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Along the Middle America Trench in southern Costa Rica, flat slab subduction of the aseismic Cocos Ridge has uplifted and exposed the outer forearc, shortened the Terraba forearc basin sequence in the inner forearc (i.e., the Fila Costena thrust belt), and uplifted the magmatic arc. The Osa Peninsula, an outer forearc high ∼20 km inboard of the Middle America Trench and ∼3 km to ∼10 km above the plate interface at its trenchward edge, is deforming in response to variations in the bathymetry of the subducting aseismic Cocos Ridge where relief locally exceeds 1 km. Modern topography of the Osa Peninsula, elevation of the basement rocks (Early to Middle Tertiary Osa melange), elevations of Quaternary marine deposits (Marenco formation), and distribution of late Quaternary uplift rates directly mirror the bathymetry on the Cocos Ridge outboard of the Middle America Trench. Rates of late Quaternary uplift are calculated from eight new radiocarbon ages, five new optically stimulated luminescence ages, and 10 previously published radiocarbon ages. Rates of uplift range from 1.7 m/k.y. to 8.5 m/k.y. The Osa Peninsula is fragmented into small (∼5 km), independently deforming blocks bounded by trench-parallel and trench-perpendicular, subvertical, normal and reverse faults that extend down to the plate interface, allowing for greatly different deformation histories over short distances. Quaternary deformation on the Osa Peninsula is modeled as a thin, outer-margin wedge that deforms in response to subduction of short-wavelength, high-relief asperities on the downgoing plate. Permanent deformation is largely accomplished by simple shear on a complex array of subvertical faults that allow the upper plate to adjust to variations in the slope of incoming ridges and seamounts. Currently, permanent deformation of the outer forearc does not appear to involve significant subhorizontal shortening of the margin wedge, although the global positioning system velocity field records elastic shortening related to locking of the plate interface. Permanent uplift and uplift rates in the outer forearc in southern Costa Rica are driven, to the first order, by the bathymetry associated with the subducting Cocos Ridge and not by the basal shear stress on the plate interface.

Upper-plate deformation in response to flat slab subduction inboard of the aseismic Cocos Ridge, Osa Peninsula, Costa Rica