Subduction of the Eastern Panama Basin and seismotectonics of northwestern South America

Spreading along the Cocos-Nazca plate boundary since the breakup of the Farallon plate in the Miocene has resulted in the formation of the Panama basin and a complex interaction of plates in and near northwestern South America. Current plate boundaries have been defined, and segments of subducted lithosphere identified through selection of hypocentral locations of earthquakes, considering only well-located events, and through focal mechanism determinations. The existence of relict plate boundaries, bathymetric features, and the Panamanian isthmus has affected the subduction process of the Nazca plate beneath South America and determined the present-day configuration of the subducting lithospheric plate. There is no single triple junction separating the Caribbean, South American, and Nazca plates. Instead, the Panamanian isthmus and surrounding areas are accommodating east-west compression (and a lesser degree of north-south compression) along a series of thrust faults striking NW to NE, and the Andean ranges of Ecuador, Colombia, and Venezeula are moving as a block NNE relative to the rest of the South American plates, along a system of faults following the front of the Eastern Cordillera. The subducted portions of the Panama basin and old Farallon plate have become segmented into three pieces recognized in this study. From north to south, they are (1) a ‘Bucaramanga’ segment continuous with the Caribbean seafloor northwest of Colombia, (2) a ‘Cauca’ segment continuous with oceanic crust (Nazca plate) currently being subducted beneath South America at the Colombia-Ecuador trench, and (3) an ‘Ecuador’ segment at the northern end of the subducted lithospheric plate which is dipping at a small angle to the east beneath northern Peru. The segmentation of the subducted plate can be explained by the buoyancy of bathymetric features which have been partially subducted.