Gofar Transform Fault

Oceanic mid-ocean ridge transform faults (RTFs) offer an ideal natural laboratory to study the evolution of stress, strength and deformation throughout an earthquake cycle: a primary reason being the relatively predictable cycle of seismicity observed with quasi-periodicities on the order of 5-6 years.

The Gofar transform, in the eastern Pacific, has been well studied and contains a region that repeatedly inhibits rupture propagation and that primarily fails aseismically. Seismic velocities show a significant difference between the seismic rupture region and the rupture barrier. We hypothesize that deep seawater penetration into the rupture barrier results in persistent high porosity at depth. The net result of these fluids is to promote dilatancy-strengthening of the fault which has been observed to inhibit dynamic deformation.

A series of CSEM profiles at different parts of the Gofar system directly measured seafloor electrical resistivity, which is indicative of the crustal porosity. Our experiment mapped porosity variations throughout the system, particularly in the damage zone of primary seismicity and within the area that has been seen to repeatedly inhibit rupture propagation. Our data reveal high conductivity within the lower crust, likely indicating the presence of saline brines, and possibly a small melt fraction. We suggest that melt, suctioned into the transform from the adjacent spreading ridge, has provided a heat source to drive differential fluid infiltration deep into the crust south of Gofar, promoting aseismic creep. We suggest that off-axis melt may play a key role in barrier zone development at OTFs.

MT data provide information on the deeper structure beneath the transform and provide insights into the thermal structure and deeper shearing patterns associated with the long-term evolution of the fault. Our survey spans both the primary fault strand and an adjacent fracture zone, allowing for comparison of these two structures.