PLATE BOUNDARY PROCESSES AND SMALL-SCALE MANTLE CONVECTION BENEATH THE REYKJANES RIDGE

Crustal features of Reykjanes Ridge and flanks have been interpreted as directly reflecting rapid mantle plume flow. Here we propose that plate boundary processes and small-scale buoyant mantle convection can instead explain these features. The Reykjanes Ridge originated as a linear unsegmented axis spreading orthogonally. It became segmented following a change in plate opening direction. Segmented crust was thinner than unsegmented crust. The ridge then systematically and progressively eliminated the just-formed segmentation to reestablish its original geometry, even though it had to spread obliquely to do this. Prominent ridge-flank V-shaped crustal ridges and troughs formed in this latest spreading stage.

The abrupt segmentation of a ridge following a change in plate opening direction has been observed at other spreading centers to be a kinematic adjustment achieved through propagating ridges and need not involve mantle plume effects. Thinner crust accreted in the segmented stage is a predicted effect of segmentation on plate-driven mantle upwelling and does not require mantle temperature changes. The reassembly of an axis back to its original configuration suggests a “memory” of the plate boundary zone due to a deep low-viscosity “wet” melting regime that remained linear following the abrupt change in opening direction and guided the reassembly of the axis. V-shaped crustal ridges can be the result of small-scale mantle upwelling instabilities propagating along the long, deep and linear wet melting regime, instead of pulsing mantle plume flow. The existence of the Iceland hotspot and associated excess melting on the Reykjanes Ridge may be explained by the slow advection of broad low-viscosity thermochemical anomalies from the deep mantle, as inferred from tomographic images. As this low-viscosity fertile mantle rises above the solidus, it may trigger shallow melt-induced small-scale convection that is the direct cause of the excess hotspot melting, rather than rapidly rising hot mantle plume flow.