DEEP-STRUCTURE OF THE NORTHERN KERGUELEN PLATEAU AND HOTSPOT-RELATED ACTIVITY

Seismic refraction profiles were carried out in 1983 and 1987 througho ut the Kerguelen Isles (southern Indian Ocean, Terres Australes and An tarctiques Francaises, TAAF) and thereafter at sea on the Kerguelen-He ard Plateau during the MD66/KeOBS cruise in 1991. These profiles subst antiate the existence of oceanic-type crust beneath the Kerguelen-Hear d Plateau stretching from 46 degrees S to 55 degrees S, including the archipelago. Seismic velocities within both structures are in the rang e of those encountered in 'standard' oceanic crust. However, the Kergu elen Isles and the Kerguelen-Heard Plateau differ strikingly in their velocity-depth structure. Unlike the Kerguelen Isles, the thickening o f the crust below the Kerguelen-Heard Plateau is caused by a 17 km thi ck layer 3. Velocities of 7.4 km s(-1) or so within the transition to mantle zone below the Kerguelen Isles are ascribed to the lower crust intruded and/or underplated by upper mantle material. The crust-mantle boundary below the Kerguelen-Heard Plateau is abrupt and devoid of an y underplated material. The difference in structure between the northe rn edge of the Kerguelen Plateau (including the archipelago) and the K erguelen-Heard Plateau may be related to variability of the time-depen dent hotspot activity. The Kerguelen-Heard Plateau was emplaced during the Cretaceous time (110 Ma) when the volcanic output rate of the Ker guelen Plateau and the Ninetyeast Ridge was high (as well as high pote ntial temperature). The northernmost Kerguelen Plateau and the archipe lago were emplaced during Tertiary time (40-45 Ma), as the volcanic ou tput rate reduced. Furthermore, intraplate volcanism continued in the Kerguelen archipelago for at least 40 Ma. The isostatic compensation o f the Kerguelen Isles and the Kerguelen-Heard Plateau is achieved by l ow-density mantle material, as shown by refraction and geoid studies. The velocity-depth structure below the Kerguelen Isles is similar to t hat found below intraplate oceanic islands such as Hawaii. Despite the differences in age, the crust below Iceland (0 Ma) and the Kerguelen Plateau (100-120 Ma) are strikingly akin. The similarity between the K erguelen-Heard Plateau and Iceland, then, strongly supports a similar origin for both structures, the Kerguelen-Heard Plateau being a fossil equivalent of present-day Iceland. Crustal thickening beneath the Ker guelen-Heard Plateau, which may result from an Iceland-type setting (i .e. an active spreading centre over a hotspot), is mostly produced by thickening of layer 3, layer 2 representing 25 per cent only of the th ickness of the igneous crust. The Kerguelen Isles, despite the initial volcanism near the active Southeast Indian Ridge, behave as a midplat e volcanic island and are definitely not representative of the whole K erguelen Plateau structure.