的数据进行测试。阵列数据应该能产生有关大陆结构的更详细信息，更好的分辨率可以表征结构的横向变化。然而，评估地震仪 1-2 年临时部署是否能够提供与根据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，额外的地球物理技术将被纳入解释中。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次要相特性的连通性的函数（例如，低阶部分熔融、流体、石墨形式的碳），从而提供有关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模型相结合，进一步增强我们对大陆岩石圈的了解。该提案的目的是开发能够根据最上地幔的物理特性对地震模型进行定量解释的方法。是否有可能准确观察异常剪切波速的深度范围，它们是否与特定的矿物学相关？研究的总体结果将极大地提高我们对大陆岩石圈现状的认识。