Crustal structure in the Society and Tuamotu Islands, French Polynesia

Summary We present seismic investigations of the crustal structure of Tahiti (Society Islands), Rangiroa (Tuamotu Islands), and neighbouring areas. Records from a series of 96 explosions at the short‐period stations of the Polynesian Seismic Network form a seismic refraction dataset from which we recover the crustal structures. The crust in the vicinity of Tahiti has the classic structure of 70 Myr‐old ocean floor, and features an 8.8 km‐thick crust (below sea floor) with 3 7.64‐8.25 km s ‐1 jump at the Mohorovičić (Moho) discontinuity. On the other hand, the Tuamotu plateau shows a very thick crust, reaching 31 km below sea floor, with a 6.83‐8.10 km s ‐1 jump at the Moho. In addition, Rangiroa atoll itself features a 2 km‐thick layer of slow material (3.3 km s ‐1 ) which we interpret as limestone; comparable structures have been reported at Enewetak and Bikini. We further use the short‐period seismic arrays on Rangiroa and Tahiti to recover the slowness vectors, and, hence, the dispersion characteristics, of Rayleigh waves in the 15‐40 s period range recorded from teleseismic events. Rayleigh dispersion under the Society Islands is compatible with published oceanic models of adequate age. We also use long‐period records in the 20‐90 s range for the path Rangiroa‐Hao orientated along the Tuamotu plateau. The Rangiroa‐Hao dispersion is slower than previously determined in neighbouring oceanic areas, and compatible with a crust 22‐30 km thick under the Tuamotu plateau. Dispersion under Rangiroa is even slower, and requires a thicker (∼35 km) crust. Any crust of standard thickness can be ruled out, as it leads to unacceptably low (< 3 km s ‐1 ) crustal S ‐velocities. These results are comparable to structures published for the Iceland‐Færœ and Walvis Ridges, and confirm that the Tuamotus were formed on‐ridge, in agreement with their very weak geoid signature.