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

Abstract

[1] 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 pressuredependent 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 alongaxis 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.

Primitive off-rift basalts from Iceland and Jan Mayen: Os-isotopic evidence for a mantle source containing enriched subcontinental lithosphere

“…The Jan Mayen Fracture Zone, however, represents a major geochemical boundary between the Kolbeinsey Ridge and the southern part of the Mohns Ridge in terms of 206,207,208 Pb/ 204 Pb, 143 Nd/ 144 Nd and 176 Hf/ 177 Hf systematics (Blichert-Toft et al, 2005). This seems in agreement with the large offset (around 200 km) between the Kolbeinsey and Mohns Ridges, following the demonstration that transform faults reduce along-axis plumedriven flux in proportion to increasing transform offset length (Georgen and Lin, 2003).…”

Uppermost mantle velocity from Pn tomography in the Gulf of Aden

“…narrow oceanic basin segmented by numerous transform faults (Fig. 8) (Leroy et al, , 2012 T T T T T TI TI TI TI TI TI I on October 2, 2014 geosphere.gsapubs.org Downloaded from (Georgen and Lin, 2003). Our results indicate this may occur in several locations in the Gulf of Aden: in the central Gulf of Aden, the Kanshar Irqah, and the Xiis Mukalla fracture zones (Fig.…”

Vitriclastic lithofacies from Macquarie Island (Southern Ocean): compositional influence on abyssal eruption explosivity in a dying Miocene spreading ridge

“…1b; Griffin 1982). Several factors may account for this and include off-axis volcanism, seamount shoaling, tectonic uplift, low magma-rate supply, short spreading-segments along the PMSR, and proximity to an amagmatic inside corner structure (Staudigel and Schmincke 1984;Batiza 1996;Naumann and Geist 1999;Goscombe and Everard 2001;Georgen and Lin 2003;Wertz et al 2003;Mosher and Massell-Symons 2008). These factors are partly dependent upon one another and reflect the eruption of the Macquarie Island extrusive sequence during the demise of the slow-spreading PMSR.…”