We report new geochemical data for marine sediments sampled in the frontal prism associated with the Costa Rica subduction zone during Leg 205 of the Ocean Drilling Program (ODP). We describe variation in sediment geochemistry with depth as the décollement zone, the interface between overriding and downgoing tectonic plates, is approached. This variation can be explained by three-component mixing of ash, lower plate sediments (LPS), and frontal prism or upper plate sediments (UPS). We detect in-mixing of LPS in localized sediment intervals, amounting to tens of vertical meters of LPS incorporation; no persuasive evidence of LPS transfer into the prism has been shown until this contribution. This inference of fine structure in the prism provides new insight into how tectonic kneading of sediments occurs in décollement zones.