ACTIVELY EVOLVING MICROPLATE FORMATION BY OBLIQUE COLLISION AND SIDEWAYS MOTION ALONG STRIKE-SLIP FAULTS - AN EXAMPLE FROM THE NORTHEASTERNCARIBBEAN PLATE MARGIN

The pattern of folding, faulting, and late Quaternary coral-reef uplif t rates in western and central Hispaniola (Haiti and Dominican Republi c) suggest that the elongate Gonave microplate, a 190,000-km(2) area o f the northeastern Caribbean prate, is in the process of shearing off the Caribbean plate and accreting to the North American plate. Late Ce nozoic transpression between the southeastern Bahama Platform and the Caribbean plate in Hispaniola has inhibited the eastward motion of the northeastern corner of the plate. Transpression is manifested in west ern and central Hispaniola by the formation of regional scare folds th at correspond to present-day, anticlinal topographic mountain chains c ontinuous with offshore anticlinal ridges. Areas of most rapid Quatern ary uplift determined from onland coral reefs 125 ka and younger, coin cide with the axial traces of these folds. Offshore data suggest recen t folding and faulting of the seafloor. Onshore reef data do not concl usively require late Quaternary folding, but demonstrate that tectonic uplift rates of the axial areas of the anticlines decrease from the N orthwest Peninsula of Haiti (0.37 mm/yr) to to the central part of the coast of western Haiti (0.19 mm/yr) to the south-central part of west ern Haiti (0 mm/yr). Formation of the 1200-km-long Enriquillo-Plantain Garden-Walton fault zone as a 'bypass' strike-slip fault has isolated the southern edge of the Gonave microplate and is allowing continued, unimpeded eastward motion of a smaller Caribbean plate past the zone of late Neogene convergence and Quaternary uplift of coral reefs in Hi spaniola. Offshore seismic reflection data from the Jamaica Passage, t he marine strait separating Jamaica and Haiti, show that the Enriquill o-Plantain Garden fault zone forms a narrow but deep, active fault-bou nded trough beneath the passage. The active fault is continuous with a ctive faults mapped onshore in western Haiti and eastern Jamaica; the bathymetric deep is present because the Jamaica Passage fault segment represents a 50-km-wide, transtensional left-step of the fault trace b etween Haiti and Jamaica. Onshore satellite imagery and field observat ions suggest that the Enriquillo-Plantain Garden fault forms a continu ous trace extending from central Hispaniola east of Lake Enriquillo, D ominican Republic, to the westernmost end of the southern peninsula of Haiti. The regional lineament corresponds to a recent fault scarp in Quaternary alluvium of the Clonard pull-apart basin in the central par t of the southern peninsula of Haiti and suggests that at least this p art of the lineament has undergone recent slip. Calmus (1983) has sugg ested a total offset of 30-50 km of the EnriquilIo-Plantain Garden fau lt zone in the southern peninsula of Haiti using the apparent offset o f lithologic units. Seismic reflection data from Lake Enriquillo docum ent recent deformation of Quaternary lake sediments where the lineamen t crosses the lake. Leveling of the crest of a late Holocene coral ree f and associated algal tufa around Lake Enriquillo demonstrate late Ho locene vertical movement and tilting in a 1500-m-wide zone parallel to the fault trend. Lateral offset is difficult to show in the Enriquill o Valley area because of rapid recent sedimentation into the valley. T he pattern of inactive strike-slip faults and fold belts of Cenozoic a ge in Cuba and the Yucatan basin suggest that two elongate microplates were sheared off the proto-Caribbean plate and accreted to the North American plate by a similar process in Paleocene and Eocene times. Age of terminal deformation in western, central and eastern Cuba is consi stent with southeastward younging and migration of are collision. The similarity of the size and sequence of events in Cuba suggests that th e process of oblique collision and sideways motion of the plate along a new strike-slip fault towards a free face may be an important proces s of microplate formation and interplate transfer in other areas.