EFFECT OF SUBDUCTING SEA-FLOOR ROUGHNESS ON FORE-ARC KINEMATICS PACIFIC COAST, COSTA-RICA

Fault kinematics and uplift in the Costa Rican fore me of the Middle A merica convergent margin are controlled to a large extent by roughness on the subducting Cocos plate. Along the northwest flank of the incom ing Cocos Ridge, seafloor is characterized by short wavelength roughne ss related to northeast-trending seamount chains. Onland projection of the rough subducting crust coincides with a system of active faults o riented at high angles to the margin that segment the fore-are thrust belt and separate blocks with contrasting uplift rates. Trunk segments of Pacific slope fluvial systems typically follow these margin-perpen dicular faults. Regionally developed marine and fluvial terraces are c orrelated between drainages and across faults along the Costa Rican Pa cific coast. Terrace separations across block-bounding faults reveal a pattern of fore-are uplift that coincides roughly with the distributi on of incoming seamounts. Magnitude and distribution of Quaternary upl ift along the Costa Rican Pacific coast suggests that, despite a thin incoming sediment pile, the inner fore are shows an accumulation of ma ss-a characteristic that may be due to underplating of seamounts benea th the fore-are high.