Hydrothermal circulation within the Earth's oceanic crust is a fundamental process that affects the chemical and physical exchange between the solid Earth and the oceans. Sea water penetrates down the ocean crust several kilometers through cracks, becomes heated up as it encounters magma reservoirs and hot rocks, reacts with the surrounding rocks, and rises back to the seafloor carrying minerals that are expelled into the oceans and used as nutrients by the vast biological communities that form deep-sea ecosystems. In addition, the interaction between the hydrothermal fluids and the rocks affects the chemical and physical properties of the lithosphere, therefore having important implications for processes like the formation of deep-sea mineral deposits. Within this general framework, this project will address the issue of what are the physical impact, depth extent, and horizontal spatial scales of hydrothermal alteration in the oceanic crust during its formation along a mid-ocean ridge, and its subsequent evolution and maturation. In particular, we want to understand (1) what is the relationship between high-temperature hydrothermal convection in a mid-ocean ridge and the underlying structure (location of magma reservoirs, porosity and permeability of the crust, etc.)? (2) How deep does the alteration penetrate as the crust matures? (3) Are there fine-scale variations in crustal structure related to topographic controls on low-temperature hydrothermal flow control? (4) What are the links between hydrothermal cooling and the observed variations in the structure of the oceanic crust near a mid-ocean ridge? Hydrothermal alteration has a significant impact on the speed at which seismic waves propagate through the crust (seismic velocity). Thus measuring in detail the seismic velocity of the upper ~1-2 km of the crust can give us a wealth of information about the processes described above. To answer the above-mentioned questions we will analyze marine seismic reflection data collected in 2002 as part of a NSF-funded project along the Juan de Fuca mid-ocean ridge and tectonic plate in the northeast Pacific, off the coast of Oregon, Washington, and British Columbia. Seismic waves generated from the research vessel M. Ewing were recorded by sensors located in a 6-km-long streamer towed by the ship. By measuring the time that waves traveled through the Earth's crust from the sources to the receivers, as well as their amplitude changes along their paths, we will be able to construct high resolution images of the elastic properties of the crust using state-of-the-art computing techniques known as travel-time and waveform seismic tomography. The resulting seismic tomography images will then be interpreted in terms alteration due to hydrothermal convection. We will also use gravity measurements to infer the density structure of the ocean crust, and together with the seismic observations, investigate the impact of hydrothermal cooling on the apparent variability of crustal thickness. Understanding the causes and consequences of hydrothermal circulation and alteration that takes place under the world's oceans requires a multidisciplinary approach, including biological, geochemical, geological, and geophysical studies, as well as numerical computer models and in-situ and remote observations. Much of what we know to date about hydrothermal circulation within oceanic lithosphere comes from studies at the Juan de Fuca ridge and tectonic plate during the past decades. This study will fill a gap by quantifying the impact and scales of hydrothermal alteration on the seismic structure of the lithosphere, and relate it to geological and environmental variables such as topography and sedimentation history. The objectives of this project are directly linked to the science goals of, and will benefit, several large initiatives and ongoing programs within Ocean Science. Our studies to characterize the structure beneath hydrothermal vent sites are directly linked to objectives of the NSF RIDGE-2000 program at the Endeavour ISS. CanNeptune and the Regional Cabled Observatory component of the Ocean Observatory Initiative will place a fiber optic cable spanning the Juan de Fuca plate to facilitate long-term monitoring of ridge axis and flank processes. The tomographic studies of axial structure of this project will provide new constraints on physical context at appropriate scales for these monitoring studies. Our ridge flank work will benefit plate-scale experiments envisioned here under ORION, and hydrogeologic objectives of ongoing ODP-IODP studies. This project will be part of the Ph.D. research of two graduate students, who will be trained in marine seismic reflection and tomography techniques. This will contribute to the development of a workforce with expertise in these methods, benefiting both the US academic research and industry communities.
