Being a geophysicist can sometimes feel like being a detective —uncovering clues, and then building a case based on the evidence.
In a new article published in Science Advances, a collaborative team led by the Woods Hole Oceanographic Institution (WHOI), presents a never-before-seen image of an oceanic transform fault from electromagnetic (EM) data collected at the Gofar fault in the eastern Pacific Ocean. The National Science Foundation funded work reveals unexpected brine deposits beneath the seafloor near the fault, which could change the way we conceptualize oceanic transform faults.
The Gofar fault operates much like the San Andreas, in that two tectonic plates slide sideways past each other. Unlike the San Andreas, large earthquakes on this fault have been strangely predictable, with large ruptures occurring every five to six years. That predictability has made Gofar an ideal place to study earthquake mechanisms, with a variety of data collected at the fault, including a number of small earthquakes measured on ocean bottom seismographs.
In contrast to seismic data, EM measurements tell researchers how well a material can conduct electricity. This is useful because one of the models for why Gofar behaves as it does is related to differences in the amounts of seawater present in the seafloor: fluids influence how faults stick, slide, and slip, causing earthquakes of various magnitudes. The salt in seawater makes it conduct electricity well, far better than the surrounding rocks, and so EM data provide clues as to where seawater or other fluids are hiding beneath the seafloor.
Using state of the art instruments, the study’s authors were able to create a snapshot of the electrical properties beneath the Gofar fault. They expected that one portion of the fault would be slightly more conductive than its surroundings based on prior models of such faults. Instead, the team was surprised to find that extremely conductive blobs reside beneath the seafloor on one side of the fault but not the other. To make matters more perplexing, other geophysical data from the area did not reveal similar anomalies.
“It was shocking to see such a stark contrast across the fault,” said Christine Chesley, a WHOI postdoc in Geology & Geophysics, and lead author of the study. “The conductivity structure defied all of our expectations based on what we thought we knew about oceanic transform faults.”
Oceanic transform faults have historically been thought of as simple, predictable features. They represent the least well-studied of the three major plate boundaries, which include divergent boundaries, like East Africa, where plates move apart forming new crust; and convergent boundaries, like the Himalayas, where two plates collide and recycle crust. However, recent findings like this necessitate a new framework for understanding oceanic transform faults.
“Whenever we go out and make these kinds of EM measurements, we see the seafloor through a different lens, and it almost always changes our views on the processes that shape the earth,” explained Rob Evans, Senior Scientist at WHOI in Geology & Geophysics and co-author of the study.
Determining why the conductive blobs appeared in the EM data, but did not present as other kinds of geophysical anomalies, required some deductive reasoning.
“We needed a self-consistent mechanism that could help explain why these conductive masses are existing under only one side of the fault and where seismic velocities seem unaffected,” Chesley explained. “Something with conductivities this high isn’t normally seen beneath the seafloor, except where magma is involved.”
Working with these puzzle pieces, the authors realized that the conductive blobs required salt—a lot of salt—to account for their high conductivity values. This suggested the anomalies represented brine accumulations.
“And in order to create brines, you need a source of heat,” added Chesley. “We think this heat source is magma near the transform fault.”
The authors hypothesized that some magma is present on the side of the fault where the conductive blobs of brine are found. This would be a remarkable shift in our understanding of transform faults, which have generally not been considered to host magmatic or hydrothermal activity.
“We have this amazing image of this particular section of the Gofar fault, but we haven’t yet been able to see how it connects to the adjacent mid-ocean ridge. We are hopeful that additional project funding will support additional research,” said Evans.
The National Science Foundation’s Division of Ocean Sciences supported this project.
The following institutions contributed to this research: University of Delaware; Boise State University; Scripps Institution of Oceanography, University of California San Diego; Western Washington University; University of Texas Austin; MIT-WHOI Joint Program in Oceanography/Applied Ocean Science and Engineering; University of Southern Maine; Columbia University; University of New Hampshire.
