USE OF WEATHERING RINDS IN FLUVIAL TERRACE CORRELATION ALONG THE DIFFERENTIALLY UPLIFTED COASTAL FORE ARC, PACIFIC COAST, COSTA RICA

Along the Pacific Coast of Costa Rica, the Cocos plate subducts under the Caribbean plate at the Middle America Trench. The bathymetry of the Cocos plate is dominated by the Cocos Ridge and subsidiary seamounts, which cause differential uplift along the coast as they subduct, segmenting the fore arc into distinct tectonic blocks. This study correlates fluvial terraces across six different drainage basins along the fore arc. Terrace correlation is based on the development of weathering rinds on basaltic andesite clasts in uplifted river gravels. Basaltic andesite was chosen because it weathers slowly and is still present in even the oldest terraces. Rinds vary in thickness from <1mm on the youngest terraces to as much as 11cm on the oldest terraces, and have a mean bulk density of 1.5g/cm³. The unweathered basaltic andesite cores have a bulk density of 2.8 g/ cm³. Bulk chemistry indicates that the elements Mg, Na, Ca, and Si decrease in abundance away from the core/rind boundary, while Al, Ti, and K remain relatively constant throughout the core and rind. Fe⁺³ is more abundant in the rind than in the core. X-ray diffraction suggests that kaolinite forms early in rind development and then is replaced by gibbsite as the rind matures. Terrace correlation, a composite paleo-sea level curve and previously determined radiometric ages constrain uplift rates of the different fore arc blocks. The Ar/Ar dates of a volcanic flow and C-14 dates establish age limits for several terraces. We conclude that the terraces range in age from oxygen isotope stage (OIS) 1-7, and may include OIS-9. Based on the regionally most extensive OIS-5 terrace (120 ka), uplift rates range from 0.1 m/ka to 1.8 m/ka across the differentially uplifted the fore arc.