Competing models for controls on the segmentation and intensity of ridge crest processes are at odds on the scale of mantle and crustal magmatic segmentation, the distribution of hydrothermal venting with respect to a volcanic segment and the properties of the thermal boundary layer that transports energy between the magmatic and hydrothermal systems. The recent discovery of an axial magma chamber (AMC) reflector beneath the Endeavour segment of the Juan de Fuca ridge, as well as systematic along axis changes in seafloor depth, ridge crest morphology and hydrothermal venting provide an ideal target for testing conflicting hypotheses. The scientific objectives of this project are to: (1) Determine if the segmentation and intensity of the magma-hydrothermal systems at the Endeavour ridge are related to magma supply or to the magma plumbing between the mantle and crust, and (2) Constrain the thermal and magmatic structure underlying the Endeavour hydrothermal system in order to understand the patterns of energy transfer. An active source seismic tomography experiment, using an array of 64 three-component ocean-bottom seismometers, will image the 3-D seismic structure of the crust and topmost mantle along an 80-km-long section of the Endeavour ridge. The experiment will image four targets: (1) crustal thickness variations within 25 km of the axial high (0 to 900 kyr); (2) the 2-D (i.e., map view) structure of the uppermost mantle beneath the spreading axis; (3) the 3-D structure of the crustal magmatic system and (4) the detailed 3-D, shallow crustal thermal structure beneath the Endeavour vent field. The results of imaging will define the recent history of magma supply, the pattern of melt delivery from the mantle to the crust and the structure and segmentation of the subseafloor magmatic and hydrothermal systems. These measurements are essential to testing critically competing hypotheses for what regulates the intensity of ridge crest magmatic and hydrothermal processes.
R/V Marcus G. Langseth leg MGL0910 conducted a multi-scale seismic tomography experiment on the Endeavour segment of the Juan de Fuca Ridge (ETOMO). The study was supported by the National Science Foundation. During the 30-day leg, 68 four-component, ocean bottom seismometers (OBSs) were deployed at 64 sites throughout a 90x50 km2 area to record seismic energy from the 36-element, 6600 cu. in. airgun array of the R/V Marcus G. Langseth. At the segment scale (~90 km along axis), the ETOMO experiment constrains the nature of sub-ridge mantle flow and the pattern of melt transport from the topmost mantle to the crust beneath the entire Endeavour segment. These ‘undershoot’ data provide a direct test of competing models for the origin of ridge crest segmentation and, in particular, provide critical data for determining if skew of mantle upwelling and melt transport is a common phenomena beneath spreading centers. Also at this scale, the ETOMO data provides direct measurements of crustal thickness which are being used to understand the history of segment-scale magma supply. At an intermediate scale of approximately 60 km along axis and 20 km across axis, the ETOMO experiment constrains the size, shape, and distribution of crustal magma bodies that fuel the Endeavour hydrothermal system. Data from the ‘crustal grid’ of seismic surveying provides one of the largest 3-D images of a crustal magmatic system for any volcano on Earth. At a smaller scale that is focused on the vent fields themselves, data from ETOMO constrain the physical properties of the reaction zone between the magmatic and hydrothermal systems. Taken as a whole, data from the ETOMO experiment track the seismic signature of heat and mass transport from the mantle to the seafloor at the Endeavour segment, thereby achieving one of the primary goals of the RIDGE 2000 program for the Endeavour Integrated Study Site. Intellectual Merit: Magmatic and hydrothermal processes along spreading centers are driven by upwelling and melt generation in the mantle. Understanding the pathways and mechanisms of mass and energy flow from the Earth’s mantle to its surface is thus a central goal of mid-ocean ridge studies. The ETOMO experiment addresses this goal by investigating two fundamental boundary layers in the system: 1) the boundary layer that connects plates to the underlying asthenosphere, and 2) the boundary layer that connects hydrothermal processes to magmatic complexes. Along with several other recent studies, the ETOMO data set is challenging the commonly held view that mantle flow beneath oceanic spreading centers is a passive response to plate spreading. Instead it appears that mantle flow direction and delivery of melt is significantly decoupled from plate motions. The Endeavour data sets provide an ideal opportunity to understand the implications of decoupled mantle upwelling for crustal accretion and to assess the roles of off-axis melt delivery and crustal tectonics in controlling the distribution of off-axis crustal accretion. Broader Impacts: Results of our research contribute to the large-scale research infrastructure of US-OOI and NEPTUNE Canada as well as to the RIDGE 2000 Endeavour Integrated Studies Site. It complements the PIs’ participation in programs at the University of Oregon (UO) and University of Washington (UW) aimed at enhancing STEM teaching and research opportunities. It facilitates ongoing efforts to incorporate software, data and techniques into classes that introduce students to current geosciences research and its societal applications for understanding geohazards. The research makes use of the UO’s new NSF-funded cloud computing facility and fosters a long-standing collaboration between Toomey’s group and computer scientists at the UO. The adaption of codes for large parallel computing environments, and the continuing code development that occurred in parallel to this project, enhances infrastructure for research. The research grant has supported graduate students at UO and UW and provided opportunities for undergrads and grad students to participate in an oceanographic expedition.
