Quaternary convergent margin tectonics of Costa Rica, segmentation of the Cocos Plate, and Central American volcanism

Along Costa Rica, new geophysical data indicate considerable control of Qua ternary convergent margin tectonics by the subducting lower plate. Three ty pes of ocean crust enter the subduction zone: (1) Cocos Ridge with its unde rlying 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 und erthrust crust has a smooth sea floor. A 3- to 10-km-wide base of slope fro ntal 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 ma rgin structure is most evident in the middle and upper slope. Where Cocos R idge and its flanking seamounts subduct, erosion is pronounced relative to the stable slope where smooth lower plate subducts. Aligned upper plate fea tures above lower plate segment boundaries extend more than 120 km landward of the trench axis and correspond in varying degrees with volcanic are seg mentation. The offset of volcanoes across the Costa Rica/Nicaragua border c orresponds with a change in crustal structure and depth of the lava source. Subducted sediment shows little correlation with the slab signal in volcan ic are lavas but the magnitude of faulting associated with ocean plate flex ure adjacent to the trench axis parallels it well. Thus fluids in ocean cru st fractures and bound water in serpentinite may have a recognizable geoche mical effect in are lavas.