The Ultraslow Spreading Southwest Indian Ridge

Abstract

The Southwest Indian Ridge (SWIR) is among the world's slowest spreading ridges with a full spreading rate of ~14 mm a-1 (at 64°E/28°S). The compilation of geophysical and geochemical data along the SWIR reveals a large-scale variation of the density and thermal structure of the axial region. The easternmost part of the SWIR appears to be among the deepest part of the oceanic ridge system, and it is thus inferred to represent a melt-poor end-member for this system. Both the easternmost and westernmost parts of the SWIR reveal a ridge segmentation that differs greatly from what is observed at the faster spreading ridges like the Mid-Atlantic Ridge. The apparent absence of volcanic activity on >100-km-long stretches of the ridge axis, where large expanses of mantle-derived peridotites are exposed in the axial valley, is one of the striking contrasts between the SWIR and these faster spreading ridges. Off-axis geophysical data in the deep magmapoor part of the SWIR reveal a new type of seafloor, which corresponds to thin crust with little to no volcanism. Very large volcanic centers with thick crust occur between these nonvolcanic sections of the ridge and have much higher relief to length ratios than the segments of the MAR. Melt supply appears to be more highly focused beneath these volcanic centers of the SWIR. Melt focusing could result from a combination of melt migration near the base of the lithosphere and rapid melt extraction through dikes rooted in melt-rich regions.