THE PIRATE MODEL FOR DERIVATION OF THE CHORTIS AND CHIAPAS BLOCKS FROM THE WESTERN GULF OF MEXICO IN THE LATEST CRETACEOUS-CENOZOIC: IMPLICATIONS FOR THE SIERRA MADRE ORIENTAL AND ZONGOLICA FOLDBELTS

There are currently two basic models for the genesis of the Caribbean Plate: (i) a Pacific model (including the Chortis block), and (ii) an in-situ model. However, there is no E-W fault connecting the Middle America Trench and the Motagua fault zone that passes eastwards into the Cayman Trough, which records ca. 1100 km of Cenozoic sinistral displacement. Although the juvenile, 120-80 Ma evolution of the Caribbean Plate probably involved backarc extension, it cannot account for the post-80 Ma, 1100 km sinistral offset. A way through this impasse is indicated by the northwestward curvature of active oblique reverse to sinistral transcurrent faulting. Extending this potential solution back in time through the Cenozoic-latest Cretaceous (ca. 80 Ma) forms the basis for the Pirate model, in which the Chortis and Chiapas blocks are derived from the western part of the Gulf of Mexico by anticlockwise rotation during the Cenozoic and latest Cretaceous. Space for the Chortis block is created by separating the northern, central and southern (≈Chiapas block) parts of the Yucatan block: Middle Miocene collision between the blocks produced the 11-9 Ma Chiapas foldbelt. Impingement of the East Pacific Rise on the Middle America Trench led to modification of the Pirate model involving subduction erosion of the Eocene-Oligocene forearc at ca. 20 Ma, opening of the Gulf of California at ca. 6 Ma, birth and ESE-movement of the South Mexico block (<5 Ma) followed by its fragmentation. The origin of the Laramide Sierra Madre Oriental foldbelt has been attributed to terrane accretion, flat-slab subduction, and variations in the convergence rate, all related to Cocos-Farallon subduction beneath western Mexico. However, in the context of the Pirate model the N-trending segment of the Sierra Madre Oriental and the Zongolica foldbelts could have formed as a result of the southward motion of the Chortis and Chiapas blocks. Such motion also explains paleocanyons and the >2 km excess depth in the western Gulf of Mexico, the formation of the magmatic, S-younging Eastern Alkaline Province down the western flank of the Gulf of Mexico. and post-10 Ma extension in the Chortis block (Chortis-Sula rift province). The Pirate mechanism indicates that the North American Plate is relatively weak, and so tears and rotates into the trailing edge of the Caribbean Plate.