Offshore evidence for a huge landslide of the northern flank of Tahiti-Nui (French Polynesia)

A bathymetric analysis of the flanks of Tahiti-Nui (French Polynesia) reveals the existence of a major catastrophic north-directed landslide during its eruptive history. The related debris-avalanche (DA) deposits extend north 70 km from the island shore and have a total volume of about 800 km(3). Two contrasted facies of the debris are recognized: (1) relatively small-sized material, including a few blocks with well-developed flow structures (elongate downslope-directed ridges and transversal pressure ridges), and (2) a scattering of large kilometer-sized individual blocks that form, in part, a prominent bathymetric hummock in the proximal central part of the landslide deposits. Bathymetric analyses suggest that the Tahitian failure evolved through two main successive shots. The DA debris were rapidly overlain by subsequent volcanic material, producing a smooth surface down to 1500 m below sea level. The on-land and offshore structures indicate a listric fault geometry for the landslide scar. The volume removed by the landslide is thus estimated between 400 km(3) and 450 km(3). Discrepancy in the volume of the scar, and the debris derived from it, is due in part to expansion of the debris, to fragmentation of the avalanche, and to incorporation of sediments. Comparison with a recently recognized southern Tahitian landslide (Clouard et al., 2001) and with landslides recognized around the Hawaiian Islands and the Canary Islands shows that the huge northern Tahitian landslide described here has a reduced mobility in its central part. Perhaps the older Tetiaroa eruptive complex to the north acted as a buttress to inhibit movement of the DA products. The northern and southern Tahiti-Nui landslides moved in opposite directions away from the main central E-W directed rift zone that served as the main magma feeding structure. Then, the huge northern landslide strongly controlled the later volcano-structural evolution by capturing subsequent volcanic activity to the north, despite the northwestward displacement of the Pacific plate over the Society hot spot.