The Varying Expression of Oceanic Detachment Faulting from Termination to 1.5 Ma

Abstract:

The style of oceanic detachment faulting varies both along and away from the ridge axis. We examine the characteristics of detachment footwalls using regional bathymetry data and high-resolution data collected by AUV Sentry near 16.5N at the Mid-Atlantic Ridge. The distance of the fault termination from the volcanic axis combined with the slope of the emerging detachment footwall has been used to infer the subsurface shape of the fault. In fact, the location of the termination and slope of the fault near the termination reflect the volume of volcanic infill at the ridge axis. The termination will be far from the volcanic axis where the volume of lava is sufficient to cover the fault surface and conversely, the termination will be closer to the axis in areas where volcanic eruptions are small. In areas of large volcanic infill where lavas have flowed onto the shallow-dipping fault surface, Sentry bathymetry data show that the volcanic morphology of the hanging wall is extremely deformed. As a detachment extends and moves away from the axis the high-resolution bathymetry data show that landslides develop on some detachments but not others, possibly reflecting the underlying lithology. We observe that a massif with km-scale corrugations, probably underlain by competent gabbros, does not show significant mass wasting. At another detachment, where altered lower crustal and upper mantle rocks were sampled, headwall scars with relief of < 30 m are observed only a few km from the termination. Farther off-axis on crust > 1 Ma, high-resolution bathymetry data show a landscape profoundly affected by landslides. Large sections of the surface have collapsed producing large headwall scars with relief of up to 200 m. Adjacent semicircular collapses leave spurs between them that are elongated close to the slip direction. The spurs have wavelengths of a few km and could be misinterpreted as relict corrugations from the regional bathymetry.