Plume‐transform interactions at ultra‐slow spreading ridges: Implications for the Southwest Indian Ridge

Abstract

We explore a potentially important variable in controlling ridge‐hot spot interaction, the effect of transform offsets in limiting along‐axis flow of plume material. We focus on the Southwest Indian Ridge (SWIR), where the “transform damming” effect is likely to be pronounced because of both long offset lengths and large contrasts in lithospheric thickness across the transform faults due to the ultra‐slow spreading rate. We investigate the degree to which transform faults affect axial asthenospheric flow by performing a series of three‐dimensional (3‐D) numerical experiments with simplified channel‐flow geometry and extrapolating their results to the SWIR. 3‐D mantle viscosity structure for a ridge‐transform‐ridge system is determined based on temperature‐ and pressure‐dependent viscosity laws. We consider six transform lengths, spanning 0 to 250 km in increments of 50 km. We then calculate the 3‐D viscous flow in response to an along‐axis pressure gradient corresponding to a ridge‐centered hot spot. Modeling results predict that transform faults affect along‐axis asthenospheric flow in two important ways. First, transforms reduce along‐axis flux. The longer the transform offset, the greater the reduction in across‐transform flux relative to the zero‐offset case. Second, transforms deflect shallow asthenospheric along‐axis flow. The predicted transform damming effect is most pronounced for a viscosity structure that is strictly pressure‐ and temperature‐dependent. Flux reduction effects could be less significant for viscosity laws that additionally consider dehydration, melting, and change in deformation mechanism. This model predicts that the waist width of an on‐axis plume is dependent not only on such previously explored factors as buoyancy and spreading rate, but also on the geometry of ridge segmentation. Along the SWIR, axial flow driven by the Marion plume is likely curtailed by the long‐offset Andrew Bain and Discovery II fracture zones, severely limiting its lateral extent.