Holocene forearc block rotation in response to seamount subduction, southeastern Peninsula de Nicoya, Costa Rica

The southeastern tip of the Peninsula de Nicoya, Costa Rica, on the Caribbe an plate margin lies inboard of the rough bathymetric terrain on the subduc ting Cocos plate and along the landward projection of the convergence vecto r for the Fisher seamount group. The southern tip of the peninsula has near ly orthogonal coastlines and extensive, well-preserved. Holocene marine ter races, and is ideally situated to evaluate the spatial distribution of fore arc deformation in response to seamount subduction. Two marine terraces that yielded 35 radiocarbon dates give information on t he rates, style, and timing of deformation along 40 km of coastline. Ages r ange from 3.5 to 7.4 ka for a higher terrace and from 0.3 to 2.9 ka for a l ower terrace. A maximum uplift rate is similar to6.0 m/k.y. along the south eastern 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 <similar to>400 km(2) region along the tip of the peninsula can be approximated as a rotat ing block with an angular rotation rate of 0.02 degrees /k.y. about an axis with an azimuth of 80 degrees. Given the modern elevation and dip of the l ate Quaternary Cobano surface, this style of deformation is limited to a du ration of 100-200 k.y. Deformation is occurring in response to seamount byp ass or underplating onto the Caribbean plate margin.