Seismic Velocity Structure Along and Across the Ultraslow‐Spreading Southwest Indian Ridge at 64°30′E Showcases Flipping Detachment Faults

We present two ∼150-km-long orthogonal 2D P-wave tomographic velocity models across and along the ridge axis of the ultraslow-spreading Southwest Indian Ridge at 64°30′E. Here, detachment faults largely accommodate seafloor accretion by mantle exhumation. The velocity models are constructed by inverting first arrival traveltimes recorded by 32 ocean bottom seismometers placed on the two profiles. The velocities increase rapidly with depth, from 3 to 3.5 km/s at the seafloor to 7 km/s at depths ranging from 1.5 to 6 km below the seafloor. The vertical gradient decreases for velocities >7 km/s. We suggest that changes in velocity with depth are related to changes in the degree of serpentinization and interpret the lithosphere to be composed of highly fractured and fully serpentinized peridotites at the top with a gradual downward decrease in serpentinization and pore space to unaltered peridotites. One active and five abandoned detachment faults are identified on the ridge-perpendicular profile. The active axial detachment fault (D1) shows the sharpest lateral change (horizontal gradient of ∼1 s–1) and highest vertical gradient (∼2 s–1) in the velocities. In the western section of the ridge-parallel profile, the lithosphere transitions from non-volcanic to volcanic over a distance of ∼10 km. The depth extent of serpentinization on the ridge-perpendicular profile ranges from ∼2 to 5 km, with the deepest penetration at the D1 hanging wall. On the ridge-parallel profile, this depth (∼2.5–4 km) varies less as the profile crosses the D1 hanging wall at ∼5–9 km south of the ridge axis