Crustal and upper mantle contribution to the axial gravity anomaly at the southern East Pacific Rise

Abstract

This paper reassesses the crustal and upper mantle contribution to the axial gravity anomaly and isostatic topography observed at two segments (14°S and 17°S) of the southern East Pacific Rise (SEPR) in order to determine what constraints these data place on the amount of melt present in the underlying mantle. Gravity effects due to seafloor topography and relief on the Moho (assuming a constant crustal thickness and density) overpredict the amplitude of the gravity high at the EPR by 8–10 mGal. About 70% of this mantle Bouguer anomaly (MBA) low (6–7 mGal) can be explained by a region of partial melt and elevated temperatures in the mid‐to‐lower crust beneath the rise axis. Compositional density reductions in the mantle due to melt extraction are shown to make a negligible contribution to the amplitude of the observed MBA. Temperature‐related mantle density variations predicted by a simple, plate‐driven, passive flow model with no melt retention can adequately account for the mantle contribution to the observed MBA within the experimental uncertainty (±1 mGal). However, the retention of a small amount of melt (≤1–2% at 14°S; ≤4% at 17°S) in a broad region (tens of kilometers wide) of upwelling mantle is also consistent with the observed gravity data given the uncertainty in crustal thermal models. The anomalous height of the narrow, topographic high at the EPR provides the strongest evidence for the existence of significant melt fractions in the underlying mantle. It is consistent with the presence of a narrow (∼10 km wide) partial melt conduit that extends to depths of 50–70 km with melt concentrations up to 2% higher than the surrounding mantle. Along‐axis variations in mantle melt fraction that might potentially indicate focused upwelling are only marginally resolvable in the gravity data due to uncertainties in crustal thermal models. The good correlation between along‐axis variations in depth, and changes in axial volume and gravity, argue against the mantle melt conduit as being the major source of this along‐axis variation. Instead, this variability can be adequately explained by a combination of along‐axis changes in crustal thermal structure and/or along‐axis crustal thickness changes of a few hundred meters.