TWO STYLES OF ACTIVE, TRANSTENSIONAL DEFORMATION ALONG THE WESTERN NORTH AMERICA-CARIBBEAN PLATE BOUNDARY ZONE

Along-strike variability of active, interplate deformation along the North America – Caribbean plate margin is controlled by the angle between the GPS-derived Caribbean plate motion vector and the azimuth of plate-bounding transform faults. We present geologic, earthquake, GPS, marine geophysical, and remote sensing data to show that late Cenozoic transtensional deformation south of the North America – Caribbean margin in Northern Central America exhibits two distinct styles: plate boundary-normal extension (north-trending rifts) in the west and plate boundary-parallel transtension (ENE-trending rifts) in the east. Plate boundary-normal rifting with basin and range morphology occurs in western Honduras and southern Guatemala along the 375-km-long plate boundary segment south of the main plate boundary Motagua fault when the angle of divergence between the fault and the GPS-derived Caribbean plate vector is equal to or greater than 10º. Plate boundary-parallel transtension with margin-parallel ridges and basins occurs in northern Honduras and the Honduran borderlands along the 600-km-long plate boundary segment south of the Swan Islands Fault when the angle of divergence between the fault and the Caribbean motion vector is between 5 and 10º. A 35-km-wide transition area separates the western plate boundary-normal rifts from the plate boundary-parallel rifts to the east. Offshore faults of the Honduran borderlands extend onshore into the Nombre de Dios range and Aguan Valley of northern Honduras where geomorphic features show pervasive oblique-slip faulting with evidence for late Quaternary left-lateral offsets and active uplift of stream networks. Seismic data tied to wells in the Honduran borderlands shows that submarine faults are active, transtensional features that initiated in the middle Miocene with filling of asymmetric half-grabens and continued through the Pliocene-Pleistocene. Initial results from the Honduras GPS network are consistent with observed strain partitioning along the plate margin. Plate reconstructions are consistent with the north trending rifts of western Honduras developed in response to increased interplate divergence (>10º) as the western margin of the Caribbean plate shifted from the Polochic to the Motagua fault about 8 Myr.