Abstract

The southeastern tip of the Península de Nicoya, Costa Rica, on the Caribbean plate margin lies inboard of the rough bathymetric terrain on the subducting Cocos plate and along the landward projection of the convergence vector for the Fisher seamount group. The southern tip of the peninsula has nearly orthogonal coastlines and extensive, well-preserved, Holocene marine terraces, and is ideally situated to evaluate the spatial distribution of forearc deformation in response to seamount subduction.

Two marine terraces that yielded 35 radiocarbon dates give information on the rates, style, and timing of deformation along 40 km of coastline. Ages range from 3.5 to 7.4 ka for a higher terrace and from 0.3 to 2.9 ka for a lower terrace. A maximum uplift rate is ∼6.0 m/k.y. along the southeastern tip of the peninsula. Uplift rates decrease linearly to <1.0 m/k.y. along both orthogonal coastlines and thus landward from the Middle America Trench and away from the line of subducting seamounts. The ∼400 km2 region along the tip of the peninsula can be approximated as a rotating block with an angular rotation rate of 0.02°/k.y. about an axis with an azimuth of 80°. Given the modern elevation and dip of the late Quaternary Cobano surface, this style of deformation is limited to a duration of 100–200 k.y. Deformation is occurring in response to seamount bypass or underplating onto the Caribbean plate margin.

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