T31B-2868
Crustal structure of the lesser Antilles Island arc south of Guadeloupe : new insights from the ANTITHESIS cruise

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Frauke Klingelhoefer1, Muriel Laurencin2, Boris Marcaillou3, Mikael Evain1, David Graindorge4, Arnauld Heuret5, Lebrun Jean-Frederic6, Francis Lucazeau7, Thibaud Pichot8, Frederique Rolandone9 and Helene Bouquerel10, (1)IFREMER, Plouzané, France, (2)IUEM Institut Universitaire Européen de la Mer, Plouzané, France, (3)University of Nice-Sophia Antipolis, Nice, France, (4)University of Western Brittany, Brest, France, (5)GEOSCIENCES MONTPELLIER, Montpellier, France, (6)Université des Antilles, Département de géologie, Pointe à Pitre, France, (7)Institut de Physique du Globe, Paris, France, (8)Ecole Normale Supérieure Paris, Paris, France, (9)University Pierre and Marie Curie Paris VI, istep, Paris, France, (10)Institut de Physique du Globe de Paris, Paris, France
Abstract:
One of the goals of the Antithesis cruise, conducted in 2013, was the study of the combined deep structure and thermal regime of the lesser Antilles subduction zone in order to quantify its seismic hazard. One combined reflection and wide-angle seismic profile has been acquired in the scope of the Antithesis cruise North of Barbuda at around 18°N using 26 ocean bottom seismometers, a 4.5 km digital streamer and a 7800 cu. inch seismic source. 13 measures of heat flow were taken in the vicinity of the profile. Forward modelling of the wide-angle seismic data reveals the deep structure of the subduction zone in the study area.

- The subducting Atlantic oceanic plate is about 5-7 km thick, modelled by one single layer.

- Sedimentary thickness on the oceanic plate is low with maximal some hundreds of meters of sediments at the trench.

- The thickness of combined two sedimentary layers is ~5 km near the trench and decreases toward the arc, based on reflection seismic data.

- The ~18-km-thick Caribbean crust at the arc is subdivided in 3 layers, overlain by a 2-to-4-km-thick layer with velocities ranging from 4.5 to 5.5 km/s that might correspond to volcanic products or high velocity sediments.

- The velocities in the lower crustal layer reach 7.4 km/s probably corresponding to mafic rocks and plutons.

- The velocities in the upper mantle were constrained to be 8.00-8.20 km/s underneath the arc as well as underneath the oceanic crust, thereby excluding the existence of extensive serpentinization of the mantle through water entering the subduction zone.

 The anomalously low heat flow indicates cold calculated temperatures along the interplate contact, suggesting a hydrothermal cooling that is possibly restricted to the shallow crustal aquifer. Alternatively, these low temperatures may be due to a depressed thermal structure of the oceanic crust related to the slow-spreading at the medio-Atlantic ridge.

A wide-angle seismic profile about 150 km to the South in the vicinity of Guadeloupe Island shows a higher sedimentary thickness at the accretionary wedge. However, the crustal thickness and slab geometry are similar indicating that the subduction process is homogeneous along this segment. Wide-angle seismic profiles acquired during the Antithesis cruise further north at the Anegada Passage show a more shallow situated slab.

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