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The role of a geophysicist can often resemble that of a detective, piecing together clues to assemble a compelling narrative based on collected evidence.
A recent publication in Science Advances showcases groundbreaking work led by the Woods Hole Oceanographic Institution (WHOI) that presents an unprecedented image of an oceanic transform fault. Utilizing electromagnetic (EM) data from the Gofar fault in the eastern Pacific Ocean, this study, funded by the National Science Foundation, uncovers surprising brine deposits beneath the seafloor in proximity to the fault, potentially reshaping our understanding of oceanic transform faults.
The Gofar fault operates similarly to the well-known San Andreas fault, where two tectonic plates slide laterally past one another. However, a notable difference is that large earthquakes at the Gofar fault occur in a remarkably predictable pattern, with significant ruptures happening every five to six years. This characteristic makes Gofar a prime location for examining earthquake dynamics, with extensive data gathered at the site, including multiple small seismic events detected via ocean bottom seismographs.
Unlike traditional seismic measurements, EM data informs researchers about the electrical conductivity of materials, which is significant for understanding fault behavior. A proposed explanation for the predictable nature of the Gofar fault relates to varying seawater concentrations in the seafloor: fluids can significantly affect how faults behave in terms of sticking, sliding, and slipping, leading to earthquakes of differing magnitudes. Because seawater is rich in salt, it conducts electricity more effectively than surrounding rock formations, making EM data crucial for identifying areas where seawater or other fluids may be located below the ocean floor.
This study utilized advanced instruments to develop a detailed picture of the electrical characteristics beneath the Gofar fault. Researchers anticipated finding a section of the fault that was slightly more conductive than its surrounding area based on existing models. However, they were astonished to discover highly conductive anomalies beneath one side of the fault that were absent on the other. This finding was particularly perplexing because other geophysical data did not indicate similar irregularities.
“The stark contrast we observed across the fault was astonishing,” remarked Christine Chesley, a WHOI postdoctoral researcher in Geology & Geophysics and the study’s lead author. “The conductivity structure challenged all of our preconceptions about oceanic transform faults.”
Traditionally, oceanic transform faults have been perceived as straightforward, predictable features and are among the least explored of the three primary plate boundaries, which also include divergent boundaries, like those forming new crust in East Africa, and convergent boundaries, like the Himalayas, where plates collide and recycle crust. The recent findings from Gofar compel researchers to reconsider the existing framework surrounding the comprehension of oceanic transform faults.
“Each time we embark on EM measurements, we gain a new perspective of the seafloor, leading to a shift in our understanding of the Earth’s processes,” explained Rob Evans, a senior scientist at WHOI specializing in Geology & Geophysics and a co-author of the study.
To understand why the EM data indicated conductive anomalies without corresponding geophysical irregularities, a thoughtful analysis was required.
“We sought a coherent explanation to clarify why these conductive masses appeared only on one side of the fault while seismic velocities appeared unaffected,” Chesley stated. “Conductivities this high below the seafloor are not commonly observed unless magma is involved.”
From their research, the authors deduced that a significant amount of salt was necessary to justify the high conductivity readings, leading them to hypothesize that the conductive anomalies might represent concentrated brine deposits.
“A heat source is essential for the formation of brine, and we believe this heat may originate from magma near the transform fault,” Chesley added.
The researchers posited that there is magma present on the side of the fault corresponding to the brine’s conductive blobs. This could mark a significant expansion of our understanding of transform faults, which have historically been considered to lack magmatic or hydrothermal activity.
“We have acquired extraordinary insights into this segment of the Gofar fault, yet we still need to explore how this aspect connects to the adjacent mid-ocean ridge. We remain optimistic that future funding will facilitate further investigations,” stated Evans.
The project received support from the National Science Foundation’s Division of Ocean Sciences. Several institutions contributed to this research, including the University of Delaware, Boise State University, Scripps Institution of Oceanography at the University of California San Diego, Western Washington University, University of Texas at Austin, MIT-WHOI Joint Program in Oceanography/Applied Ocean Science and Engineering, University of Southern Maine, Columbia University, and the University of New Hampshire.
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