A FRAMEWORK FOR FLUVIAL TERRACE CORRELATION ALONG A SEGMENTED TECTONIC COASTLINE, COSTA RICA, CENTRAL AMERICA

Along the Middle America convergent margin, pronounced changes in subduction parameters (crustal age, slab dip, roughness) produce sharp contrasts in coastal tectonism along a 500 km trend from central Nicaragua to southern Costa Rica. The correlation of late Quaternary fluvial and marine terraces along this trend allows for evaluation of differing rates and styles of fore arc deformation. Fluvial terraces on the Orotina-Esparza coastal piedmont of central Costa Rica serve as a critical link across the rough-smooth segment boundary (RSB) on the subducting Cocos plate. Three terrace groups (I, II, III) are identified and correlated along the coast based on isotopic ages, soil development, depositional characteristics, and spacing between terrace flights. Group I consists of a broad, composite upland (El Diablo surface) formed on a deeply-weathered alluvial fill prism along most Pacific coast drainages. Group II includes moderately weathered terraces inset below the El Diablo upland along the lower reaches of river profiles. Group III includes narrow, low-elevation surfaces with poorly developed soils adjacent to active flood plains. Along this tectonic coastline, base-level fluctuations are controlled by the interaction between eustatic sea level and spatially variable rock-uplift. Terraces form by aggradation during sea level maxima, with terrace groups I-III linked to major high stands (oxygen isotope stages 7-1). The number of terraces within each group varies between drainages with respect to local rock uplift. Where uplift rates are low (north of RSB), each group consists of a single terrace. Where moderate uplift rates occur onshore of the seamount domain (south of RSB), coastal blocks preserve a maximum number of terraces, reflecting better resolution of sub-stage high stands (e.g., 5a - 5e). Where uplift rates are high (e.g., Herradura and Osa blocks), older terraces (Group I) are poorly represented, having reached elevations above the terrace preservation fringe.