Thermal Models of the Costa Rica – Nicaragua Subduction Zone: the Effect of a Three-Dimensional Oceanic Plate Structure and Hydrothermal Circulation in the Temperature Distribution and Mantle Wedge Dynamics

Abstract:

Over the last years several 2D thermo-mechanical models of the Costa Rica - Nicaragua Subduction Zone (CNSZ) have studied the thermal distribution of sections of the fault. Such investigations allow us to understand temperature-related aspects of subduction zones, like volcanism and megathrust earthquake locations. However, certain features of the CNSZ limit the range of applicability of 2D models. In the CNSZ, geochemical trends and seismic anisotropy studies reveal a 3D mantle wedge flow that departs from the 2D corner flow. The origin of this flow are dip variations (20^{o to} 25^o between Nicaragua and Costa Rica) and the presence of a slab window in Panama that allows material to flow into the mantle wedge. Also, the Central America trench has abrupt variations in surface heat flux that contrasts with steady changes in plate age and convergence rate. These variations have been attributed to hydrothermal circulation (HC), which effectively removes heat from the oceanic crust.

In this project we analyze the thermal structure of the CNSZ. The objective is to study dehydration and metamorphic reactions, as well as the length of the megathrust seismogenic zone. We created 3D finite-element models that employ a dislocation creep rheology for the mantle wedge. Two aspects make our models different from previous studies: an up-to-date 3D slab geometry, and an implementation of HC by introducing a conductive proxy in the subducting aquifer, allowing us to model convective heat transport without the complex, high-Rayleigh number calculations. A 3D oceanic boundary condition that resembles the along-strike changes in surface heat flux is also employed. Results show a maximum mantle wedge flow rate of 4.69 cm/yr in the along-strike direction, representing more than 50% of the slab convergence rate. With respect to 2D models, analysis shows this flow changes temperatures by ~100 C in the mantle wedge near areas of strong slab curvature. Along the subducting interface, there is also a change of 10-40 C, which can have a significant impact on dehydration and metamorphic reactions. Also, 2D models have proven that HC controls temperatures along the subduction thrust, which controls the length of the seismogenic zone. In general, the combined effect of 3D mantle wedge flow and HC is expected to have a significant impact on the thermal structure.