Abstract

We present kinematic data for late Cenozoic deformation in central Costa Rica that marks the western margin of the Panama microplate (i.e., the Caribbean-Panama boundary). This boundary extends from the North Panama deformed belt, west through the Valle Central in Costa Rica, and then southwest along the East Nicoya Fracture Zone to intersect the Middle America Trench. Terrace correlation and basin asymmetry indicate a major change in tectonic evolution across the boundary, where three regional northeast-striking faults intersect the Pacific coast. Mesoscopic fault populations are consistent with transtension across these north-east-striking faults and with transpression within the east-trending Valle Central. This late Tertiary and Quaternary transcurrent deformation links the North Panama deformed belt to the east with the Middle America Trench to the west. Earthquake focal mechanisms are consistent with mesoscopic fault data, suggesting that fault populations characterize the present-day stress field. This deformation marks the western extent of the Panama microplate, a fragment of volcanic arc that separated from the Caribbean plate in the late Tertiary or early Quaternary and is currently advancing northward due to collisions with South America to the east and the indenting Cocos Ridge on the Cocos plate to the south.

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