3. Site 1135¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1135 is situated on the southern Kerguelen Plateau (SKP) (Fig. F1), approximately midway between Site 738 (Leg 119) and Site 750 (Leg 120). At Sites 738 and 750, the uppermost basement is tholeiitic basalt formed at ~110 Ma (⁴⁰Ar/³⁹Ar data from Pringle et al., 1994; Storey et al., 1996). Although basalts from these sites are broadly similar in major element composition, those from Site 738 have distinctive geochemical characteristics indicating that they contain a component derived from the continental lithosphere (Mahoney et al., 1995), perhaps amphibolite-facies Archean crust. Specifically, they have relative depletions in abundance of Nb (and Ta), unusually high initial ⁸⁷Sr/⁸⁶Sr (>0.7090), low initial ¹⁴³Nd/¹⁴⁴Nd (<0.5122), and high ²⁰⁸Pb/²⁰⁴Pb relative to ²⁰⁶Pb/²⁰⁴Pb (see Figs. F9, F10, F11A, and F12A, all in the "Leg 183 Summary" chapter). Two principal reasons for recovering basement from a SKP site in the vicinity of 59°S are to determine (1) if the uppermost basement of the SKP is uniformly ~110 Ma, and (2) the areal extent of the continental component, which is present in Site 738 basalt, but not in Site 750 basalt.

Lying on the northeastern flank of the SKP rift zone (Rotstein et al., 1992; Könnecke and Coffin, 1994; T.P. Gladczenko and M.F. Coffin, unpubl. data.), Site 1135 was one of two sites drilled on the SKP during Leg 183. It is ~110 km east of the axis of the SKP rift zone and 30 km west of the steep scarp that marks the boundary between the SKP and the Labuan Basin. We located Site 1135 on Australian Geological Survey Organisation (AGSO) Rig Seismic multichannel seismic (MCS) line RS180/201 (Fig. F2) ~6 km northwest of an intersection with Marion Dufresne MCS line MD47/10. The site lies at a depth of 1567 m on a coherent basement edifice; no major basement faults are evident proximal to the site. We chose this location as representative of the entire SKP south of the Raggatt Basin and east of the SKP rift zone on the basis of its relatively simple structural setting and seismic stratigraphic section. Interpreted igneous basement contains few internal reflections and is downthrown along normal faults to the south and east. We observe two seismic sequences overlying the basement: a lower sequence characterized by a velocity of 2.7 km/s and an upper sequence with a velocity of 1.9 km/s (Fig. F3). Employing these velocities, we estimate that the lower sequence is ~250 m thick (0.18 s TWT) and the upper sequence is ~350 m thick (0.37 s TWT).

The main objectives at this site were to

1. Characterize the petrography and compositions of the lavas, with particular focus on testing for the presence of a continental lithospheric component found in Site 738 basalts to the south but not identified in Site 750 basalts to the north (Fig. F1);
2. Determine the age of the lavas, testing the hypothesis that the uppermost igneous basement of the SKP is ~110 Ma, the age yielded by basalts from Sites 738, 749, and 750;
3. Determine the physical characteristics of the lava flows;
4. Identify the environment of eruption (subaerial or submarine);
5. Obtain minimum estimates for the age of basement from overlying sediment;
6. Estimate the duration of possible subaerial and shallow marine environments from the sedimentary and igneous record;
7. Determine the facies of the two seismic stratigraphic sequences;
8. Define the ages of seismic sequence boundaries; and
9. Determine the paleoceanographic history of this high latitude site.