A different pattern of ridge segmentation and mantle Bouguer gravity anomalies along the ultra-slow spreading Southwest Indian Ridge (15°30′E to 25°E)

Abstract

The results of a recent bathymetric and geophysical investigation of a ∼650 km-long portion of the very slowly opening (16 mm/yr full rate) Southwest Indian Ridge (SWIR) between 15°30′E and 25°E are presented. Bathymetry and mantle Bouguer gravity anomalies (MBA), caused by variations in crustal thickness and/or crustal and upper mantle densities, show different characteristics from those observed at faster spreading centers like the Mid-Atlantic Ridge (MAR) (20–30 mm/yr full rate). With the exception of the Du Toit Transform, none of the ridge-axis discontinuities have offsets greater than 10 km and few of the discontinuities have clearly defined off-axis traces. The MBA patterns associated with individual segments are much more complex than the simple circular bull's eyes lows reported along the MAR. While the short wavelength ridge segment length is comparable to that of the MAR, there is little correlation with MBA amplitude and segment length and axial relief. Furthermore, an eastward propagating magma source and an ∼84 km-long zone of oblique spreading appears to define a fundamental boundary along the SWIR between two 250–300 km-long sections characterized by distinctly different axial morphology and gravity signatures. We interpret these results to indicate a long-wavelength segmentation pattern of the underlying upwelling mantle. Melt separates from the upwelling mantle at the base of the lithosphere and is channeled to the surface along dikes. Fissure eruptions within the rift valley build linear ridges defining a short-wavelength spatial pattern of ridge segmentation that is not directly related to the segmentation pattern of the upwelling mantle. Our results and interpretation are quite different than that predicted by extending current models of the faster spreading MAR to these ultra-slow spreading rates.