Skew of Mantle Upwelling Beneath the EPR: A Reconsideration of Data and Models

学术价值：本项目的总体目标是进一步发展海洋地壳的形成和演化模型，山脊分割的起源模型，以及地幔上升流与海底热液和火山活动之间的联系模型。地幔上升流是洋中脊新洋壳形成的必要条件，一般认为这种上升流在活动脊下是对称的。以前的洋中脊模型通常假设岩浆供应控制着洋中脊的分割，软流圈内的熔融体运输与上覆构造板块的扩张方向平行，地幔上涌和熔融体运输是围绕洋中脊轴线对称的。然而，地球物理研究表明，在RIDGE 2000东太平洋隆起综合研究基地（EPR ISS）下，地幔上涌和地幔向地壳的熔融输送既不对称，也不以上升轴为中心。这些结果首次显示了大洋中脊下地幔上涌的大规模倾斜和不对称。这些令人惊讶的发现重新引起了关于扩展中心分割和脊峰活动的起源和意义的争论，以及关于控制海洋地壳结构和莫霍地震过渡带（地壳底部）的过程的争论。为了解决这一争议，我们将重新检查现有的EPR地震、水深、重力和岩石学数据，并将我们的结果与之前的研究结果相结合。我们的工作计划包括：“挖掘”高质量地震数据，发展地震方法，研究地幔上涌倾斜对地壳和莫霍过渡带结构的影响，包括在epr侧翼绘制地壳级岩浆房和热异常图。分析现有的水深、地震和重力数据，了解轴向和离轴海底深度和重力的区域尺度变化，并解决轴向深度和地壳厚度在快速扩张脊上不相关的悖论。将我们的研究结果与现有的岩石学和地质学研究相结合，包括对活动脊和蛇绿岩（陆地上暴露的“化石”洋中脊地质剖面）的研究，以建立海洋地壳的形成和演化以及洋中脊分割的起源模型。更广泛的影响：拟议的研究将在完成RIDGE 2000计划的目标方面发挥重要作用。该计划的一个核心领域是跨学科研究与“能量和物质从地球深部地幔，通过海洋地壳的火山和热液系统，流向深海”相关的基本问题。该项目通过展示脊顶火山和热液活动强度与地幔上涌倾斜之间的关系，直接解决了这一主题。东太平洋隆起下地幔的倾斜上涌导致了岩浆分异的分段尺度变化，分异主要发生在地壳下方和莫霍线附近。因此，发生在莫霍内部和紧接之下的岩浆过程对于理解地幔和地壳之间的质量（和能量）交换、理解沿上升轴发现的火山和热液活动的分割以及理解轴向深度异常的意义至关重要。以培养研究生的形式开发人力资源是另一个重要成果。申请的大部分资金将用于支持博士研究生的研究。

Intellectual Merit: The overarching goals of this project are to further develop models for the formation and evolution of the oceanic crust, for the origin of ridge crest segmentation and for the linkages between mantle upwelling and seafloor hydrothermal and volcanic activity. Mantle upwelling is essential to the generation of new oceanic crust at mid-ocean ridges, and it is generally assumed that such upwelling is symmetric beneath active ridges. Previous models of mid-ocean ridges have usually assumed that magma supply controls ridge crest segmentation, that the transport of melt within the asthenosphere parallels the spreading direction of the over-riding tectonic plates, and that mantle upwelling and melt delivery is symmetric about the mid-ocean ridge axis. Geophysical studies, however, reveal that mantle upwelling and the delivery of melt from the mantle to the crust beneath the RIDGE 2000 East Pacific Rise Integrated Study Site (EPR ISS) is neither symmetric nor centered beneath the rise axis. These results are the first to show large-scale skew and asymmetry of mantle upwelling beneath mid-ocean ridges. These surprising discoveries renew the debate over the origin and significance of spreading-center segmentation and ridge crest activity as well as over the processes that control the architecture of oceanic crust and the Moho seismic transition zone (at the base of the crust). To address this debate, we will reexamine existing seismic, bathymetric, gravity and petrologic data from the EPR and integrate our results with previous studies. Our work plan includes:1. "Mining" high-quality seismic data and developing seismic methods to investigate the effects of skew of mantle upwelling on crustal and Moho transition zone architecture, including mapping crustal-level magma chambers and thermal anomalies on the flanks of the EPR.2. Analyzing existing bathymetric, seismic and gravity data to understand regional scale variations in on-axis and off-axis seafloor depth and gravity and to address the paradox of why axial depth and crustal thickness are not related along fast-spreading ridges.3. Integrating our results with existing petrologic and geologic studies, both at active ridges and ophiolites ("fossil" mid-ocean ridge geological sections exposed on land), to develop models for the formation and evolution of oceanic crust and for the origin of segmentation of mid-ocean ridges.Broader Impacts: The proposed research will play an important role in accomplishing the goals of the RIDGE 2000 program. A central area of this program is in interdisciplinary investigation of fundamental problems related to "the flow of energy and material from Earth's deep mantle, through the volcanic and hydrothermal systems of the oceanic crust, to the deep ocean". This project addresses this theme directly by demonstrating a relation between the intensity of ridge crest volcanic and hydrothermal activity and the skew of mantle upwelling. The skew of mantle upwelling beneath the East Pacific Rise results in segment-scale variations in magmatic differentiation, with the differentiation occurring beneath the crust and near the Moho. Thus magmatic processes occurring within and immediately beneath the Moho may be centrally important to understanding the exchange of mass (and energy) between the mantle and the crust, to understanding the segmentation of volcanic and hydrothermal activity found along the rise axis and to understanding the meaning of axial depth anomalies. Development of human resources in the form of graduate student training is another important outgrowth. The majority of the funds requested will be used to support the research of PhD candidate.