LATE HOLOCENE MARINE TERRACE UPLIFT RATES AT THE THRUST FRONT OF THE FILA COSTEÑA, PACIFIC COAST, COSTA RICA

Along the Pacific coast of Costa Rica, shallow subduction of the Cocos Ridge beneath the Caribbean plate at the Middle America Trench creates variable forearc uplift. The Fila Costeña thrust belt lies landward of the Cocos Ridge and is oriented parallel to the Middle America Trench. The thrust front for the Fila Costeña, the Longitudinal Fault, extends along the coast for 220 km from the town of Dominical to the Panama border. Two well-exposed Holocene beach ridges along the active coastal frontal thrust of the Fila Costeña at Playa Ballena constrain inner fore arc deformation. Soil properties on these beach ridges are consistent with soil chronosequences on other dated Holocene beach ridges on the Osa Peninsula, 80 km to the east. The higher beach ridge is characterized by a >0.6 m-thick Bt horizon and a 5 YR to 10YR color, and the lower beach ridge is characterized by a >0.4 m-thick Bw horizon and a 10YR color.

A radiocarbon date of 5.54ka for the higher platform and four transects perpendicular to the coast at Playa Ballena are used to constrain surface uplift rates. Uplift rate is 0.34 ± 0.3 m/ka. Coast parallel angular rotation rate for Playa Ballena is 0.001°/ka with northwest side down, consistent with rising bathymetry on the offshore Cocos Ridge. Elevations of inferred oxygen isotope stage 5e (125 ka) highstand terraces 30 km to the east along the coast suggest a Holocene uplift rate closer to the maximum calculated uplift rate, ~0.64 m/ka.