What is the Relationship Between Mineral Fabric, Seismic Anisotropy, and Mantle Flow in Partially Molten Regions? The Ethiopian Rift as a Testing Ground

地震各向异性是测试地幔变形和流动模型最广泛使用的工具之一。由于地幔橄榄岩中的橄榄石晶体在变形时产生了相干结构，地幔流动方向与地震快速方向之间明显的简单对应导致了它的广泛使用。然而，最近的实验室研究对地幔岩石在少量水或熔体存在的情况下被拉伸时流动和快波方向之间的关系提出了怀疑。对存在水和熔体的橄榄岩的变形研究表明，地震快速方向可以从剪切方向旋转90度，而不是像通常假设的那样与剪切方向平行。这些观测结果可能会颠覆我们在融冰丰富地区的地幔流动模型，但迄今为止，这些观测结果仅来自实验室实验，并且尚未证明它们与地幔规模和/或条件有关。我们将测试在融冰丰富的地区，特别是埃塞俄比亚的东非裂谷，部分熔融和/或水对地幔结构的控制程度。这个位置对这个实验来说是理想的，因为这个明显富含融水的区域是非常明确的，并且存在丰富的地震数据。然而，目前尚不清楚现有数据是否具有足够的各向异性空间分辨率来区分竞争模式。在这项为期一年的概念验证研究中，我们将进行地震表面波分析，并测试几个简单的地幔结构和流动模型。我们的主要目的是确定裂缝下的各向异性是否表明在干燥、无熔体样品中发现的“标准”织物，或者观察到的各向异性是否与在富含熔体或潮湿条件下观察到的变形织物更一致。在此过程中，我们将大大提高我们对该地区产生伸展和火山作用的地幔动力学过程的理解。在更广泛的影响中，这项工作将为LDEO博士后研究员本·霍尔茨曼提供地震学方面的培训，他是该项目的共同负责人。此外，我们将与布里斯托尔大学的Michael Kendall教授和利兹大学的Graham Stuart博士合作。

Seismic anisotropy is one of the most widely utilized tools for testing models of mantle deformation and flow. Its widespread use results from the apparently simple correspondence between the orientation of mantle flow and seismic fast-direction, due to the coherent fabric that is produced in olivine crystals in mantle peridotites when they deform. However, recent laboratory studies have cast doubt on this relationship between flow and fast-wavespeed directions when mantle rocks are strained in the presence of a small amount of water or melt. Deformation studies of olivine rocks in the presence of water and melt suggest that the seismic fast direction can rotate to 90 degrees from the shear direction, rather than parallel to the shear direction as is commonly assumed. These observations potentially turn on their heads our models of mantle flow in melt-rich regions, but to date the observations are only from laboratory experiments, and it has not been demonstrated that they are relevant to mantle scales and/or conditions. We will test the degree to which partial melt and/or water may control the mantle fabric in a melt-rich region, specifically the East African Rift in Ethiopia. This location is ideal for this experiment, because the apparently melt-rich region is very well defined, and abundant seismic data exists. However, it is unclear whether the existing data have sufficient spatial resolution of anisotropy to differentiate between competing models. In this one-year proof-of-concept study, we will perform the seismic surface-wave analysis and test several simple models of mantle fabric and flow. Our primary aim is to determine wither the anisotropy beneath the rift is indicative of the "standard" fabric found in dry, melt-free samples, or whether the observed anisotropy is more consistent with deformation fabrics observed under melt-rich or wet conditions. In the process, we will greatly improve our understanding of the mantle dynamic processes that are producing extension and volcanism in this region. Among the broader impacts are that the work will provide training in seismology for LDEO Post-Doctoral Fellow Ben Holtzman, who is serving as a co-PI on the project. In addition, we will be collaborating with Prof. Michael Kendall of the University of Bristol, and Dr. Graham Stuart of University of Leeds.