Passive obduction and gravity-driven emplacement of large ophiolitic sheets: The New Caledonia ophiolite (SW Pacific) as a case study?

– The 300 km long allochthonous sheet of oceanic mantle forming the New Caledonia ophiolite displays three specific characters: 1) the ophiolite pile lacks concordant sheeted dykes and pillow basalt layers; 2) the ophiolite, refered to as the Peridotite nappe, is thrusted over the basaltic formations of the Poya terrane which are classicaly thought to originate from a different oceanic environment; 3) The basal contact of the ultramafic sheet is remarkably flat all along New-Caledonia and the Peridotite nappe has not been thickened during obduction, rather it experienced significant extension. This suggests that the peridotites have not been emplaced by a tectonic force applied to the rear. New petrological and geochemical results obtained from mantle rocks finally show that the Poya terrane may originate from the same oceanic basin as the peridotites. In this article, we consider such possible cogenetic links and we propose a simple model for the obduction of the New Caledonia ophiolite in which the Poya basalts represent the original cover of the Peridotite nappe. We infer that continuous uplift of the subducted units buried beneath the oceanic lithosphere in the northern part of New Caledonia drove passive uplift of the ophiolite and led to erosion and to initiation of sliding of the basaltic layer. During the Priabonian (latest Eocene), products of the erosion of the basaltic layer were deposited together with sediments derived from the Norfolk passive oceanic margin. These sediments are involved as tectonic slices into an accretionary wedge formed in response to plate convergence. The volcaniclastic sedimentation ends up with the emplacement of large slided blocks of basalts and rafted mafic units that progressively filled up the basin. Obduction process ended with the gravity sliding of the oceanic mantle sheet, previously scalped from its mafic cover. This process is contemporaneous with the exhumation of the HP-LT units of Pouebo and Diahot. Gravity sliding was facilitated by the occurrence of a continuous serpentine sole resulting from metasomatic hydratation of mantle rocks, which developed during the uplift of the Norfolk basement and overlying Diahot and Pouebo units. Progressive emersion of the obducted lithosphere allowed subsequent weathering under subaerial, tropical conditions. Obduction passive et mise en place gravitaire des grandes nappes ophiolitiques : les ophiolites de Nouvelle-Caledonie (SW Pacifique) sont-elles un cas d'ecole ? Resume. – La nappe de manteau de Nouvelle-Caledonie, s'etendant sur 300 km de long montre trois caracteristiques im-portantes: 1) La succession ophiolitique ne contient ni dykes, ni basaltes en coussin, 2) l'ophiolite est charriee sur des formations basaltiques (nappe de Poya) que l'on considere classiquement comme originaires d'un bassin oceanique dif-ferent de celui d'ou est issu le manteau, et 3) le contact basal de la nappe ultrabasique est remarquablement plat tout le long de l'ile et celle-ci n'a pas ete epaissie durant l'obduction. Cela suggere que les peridotites n'ont pas ete mises en place en reponse a une poussee tectonique arriere. De nouveaux resultats petrologiques et geochimiques obtenus sur l'ophiolite montrent en realite que les basaltes de Poya peuvent etre extraits de la fusion du manteau de l'ophiolite. Nous proposons donc un nouveau modele pour l'obduction, dans lequel les basaltes de Poya forment la couver-ture initiale primitive du manteau de la nappe des peridotites. La surrection progressive des unites profondes enfouies lors de la subduction de la lithosphere du bassin sud-Loyaute a l'Eocene, a permis le soulevement passif de l'ophiolite et l'erosion puis le glissement des unites basaltiques. Durant le Priabonien, l'accumulation des produits de l'erosion de la couche des basaltes et de la marge passive de Norfolk ont forme les sequences de flysch comprenant localement des debris basaltiques grossiers. Ces sediments font partie d'un prisme d'accretion tectonique construit en reponse a la convergence au front de la subduction. La sedimentation detritique se termine par la mise en place de grandes unites ba-saltiques glissees qui comblent le bassin. Le processus de l'obduction se termine par le glissement gravitaire de l'ecaille de manteau oceanique sur les basaltes. Cet evenement est contemporain de l'exhumation des unites HP-BT de Pouebo et du Diahot. Le glissement gravitaire a ete facilite par la presence sous l'ophiolite, d'une semelle continue de serpentines resultant de l'interaction du manteau avec des fluides metasomatiques remontant durant l'exhumation des unites en-fouies pendant la subduction continentale. L'emersion progressive de la lithosphere obduite a place le manteau dans des conditions d'alteration supergene tropicale.