Seismological Investigations of the Continental Upper Mantle

大陆上地幔的地震学研究

• 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

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因素是我们对大陆上地幔详细结构和性质的了解。很少有地球的地幔可以直接采样，地球物理技术提供了最全面的方法来了解大陆岩石圈的结构。在大陆之下，观测到的古老的板块区域和年轻板块区域之间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，地震速度的急剧横向变化，如在一些区域和当地的实验，不能解释的热结构的差异。表面波层析成像的最新结果表明，以及速度的快速横向变化，剪切波速的垂直变化是不兼容的简单的组成模型和超声波地热。在一些古老的大陆区域下面，莫霍面下的地震速度比预期的要慢，并且在上地幔中似乎存在正的速度梯度。为了研究这一问题，有必要开发使用不同技术进行联合解释的方法。异常结构在表面波层析成像中被突出显示，然而这些研究具有有限的垂直分辨率，不足以对物理结构中的任何非常快速的转变进行成像。这种局限性可以通过将表面波层析成像与接收器功能相结合来解决。接收器函数利用以速度对比转换的地震波，因此将突出物理特性中的不连续性。在地震台站密集的地区，不仅可以调查垂直变化，而且可以调查物理性质的任何横向不连续性。该提案将通过联合解释非洲和澳大利亚常设地震台站下的表面波色散和接收器功能，调查大陆上地幔的结构和性质。不同构造背景下速度模型的比较对于更好地理解大陆岩石圈的物理演化具有重要意义。慢速度和正梯度是在所有环境中观察到的，还是构造过程的一个特殊特征？在南部非洲，还可以用临时地震台阵的数据来检验这些方法。阵列数据应能提供关于大陆结构的更详细的资料，分辨率越高，就越能描述结构的横向变化。然而，还必须评估1-2年临时部署地震仪是否能提供与根据多年永久台站数据获得的模型同样可靠的模型。在南部非洲，将在解释中采用另一种地球物理技术。与地震学技术相反，使用大地电磁数据的方法主要是岩石中的次要相位特性的连通性的函数（例如，低阶部分熔体、流体、石墨形式的碳），从而提供关于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，进一步提高我们对大陆岩石圈的认识。这项建议的目的是发展一些方法，以便能够根据最上层地幔的物理性质对地震学模型进行定量解释。是否有可能准确地观察到异常剪切波速的深度范围，以及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。