Along Costa Rica, new geophysical data indicate considerable control of Quaternary convergent margin tectonics by the subducting lower plate. Three types of ocean crust enter the subduction zone: (1) Cocos Ridge with its underlying thick crust stands 2 km high, (2) on its north flank is normal crust covered 40% by seamounts, and (3) along the adjacent Nicoya margin the underthrust crust has a smooth sea floor. A 3- to 10-km-wide base of slope frontal prism varies little opposite different subducting crusts except where subducting seamounts eroded it. Once the breaching seamount has passed the prism it is quickly restored. The effect of oceanic crust on continental margin structure is most evident in the middle and upper slope. Where Cocos Ridge and its flanking seamounts subduct, erosion is pronounced relative to the stable slope where smooth lower plate subducts. Aligned upper plate features above lower plate segment boundaries extend more than 120 km landward of the trench axis and correspond in varying degrees with volcanic arc segmentation. The offset of volcanoes across the Costa Rica/Nicaragua border corresponds with a change in crustal structure and depth of the lava source. Subducted sediment shows little correlation with the slab signal in volcanic arc lavas but the magnitude of faulting associated with ocean plate flexure adjacent to the trench axis parallels it well. Thus fluids in ocean crust fractures and bound water in serpentinite may have a recognizable geochemical effect in arc lavas.