Repeated ridge jumps associated with plume-ridge interaction and melt migration

Observations at ridges migrating away from hotspots such as the Mid-Atlantic Ridge at Iceland and the Galapagos Spreading Center display repeat jumps of the axis toward the hotspot. The mechanisms that control the initiation of jumps and repeat jumps are poorly understood, but likely include the effects of off-axis plume-lithosphere interaction and lithospheric heating caused by magma penetration. We use the CITCOM finite-element code to examine new 2D (cross section) models of ridge jumps associated with a near-ridge plume-like upwelling in a visco-plastic mantle and magmatic heating of the lithosphere controlled by melting and melt migration. Melting of the mantle is calculated assuming an anhydrous peridotite solidus with a mantle productivity that produces 6-7 km of crust at a normal ridge. Melt is assumed to travel vertically through the mantle to the solidus, just below the lithosphere, and then to be transported along the solidus according to a simplified version of two-phase flow. In locations along the solidus with large porosities (melt pressure) and/or damage associated with previous magmatism, a narrow zone of magma penetration and heating weakens the lithosphere promoting (for off-axis locations) ridge jumps. Calculations predict ridge jumps to be promoted on younger, slower moving plates and the time to initiate a jump to decrease with smaller plume-ridge separation distances and larger heating rates. Additionally, in cases including ridge migration, our results predict jumps to be promoted for migration rates very similar to the half-spreading rate. A migration rate much greater than the half-spreading rate results in the plume and ridge separating without a jump because the plate rapidly crosses the plume melting zone decreasing the time available to heat a given portion of lithosphere. Large heating rates tend to "capture" the ridge without a jump. Finally, heating rates predicted to initiate jumps to ~3-5 Myr old lithosphere, successfully predict repeat ridge jumps. The primary process involved in initiating repeat jumps is the dependence of magmatic heating on melt transport which causes a discrete shift (instead of a smooth migration) of the heating zone from the current ridge to a new off-axis location.