Abstract
Ridge jumps alter the thermal structure and geometry of tectonic plates at diverse oceanic settings including mid-ocean ridges, back-arc basins, and microplates. One hypothesis to explain how ridge jumps occur involves thermal weakening off-axis. Off-axis lithosphere is weakened from the heat of penetrating magmas (e.g., from a near-ridge mantle plume). Weakening favors new rift propagation which competes with the already existing ridge. Previous 2-dimensional (2D) models of ridge jumps found jumps preferentially occur in younger, slower spreading conditions. 2D models also found the time to complete a ridge jump decreases as the distances between a plume and the ridge decrease, and increases at larger magmatic heating rates. Nevertheless, 2D simulations fail to account for inherently 3D processes along a ridge axis, such as new axis propagation.
In this project, I conduct 3-dimensional (3D) simulations to asses the role of heating alone on the success and evolution of a ridge jump. Using the highly parallel, finite difference code Lithosphere and Mantle Evolution Model (LaMEM), I simulate ridge jumps by solving the equations of conservation of mass, momentum, and energy in a visco-elastic-plastic continuum. Ridge jumps are promoted by imposing an off-axis heating zone spanning the height of the lithosphere. The role of heating is tested in simulations that vary four sets of parameters: (a) models with variable heat zone geometries spanning different aspect ratios (i.e. elliptical to circular), (b) models with a heat zone underlying different aged crust (7, 4, and 2 Myrs old), (c) models that vary the heating rate at the heat zone, and (d) models that vary the half spreading rate of the ridge system.
Ridge jumps evolve through magmatic heating that thins off-axis lithosphere, initiating strain and, eventually, a jump. Rifting continues coevally along the new and old axes as the new axis weakens. The original axis becomes extinct as plate spreading is increasingly accommodated along the new ridge. I find that models successfully produce ridge jumps at elongate heat zone geometries spanning ¾ and ½ the model ridge parallel length, and smaller widths increase the time it takes for the ridge to jump at the same magmatic heating rate compared to larger widths. Decreasing the heating rate increases the time it takes for a ridge to jump. Decreasing the spreading rate creates more favorable conditions for the success of a jump, whereas faster spreading rates hinder the ridges ability to jump. Nevertheless, models require initially high heating rates, closer to the magma flux values of flood basalts, suggesting there are other factors rather than just heating of the lithosphere causing ridge jumps globally. Here, I will present a suite of model results and discuss their implications for mantle-lithosphere dynamics and the evolution of divergent plate boundaries.