STUDY AREA
Physical Description
The Kerguelen Plateau is a broad topographic high in the southern Indian Ocean surrounded by deep ocean basins: to the northeast lies the Australian-Antarctic Basin; to the south, the 3500-m-deep Princess Elizabeth Trough; to the southwest, the Enderby Basin; and to the northwest, the Crozet Basin (Fig. 2). The plateau stretches ~2300 km between 46°S and 64°S in a southeast-trending direction toward the Antarctic continental margin. It is between 200 and 600 km wide and stands 2-4 km above the adjacent ocean basins. The age of the oceanic crust abutting the Kerguelen Plateau is variable (Fig. 2). As summarized by Schlich and Wise (1992), the oldest magnetic anomalies range from Anomaly 11 (~32 Ma) in the northeast to Anomaly 18 (~43 Ma) off the central part of the eastern plateau. Farther south, the east flank of the Southern Kerguelen Plateau is bounded by the Labuan Basin. Basement of the Labuan Basin has not been sampled, but its age and structure appear to be similar to the main Kerguelen Plateau (Rotstein et al., 1991; Munschy et al., 1992). To the northwest, magnetic anomaly sequences from 23 to 34 have been identified in the Crozet Basin, but on the southwest flank no convincing anomalies have been identified in the Enderby Basin, although Mesozoic anomalies have been suggested (Li, 1988; Nogi et al., 1996). An Early Cretaceous age for the Enderby Basin is assumed in most plate reconstructions (e.g., Royer and Coffin, 1992).
Beginning with early studies (Schlich, 1975; Houtz et al., 1977), the Kerguelen Plateau province has been divided into distinct domains that currently total five: northern, central, and southern portions; Elan Bank; and the Labuan Basin (Figs. 2, 4; Coffin et al., 1986; in prep.). The Northern Kerguelen Plateau (NKP), ~46°S to 50°S, has shallow water depths, <1000 m, and basement elevations 3000-4000 m above adjacent seafloor with maximum elevations forming the Kerguelen Archipelago. A lack of any rocks older than ~40 Ma from the Kerguelen Archipelago and the submarine NKP, as well as plate reconstructions (Royer and Sandwell, 1989; Royer and Coffin, 1992) suggest that the NKP is =40 Ma in age, whereas the remainder of the Kerguelen Plateau province appears to be of Cretaceous age. The Central Kerguelen Plateau (CKP), ~50°S to 55°S, is also relatively shallow, contains a major sedimentary basin (Kerguelen-Heard basin), and includes Heard and McDonald Islands, which are dominated by active volcanoes (P. Quilty, pers. comm., 1998). Broken Ridge and the CKP are conjugate Late Cretaceous (see below) provinces (Fig. 1) that were separated by seafloor spreading along the Southeast Indian Ridge (SEIR) during Eocene time (Mutter and Cande, 1983).
The Southern Kerguelen Plateau (SKP) is generally characterized by deeper water, 1500 to 2500 m, and is tectonically more complex (Figs. 2, 4). Of Early Cretaceous age (see below), it is characterized by several large basement uplifts and has experienced multiple stages of normal faulting, graben formation, and strike-slip faulting (e.g., Coffin et al., 1986; Fritsch et al., 1992; Rotstein et al., 1992; Royer and Coffin, 1992; Angoulvant-Coulon and Schlich, 1994; Könnecke and Coffin, 1994; Gladczenko et al., 1997). Elan Bank, a salient extending westward from the boundary between the CKP and SKP, is characterized by water depths of <1000-2000 m. Basement has not been sampled from Elan Bank, and consequently, its age is unknown. Labuan Basin, east of and adjacent to the CKP and SKP, is an extensively faulted, thickly sedimented (>2 s two-way traveltime [TWT] in places) deep ocean basin. Its crust has not been sampled by drilling, but dredging has recovered metamorphic and granitic rock (Montigny et al., 1993). The basin's crustal origin and age are uncertain.
As noted above, Broken Ridge and the Kerguelen Plateau formed as one entity in Cretaceous time and began separating at ~40 Ma. Broken Ridge currently lies ~1800 km north of Kerguelen and forms a narrow and elongated oceanic plateau (100-200 km by ~1000 km at ~2000 m water depth) that trends west-northwest (Fig. 3). It is markedly asymmetric in cross-section, dipping gently (<2°) toward the north but with a steeply dipping (>10°) southern face (Fig. 3). This southern flank was uplifted, perhaps >2 km, by flexural rebound following rift-related Early Tertiary extension (Weissel and Karner, 1989; Peirce et al., 1989).
Crustal Structure
On the basis of drilling results from ODP Legs 119 and 120 (Barron, Larsen, et al., 1989; Schlich, Wise, et al., 1989), together with multichannel seismic reflection data (Coffin et al., 1990; Schaming and Rotstein, 1990; Schlich et al., 1993), the crust of the Kerguelen Plateau and conjugate Broken Ridge is believed to be overwhelmingly basaltic. Numerous dipping basement reflections that are interpreted as flood basalts have been identified in the crust of the CKP and SKP and on Elan Bank (Könnecke et al., 1997). Wide-angle seismic data from the Kerguelen Archipelago, on the NKP, show an upper igneous crust ~10 km thick and a lower crust 7-9 km thick (Recq et al., 1990, 1994; Charvis et al., 1995). Ocean-bottom seismometer wide-angle reflection and refraction experiments have been undertaken recently on both the CKP and SKP (Charvis et al., 1993, 1995; Operto and Charvis, 1995, 1996; Könnecke et al., in press; Charvis and Operto, in press). Crustal structure beneath the Kerguelen Archipelago differs significantly from that of the CKP. Igneous crust of the CKP is 19-21 km thick and is composed of three layers. The upper layer is 1.2- to 2.3-km thick, and velocities range from 3.8 to 4.9 km/s. It could be composed of either lava flows or interlayered volcanic and sedimentary beds. The second layer is 2.3 to 3.3 km thick, with velocities ranging from 4.7 to 6.7 km downward. In the lower crust, velocities increase from 6.6 km/s at ~8.0 km depth (near the top of the layer) to 7.4 km/s at the base of the crust, with no internal discontinuity. On the southern plateau, igneous crust can be divided into three layers: (1) an upper crustal layer ~5.3 km thick with velocities ranging from 3.8 to 6.5 km/s; (2) a lower crustal layer ~11 km thick with velocities of 6.6 to 6.9 km/s; and (3) a 4 to 6-km thick transition zone at the base of the crust characterized by an average velocity of 6.7 km/s (Operto and Charvis, 1995, 1996). This low-velocity, seismically reflective transition zone at the crust-mantle interface has not been imaged on the NKP or CKP, and it is the basis for the hypothesis that parts of the SKP are fragments of a volcanic passive margin, similar to the Rockall Plateau in the North Atlantic Volcanic Province (Schlich et al., 1993; Operto and Charvis, 1995, 1996).
Previous Sampling of Igneous Basement
What has been learned from previous sampling (ODP Legs 119 and 120 and dredging) of the Kerguelen Plateau/Broken Ridge LIP? Based in large part on ODP-related studies, there is a consensus that decompression melting of the Kerguelen plume was a major magma source for the Kerguelen Plateau/Broken Ridge system. Although sampling and age dating for the entire LIP are grossly insufficient, sampling of the southern Kerguelen Plateau at four spatially diverse locations (ODP Sites 738, 749, and 750 and dredge site MD 48-05 [Fig. 4]) shows that the uppermost parts of the SKP formed over a relatively short interval at ~110 Ma (Leclaire et al., 1987; Whitechurch et al., 1992). This is corroborated by recent 40Ar/39Ar studies (Pringle et al., 1994; Pringle et al., 1997; Coffin et al., in prep.), which report ages ranging from 108.6 Ma to 112.7 Ma for basement basalts from Sites 738, 749, and 750. Therefore, south of 57°S the uppermost Kerguelen Plateau formed at ~110 Ma. In contrast, basalts from Site 747 on the Central Kerguelen Plateau are much younger, ~85 Ma. This age is similar to the 83-88 Ma age for lavas from Broken Ridge dredge sites 8 and 10 (Fig. 3; Duncan, 1991), which is consistent with the pre-rifting position of Site 747 relative to these Broken Ridge dredge sites. Also, piston coring of sediments on the northeast flank of the plateau between the Kerguelen Archipelago and Heard Island (Fig. 4) recovered Upper Cretaceous cherts and calcareous oozes of probable Santonian age (Fröhlich and Wicquart, 1989). In summary, we have very few high-quality age data for the 2.3 x 106 km2 (equivalent to approximately eight Icelandic plateaus) of the Kerguelen Plateau/Broken Ridge LIP. Nevertheless, the available data show that large magma volumes erupted over short time intervals, possibly as two or even three pulses (Coffin et al., in prep.): the SKP at ~110 Ma; the CKP, Broken Ridge, and Elan Bank at ~85 Ma; and much of the Kerguelen Archipelago and perhaps the northernmost portion of the Kerguelen Plateau at ~40-23 Ma (Nicolaysen et al., 1996). Sampling by drilling at other sites throughout the plateau is required to determine if formation of this LIP was truly episodic or if there was a continuous south to north decrease in age of volcanism.
Although the Kerguelen Plateau is a volcanic construction formed in a young oceanic basin (Royer and Coffin, 1992; Munschy et al., 1994; Coffin et al., in prep.), evidence is equivocal as to whether it was emplaced at a spreading center (e.g., Iceland) or off-ridge (e.g., Hawaii) (Coffin and Gahagan, 1995). In contrast, there is unambiguous evidence that much of the uppermost basement of the southern and central Kerguelen Plateau was emplaced in a subaerial environment. The evidence includes: (1) oxidized flow tops and the vesicularity of lava flows at ODP Sites 738 and 747, (2) nonmarine, organic-rich sediments (containing up to 5-cm pieces of charcoal) overlying the basement at Site 750, and (3) claystone topped by a basalt cobble conglomerate and glauconitic sediment with wood fragments in the lowermost core at Site 748 (Schlich et al., 1989). Coffin (1992) concluded that the drill sites in the SKP had long (>10 to =50 m.y.) histories of subaerial volcanism and erosion followed by subsidence caused by cooling. Higher temperature metamorphism, based on zeolite mineralogy, of the basaltic basement at Site 749 compared to Sites 747 and 750 may indicate erosion to deeper levels at Site 749 (Sevigny et al., 1992).
The islands on the NKP are dominantly formed of <40 Ma transitional and alkaline lavas (e.g., Barling et al., 1994; Yang et al., 1998). In contrast, dredging along the 77°E graben of the Kerguelen Plateau and from Broken Ridge (Figs. 3 and 4) recovered basaltic rocks whose compositions are tholeiitic; however, their incompatible element abundances are similar to those of ocean island tholeiitic basalts rather than typical mid-ocean-ridge basalt (MORB) (e.g., Davies et al., 1989; Weis et al., 1989; Mahoney et al., 1995). Tholeiitic basalts also form the igneous basement of the Kerguelen Plateau at drill Sites 738, 747, 749, and 750 (Fig. 5A). Except for an alkaline basalt flow 200 m above basement at Site 748, all samples derived from the Kerguelen plume (i.e., the Kerguelen Plateau and Ninetyeast Ridge) over the interval from ~110 to 38 Ma are tholeiitic basalt (e.g., Frey et al., 1991; Saunders et al., 1991; Storey et al., 1992; Frey and Weis, 1995). The significance of this result is that tholeiitic compositions are derived by relatively high extents of melting (e.g., Kent and McKenzie, 1994), which suggests that the Kerguelen plume was a high-flux magma source for a long time. However, the MgO-rich melts (picrites) expected from unusually large extents of melting of high-temperature plumes (Storey et al., 1991) have not been recovered. In fact, in contrast to tholeiitic lavas forming the Hawaiian shields, there is no evidence in lavas associated with the Kerguelen plume for melt segregation at relatively high pressures within the garnet stability field (Frey et al., 1991).
Most lavas from the Kerguelen Plateau and Broken Ridge have Sr and Nd isotopic ratios that range from those typical of enriched MORB from the SEIR to those proposed for the Kerguelen plume (Fig. 5B). In Pb-Pb isotopic ratios, most Kerguelen Plateau lavas define an elongated field that is subparallel to that for SEIR MORB. However, like lavas forming the Kerguelen Archipelago, the Kerguelen Plateau lavas are offset from the MORB field to higher 208Pb/204Pb at a given 206Pb/204Pb ratio (Fig. 5C). These Sr, Nd, and Pb isotopic data have been interpreted as resulting from mixing of the Kerguelen plume with entrained depleted asthenosphere (e.g., Weis et al., 1992). In contrast, basalts from ODP Site 738 on the southernmost SKP and dredge site 8 from eastern Broken Ridge (Fig. 1) have atypical geochemical characteristics for oceanic lavas. These lavas have very high 87Sr/86Sr, low 143Nd/144Nd, and high 207Pb/204Pb ratios, which accompany relatively low 206Pb/204Pb compositions (Fig. 5B, C). Although sampling is sparse, Mahoney et al. (1995) have shown that lavas from plateau locations closest to continental margins (e.g., Site 738 in the far south, dredge site 8 from eastern Broken Ridge, and lavas from the Naturaliste Plateau [Fig. 1]) have the most extreme isotopic characteristics (e.g., 87Sr/86Sr >0.7090), which are accompanied by relative depletions in Nb and Ta and relatively high 207Pb/204Pb ratios. They conclude that these geochemical features arose from a continental lithosphere component (e.g., Storey et al., 1989) that contributed to magmatism near the edges of the Kerguelen Plateau/Broken Ridge system. The geochemical evidence for this continental component is consistent with geophysical evidence suggesting the SKP contains a passive margin fragment (Schlich et al., 1993; Operto and Charvis, 1995, 1996).