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03-Cottin [Kerguelen] The Kerguelen volcanic Plateau: the second largest oceanic Igneous Province (LIP) on earth and a witness of the Indian Ocean opening by Jean-Yves COTTIN* (1), Gilbert MICHON (1) & Guillaume DELPECH (2) ABSTRACT. - The geological evolution, geomorphological and geophysical data of the Kerguelen Plateau are summa- rized from the results of Indian Ocean Drilling Program (ODP) and Institut Paul Emile Victor (IPEV) scientific programs conducted during the last twenty years. This plateau is located in the Antarctic plate oceanic domain between 46 and 64°S, approximately equidistant from Africa and Australia. It is a Large Igneous Province (LIP) made up of thick sequences of lava flows rising 2000 to 3000 m above the seafloor, and generated by the activity of a hot spot, which seems to be pres- ently located beneath Heard Island (where the active volcano Big Ben is located). The birth of the Plateau occurred in a mid-oceanic ridge setting (South East Indian Ridge, SEIR) 115 Ma ago. Since 43 Ma, the Kerguelen Plateau and Broken Ridge separated and the SEIR moved away northward from the plateau to reach its present intraplate setting. The plateau is subdivided into three parts: (1) the Northern Part, which carries the Kerguelen Islands, (2) the Central Part, which rep- resents a basin located between Kerguelen and Heard islands with Upper Cretaceous sediments, and (3) the Southern Part which is characterized by a thick crust and which may contain old continental crust fragments. Available ages on sediments and lavas from several ODP legs indicate a short interval (110-115 Ma) for the formation of the uppermost igneous crust constituting the Kerguelen plateau. RÉSUMÉ. - Le plateau volcanique de Kerguelen : la deuxième plus grande province magmatique océanique terrestre et un témoin de l’ouverture de l’océan Indien. L’histoire géologique et les données géomorphologiques et géophysiques acquises sur le Plateau de Kerguelen sont résumées à partir des résultats des vingt dernières années des programmes Ocean Drilling Program (ODP) et des pro- grammes de l’Institut Polaire Français Paul Emile Victor (IPEV) sur les îles subantarctiques de l’océan Indien. Ce plateau appartient à la partie océanique de la plaque Antarctique, entre le 46e et le 64e parallèle Sud, et il est situé à mi chemin de l’Afrique et de l’Australie. C’est une grande province ignée engendrée par l’activité d’un point chaud qui pourrait être actuellement localisé sous l’île de Heard (volcan actif Big Ben). La naissance de ce plateau remonte à 115 Ma à proximité de la ride sud-est indienne, alors que depuis 43 Ma il s’est séparé de Broken Ridge pour acquérir la position intraplaque actuelle. Ce plateau comprend trois parties : (1) le plateau océanique nord qui porte les îles Kerguelen, (2) la partie centrale qui est un basin sédimentaire d’âge Crétacé séparant les îles Kerguelen et Heard, et enfin (3) la partie sud, caractérisée par une croûte épaisse marquant la transition avec la partie continentale de la plaque Antarctique. Les âges des sédiments et des laves de différents forages ODP indiquent un intervalle de temps très court (110-115 Ma) pour la formation de la partie magmatique supérieure de la croûte. Key words. - Kerguelen Plateau - Volcanism - Ocean drilling - Geology - Plate tectonics - Gondwana break-off. The Kerguelen Plateau, located in the Antarctic plate Plateau is a Large Igneous Province (LIP) made up of thick oceanic domain, extends at 2300 km between 46 and 64°S sequences of lava flows rising 2000 to 3000 m above the (Fig. 1) and represents a large structural height in the south- seafloor. Seismic data reveal that this part of the Kerguelen- ern Indian Ocean. With dimensions of 500 km by 2100 km, it Gaussberg ridge has a 14-23 km thick oceanic crust, thicker covers around 2 x 106 km2, and is the second largest oceanic than that of the Northern Kerguelen Plateau (Recq and Char- plateau after Ontong-Java in the Pacific Ocean (Coffinet al., vis, 1986; Charvis et al., 1993). 1999; 2002; Delpech, 2004). The combined magmatic vol- ume of the Kerguelen Plateau and the Broken Ridge Plateau Geological evolution: geomorphological, geophysical and is approximately 57.106 km3. The Kerguelen Plateau corre- geochemical data sponds to the northwest extension of the Kerguelen-Gauss- berg aseismic ridge, which extends southward close to the Structure, stratigraphy and age of the Kerguelen Plateau Antarctic continent with a NW-SE trend parallel to that of The Kerguelen Plateau is subdivided into three domains the South East Indian Ridge (SEIR) (Fig. 1). The Kerguelen and radiometric ages young northward (Figs 2 and 3): (1) Université Jean Monnet Saint-Etienne, membre PRES Université de Lyon, UMR CNRS 6524 “Magmas et volcans”, 42023 Saint-Etienne CEDEX, France. [[email protected]] (2) UMR CNRS 8148 IDES, Université Paris Sud-XI, 91405 Orsay CEDEX, France. [[email protected] ] * Corresponding author [[email protected]] The Kerguelen Plateau: marine ecosystem and fisheries: 29-42. Geology of the Kerguelen Plateau COTTIN ET AL. Figure 2. - Bathymetry of the Kerguelen Plateau (Coffin et al., 1999; Proceedings of the ODP Leg 183 Initial Reports). started about 120 Ma ago (Coffin et al., 2002). However, Figure 1. - Geographic map of the Indian Ocean showing the geo- Gagnevin et al. (2003) described explosive volcanic prod- dynamic position of the Kerguelen Plateau and related geological ucts in the Rallier du Baty peninsula dated at 26,000 years features (after Kent et al., 2002). BP. The recent discovery of pumice layers covering glacial deposits allows the estimation of the last volcanic activity of I. The Northern Part of the Kerguelen Plateau (NKP) the Kerguelen Archipelago to be historical. extends from 46 to 53°S. The altitudes of the Kerguelen II. The Central Part of the Kerguelen Plateau (CKP) is Islands, which are the apex of this part of the Plateau, are a basin located between Kerguelen and Heard-McDonald mostly lower than 1000 m and the highest summit reaches Islands (~50-55°S). Sediment ages are Cretaceous (Schlich, 1852 m above sea level (Mount Ross) (Fig. 2). 1982; Quilty et al., 1983; Munschy and Schlich, 1987), but On the northernmost edge of the NKP, at site 1140 (LEG younger than the southern part of the Kerguelen Plateau with ODP 183), 40Ar/39Ar radiometric dating on drilled sub- Santonian age (~85 Ma) for cherts and calcareous oozes marine basaltic rocks yielded ages of 34-35 Ma (Duncan, (Frölich et al., 1983; Frölich and Wicquart, 1989). This age 2002), these ages can be considered as the youngest mag- is similar to those of the 83-88 Ma old lavas from Broken matic activity for the northern part of the Plateau. The oldest Ridge (Duncan and Richards, 1991), an oceanic EW-trend- 40Ar/39Ar ages are given by basalts from Skiff Bank, located ing plateau located east of the Kerguelen Plateau, although west of the Kerguelen Islands, which are 68-69 Ma (Duncan, more recent 40Ar/39Ar dating on Broken Ridge basalts (Dun- 2002). Therefore, the timing of formation of the NKP can be can, 2002) yielded an older age of 95 Ma at site 1141/1142. constrained between 34 and 69 Ma. On the southern part of the CKP, at site 1138 (Leg ODP 183), Radiometric ages for the exposed Kerguelen Islands 40Ar/39Ar ages on drilled basalts underlying the sediments basalts are younger and range from 30 to 0.1 Ma (Nico- yielded an age estimate of ~100-101 Ma (Duncan, 2002). laysen et al., 2000). These volcanic rocks are related to the Seismic data reveal that this part of the Kerguelen-Gauss- youngest magmatic activity of the Kerguelen plume, which berg ridge has a 14-23 km thick oceanic crust, thicker than 30 The Kerguelen Plateau: marine ecosystem and fisheries COTTIN ET AL. Geology of the Kerguelen Plateau Figure 3. - Gravity map of the different parts of the Kerguelen Plateau and asso- ciated magmatism with the Kerguelen plume. The Kerguelen Islands belongs to the northern part of the plateau and Heard Island to the central part of the plateau. Map from Weis et al. (2002), after Coffin et al. (1999, 2002). The locations of the ODP legs (sites 738, 744, 745, 746, 747, 749, 1137, 1138, 1140) are also shown. that of the Northern Kerguelen Plateau (Recq and Charvis, by 40Ar loss, resulting in an apparent older age. A reliable 1986; Charvis et al., 1993). conservative estimation on the timeframe of emplacement III. The Southern part of the Kerguelen Plateau (SKP) is of the southernmost SKP would thus be between 110 and located between 55 and 64°S. The crust beneath this part of 120 Ma (Coffinet al., 2002; Duncan, 2002). the Plateau is about 25 km thick (Goslin and Diament, 1987) Evidence from basalts and overlying sediments at sites and may contain old continental crust fragments (Ramsay et 738, 747, 748, 749 and 750 (Fig. 5), combined with the al., 1986; Alibert, 1991; Operto and Charvis, 1995, 1996). results of subsidence modelling (Coffinet al., 1999) shows Available ages from sediments and lavas from several ODP that much of the magmatic rocks of the SKP and CKP were legs show that the uppermost igneous crust in this region erupted in a sub-aerial environment. Multi-channel seis- formed during a relatively short interval at ~110-115 Ma mic reflection profiles indicate that the volcanic rocks at all (Leclaire et al., 1987; Schlich et al., 1989), although more sites except site 748 (Raggat Basin) were recovered from recent 40Ar/39Ar dating by Duncan (2002) gave an age of the uppermost igneous basement of the Plateau, which lies ~118-119 Ma.
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