Investigating the long-term spatial stability of LLSVPs

GSTDTAP

项目编号	1722623
	Investigating the long-term spatial stability of LLSVPs
	Allen McNamara
主持机构	Michigan State University
项目开始年	2017
	2017-07-01
项目结束日期	2019-06-30
资助机构	US-NSF
项目类别	Continuing grant
项目经费	213618(USD)
国家	美国
语种	英语
英文摘要	The Earth's surface is highly dynamic, characterized by earthquakes, volcanoes, and active faulting caused by the movement of tectonic plates at the surface. It is well-understood that the driver of plate tectonics is convection within Earth's mantle, the layer between the core and the crust. Over the past several decades, there has been much advancement toward increasing our understanding of how tectonic plates form, move about the surface, and sink back into the Earth's interior; however, we still have fundamental, first-order questions regarding how the mantle convects and drives plate tectonics. Seismology provides the best clues to answer these questions, by using seismic waves caused by earthquakes to generate a 3D image of the interior. One puzzling observation that we find from seismology is the presence of 2 large structures in the lowermost mantle, beneath Africa and the Pacific. These structures rest directly on the Earth's core, and extend several hundred kilometers into the mantle. Seismic waves travel slower than usual through these structures, and they seem to have sharp boundaries, indicating that they may be composed of something somewhat different than the surrounding mantle rock. Interestingly, these structures underlie a large number of anomalous active volcanoes (called hotspots), such as Hawaii. By reconstructing plate motions back through time, we find that many extinct, ancient volcanoes were also formed above these same locations. Therefore, if the large structures beneath Africa and the Pacific are the cause of these hotspot volcanoes, they must have been in their present location for a long time, perhaps for the past 500 million years. This is contradictory to computational models of mantle convection that predict that such structures would be easily moved by changing tectonic plate motions. The PI hypothesizes that these deep mantle structures may have a different mineralogical grain size than the rest of the mantle, which could cause them to have a higher viscosity. The team will perform numerical fluid dynamics to examine whether the higher viscosity associated with these structures could lead to a stabilizing of mantle convection currents over geologic time to explain the long-lived creation of volcanoes over these geographic areas. More importantly, the investigators will examine how the dynamics of these African and Pacific structures can guide and control the long term patterns of mantle convection. Results from this study will provide critical insight into understanding how mantle convection causes and controls plate tectonics. Seismic tomography reveals the presence of large regions in the lowermost mantle (beneath Africa and the Pacific) that exhibit lower-than-average shear wave velocity, commonly referred to as the Large Low Shear Velocity Provinces (LLSVPs). It has been hypothesized that they are caused by large-scale compositional heterogeneity (e.g., thermochemical piles). Discovering the properties and dynamical implications of the LLSVPs has involved much active research in recent years due to their critical role toward understanding large-scale mantle convection, heat transport, and thermal and chemical evolution of the Earth. Interestingly and somewhat paradoxically, recent paleomagnetic research has hinted that LLSVPs may have remained in the same positions for hundreds of millions of years. This is perplexing because dynamical calculations indicate that they should be easily swept around by changing subduction patterns. This is a critically important issue to address, and here, the following question is explored: Can a reasonable dynamical/rheological conceptual model of thermochemical mantle convection explain how LLSVPs could remain spatially fixed (or very slow moving)? Most geodynamical calculations employ a temperature-dependent rheology, resulting in LLSVPs being weaker (due to their high temperature) and therefore, easier to be laterally pushed around by downwelling slabs. However, diffusion creep rheology is highly sensitive to mineral grain-size (in a power-law manner), so small changes in grain-size can lead to large changes in viscosity. If LLSVPs have remained compositionally distinct from the background mantle over geologic timescales, there is no reason to expect that they would have the same average grain-size as the surrounding mantle. If the average grain-size of LLSVPs is only slightly larger than that of the background mantle, LLSVPs would have an increased compositional viscosity. Preliminary work has shown that when combined with temperature-dependence of viscosity, this leads to a strong rind or envelope surrounding the LLSVPs, producing unusual thermochemical structures that dynamically behave quite differently than conventional, passive thermochemical piles. This type of thermochemical convection has not been explored, and the proposed work will test the hypothesis that grain-size induced compositional dependence of viscosity can lead to LLSVP structures that are less easily pushed around by subducting slabs and therefore, become more spatially-stable over geologic timescales. An extensive geodynamical study (through numerical modeling) will be performed to explore how grain-size dependence of rheology influences the fluid dynamical properties of the system, compared to traditional models. Additionally, grain-size evolution will be employed in a subset of models, to examine whether it is feasible for LLSVPs to have larger grain size than background mantle over the relevant geological timescales. In summary, numerical experiments will be developed and performed to determine how grain-size dependence of viscosity influences the spatial stability of thermochemical piles and whether this is a dynamically feasible mechanism to produce long-lived, stable positions of LLSVPs.
文献类型	项目
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/71206
专题	环境与发展全球科技态势
推荐引用方式 GB/T 7714	Allen McNamara.Investigating the long-term spatial stability of LLSVPs.2017.