La Desirade Research Articles

Jurassic and Early Cretaceous radiolarians in Puerto Rican ophiolite—Tectonic implications

The Mariquita Chert of the Bermeja Complex in southwestern Puerto Rico contains radiolarians ranging in age from early Tithonian to late Aptian. Local stratigraphic and structural relations indicate that Puerto Rican ultramafic rocks are as old as or older than the chert; they are thus probably Jurassic or older. These rocks are the first pre-Cretaceous rock assemblages between Cuba and La Desirade (Lesser Antilles) to be identified, and they document the presence of pre-Cretaceous oceanic crust in the northeastern Caribbean. Subduction of the Caribbean plate beneath the North American plate, ending in the Early Cretaceous, destroyed most of this old oceanic crust and created a belt of metamorphosed volcanic and ophiolitic rocks now exposed in Cuba, Hispaniola, and Puerto Rico. The maximum radiometric ages from this belt are 127 m.y. Younger rocks were superimposed on the older belt when subduction polarity reversed and the North American plate was subducted beneath the Caribbean plate from 110 to 45 m.y. ago.

K-Ar geochronology of the Limestone Caribbees and Martinique, Lesser Antilles, West Indies

Twenty-seven K-Ar ages have been measured on igneous rocks from islands of the Limestone Caribbees (St. Martin, St. Bartholomew, Antigua, and Grande Terre, Guadeloupe) and Martinique in the Lesser Antilles arc. Earlier paleontological data indicated that rocks on several of these islands are Eocene in age and among the oldest known in the arc. The oldest igneous activity on the islands studied is 35–40 m.y. Bifurcation of the Lesser Antilles arc north of Guadeloupe took place between 7 and 20 m.y. ago. A distinct physical overlap of older and younger volcanic arcs exists on Martinique. Our results do not support previous suggestions that the pre-Tertiary basement of the Greater Antilles extends through the Limestone Caribbees to the anomalously old, Jurassic-Cretaceous, igneous rocks of La Desirade.

Continental Drift, IV: The Caribbean "Plate"

According to "the new global tectonics," the Caribbean Sea and the marginal lands surrounding it comprise one "lithosphere plate." Its eastern margin includes the Barbados Ridge, Lesser Antilles arc, and Aves Ridge; the western margin is composed of Central America and the Middle America Trench. These two margins are interpreted as subduction zones by "new global tectonics" advocates. The northern and southern margins are the alleged North Caribbean and South Caribbean faults, respectively. According to recent JOIDES drilling data, the Caribbean "plate" may be underlain by Late Cretaceous (75-85 m.y.) basaltic crust, and therefore is younger than both the Americas plate on the north, east, and south and the Cocos plate on the west. From this, some workers have inferred that the Caribbean formed by sea-floor spreading which separated North and South America. However, there is no spreading center in the Caribbean. Moreover, geologic and seismic data prove that the proposed North Caribbean and South Caribbean faults do not exist. La Désirade is a Jurassic or older island east of the Lesser Antilles. Barbados Ridge is an aseismic, pre-Late Jurassic salient of South America (that is, part of the so-called Americas plate) that parallels and lies just east of the alleged Lesser Antilles subduction zone. The stratigraphic units of Barbados are correlat-able with those of northern South America. Structural trends on Barbados strike east-northeast, are Late Cretaceous-early Eocene, and parallel those of South America. These facts show that the position of Barbados with respect to that of South America has not changed for more than 140 m.y. Yet "the new global tectonics" require that La Desirade and the Barbados Ridge must have been subducted more than 70 m.y. ago. On the basis of the facts presented here, the so-called Caribbean plate and the regions surrounding it have held the same positions relative to one another since pre-Jurassic (probably pre-Paleozoic) time. "The new global tectonics," therefore, are not applicable in the Caribbean area.