Nature of a Low Velocity Anomaly in the Mantle Transition Zone Beneath the Western Great Plains

几十年的地质科学深入研究令人信服地表明，大尺度的地表特征，如山带、海洋盆地和裂谷；以及地震、火山爆发等自然灾害；是发生在地球深处的动态过程的表现。其中一个过程是地幔中温度高于正常水平的部分融化岩石的上涌。近年来，许多地球物理研究已经在大平原西部发现了这种上升流，但这种特征的几何形状、深度范围，更重要的是，形成机制仍然是一个谜。这项研究将采用一套计算密集型地球物理技术来研究这一有趣的特征。这项研究的发现将揭示地球上其他地方的类似特征，最终提高我们理解、预测和减轻自然灾害的能力，并有效地勘探和开发自然资源。提出的科学调查将对各个领域和职业阶段的地球科学家产生重大影响。一名研究生、两名本科生和一名暑期实习生将参与这项研究。该项目开发的材料将整合到大学的各种教育计划中，旨在招募少数民族本科生和研究生从事科学事业。研究小组将使用一系列地震学方法，通过测试三种假设，来调查大平原西部地下一个显著低速异常的存在和性质，包括它是否代表1)与法拉龙板块俯冲有关的异常高温、低密度、上升流特征；2)起源于下地幔的新兴地幔柱；3)地幔过渡带中高含水带。验证假设的研究任务包括：利用P-to-S接收函数对过渡带不连续深度进行速度修正；利用多重反射p - s转换成像地幔过渡带；地震各向异性分层研究；并利用综合测试对异常特征进行约束。验证这些假设可以为地球动力学模型提供一级约束，从而在理解北美构造演化方面取得重大进展。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Decades of intensive research in the geological sciences have convincingly shown that large-scale surface features such as mountain belts, ocean basins, and rifted valleys; and natural hazards such as earthquakes and volcanic eruptions; are manifestations of dynamic processes occurring in the deep interior of the Earth. One of these processes is the upwelling of hotter-than-normal, partially melted rocks in the Earth’s mantle. In recent years, a number of geophysical studies have identified such upwelling beneath the western Great Plains, but the geometry, depth extent, and more importantly, formation mechanism for this feature remain enigmatic. This study will employ a suite of computationally-intensive geophysical techniques to investigate this intriguing feature. Findings from this study will shed light on similar features elsewhere on Earth, ultimately improving our capability to understand, predict, and mitigate natural hazards and effectively explore and develop natural resources. The proposed scientific investigations will have significant impacts for geoscientists from a wide range of fields and career stages. A graduate student, two undergraduate students, and a summer intern will be involved in the study. Materials developed from this project will be integrated into various educational programs at the university aimed at recruiting minority undergraduate and graduate students into scientific careers.The research team will use an array of seismologic approaches to investigate the existence and nature of a prominent low velocity anomaly beneath the western Great Plains by testing three hypotheses, including whether it represents 1) an anomalously hot, low-density, upwelling feature associated with the subduction of the Farallon slab; 2) an emerging plume originating from the lower mantle; and 3) a zone of high water content in the mantle transition zone. Research tasks to test the hypotheses include velocity corrections to the apparent transition zone discontinuity depths, obtained using P-to-S receiver functions; imaging the mantle transition zone using multiply reflected P-to-S conversions; investigation of seismic anisotropy layering; and constraining the anomaly feature using synthetic tests. Testing these hypotheses may provide first-order constraints for geodynamic models, leading to significant progress in understanding the tectonic evolution of North America.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.