Asymmetric generation of oceanic crust at the ultra‐slow spreading Southwest Indian Ridge, 64°E

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Abstract
We describe topographic, gravity, magnetic, and sonar data from a Southwest Indian Ridge spreading segment near 64°E, 28°S. We interpret these to reveal crustal structure, spreading history, and volcanic and tectonic processes over the last 12 Myr. We confirm that the crust is some 2 km thicker north of the ridge axis, though it varies along and across axis on scales of ∼10 km and 4 Myr. The plate separation rate remained approximately constant at 13 ± 1 km Myr−1, but half‐spreading rates were up to 40% asymmetric, varying between faster‐to‐the‐north and faster‐to‐the‐south on a 4 Myr timescale. Topography shows a dominant E–W lineation normal to the N–S spreading direction. This is superficially similar to faulted abyssal hill terrain of the Mid‐Atlantic Ridge (MAR), but inferred fault scarps are 3–4 times more widely spaced and have greater offsets. Conjugate pairs of massifs on either plate are interpreted as volcanic constructions similar to the large volcano currently filling the median valley at the segment center. They have temporal spacings of ∼4 Myr and are thought to reflect episodic melt focusing along an otherwise melt‐poor ridge. Additionally, there are places, mainly on the southern plate, where lineated topography is replaced by a much blockier topography and embryonic ocean core complexes similar to those recently reported on the MAR near 13°N. There is generally more extrusive volcanism on the northern plate and more tectonism on the southern one. Extrusive volcanism has propagated westward from the segment center since 2 Ma. The FUJI Dome core complex and adjacent seafloor to its east and west appear to be part of a single coherent block, capped by extrusive rock near the segment center, exposing gabbro via a detachment fault over the Dome and probably exposing deeper crust or upper mantle farther west near the segment end. Magnetic anomalies are continuous along this block. We suggest that at its eastern boundary the detachment is simply welded onto magmatically emplaced crust to the east in a similar way to young crust being welded to the old plate at ridge‐transform intersections.

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