Reconstructing the geological evolution of small tropical volcanic islands: Insights from the study case of Moorea (French Polynesia)

Studying extinct volcanic islands is critical to constrain the geological evolution of intra-plate volcanoes and shed light on related natural hazards. From new fieldwork, geomorphological and new K/Ar geochronological data, we revisited the main stages of growth and destruction of Moorea, a small and highly eroded tropical volcanic island in French Polynesia. We show that: (1) the southern part of the island comprises the remnants of a main former basaltic volcano that was active between ca 1.85 Ma and 1.70 Ma; (2) the northern flank of this early edifice was removed around 1.64 Ma by a main flank-collapse that produced debris-avalanche partly exposed on the ocean floor up to 30 km from the island shore; (3) the sudden subsequent decompression was followed by vigorous volcanic activity witnessed by widespread strombolian deposits including peridotite xenoliths; (4) a second main volcanic edifice rapidly grew inside the collapse depression between 1.64 Ma and ca 1.56 Ma, followed by less-voluminous sporadic lava eruptions until ca 1.36 Ma; (5) late volcanic units were erupted from vents along the E-W direction, and/or small parasitic cones around the rims of the collapse scar; (6) related lava flows covered unconformably the weathered slopes of the older edifice and filled existing paleo-valleys; (7) subsequent erosion enlarged the former landslide scar and led to significant topographic inversion. Successive construction and destruction episodes on small volcanic islands like Moorea lead to strong geometric imbrication of geological units, which has important societal implications, e.g., for volcanic aquifers management or land occupation.