THE CRETACEOUS MARGINS OF THE EXTREME SOUTHWEST CORNER OF THE NORTH AMERICAN PLATE

The northern Central American Chortis block was the southwest corner of the North American plate during the Cretaceous prior to Tertiary translation into the Caribbean. Field studies on the Chortis block define an intra-arc setting for the Pacific-facing margin while the southern margin of the block experienced the collision of the Caribbean island arc. Major and trace element geochemistry of volcanic rocks and paleontological dates from sedimentary units of the Olancho basin in Honduras confirm an Aptian-Cenomanian intra-arc origin. Thickness and facies variations indicate a 3.5-km-deep, fault-bounded basin with pre-Cenomanian syn-sedimentary erosion of the basin margins. The basin trends west-northwest in present geography and north-south when reconstructed against the southwest margin of Mexico. The Olancho basin was deformed and inverted by thick-skinned style, south dipping reverse faults in post-Cenomanian time. The age of deformation correlates with early Laramide-age shortening in southwestern Mexico. Geochemical trends from the displaced Olancho arc are most similar to trends reported for the Teloloapan arc in Mexico providing a geochemical piercing point for plate reconstruction. Mapping and paleontological dates from sedimentary units in the eastern Honduran Montañas de Colon fold-thrust belt confirm a pre-Cenomanian south-facing continental margin setting. Shallow-water carbonate rocks and subareal clastic strata totaling 4 km in thickness were deformed by thin-skinned style, northwestward-directed thrusting in Campanian time. Deformation is attributed to oblique collision of the Caribbean oceanic plateau and emplacement of the Nicaraguan Siuna oceanic terrane as these oceanic elements entered from the eastern Pacific.