Géologie des Marquises : des volcans boucliers intra-océaniques effondrés issus d’un point chaud atypique

The Marquesas archipelago is the northernmost of thefour Neogene-Quaternary linear intraplate chains of FrenchPolynesia. We present the first detailed geological maps ofits eight main volcanic islands (Eiao, Nuku Hiva, Ua Huka,Ua Pou, Hiva Oa, Tahuata, Motane, Fatu Hiva). The islandages decrease from northwest (5.5 Ma at Eiao) towardssoutheast (1.1 Ma at Fatu Hiva). The Marquesas shieldvolcanoes experienced important sector collapse eventslinked to caldera formation, which led to the sinking of half ormore of their initial shields. Most islands are made up of anouter remnant of such a shield, in the caldera of which liesan inner, younger and smaller post-shield volcano. The totalvolume of Marquesas volcanic edifices (42,500 km3 abovethe -3 000 m isobath) corresponds to a rather low magmaticproduction (7,700 km3 per million years, i.e. less than 4 % ofthat of Big Island, Hawaii). Magmas uprising from theMarquesas hotspot reach the surface episodically, hence theuncertainties regarding its location, generally thought to beclose to the Marquesas Fracture Zone. These magmasderive from low to moderate melting rates of a highlyheterogeneous (at a 10 km scale) “weak” mantle plumewhich includes two adjacent filaments of slightly differentisotopic compositions. These produced two rows of islandslabelled the northeastern Ua Huka group and thesouthwestern Fatu Hiva group, respectively. The Marquesasvolcanic successions depart from the Hawaiian model, astholeiitic magmas are not restricted to the shield stage, nor alkali basaltic/basanitic magmas to the post-shield stage.These two types are thought to derive from relativelydepleted (peridotitic) and enriched (pyroxenitic) componentsof the plume, respectively. L’archipel des Marquises comporte huit îlesprincipales (Eiao, Nuku Hiva, Ua Huka, Ua Pou, Hiva Oa,Tahuata, Motane, Fatu Hiva), dont nous présentons lespremières cartes géologiques détaillées. L’âge de cesîles diminue du Nord-Ouest (5,5 Ma à Eiao) au Sud-Est(1,1 Ma à Fatu Hiva). Les volcans boucliers marquisiensdiffèrent considérablement de leurs équivalentshawaiiens. Ils ont fréquemment subi des effondrementssectoriels, responsables de la disparition en mer de lamoitié, voire davantage des boucliers initiaux, dans lacaldeira desquels est apparu un volcan interne plusrécent et plus petit. Le volume total des édificesvolcaniques (42 500 km3 au-dessus de l’isobathe -3 000 m) correspond à une production magmatiquefaible (7 700 km3/Ma, soit moins de 4 % de celle deHaw