The Chort¿s block is undergoing deformation as a direct result of large-magnitude sinistral strike-slip faulting on the North America-Caribbean plate boundary. We have studied the Chort¿s block using geologic mapping, remote imagery, and earthquake seismology to evaluate the effect that large-magnitude strike-slip faulting has on an adjacent microcontinental block. In the eastern part of the Chort¿s block, recent geologic mapping has shown that the northeast striking Guayape fault is dextral instead of sinistral as previously assumed. Because this is a major fault that cuts most of the Central America isthmus, the new evidence for dextral slip necessitates a reinterpretation of the active tectonics of the Chort¿s block. The presence of several north trending rifts in the western part of the Chort¿s block has led previous workers to propose that the Chort¿s block is divided into smaller, independently rotating blocks. We use earthquake seismology and remote imagery data to extend this interpretation to the central portion of the Chort¿s block. However, we have modified their model to account for dextral slip on the Guayape fault. We believe that rotation of the Chort¿s block causes dextral slip on the Guayape fault even though Central America is undergoing large sinistral displacement with respect to North America. ¿ American Geophysical Union 1994