Northern Lesser Antilles Forearc Strain Pattern: New Insights from Antigua Island, Lesser Antilles Forearc.

The active arc of the Lesser Antilles subduction zone shows a specific pattern with a northern part exposing the remnant of a extinct volcanic arc in the forearc region. Indeed, in this region, the Anguilla and Antigua banks are shallow bathymetry banks blanketed by holocene reefs from which emerge Eo to Miocene volcanic arc rocks in the islands of Anguilla-St Martin-St Barthélemy and Antigua, respectively.

In the frame of the GAARAnti project, aiming at understanding the South American mammal fauna dispersal along the eastern boundary of the Caribbean plate "en route" to colonized North America, two crucial constrains are needed: to identify the Lesser Antilles strain record and to identify and map emerged area. To reach this goal, a novel an extensive structural dataset (consisting of bedding plane, fault, kinematics indicators, shear zones, folds measurements) has been acquired in the island of Antigua.

The island shows a southwest-northeast structure as follows: a Basal Volcanic Group, a Central Plain Group consisting of volcanic and volcaniclastic rocks, interbedded with lacustrine and shallow marine limestones and an upper limestone Group . Based on the already available lithological maps, we propose a new geological map and discuss the strain pattern of the island integrating the analysis of the brittle and ductile strain. We show that the central plain deposits are conformably lying on the Basal Volcanics and found an angular unconformity within the limestones. Our thorough field investigations and the establishment of geological cross section reveal that the sequence is largely affected by 140° trending normal faults formed in a large scale left lateral strike slip system affecting the island. We identify at least two phases of deformation affecting the volcanic part and the base of the limestones of which one was in a compressional stress field creating folds and reverse faults. The last deformation phase was in an extensional stress field creating the 140° trending normal faults through the upper part.

We then compare our findings with those of the islands of the Anguilla bank presented by Cornée et al., in this session and discuss the perspectives opened by this field based study.

This study as been funded by the Molengraaff Fund and the ANR GAARAnti.