Small-scale crustal variability within an intraplate structure: the Crozet Bank (southern Indian Ocean)

Geophys. J. Int. (1998) 134, 145–156 Small-scale crustal variability within an intraplate structure: the Crozet Bank (southern Indian Ocean) Maurice Recq,1 Jean Goslin,1 Philippe Charvis2 and Stéphane Operto2 1UMR 6538 du CNRS ‘Domaines océaniques’, Université de Bretagne Occidentale, BP 809, 6, Avenue L e Gorgeu, F-29285 Brest cedex, France. E-mail: ; 2 ORST OM (UR14), UMR 6526 du CNRS ‘Géosciences Azur’, Observatoire Océanologique de V illef ranche-sur-mer, BP 48, F-06235 V illefranche-Sur-Mer France. E-mail: [email protected] r; [email protected] r Accepted 1998 January 8. Received 1997 December 4; in original form 1997 July 3 Key words: aseismic ridge, Crozet Islands platform, crustal structure, gravity anomalies, hotspots, seismic refraction. T HE S U BM A R IN E C R O ZE T P LAT EA U A N D T HE CR O Z ET B AN K The submarine Crozet Plateau (southern Indian Ocean) is a composite E–W-trending, shallow bathymetric feature, which extends roughly between 43°E and 53°E and is 300 km across (Fig. 1). To the west this feature abuts the Southwest Indian Ridge. The Madagascar Ridge is the symmetrical structure to the Crozet Plateau with respect to the Southwest Indian Ridge. Both features overlap in pre-anomaly 24 reconstructions © 1998 RAS (Goslin 1981; Goslin & Patriat 1984). The western region of the submarine Crozet Plateau, the Del Cano Rise, has a rather smooth completely submerged topography, and culminates on average at a depth of roughly 1500 m below sea level. The Crozet Bank (Fig. 2) sits at the eastern end of the Crozet Plateau on oceanic crust emplaced sometime before anomaly 31 at the axis of the Southeast Indian Ridge (McKenzie & Sclater 1971). The Crozet Bank is on average shallower than the Del Cano Rise and is steep-sided and capped by two separate groups of volcanic islands. Between 145 SU MM A RY The Crozet Bank, the easternmost region of the Crozet Plateau (austral Indian Ocean), is capped by two groups of islands which form the Crozet Archipelago (Terres Australes and Antarctiques Françaises). A N–S-trending 2 km deep basin, the Indivat Basin, extends between the two groups of islands and bisects the Crozet Bank into two separate domains. The interpretation of the KeOBS8 seismic refraction profile shot during the KeOBS/MD66 cruise (January–February 1991) over the eastern Crozet Plateau was carried out by ray tracing and the computation of synthetic seismograms. This interpretation leads to a rather complex seismic structure and outlines a pronounced eastward crustal thinning from 16.5 to 10 km at the expense of layer 2. The thinning of the crust is abrupt east of the Indivat Basin. Unlike below the Hawaiian Islands and the Marquesas Islands, no underplated mantle material underlies the crust of the Crozet Bank. Moreover, this deep seismic sounding has further revealed that a high-velocity feature outcropping the seabed 30 km east of Ile aux Cochons could be a major structural feature, deeply rooted within the lower crust. The occurrence of this feature (a submarine volcano?) is associated with a mantle dyke causing a steep shallowing of the lower crustal interfaces. Gravity modelling was performed along line KeOBS8, with a density structure deduced from the seismic section, to model free-air anomalies derived from altimetry data. This modelling confirms that the Indivat Basin, underlined by a gravity low running roughly N–S between the two groups of islands, is a major structural boundary. As the model generates medium-wavelength anomalies of adequate amplitude, it also confirms that the volcano, located west of the Indivat Basin, is a deeply rooted feature. The Crozet Bank clearly appears as a plume-affected structure, which may have originated from a deep thermal anomaly within the lithosphere. More recent volcanic episodes, related to a still active plume activity under the Crozet Bank, could have uplifted upper-mantle material and caused the emplacement of the newly discovered feature and of the western group of islands. 146 M. Recq et al. Figure 2. Simplified bathymetric map of the Crozet Plateau contoured from ETOPO5 data. The 250 m contours are shown as plain lines. The contours are labelled every 500 m. The locations of OBSs are indicated by white stars. The names of the islands have been abbreviated, from east to west: ‘Est’: Ile de l’Est; ‘Pos’: Ile de la Possession; ‘Pin’: Ile des Pingouins (Penguin Island); ‘Coc’: Ile aux Cochons (Hog Island); ‘Apo’: Iles des Apôtres (Apostles’ Rocks). English island names are those quoted in the ‘Challenger Reports’. the two groups of islands is the Indivat Basin, which reaches depths of 2000 m. The western group of islands comprises the small stratovolcano of Ile aux Cochons, culminating at 770 m above sea level, and the reefs of Iles des Pingouins and Iles des Apôtres. The eastern group, 120 km to the east, comprises Ile de la Possession (19 km long, 15 km across) and the easternmost arid Ile de l’Est (19 km long, 10 km across). Ile de la Possession, the largest island of the archipelago, rises over 1000 m above sea level, and Ile de l’Est culminates at 900 m. There is no sedimentary material on the islands except some detrital Quaternary deposits (Bellair 1964). Ile de la Possession and Ile de l’Est are predominantly composed of alkaline basaltic rocks with fragmental deposits and lava flows (Philippi 1912; Reinisch 1912; Lacroix 1923, 1940; Tyrrell 1937; Dreux & Rémy 1963; Girod & Nougier 1971; Bellair 1964; Gunn et al. 1972; Chevallier 1981). The earliest volcanism occurred 8 Ma ago on Ile de la Possession, while Iles aux Cochons, Ile des Pingouins and Ile des Apôtres were formed by more recent volcanism, only 1 Ma ago (Chevallier & Nougier 1981; © 1998 RAS, GJI 134, 145–156 Figure 1. General topography of the Southwest Indian Ocean contoured from the ETOPO5 gridded data set. The Crozet Bank is outlined by a rectangular white frame, which delineates the bounds of Figs 2 and 7. T he Crozet Bank, an intraplate structure © 1998 RAS, GJI 134, 145–156 & Sclater 1977) to higher values, 96 mW m−2 and even 165 mW m−2 at a less reliable station on the island platform. This suggests a thermal origin for the uplift of the Crozet Bank, a theory substantiated by the high geoid anomaly, +9 m (Courtney & Recq 1986). Neither a mere crustal thickening with Airy local compensation, nor an increase of the plate’s elastic thickness (Goslin & Diament 1987) can account for the observed geoid/topography ratio (G/T). Indeed, the geoid anomaly, 1.7 m, and the G/T, 0.8 m km−1 computed for an Airy-type isostatic response do not match the values of 9 m and 4.9 m km−1, respectively, measured over the Crozet Bank. The ratio of 4.9 m km−1 is similar to that seen over other oceanic hotspot swells (Courtney & White 1986; Detrick et al. 1986). Courtney & Recq (1986) postulated that a convective plume would cause dynamic uplift of the Crozet Bank and would cause the low density of the mantle material (...truncated)