Relationship among Rough Crust Subduction, Forearc Kinematics, and Quaternary Uplift Rates, Costa Rican Segment of the Middle America Trench

Orthogonal subduction of rough oceanic lithosphere along the northwestern flank of the Cocos Ridge imprints a distinctive style of deformation on the overriding Costa Rican forearc. We divide the Costa Rican forearc into three 100–160 km-long, deformational domains based on the bathymetry and thickness of the Cocos plate entering the Middle American trench, the dip of the subducting plate, the variation in surface uplift rates of late Quaternary coastal deposits, and the orientations and types of faults deforming Paleogene and Neogene sedimentary rocks. In the ~100-km-long Nicoya forearc domain, steep (~75°) subduction of smooth oceanic crust, coastal deposits show localized surface uplift and arcward tilting above the downdip projections of the COC-NAZ-PAC triple junction ridge and the Fisher seamount/ridge. In the ~120-km-long Central Pacific forearc domain between the Nicoya Peninsula and Quepos, shallower (~60°) subduction of seamounts and plateaus is accompanied by trench-perpendicular late Quaternary normal faults. Steeply dipping, northeast-striking, margin-perpendicular faults accommodate differential uplift associated with seamount subduction. Uplift and faulting differ between the portions of the forearc opposite subducting seamounts and ridges. Inner forearc uplift along the seamount-dominated segment is greatest inboard of areas of greatest offshore subsidence. Coastal deposits show localized uplift and arcward tilting along some seamounts but are lacking opposite the Quepos Plateau. In contrast, inboard of the underthrusting aseismic Cocos Ridge along the ~160-km-long Fila Costeña domain between Quepos and the Burica Peninsula, mesoscale fault populations record active shortening related to an extensive thrust belt. These differences in patterns of faulting and permanent uplift are best explained by differences in the mechanism of crustal thickening. The uplifted terraces provide a first order estimate of permanent strain along the forearc in Costa Rica. The permanent strain recorded by these Quaternary surfaces exceeds rebound of stored elastic strain released during subduction zone earthquakes.