The Role of On- and Off-Axis Faults and Fissures During Eruption Cycles and Crustal Accretion at 9 degrees 50'N, East Pacific Rise

Fissures and faults provide insight into how plate separation is accommodated by magmatism and brittle deformation during crustal accretion. Although fissure and fault geometry can be used to quantify the spreading process at mid-ocean ridges, accurate measurements are rare due to insufficiently detailed mapping data. Here, fissures and faults at the fast-spreading 9 degrees 50'N segment of the East Pacific Rise were mapped using bathymetric data collected at 1-m horizontal resolution by autonomous underwater vehicle Sentry. Fault dip estimates from the bathymetric data were calibrated using co-registered near-bottom imagery and depth transects acquired by remotely operated vehicle Jason. Fissures are classified as either eruptive or non-eruptive (i.e., cracks). Tectonic strain estimated from corrected fault heaves suggests that faulting plays a negligible role in the plate separation on crust younger than 72 kyr (<4 km from the ridge axis). Pre-and post-eruption surveys show that most fissures were reactivated during the eruptions in 2005-2006. Variable eruptive fissure geometry could be explained by the frequency with which each fissure is reactivated and partially infilled. Fissure swarms and lava plateaus in low-relief areas >2 km from the ridge are spatially associated with off-axis lower-crustal magma lenses identified in multichannel seismic data. Deep, closely spaced fissures overlie a relatively shallow portion of the axial magma lens. The width of on-axis fissures and inferred subsurface dike geometry imply a similar to 9-year long diking recurrence interval to fully accommodate plate spreading, which is broadly consistent with cycle intervals obtained from estimates of melt extraction rates, eruption volumes, and spreading rate. Plain Language Summary New oceanic crust is created by seafloor spreading at mid-ocean ridges, where lava erupts as the tectonic plates spread apart. Plate separation is accommodated by a combination of slip along dipping faults and the opening of magma-filled cracks, called dikes. We present bathymetric and profile mapping data collected from near the seafloor on a volcanically active portion of the East Pacific Rise near 9 degrees 50'N. The data show faults and fissures (i.e., open cracks) in remarkable detail, with meter-scale resolution. A total of 707 fissures and 42 faults were identified and measured, suggesting that the amount of plate separation accommodated by faulting is minimal compared to that by dike intrusion causing open cracks. About one-third of the fissures mapped are located within the region covered by the most recent eruptions in 2005-2006, and most of these fissures seem to have been reactivated from previous eruptions. Based on measurements of fissure width, we estimate that the interval between diking events is similar to 9 years, which agrees with previous independent estimates. The analysis in this study reveals the relative importance of faults and fissures in seafloor spreading, and in the magmatic cycles that continuously re-pave the ocean floor.