The lithosphere-asthenosphere boundary: integrated modeling of scattered wave observations and mantle dynamics

区分地球岩石圈和软流圈的物理和化学因素并不是唯一已知的，它们在不同构造环境之间的变化也不是唯一已知的。然而，在岩石圈-软流圈边界（LAB）的地震速度梯度的高分辨率成像有可能限制的组合的温度，成分（亏损和挥发分含量），和熔体分数，可以创建LAB。 例如，在北美东部的宽带台站观测到的P到S散射波（Ps）揭示了LAB速度梯度（在不到11公里的范围内，剪切波速度下降了3%到11%），这些变化太强烈和尖锐，无法单独用温度来解释，这表明了成分效应（一个更加枯竭，干燥的岩石圈在一个不那么枯竭，水化软流层）或者可能是软流圈中部分熔融的存在。这个项目的目标是更好地确定从岩石圈到软流圈过渡的尖锐程度，这些因素的组合决定了这种转变，以及它们在不同构造环境之间的相对影响的差异。 该方法是集成的速度梯度约束在LAB从散射波观测模型的地幔流动和熔融。 具体地说，PI是：使用Ps和Sp相位来限制LAB剪切速度梯度在各种大陆和海洋设置，数值模拟地幔流动和熔融，以计算温度范围，主要元素贫化，挥发物富集，和部分熔融附近的LAB;确定哪些条件与观测到的地震速度梯度一致，利用地震速度与地幔成分和温度之间的现有关系。一个有待检验的过度假设是，LAB下方异常陡峭的地震速度梯度可能是由绝热减压熔融形成的少量部分熔融造成的，并被渗透性较低的岩石圈捕获。不太陡的梯度可以单独用温度或挥发物含量来解释。 该项目最初将侧重于三个案例：北美东部、南部非洲和不同年龄的海洋岛屿。 选择这些区域是因为它们包含了LAB梯度成像所需的长期宽带站的分布，并且因为它们允许评估地幔流，成分，温度和熔融之间的特定相互作用。 在北美东部和非洲南部，有待检验的观点是，岩石圈厚度迅速变化驱动的地幔上涌通过绝热减压产生了少量的部分熔融。 在海洋岛屿的情况下，该项目将检查是否LAB梯度需要融化发生在年轻的海洋岛屿附近，LAB梯度与岩石圈的脱水和消耗一致，或仅与温度一致，更有可能在最古老的海洋岛屿之下。这项工作受益于研究生和本科生研究人员的参与，反过来又有助于他们的教育。

The physical and chemical factors that distinguish the Earth's lithosphere from its asthenosphere are not uniquely known, and neither is their variation between different tectonic environments. However, high resolution imaging of seismic velocity gradients at the lithosphere-asthenosphere boundary (LAB) has the potential to constrain the combinations of temperature, composition (depletion and volatile content), and melt fraction that could create the LAB. For example, P-to-S scattered waves (Ps) observed at broadband stations in eastern North America have revealed LAB velocity gradients (a 3 to 11% shear-wave velocity drop over less than 11 km) that are too strong and sharp to be explained by temperature alone, suggesting compositional effects (a more depleted, dry lithosphere over a less depleted, hydrated asthenosphere) or perhaps the presence of partial melt in the asthenosphere.The goal of this project is to better determine the sharpness of the transition from the lithosphere to the asthenosphere, the combination of factors dominate this transition, and the variance of their relative influence between different tectonic settings. The approach is to integrate constraints on velocity gradients at the LAB from scattered wave observations with models of mantle flow and melting. Specifically, the PI's are: using Ps and Sp phases to constrain LAB shear velocity gradients in a variety of continental and oceanic settings; numerically modeling mantle flow and melting to calculate ranges of temperature, major element depletion, volatile enrichment, and partial melting near the LAB; determining which conditions are consistent with the observed seismic velocity gradients, using existing relationships between seismic velocity and mantle composition and temperature.An over-arching hypothesis to be tested is that exceptionally steep seismic velocity gradients beneath the LAB may be caused by a small degree of partial melt formed through adiabatic decompression melting, and trapped by a less permeable lower lithosphere. Less steep gradients may be explained by temperature or volatile content alone. The project will initially focus on three cases: eastern North America, southern Africa, and ocean islands of varying age. These regions were chosen because they contain the distributions of long-term broadband stations necessary for imaging LAB gradients, and because they permit the evaluation of specific interactions between mantle flow, composition, temperature, and melting. In eastern North America and southern Africa, the idea to be tested is that mantle upwelling driven by rapid changes in lithospheric thickness generates a small degree of partial melt through adiabatic decompression. In the case of ocean islands, the project will examine whether LAB gradients requiring melt occur near young ocean islands, and LAB gradients consistent with dehydration and depletion of the lithosphere, or with temperature alone, are more likely beneath the oldest ocean islands.This work is benefiting from the participation of a graduate student and undergraduate researchers and is in turn contributing to their education.