Finite-element modeling of syn-rift magmatism in a multi-layer Earth using ASPECT

Understanding interactions between rifting and magmatism require geodynamic models that couple Stoke's and heat equations with strain-rate and temperature dependent rheologies and the equations governing melting and melt transport. We adapted the finite-element program ASPECT (Advanced Solver for Problems in Earth's Convection) to simulate deformation in a multi-layer viscoplastic Earth undergoing melting, freezing, and melt transport. We developed a new Material Model for ASPECT where deformation is governed by dislocation or diffusion creep with a plastic yield limit. Melt fraction is calculated with a user-supplied melt function, and advected according to Darcy's Law. Heat advects with the melt while depletion is tracked and used in the melt function. The material viscosity includes an exponential melt weakening term. We tested the Material Model using a simple rifting scenario having separate layers for the upper and lower crust, lithospheric mantle, and asthenosphere. Each layer has its own rheological parameters and melting relation. The model is extended on the sides, with an inflow boundary condition at the bottom. Our preliminary models have a moderately thin pre-rift crust (30 km) and a geotherm slightly warmer than typical cratonic lithosphere. In these models, a layer of low percent melt (<1%) forms in the upper asthenosphere as the lithosphere thins. The top of the melt zone and the percent melt rise as extension progresses. As the lithosphere thins, convective mixing in the mantle develops and forms a sharp temperature gradient at the top of the asthenosphere, coinciding with the melt front. As extension progresses, a lithospheric neck and central graben form. Melt fraction increases and melting depth decreases as the asthenosphere rises beneath the axis of the rift, and convective mixing enhances melt production beneath the rift flanks. Eventually the upper crust separates, exposing the rising mantle and melt zone and marking continental breakup.