Crustal and upper mantle seismic structure and lithospheric flexure along the Society Island hotspot chain

Abstract

Summary Four wide-angle and refraction lines have been shot along the Society Island hotspot track. A 280 km long profile covers oceanic crust to the east of the islands, the active hotspot region, and the apron of Tahiti. The other profiles cover the region between Moorea and Huahine and the western archipelago of Huahine, Raiatea, Bora Bora, and adjacent oceanic crust. Traveltime modelling provided lateral variations in crustal structure along the hotspot chain. Removal of the contribution of the uppermost low-velocity layer, which represents the volcanic edifices and debris, suggests that the islands and atolls sit on normal oceanic crust and are surrounded by a large volcaniclastic apron. Seismic profiles running between Raiatea and Huahine, and between Raiatea and Moorea cover a region where extreme down-bending due to volcanic loading is expected. A nearly flat crust–mantle boundary indicates regional isostasy and clearly supports a stiff plate. Seismic structural models have been compared with calculated boundaries based on simple 3-D elastic plate models. A good fit to the seismic crust–mantle boundary was obtained for models with an effective elastic thickness of the lithosphere of 20–25 km. These values are in accordance with a model in which the elastic thickness depends on the thermal age of the lithosphere, and therefore contradict models that suggest that the region of French Polynesia has been thermally rejuvenated. Amplitude modelling of crustal and upper mantle arrivals suggests that crust on the hotspot trail has been modified with respect to the pre-hotspot crust. To explain the observations we suggest that, during the ongoing late-stage volcanic activity, the lower crust has been intruded by sills, and some melts are trapped in the crust–mantle boundary region. If late-stage volcanism and plutonism continue, prolonged crustal growth may form a large underplate or subcrustal plutonic core complex along the hotspot chain.