Upper oceanic crust within the Cocos Plate offshore northwestern Costa Rica is dominated by sill intrusions of various ages, recognized regionally as smooth, high amplitude seismic reflections at the base of the sedimentary sequence and locally by dolerites and microgabbros recovered by drilling. Earlier interpretation of seafloor magnetic anomalies documented a set of spreading ridge jumps at 19.5 and 14.5 Ma, continuing to the present. In addition to these events, we present evidence for a widespread tectonic event in the period 8--10 Ma displayed in reflection seismic data recorded during the Ticoflux 1 and 2 experiments and dated by seismic correlation to ODP Site 1039. The 8--10 Ma events may have been triggered by collision of the Cocos Ridge at this time and facilitated by widespread sill intrusion associated with the Galapagos Hot Spot.