Submarine landslides in French Polynesia SUBMARINE LANDSLIDES IN SOCIETY AND AUSTRAL ISLANDS, FRENCH POLYNESIA: EVOLUTION WITH THE AGE OF EDIFICES V. CLOUARD Departamento de Geofísica, Univ. de Chile, Blanco Encalada, 2085, Casilla 2777, Santiago, Chile A. BONNEVILLE Géosciences Marines, CNRS, Institut de Physique du Globe, 4 place Jussieu, 75252 Paris Cedex 05, France Abstract This paper presents descriptions of numerous submarine landslides in French Polynesia. This inventory shows an evolution of the landslide type with the age of oceanic islands. Submarine active volcanoes are subject to superficial landslides of fragmental material whereas young islands exhibit marks of mass wasting corresponding to giant lateral collapses due to debris avalanche that occurred during the period of volcanic activity. Later, erosional processes generate sand-rubble flows and lead the islands to the stellate morphology known on atolls and guyots. In addition, Tupai atoll and Rurutu Island have been subject to giant slump that deeply modify their shape. Keywords: Submarine landslide, French Polynesia, Austral, Society, classification, oceanic islands 1 Introduction Landslides are common feature on oceanic island and play a key role in their building (Moore et al., 1989). Collapses can originated from different phenomena (Walker, 1988), which define four major types: (1) caldera subsidence, (2) superficial landslides, (3) landslides due to a deep listric fault, (4) side-slip along a deep decollement. Superficial landslides are mainly related to erosional processes of the subaerial parts of the island. They produce lobate debris tongue termed sand-rubble flow by Fornari et al. (1979). Debris are less than 1 m in size, and the seafloor is rippled. Lateral collapses of type (3) and (4) produce giant submarine landslides, with a amphitheatre at their head (Moore et al., 1989). Landslides due to a deep listric fault are cataclysmic events producing fast moving debris avalanche. Deposits can stretch out on several hundreds kilometers offshore the island and are characterized by a thickness less than 2 km and hummocky terrain in their lower part. Side-slip along a deep decollement are termed slump. Slumps are slow-moving slope instabilities. The thickness of the deposits can reached 10 km as the primitive volcano flank is less shattered and disrupted than in the case of a debris avalanche. Cause of major lateral collapses are still matter of debate, but are supposed to be related to the magma intrusion in the rift zones (e.g., Denlinger and Okubo, 1995). We present in this paper evidence for 36 submarine landslides in Society and Austral islands, French Polynesia, from a systematic study of submarine island slopes. The detailed bathymetry of the seafloor around Society and Austral islands were acquired during a geological survey conducted in July 1999 with R/V L'Atalante (ZEPOLYF2, 2001). The in-land origin of the landslides is deduced from geomorphological analysis of the aerial topography of the edifices. 1
Clouard and Bonneville 2 Geological setting Society and Austral islands are located on the south central part of the Pacific plate (Figure 1). The age of the oceanic crust ranges between 50 and 85 Ma (Mayes et al., 1990; Munschy et al., 1996}. The Society and Austral island chains trend N120º, the same direction of the present Pacific Plate motion, and are associated with recent hotspot volcanism. Society island chain extends 750 km from the present hotspot location over Meetia to three western atolls. It is composed of five atolls and nine islands. Ages increase from the southeast to the northwest and are reported figure 1. Figure 1: Bathymetric map of the Society and Austral islands. FZ stands for fracture zone. Black stars represent the active areas of Society, Aragoand Macdonald hotspots. Ages of the volcanoes are reported in Ma between parentheses. Society Islands exhibits every transition stage of the oceanic islands in low latitudes, from elevated island with a growing coral reef to the atoll stage. The Austral Islands chain is more than 1500 km long and composed of five small islands and one atoll. Two hotspots are presently active within the Austral alignment, Macdonald hotspot to the 2
Submarine landslides in French Polynesia eastern extremity (Norris and Johnson, 1969) and Arago hotspot 120 km to the southeast of Rurutu island (Bonneville et al., 2002). 3 Landslide distribution From the systematic study of Austral and Society islands slopes, a classification of the landslide type appears, depending of the development stage of the island. Hence, our results are presenting following this classification, and only one example of each type is described with some details. 3.1 LANDSLIDE RELATED TO SUBMARINE ACTIVE VOLCANOES Mouaa Piha (Figure 2) is the shallowest active seamount of the Society hotspot. It is located to the south-west of Meetia. The upper submarine slopes of Mouaa Piha are scalloped by three indentations, each of which corresponding to the headwall of landslides. The lower slopes are characterized by a ripple seafloor, due to sand-rubble flows. The deposits are made of small size fragmental material with no trace of hummocks, except for the north-western landslide where few blocks are present. The scar heads range at depth comprised between 500 m for the western landslide, and 1500 m for the north-western one. The two active seamounts of the Austral Islands, Arago and Macdonald seamounts present the same landslide characteristics, showing only sand-rubble flow deposits, with very few isolated blocks of removed material. Figure 2: Three-dimensional shaded view of the bathymetry of Mouaa Piha seamount (summit depth: 143m). Three sand-rubble flows can be observed. Hence, sand-rubble flows seem to characterize landslides that occur on submarine active volcanoes: deposits are made of small size fragmental material with no trace of hummocks. The pattern of the deposits observed indicates that the sediments were laid down in smooth fashion, and there is no sign for any cataclysmic events. 3
Clouard and Bonneville 3.2 LANDSLIDES RELATED TO YOUNG OCEANIC ISLANDS (< 4 Ma) Debris avalanches characterize young polynesian island landslides. For instance, Meetia, an active island to the south-eastern boundary of the Society hotspot, has all its flanks mantled by mass wasting that present a homogenous radial distribution. Basically, submarine slopes are covered by sand-rubble flows as it was the case for active submarine volcanoes, but large rocky blocks also exist on the north-eastern and western flanks, and deposits of a debris avalanche can be observed on the southern flank. On Tahiti Island, a giant landslide of the southern flank (Figure 3) originating from a debris avalanche has been dated to the beginning of the late shield field (Clouard et al., 2001). It is in good agreement with Hawaian landslide study of Moore et al (1989), which proposed that main cataclysmic events happened at the end of the shield building phase. Figure 3: Three-dimensional shaded view of the bathymetry and topography of the south of Tahiti and Moorea islands (after Clouard et al., 2001). Raiatea, Tahaa and Bora Bora also present evidence of debris avalanches. In all the Society Islands except Meetia, there are no trace of sand-rubble flow. They could be covered by subsequent debris avalanche deposits that occurred on same areas or because their small-size deposits have been removed by submarine currents. 3.3 LANDSLIDES RELATED TO OLDEST OCEANIC ISLANDS (> 5 Ma) Volcanism in Austal Islands vanished more than 5 myr s ago (except in Rurutu, see 3.4). The slopes of the islands of Rapa, Tubuai, Raivavae and Rimatara present evidence of both debris avalanche and sand-rubble flow. If, as the study of the Society landslide suggests, the trace of sand-rubble flow rapidly disappear, then Austral Island sand-rubble flows are recent. They should correspond to erosional processes of old volcanic material between rift zones, which lead to the stellate characteristics of atolls and guyots. 3.4 LANDSLIDE RELATED TO TECTONIC EVENTS Tupai is a small atoll of the Society Islands, north of Bora Bora. Trace of a large landslide exists on the east side of the edifice (Figure 4). It is characterized by large debris in the lower part of the slope, their widths reaching several kilometers (3 km x 2.5 km for the greater), and it presents steep toe. These are the characteristics of a 4
Submarine landslides in French Polynesia slump. Tupai slump heads in a giant scarp, which reaches 1000 m height and 7 km length. It might correspond to the scar of the mega blocks. We propose that this landslide is the result of the collapse where the whole aerial and submarine eastern flank of the island slided down to the sea. This explains the non circular shape of the presentday coral reef. An indication of the age of this landslide is given by the sediment coverage of the deposits: Figure 4 indicates that Bora Bora sediment drifts overlap the southern hummocks. On another hand, the displacement of the landslide was not linear but diverted to the north, probably by the submarine volcanic slope of Bora Bora. Therefore, we can supposed that this collapse occurred at the end of Bora Bora building stage. Figure 4:Three-dimensional shaded view of the bathymetry of Tupai atoll. The average height of the scarp is 700 m over 7 km wide. Rurutu Island, in the Austral Islands, has a shape similar to that of Tupai. Rurutu is characterized by two periods of volcanic activity separated by 11 myr, and by uplifted carbonate plateaus. This uplift has been explained by thermal rejuvenation of the lithosphere due to the second hotspot (Calmant and Cazenave, 1986). A large landslide exists on the western flank of Rurutu, and its origin corresponds to the western bay. Unfortunately, data do not cover the lower part of the deposits. However by analogy with Tupai case, this landslide must correspond to a slump, where the south-western part of the island disappeared into the sea. The second volcanic episode associated to vertical motions of the island would be responsible for this slump. Conclusion Our study reveals 36 major landslides that mantled the submarine slopes of Society and Austral Islands. These submarine landslides can be classified in three types that are related to three stages of the edifice life. Submarine active volcanoes are subject to numerous mass wastings characterized by small volumes, small size deposits, visible 5
Clouard and Bonneville scars in the upper slopes of the edifices and a radial distribution of the landslides. During the subaerial volcanic stage, islands are subjects to giant collapses associated with debris avalanches. It correspond to isolated events, with large subaerial volume removed. The end of the volcanic activity is marked by the end of giant lateral collapse. Mass wasting continues, but are mostly sand-rubble flows. These erosional processes occured between the rift zones and produced the stellate morphology observed on old island and on Pacific guyots. In addition, two islands submitted to important vertical tectonic motions have been the locus of giant slump that re-shape their aerial aspect. ACKNOWLEDGEMENTS REFERENCES Bonneville, A., R. Le Suavé, L. Audin, V. Clouard, L. Dosso, P.-Y. Gillot, P. Janney, K. Jordhal, and K. Maamaatuaiahutapu, 2002, Arago seamount, the missing hotspot found in the Austral Islands, Geology, 30:1023-1026. Calmant, S. and A. Cazenave, 1986, The effective elastic thickness under the Cook- Austral and Society Islands, Earth Planetary Science Letter, 77:187-202. Clouard, V., A. Bonneville and P.-Y. Gillot, 2001, A giant landslide on the southern flank of Tahiti island, French Polynesia, Geophysical Research Letter, 28: 2253-2256; Denlinger, R.P. and P. Okubo, 1995, Structure of the mobile south flank of Kilauea Volcano, Hawaii, Journal of Geophysical Research,100: 24499-24507. Duncan, R.A. and I. McDougall, 1976, Linear volcanism in French Polynesia, Journal of Volcanology and Geothermal Research, 1: 197-227. Fornari, D.J., J.G. Moore and L. Calk, 1979, A large submarine sand-rubble flow on the Kilauea volcano, Hawaii, Journal of Volcanology and Geothermal Research, 5: 239-256. Hampton, M.A., H.J. Lee and J. Locat, 1996, Submarine landslides, Review of Geophysics, 34: 33-59. Mayes, C.L, L.A. Lawver and D.T. Sandwell, 1990, Tectonic history and new isochron chart of the South Pacific, Journal of Geophysical Research, 95: 8543-8567. Moore, J.G., D.A. Clague, R.T. Holcomb, P.W. Lipman, W.R. Normark and M.T Torresan, 1989, Prodigeous submarine landslides on Hawaiian ridge, Journal of Geophysical Research, 94: 17645-17484. Munschy, M., C. Antoine and A. Gachon, 1996, Evolution tectonique de la région des Tuamotu, océan Pacifique Central, Comptes Rendus de l'académie des Sciences de Paris, série IIa, 323: 941-948. Norris, A. and R.H. Johnson, 1969, Submarine volcanic eruptions recently located in the Pacific by SOFAR hydrophones, Journal of Geophysical Research, 74: 650-664. Walker, G.P.L., 1988, Three Hawaiian calderas: An origin through loading by shallow intrusions?, Journal of Geophysical Research, 93: 14773-14784. ZEPOLYF2, 2001, Rapport de campagne, Documents et Travaux ZEPOLYF, 2, Université de la Polynésie Francaise, Tahiti, French Polynesia, 174 p. (+30 maps). 6