Reconnaissance de structures tectoniques et volcaniques sous-marines de l'arc recent des Petites Antilles (Kick'em Jenny, Qualibou, Montagne Pelee, nordouest de la Guadeloupe)

Abstract

A multibeam bathymetric survey (SEACARIB 1) was carried out in April 1985, on board the R.V. Conrad (Lamont-Doherty Geological Observatory) off the western coasts of several islands of the Lesser Antilles, complementing a previous survey of the same type (ARCANTE 2-THERMOSITE, 1980) devoted to adjoining areas in Guadeloupe, Martinique and St. Lucia. It allowed us to precisely map submarine volcanoes in a marginal setting behind the recent arc, the main volcanic axis of the archipelago, emplaced during the Neogene. The Kick'em Jenny volcano (south of the Grenadines) is the only active submarine volcano of the Lesser Antilles. It is a small and very young cone (probably only tens or hundreds of years old) which was hydrothermally active during the survey. The Directeur volcano (Guadeloupe) is the largest submerged volcanic centre thus far detected in the Lesser Antilles having been formed between 3.3 and 2.9 Ma. During the 1980 survey, two other large volcanoes were discovered: off southern Basse-Terre, Guadeloupe (Vieux Fort volcano, 2.5–2.0 Ma) and in front of Fort de France Bay, Martinique (Schoelcher volcano, older than 4.2 Ma). Several other much smaller volcanoes have been mapped off the same coasts, particularly the Bouillante seapeaks (Guadeloupe).Moreover, large tectonic structures cut the recent arc transversally and obliquely. The well-known Qualibou depression, on the island of St. Lucia, was interpreted in various ways; neither the hypothesis of a large caldera, the western side of which collapsed offshore, nor the suggestion of a huge rotational gravity slide, are confirmed. The depression seems more likely to have been controlled by tectonic factors shaping a complex graben structure. The seaward extension appears to be restricted to a northern channel, the Soufrière Channel. The St. Pierre fault, in Martinique, is a large transverse feature, extending seaward down to some 2000 m depth. To the south, it bounds the Mount Pelée active volcanic system, and the escarpment of the fault forms a “dam” against subaqueous pyroclastic flows running down from the volcano. These flows are responsible for the unusually smooth morphology of the submarine flanks. The Zombi fault of northwest Guadeloupe is part of a first-order feature, extending along at least 130 km, from Montserrat in the north, to Marie-Galante in the east. This curved fracture cuts across the northern central part of the Lesser Antilles, passing progressively from a subparallel to a transverse orientation with respect to the axis of the island arc.These types of surveys are important, because they provide better understanding of the structure and recent geological evolution of the recent volcanic arc. They are also a useful complementary tool for geothermal and seismological studies.