Fracturation Pattern in the Limestone Loyaute Islands and its Relation to the Neighbouring Vanuatu Subduction Zone (SW PAcific)

The Loyauté Islands are a series of limestone karstified islands that are currently uplifted and deformed on the elastic bulge of the Australian plate before its subduction at the Vanuatu Trench (SW Pacific). As part of the SAGE program of the New Caledonian Province des Iles, they have been extensively surveyed for geology and hydrogeology. As part of this project, a map of fracturation deduced from aerial photos, and from SPOT4 and ENVISAT satellite data has been produced and a field trip allowed to verify that the main fracture orientations were also present on the most recent terranes bordering the islands. Since their formation during the Miocene, these islands are in a tectonically stable area. Thus, they provide a unique opportunity to study their fracture distribution in relation with their recent tectonic context. We will present the results of a statistical analysis of fracture distribution both in number and in fracture length and an attempt to model the fracture orientations as resulting from the elastic deformation of the Australian lithosphere before its subduction. Three main fracture families have been determined for the three island, with very few differences if fracture number of fracture length statistic is considered. These families are N62.5, N107.5, and N152.5 for Lifou, which is the largest and central island, which are further termed as F1, F2, F3. F2 is at least 5 times more important than F1 and F3, which are 45° apart on both sides of F2. The orientation of families F1-F3 are N 65, N110, and N155 in Maré, which located less than 100 km apart from the subduction zone, and N60, N105, and N150 in Ouvéa , which is the most distant island from the subduction and is only uplifted in its NorthEastern part. The main family F2 does not correspond either to the subduction zone orientation (N150) nor to that of the Loyauté ridge (N135) on which the three islands are located. Thus, the fracture pattern of the three island cannot be explained by a 2-dimensional bulging of the Australian plate approaching the Vanuatu subduction zone. We will present two new analytical models for the elastic deformation of the Australian lithosphere. The first one takes into account the curvature of the subduction zone while the second one introduces a punctual force which account the first stages of a collision between the Loyalty ridge and this subduction zone. The directions of principal stresses deduced from these models are compared to the deformation recorded in the fracture netword of the three islands