4. Site 1136¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

Site 1136 was one of two sites drilled on the southern Kerguelen Plateau (SKP) during Leg 183 (Fig. F1). The collapse of Hole 1135A (see "Operations" in the "Site 1135" chapter) before reaching igneous basement forced us to consider alternatives for sampling basaltic crust of the SKP. Because the sediment formation responsible for making us abandon Site 1135 appeared to be of similar thickness and reflection character at the JOIDES Pollution Prevention and Safety Panel-approved alternate site, we used multichannel seismic (MCS) data provided by the Ocean Drilling Program (ODP) site-survey data bank to select a new alternate site. Site 1136 is on the SKP approximately midway between two previous sites where basaltic basement was recovered, Site 738 (Leg 119) and Site 750 (Leg 120). Site 1136 is 8 km west of the fault scarp marking the eastern boundary of the SKP with the Labuan Basin (Fig. F1). To maximize our chances of sampling igneous basement, we located Site 1136 on Marion Dufresne MCS line MD47/10 (Fig. F2), where the water depth is 1931 m and sediment thickness is only 133 m. Here, ~32 km from Site 1135, the chert-rich formation that created hole instability at Site 1135 appeared to be either thin or absent on the MCS data. The sedimentary section at Site 1136 is characterized by moderate- to high-amplitude continuous reflections, and the reflection from the top of basaltic basement is strong and continuous and has an apparent 1° dip to the west (Fig. F3). West of the site, a wedge of intrabasement reflections also has an apparent westward dip. The wedge is disrupted by normal faults and terminates against an interpreted normal fault 12 km west of Site 1136. To the east of the site, one major fault and several minor faults offset igneous basement >3000 m downward into the Labuan Basin (Rotstein et al., 1991). Although most motion on these faults delineating the northeastern boundary of the SKP probably occurred in Cretaceous time, earthquake activity along the scarp continues today (Okal, 1981; Adams and Zhang, 1984).

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 basalt to the south, but not identified in Site 750 basalts to the north (Fig. F1) (see "Background and Objectives" in the "Site 1135" chapter);
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 the 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;
9. Determine the paleoceanographic history of this high-latitude site.