Donnees sur la structure de l'arc insulaire des Petites Antilles, entre Ste-Lucie et Anguilla

Abstract

Geological and geophysical data were collected during a marine geological cruise (ARCANTE 1) carried out in 1977–1978 between St. Lucia and Anguilla, corresponding to about two thirds of the length of the Lesser Antilles archipelago. They allow of a reconsideration of the structural framework of this island arc. Considered as a whole, the Lesser Antilles ridge is characterized by a pattern contrasting sharply between its northern and southern halves. The major discontinuity axis runs to the south of the Guadeloupe archipelago. The basement of the northern half is supposed to be mainly Mesozoic and is connected with the evolution of the Greater Antilles, whilst the southern half was generated only since the Eocene. The NE flanks of the Lesser Antilles ridge are steep and dissected by large submarine valleys such as the Desirade sea trough; their recent sedimentary cover (post-lower Miocene) is lacking or very poor, enabling the outcropping of older formations: volcanic Mesozoic basement (around la Desirade), Upper Cretaceous, Eocene and Oligocene sediments. Conversely, the NW flanks are gently sloping and are draped by a rather thick overburden of Plio-Pleistocene deposits. To the south, the steep SW flanks are covered with a moderate thickness of recent sediments and display characteristics contrasting with those of the SE flanks, facing the Barbados accretionary prism. From the south up to Martinique, there is only one volcanic ridge, of Eocene to Present age. To the north of Martinique this ridge splits into two axes. The submarine landmarks of these two arcs have been disclosed during the present survey. The eastern one, or outer arc, shows a row of seamounts and islands often marked by strong magnetic (Dien Bien Phu Bank, Marie-Galante, Agouti Bank) or heavy gravimetric (Marie-Galante, Grande-Terre) anomalies. This outer arc was superimposed on the pre-Eocene basement, and is expressed in the Eocene and Oligocene volcanic terrains cropping out in the islands of Antigua, St. Bartholomew, St. Martin and Anguilla which rise from the wide northern insular shelves. A major (ca. 70 km) interruption in the continuity of this axis takes place between Dien Bien Phu Bank and Marie-Galante. La Desirade and Barbuda islands, although pertaining to the “Limestone Caribbees”, are not included in the outer arc. The western — or inner — arc, made up of upper Miocene to Present volcanics, begins with Dominica island and terminates at Luymes Bank, but an interruption of about 50 km is to be noted between Basse-Terre and Montserrat. The outer and inner arcs are separated by the narrow (ca. 50 km) and elongated (ca. 250 km) Kallinago depression. This intra-arc trough is generally filled by less than 1 km of recent sediments and is subdivided into three smaller basins; it was not initiated by extensional rifting related to back-arc spreading, as was previously assumed. It is possible that from Martinique to Guadeloupe an intermediate arc was in activity during the Miocene. Data from exploratory oil surveys provide some new insights into the geology and the structure of the wide Saba Bank, located to the west of the inner arc and near the northern end of the archipelago. Its volcanic substratum is of Upper Cretaceous—Paleocene age and is part of the pre-Eocene basement of the northern Lesser Antilles. The general characteristics of the NE flanks of the Lesser Antilles ridge may indicate some tilting toward the west of the northern part of the island arc. This may be ascribed to the arrival of Atlantic fracture zone ridges (Barracuda and Tiburon) in the subduction zone, during the Miocene. This event could explain the jump of the volcanic line from the outer to the inner arc.