Epiclastic deposits and ‘horseshoe-shaped’ calderas in Tahiti (Society Islands) and Ua Huka (Marquesas Archipelago), French Polynesia

Occurrences of debris avalanche deposits newly identified in Tahiti (Society Islands) and Ua Huka (Marquesas Archipelago) are described and interpreted here. In both islands, the breccias are located within horseshoe-shaped residual calderas. In Tahiti, the epiclastic formations, up to 500 m thick, lie on the floor of the central depression and in the valley of the northwards running Papenoo River. In Ua Huka, the breccias crop out within a depression limited by a semicircular crest in four bays along the southern coast. Their thickness is ca. 100 m. A few clasts collected in the Tahitian breccias and some rocks forming their substratum have been dated (K–Ar datings) and analysed (major and trace elements, Sr–Nd isotopes) for this study. Using these data, we show that the debris avalanche(s) occurred in Tahiti Nui at the end of the growth of the shield volcano (between 570 000 and 390 000 years ago), maybe in consequence of the emplacement of the plutonic body which occupies the central part of the caldera. In Ua Huka, the collapse took place nearly 3 Ma ago, between the construction of the shield volcano and that of the inner one. The southwards orientation of the caldera, like that of the neighbouring island Nuku Hiva, might reflect a preferential direction of weakness in the substratum of the central Marquesas.