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

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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.