YN (2009-2009): ETMO: Endeavour Seismic Tomography Experiment

• University of Oregon, United States of America

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). During the 30-day leg, 64 four-component, ocean bottom seismometers (OBSs) were deployed throughout a 90x50 km*2 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 common phenomena beneath spreading centers. Also at this scale, the ETOMO data provides direct measurements of crustal thickness which will be 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 will provide 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 will constrain the physical properties of the reaction zone between the magmatic and hydrothermal systems. Taken as a whole, data from the ETOMO experiment will be able to 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.

The Endeavour seismic tomography experiment (ETOMO) experiment will test competing models for what controls the segmentation and intensity of ridge crest processes. Existing models 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 models of the origin of segmentation at mid-ocean ridges.
The ETOMO experimental design will investigate the 3-D structure of the crust and topmost mantle beneath the Endeavour segment, a RIDGE2000 Integrated Study Site (ISS). The scientific objectives 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.

The results of our environmental research will also have other concrete science and societal benefits, including an improved understanding of the life cycle of deep-sea vents and of how the structure and hydration of oceanic crust contributes to earthquakes, tsunamis and volcanic hazards that threaten the Pacific Northwest (PNW).

The ETOMO expedition is collaborative between Professors Dr. Douglas Toomey and Dr. Emilie Hooft (University of Oregon) and Professor Dr. William Wilcock (University of Washington.

The location of the ETOMO experiment is the Endeavour segment of the Juan de Fuca ridge. This mid-ocean ridge segment is one of three integrated study sites for the NSF-funded RIDGE2000 program and it is a primary science node for NEPTUNE Canada.

*Arnoux, G. M., D. R. Toomey, E. E. E. Hooft, W. S. D. Wilcock, J. Morgan, M. Warner, and B. P. VanderBeek Seismic evidence that black smoker heat flux is influenced by localized magma replenishment and associated increases in crustal permeability, Geophys. Res. Lett., 44, https://doi.org/10.1002/2016GL071990, 2017.

*VanderBeek, B. P., D. R. Toomey, E. E. E. Hooft, and W. S. D. Wilcock, Segmentation of mid-ocean ridges attributed to oblique mantle divergence, Nature Geosciences, 9, doi: https://doi.org/10.1038/NGEO2745, 2016.

*Soule, D., W. S. D. Wilcock, D. R. Toomey, E. E. E. Hooft, and R. T. Weekly, Near-axis crustal structure and thickness of the Endeavour Segment, Juan de Fuca Ridge, Geophys. Res. Lett., 43, 5688-5695, doi: https://doi.org/10.1002/2016GL068182, 2016.

*Weekly, R. T., W. S. D. Wilcock, D. R. Toomey, E. E. E. Hooft and E. Kim, Upper crustal seismic structure of the Endeavour Segment, Juan de Fuca Ridge from travel time tomography: Implications for oceanic crustal accretion, Geochem. Geophys. Geosyst, 15, https://doi.org/10.1002/2013GC005159, 2014.

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