4CAST Gofar: New Observations of Structure, Tectonics, Magmatism, and Hydrothermal Activity within the Gofar Transform Fault

Oceanic transform faults are a first-order feature of the mid-ocean ridge system and are often host to complex, segmented magmatic and tectonic systems. Expeditions in 2019 and 2021 to the Gofar Transform Fault on the East Pacific Rise acquired new, high resolution bathymetry and backscatter mosaic imagery of three en échelon strike-slip transform faults, separated by two magmatically-active spreading centers. Bathymetry and backscatter data detail an intricate strike-slip fault system, expressed in some locations as a 50 m tall, linear ridge approximately 400 m wide, that coincides with shallow earthquakes. The northeastern intra-transform region is characterized by a deep pull-apart basin, though fresh lavas have been dredged from this basin as recently as 2006. In contrast, at the shallower southwestern intra-transform region, our new data reveal mappable lava flows unbroken by normal faults, suggesting the presence of a magmatically-robust spreading center. During the 2019 expedition, we detected what appears to have been a hydrothermal event plume in temperature, optical backscatter, and oxidation-reduction potential observations, less than 30 km from the soutwestern intra-transform spreading center. Our observations broadly constrain the timing of intra-transform spreading center migration and changing morphology of the active strike-slip faults, indicated by progressive changes in the associated fracture zones. Within the past 1 Myr, the northernmost strike-slip segment has relocated ~8 km southward, shortening the northeastern intra-transform spreading center. Meanwhile, the southwestern intra-transform spreading center exhibits relatively constant length over the same period. The intra-transform spreading fabric in both northern and southern segments is markedly rougher than spreading fabric outside the transform fault, and punctuated by occasional isolated seamounts, suggesting more dynamic magma-tectonic interactions within the intra-transform region.