Collaborative Research: Thermochemical Models of Mantle Dynamics and Plate Motions

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。这项工作的目标是了解地球的动力学。地球内部和地球的长期演化。构造板块的运动和随之而来的地震是内部大规模流动的表面表现，这是由地球冷却时的热浮力和成分浮力共同驱动的。我们在地表观察到的大多数地质过程都以这样或那样的方式与这种大规模流动有关。然而，我们从动力学角度对流的理解还远远不够。驱动水流的浮力的来源是什么？这种流动是如何与地表的构造板块相互作用的？流动如何以及为什么重组并导致（相对）突然的板块运动变化。为了解决这些问题，我们建议建立一个新的大尺度流动理论模型，并通过引入一些新的观测来测试和完善模型。我们提出了几项比以往研究更重要的进展。首先，我们计划在全球流动模型中对俯冲带进行更完整的处理。俯冲动力学用一种新的粘性薄片模型来描述，该模型明确地包括了板块弯曲的影响以及由于冷而致密的俯冲板块的重量而引起的板块内部的拉应力。对俯冲板块深处地震的观测提供了关于板块内部应力状态的宝贵信息。我们计划在全球流模型中首次使用这些信息。其次，我们建议发展粘度横向变化的自洽描述。由于重要的输运性质（如粘度）对温度有很强的依赖性，热浮力预计会引起粘度的大变化。当从地震非均质性层析模型推断浮力时，我们建议使用这种自洽模型来预测流动。地震异常到密度异常的转换是一个有争议的问题，特别是在地幔下部。热浮力和成分浮力的相对重要性尚不为人所知。我们计划利用最近探测到的地球自由振荡来约束地球内部的重力场。从地震异常到密度异常的不同转换对全球重力场有不同的影响，这可以通过地表重力测量和我们对内部重力场的新约束来验证。我们希望能更好地理解推动水流的浮力，并对板块在组织水流中的作用提供新的见解。这项提议的工作支持了两位年轻的女性研究者（Kayla Lewis博士和Melanie Gerault女士），并促进了南加州大学和加州大学伯克利分校之间的新合作。拟议的工作还将使用、调整和改进CIG存储库（nsf资助的一项倡议）中的现有计算机代码。我们打算在项目完成后将新代码贡献给CIG。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The goal of the proposed work is to understand the dynamics of the Earth?s interior and the long-term evolution of our planet. The motion of tectonic plates and the attendant earthquakes are surface expressions of a large-scale flow in the interior, which is driven by a combination of thermal and compositional buoyancy as the planet cools. Most of the geological processes we observe at the surface are related in one way or another to this large-scale flow. However, our understanding of the flow from a dynamical perspective is far from complete. What is the origin of the buoyancy that drives the flow? How does this flow interact with tectonic plates at the surface? How and why does the flow reorganize and cause (relatively) abruptly changes in plate motions. We propose to address these questions developing a new theoretical model for the large-scale flow and by introducing several new observations to test and refine the model.We propose several important advances over previous studies. First, we plan to develop a more complete treatment of subduction zones in global models of flow. The dynamics of subduction is described by a new viscous sheet model that explicitly includes the effects of plate bending as well as the tensile stresses inside the plate due to the weight of the cold and dense subducted plate. Observations of deep earthquakes in subducted plates provide valuable information about the stress state inside the plates. We plan to make use of this information for the first time in global flow models. Second, we propose to develop a self-consistent description of lateral variations in viscosity. Thermal buoyancy is expected to cause large variations in viscosity due to the strong temperature dependence of important transport properties (like viscosity). We propose to use this self-consistent model to predict flow when the buoyancy forces are inferred from tomographic models of seismic heterogeneity. The conversion from seismic anomaly to density anomaly is a controversial issue, particularly in the lower part of the mantle. The relative importance of thermal and composition buoyancy is not well known. We plan to use a recently detected free oscillation of the Earth to constrain the gravity field in the interior. Different conversions from seismic anomaly to density anomaly have different consequences for the global gravity field, which can be tested using gravity measurements at the surface and our new constraint on the gravity field in the interior. We hope to gain a better understanding of the buoyancy forces that drive the flow and provide new insights into the role of plates in organizing the flow. The proposed work supports two young female investigators (Dr. Kayla Lewis and Ms. Melanie Gerault) and fosters a new collaboration between USC and UC Berkeley. The proposed work will also use, adapt and improve an existing computer code in the CIG repository (an NSF-funded initiative). We intend to contribute the new code back to CIG when the project is completed.