Conductivity structure suggests a fluid-rich rupture barrier on the Gofar oceanic transform fault (East Pacific Rise)

Oceanic transform faults (OTFs) accommodate a substantial proportion of tectonic plate motion aseismically. Systematic along-strike coupling patterns have been observed at both fast and slow OTFs. Patches of high-coupling rupture regularly, generating earthquakes of magnitude (M) greater than 5.5. These zones are separated by regions of low coupling that act as barriers to rupture propagation. The mechanisms controlling these persistent barrier zones remain poorly understood. We used magnetotelluric (MT) methods to image a rupture barrier on the westernmost Gofar oceanic transform fault, a left-lateral fault offsetting the East Pacific Rise near 5° S. Our results reveal heterogeneity within the barrier zone: a high-conductivity western section and a low-conductivity eastern section, which also exhibit distinct patterns in seismicity and surface structure. The electrical resistivity model derived from the MT data suggests porosity on the order of 3% through much of the crust in the western section, which likely results from seawater infiltration facilitated by a highly damaged fault zone. Fluid infiltration would promote alteration and low coupling, and could explain the long-term seismic deficit of the barrier. Under such conditions, dilatancy-induced pore-pressure fluctuation could exert control on seismicity and slip mode, including arresting adjacent M 6 ruptures. Seismic behavior in the eastern barrier zone differs from the western barrier zone, possibly due to differences in fluid content. Taking the Gofar barrier as an analog, aseismic OTFs around the world may have fluid-rich fault zones with rupture behavior regulated by hydromechanical processes.