PLATE MOTIONS IN THE NORTH ANDEAN REGION

Repeated geodetic measurements with the Global Positioning System (GPS) provide direct measurements of displacements due to plate motions and active crustal deformation in Central America and northern South America, an area of complex interaction of the Nazca, Cocos, Caribbean and South American plates. The displacement rates for the period 1988-1991, obtained from the results of the first three Central And South America (CASA) GPS campaigns, are in general agreement with the predictions of the NUVEL-1 plate motion model, but there are differences in detail between the observations and the model. The Nazca-North Andes convergence rate vector measured by GPS is different from the NUVEL-1 vector at 95% confidence. The difference implies that the North Andes am moving northward relative to South America. The measured convergence between the Caribbean plate and the North Andes suggests that the southern margin of the Caribbean plate is located in the South Caribbean deformed belt. The April 1991 Costa Rica earthquake and the Cocos-Caribbean convergence rate determined by GPS suggest the possibility of significant ongoing deformation between Central America and the stable interior of the Caribbean plate. Our GPS results are consistent with deformation of the overriding plates at the convergent margins of Central and South America and confirm that active convergence is occurring around much of the southern margin of the Caribbean plate, from Colombia west to Costa Rica. Costa Rica and Panama am not part of the stable Caribbean plate. Instead, the South Caribbean deformed belt and the North Panama fold belt probably represent the southern margin of the Caribbean plate.