Seaward of the subduction zone off Nicoya Peninsula, Costa Rica, differences in the thermal state of the ocean crust occur across a transition between crust generated at the Cocos-Nazca Spreading Center and crust formed at the East Pacific Rise. This change in the thermal state of the subducting plate results in along-strike differences in subduction zone temperature. These temperature variations are significant because they modulate diagenetic reaction progress and hydraulic conductivity. A numerical simulation of fluid production from opal-to-quartz and smectite-to-illite diagenesis displays an offset along strike due to the different temperatures of subduction inputs. Additionally, fluid viscosity is lower on the warm side of the margin than on the cool side. As a result, for the same permeability, hydraulic conductivity within the first 30 km from the trench is 1.25–2.0 times greater on the warm side of the margin than on the cool side. Spatial differences in hydraulic conductivity and sources of fluid likely combine to drive some flow along strike. A coupled model of fluid flow and solute transport indicates that for margin-wedge permeability <10⁻¹⁹ m², décollement permeability must be ≥10⁻¹⁴ m² in order to drain fluid sources and maintain fluid pressures below lithostatic. A steady-state model of chloride concentrations within the frontal décollement results in values higher (more saline) than those measured at Ocean Drilling Program Site 1040. One way to explain this discrepancy is for transient fluid flow to occur in a system with temporary enhancement of décollement permeability, thereby allowing transport to Site 1040 from deep zones of dehydration reactions. The onset of microseismicity on the plate boundary coincides with locations where modeled fluid overpressures dissipate in the décollement.