High‐resolution bathymetry reveals contrasting landslide activity shaping the walls of the Mid‐Atlantic Ridge axial valley

Abstract

[1] Axial valleys are found along most slow-spreading mid-ocean ridges and are one of the most prominent topographic features on Earth. In this paper, we present the first deep-tow swath bathymetry for the axial valley walls of the Mid-Atlantic Ridge. These data allow us to analyze axial valley wall morphology with a very high resolution (0.5 to 1 m compared to ≥ 50 m for shipboard multibeam bathymetry), revealing the role played by landslides. Slow-spreading ridge axial valleys also commonly expose mantle-derived serpentinized peridotites in the footwalls of large offset normal faults (detachments). In our map of the Ashadze area (lat. 13 N), ultramafic outcrops have an average slope of 18 and behave as sliding deformable rock masses, with little fragmentation. By contrast, the basaltic seafloor in the Krasnov area (lat. 16 38 0 N) has an average slope of 32 and the erosion of the steep basaltic rock faces leads to extensive fragmentation, forming debris with morphologies consistent with noncohesive granular flow. Comparison with laboratory experiments suggests that the repose angle for this basaltic debris is > 25. We discuss the interplay between the normal faults that bound the axial valley and the observed mass wasting processes. We propose that, along axial valley walls where serpentinized peridotites are exposed by detachment faults, mass wasting results in average slopes ≤ 20 , even in places where the emergence angle of the detachment is larger.

What causes low magnetization at basalt-hosted hydrothermal sites? Insights from inactive site Krasnov (MAR 16°38′N)

“…in 2007, we collected high‐resolution bathymetric and magnetic data using ROV Victor of Ifremer. Bathymetric data were acquired by a RESON SeaBat 7125 multibeam echosounder [ Cannat et al ., ] and the magnetic data by a three‐component fluxgate magnetometer. The ROV surveyed east‐west parallel profiles, spaced 120 m apart and 50 m (±5 m) above the seafloor over an area of ∼3000 × 1500 m. No crossing line was performed.…”

“…The bathymetric grid, presented by Cannat et al . [], has a resolution of 0.5 m and extends across the axial valley wall from depth ∼3600 to ∼4400 m (Figure a).…”

“…This magnetic feature extends eastward the strong positive anomaly located ∼120 m beneath the NW‐SE fault scarp (Figure ), which may mark the top of a coherent tilted block of magnetized basalt beneath debris or hydrothermal deposits (see above, section ). Such an inference is supported by the association of the slight relative magnetic high with the coherent tilted block observed in the hydrothermal area (see above, section ) [ Cannat et al ., ]. We made no attempt to model such variations of the magnetic basement, considering the lack of constraint on its geometry.…”

Quasi‐3‐D Seismic Reflection Imaging and Wide‐Angle Velocity Structure of Nearly Amagmatic Oceanic Lithosphere at the Ultraslow‐Spreading Southwest Indian Ridge

“…The seafloor in the footwall compartment, however, dips only 20° on average (Figure ). Recently acquired high‐resolution bathymetry data and submersible observations (Cannat et al, ) indicate that this difference is best explained by extensive mass‐wasting of the exposed part of the footwall when the fault dip at emergence is more than the stability angle, as proposed for axial detachment faults at the Mid‐Atlantic Ridge (Cannat et al, ).…”

Composition and Formation of Gabbro-Peridotite Hosted Seafloor Massive Sulfide Deposits from the Ashadze-1 Hydrothermal Field, Mid-Atlantic Ridge

“…High-resolution bathymetry (pixel size: 0.5-1 m) [8] and video profiling was used to describe the local geological setting and to get visual images of the Ashadze-1 deposit. Ondreas [8] and Cannat [10] proposed that the rockslides in the region, interpreted from the detailed bathymetric data, were associated with the Ashadze-1 hydrothermal field. The topography of the site is low, with elevation reaching 6 m above the seafloor.…”