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

知识价值：该项目的首要目标是进一步发展洋壳的形成和演化、海脊分段的起源以及地幔上涌与海底热液和火山活动之间的联系的模型。地幔上升流是大洋中脊形成新洋壳的关键，一般认为这种上升流在活动海脊下是对称的。以往的大洋中脊模型通常假定岩浆供应控制着大洋中脊的分段，软流圈内熔体的输运平行于上覆构造板块的扩展方向，地幔上涌和熔体输送相对于大洋中脊轴线对称。然而，地球物理研究表明，在2000年东太平洋隆起综合考察点(EPR ISS)之下，地幔上涌和从地幔到地壳的熔体输送既不对称，也不以隆起轴以下为中心。这些结果首次显示了大洋中脊下方地幔上升流的大范围倾斜和不对称。这些令人惊讶的发现重新引发了关于扩张中心分段和海脊活动的起源和意义的争论，以及控制洋壳结构和莫霍面地震过渡带(在地壳底部)的过程的争论。为了解决这场争论，我们将重新检查来自EPR的现有地震、水深、重力和岩石学数据，并将我们的结果与以前的研究相结合。我们的工作计划包括：1.2.“挖掘”高质量的地震数据，开发地震方法，以调查地幔上涌的倾斜对地壳和莫霍面过渡带结构的影响，包括绘制地壳级别的岩浆室和地球物理研究中心侧翼的热异常。分析现有的水深、地震和重力数据，以了解轴上和轴外海底深度和重力的区域尺度变化，并解决为什么轴向深度和地壳厚度沿快速扩张的海脊不相关的悖论。将我们的成果与现有的岩石学和地质学研究相结合，在活动海脊和蛇绿岩(暴露在陆地上的“化石”中脊地质剖面)上，开发洋壳的形成和演化以及大洋中脊分段的起源模型。广泛的影响：拟议的研究将在实现海脊2000计划的目标方面发挥重要作用。该计划的一个中心领域是对与“能量和物质从地球深部地幔，通过大洋地壳的火山和热液系统，流向深海”有关的基本问题进行跨学科研究。该项目通过论证山脊顶火山和热液活动的强度与地幔上升的偏斜之间的关系，直接解决了这一主题。东太平洋隆起下方地幔上涌的偏斜导致了岩浆分异的分段尺度变化，分异发生在地壳之下和莫霍面附近。因此，发生在莫霍面内和紧邻莫霍面之下的岩浆作用对于理解地幔和地壳之间的质量(和能量)交换、理解沿隆起轴发现的火山和热液活动的分段以及理解轴向深度异常的意义可能是至关重要的。以研究生培训形式开发人力资源是另一个重要的结果。申请的大部分资金将用于支持博士生的研究。

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.