Thermal structure of the Costa Rica – Nicaragua subduction zone

Abstract

We constructed four high-resolution, finite-element thermal models across the Nicaragua – Costa Rica subduction zone to predict the (i) thermal structure, (ii) metamorphic pressure ( P)–temperature ( T) paths followed by subducting lithosphere, and (iii) loci and types of slab dehydration reactions. These new models incorporate a temperature- and stress-dependent olivine rheology for the mantle–wedge that focuses hot asthenosphere into the tip of the mantle–wedge. At P = 3 GPa (100 km depth), predicted slab interface temperatures are ∼800 °C, about 170 °C warmer than temperatures predicted using an isoviscous mantle–wedge rheology. At the same pressure, predicted temperatures at the base of 7 km thick subducting oceanic crust range from 500 °C beneath SE Costa Rica to 400–440 °C beneath Nicaragua and NW Costa Rica. The high thermal gradients perpendicular to the slab interface permit partial melting of subducting sediments while the underlying oceanic crust dehydrates, consistent with recent geochemical studies of arc basalts. Hydrous eclogite is predicted to persist to ∼120 km depth beneath Nicaragua. This is slightly less than the ∼150 km depth extent of a dipping low-seismic-velocity wave guide which may reflect deeper persistence of metastable gabbro. Along-strike variations in the calculated thermal structure are relatively minor compared to variations in the distribution of Wadati-Benioff earthquakes and arc geochemistry, suggesting that regional variations in slab stresses, crustal thickness, incoming sediment load, and the distribution of hydrous minerals in the incoming lithosphere play important roles.