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.
This collaborative project with the University of Oregon completed a seismic tomography experiment on the Endeavour Segment of the Juan de Fuca Ridge. Mid-Ocean Ridges are tectonic plate boundaries where two oceanic plates move apart and new crust is created through volcanism. Mid-ocean ridges form a linear network of extensional plate boundaries that extends 40,000 miles through all the Earth’s ocean and resurface 2/3 of the Earth’s surface on a 100-Myr timescale. Mid-ocean ridges account for about 80% of Earth’s volcanism and the hydrothermal systems these volcanoes support lead to extensive heat and chemical exchange with the ocean and may have been the site of the origin and/or early evolution of life on Earth. This project focused on a portion of a 100-km-long segment of mid-ocean ridge located about 200 miles west of the Straits of Juan de Fuca. It hosts some of the most extensive and well-studied mid-ocean ridge hydrothermal systems and is a node on the NEPTUNE Canada cabled observatory. The tomography study comprised 68 ocean bottom seismic recorders and >5000 impulsive sound sources created by the compressed air guns on the Research Vessel Marcus Langseth. Travel times between the sound sources and seismic instruments were measured and used to solve for the speed of seismic waves (sound) in the Earth’s crust and mantle across the plate boundary. The speed of sound in the Earth’s crust is affected by the composition of the crust, the presence of cracks, temperature and melt which allowed us to interpret the seismic images in terms of volcanic, tectonic and hydrothermal processes. At the Endeavour we found that the volcanic crust is thicker near the segment center where previous studies have detected a magma chamber underlying very vigorous black smoker hydrothermal systems. A big surprise of our study was the discovery of many regions of hot and partially molten crust well away from the plate boundary. It is clear that volcanism is not just confined to the plate boundary but extends ~10 km away from it. We found that the crust is much more heavily fractured near the segment ends where there are fewer volcanic eruptions. While the hydrothermal systems in these regions are not as spectacular, the hydrothermal fluids appear to circulate deeper to the base of the crust and possibly into the mantle.
