Mid-Atlantic Ridge 29oN: New Insights on Ridge-Axis Faulting From High-Resolution, Deep-Towed Swath Bathymetry

The deep-towed vehicle TOBI carries 30 kHz side-scan sonar giving 6 m resolution over a 6 km swath. A phase measuring swath bathymetry system was added in 1998. The first deployment was over the Mid-Atlantic Ridge axis at 29^oN, as part of a comprehensive study of the Broken Spur spreading segment. The system, still undergoing development, achieved partial success. We recorded and have processed two adjacent swathes of data across the ridge axis, extending 35 km either side of the axis (approximately to crustal age 2.7 Ma). The phase signals were rather noisy, but usable data were recovered in swaths up to 1.2 km wide on either side, though over significant areas coverage was much worse. Best coverage was obtained over seafloor depths greater than 3000m, especially the median valley floor, possibly because lack of sediment there provides the strongest acoustic contrast. Post-processing included conversion of acoustic phase to depth, with corrections for varying vehicle attitude. Careful analysis reveals some residual noise that seems to correlate with vehicle motion, suggesting sub-optimum calibration of its attitude sensors, and in places clear artefacts trend parallel to track or to iso-phase lines. Nevertheless we were able to grid the data with an interval of 10m following filtering. The results have been compared to both bathymetry from the ship-mounted, 12 kHz Simrad EM-12 and to the TOBI side-scan data. We were able to grid the Simrad data to 50 m horizontal resolution. Seafloor fault scarps can be reliably resolved in Simrad data where apparent seafloor gradients are greater than about 20-30^o and scarp heights are greater than several tens of metres. Many of the smaller fault scarps have apparent gradients of less than 25^o and hardly ever more than 35^o, often making it difficult to distinguish them from steep volcanic slopes. On larger scarps, slopes up to 55^o and rarely 65^o can be seen. Scarps less than 50 m wide are not resolved. By contrast, TOBI reliably resolves scarps down to 10 m wide, and shows that even these have slopes of at least 30^o, with slopes up to 50^o being common and maximum slopes reaching more than 60^o. Major escarpments can sometimes be resolved into bundles of smaller faults. Faults are thus imaged with comparable resolution to the TOBI side-scan, but with the addition of quantitative depth information. This has important consequences for estimates of tectonic extension. While we have relatively little coverage of the axial neovolcanic zone, volcanic forms are well resolved at the 10 m grid interval, showing the typical hummocky terrain revealed with high-resolution side-scan, as well as larger mounds and cones.