4. Site 11841

Shipboard Scientific Party2

BACKGROUND AND OBJECTIVES

Site 1184 lies at a water depth of 1661.5 m on the unnamed northern ridge of the eastern lobe or salient of the Ontong Java Plateau (Fig. F1). The eastern lobe had not been drilled before Leg 192. As with the dome of the high plateau (see "Background and Objectives" in the "Site 1183" chapter), we thought that this site near the summit of the ridge might be in an area that originally was at relatively shallow water depths. The site is 570 km from central Malaita, 807 km from Deep Sea Drilling Project (DSDP) Site 289, 907 km from Ocean Drilling Program (ODP) Site 1183, 918 km from ODP Site 803, and 1274 km from ODP Site 807.

The relationship of the eastern lobe to the high plateau is unknown. It could be contemporaneous with the high plateau or be the trace of the postulated plume tail following the emplacement of the high plateau and, specifically, may be the main locus of 90-Ma eruptions (Tejada et al., 1996). Also, the eastern lobe appears to have been rifted into northern and southern portions that were separated by nearly 300 km of seafloor spreading in the Stewart Basin (Kroenke and Mahoney, 1996). The southern portion, known as Stewart Arch, is the proposed conjugate feature to the northern ridge. Lavas associated with this poorly understood rifting event may have been preserved along both the northern and southern rift-facing sides of the salient. Furthermore, this part of the plateau passed over the estimated position of the Samoan hotspot ~35-40 Ma (Yan and Kroenke, 1993), and volcanic evidence of this passage might be present.

Site 1184 is on the faulted crest of the northern ridge, which trends northwest-southeast, as does a corresponding free-air gravity high (Figs. F1, F2). The ridge's southwestern flank is cut by numerous normal faults that are likely to be related to the opening of the Stewart Basin. The ends of four northeast-southwest trending seamount chains intersect the ridge, one in the vicinity of Site 1184. Several distinct, semicircular bathymetric and free-air gravity highs, possibly related to these chains, are evident along the ridge; three lie within 35 km of Site 1184, and the closest is 15 km west of the site (Fig. F3).

Geophysical Background

Site 1184 is located at a water depth of ~1662 m (drill pipe measurement) on multichannel seismic (MCS)-reflection Line 101 of Leg 2, cruise KH98-1 of the Hakuho Maru, ~2.5 km southwest of the intersection with Line 102 (Figs. F2, F3). The site is ~2.5 km north-northeast of the bounding fault of a 15-km-wide graben (Figs. F4, F5). As seen on the Line 101 profile, this graben contains >1.0 s two-way traveltime (TWT) of sediment, which, assuming an average P-wave velocity of 2000 m/s for the entire sediment package, corresponds to a thickness of 1000 m. The upper surface of the fault block on which Site 1184 is located has an apparent dip of 4° in the direction of 32° (using a sediment velocity of 1650 m/s for the portion of the sediment package cored; see "Physical Properties"). On the intersecting Line 102 profile, the upper surface of the block has an apparent dip of 1° in the direction of 302° (Fig. F6). We calculate the true dip of this surface to be 4° in the direction of 18°. Approximately 10 km southeast of Site 1184, a major fault offsets the seafloor downward to the southeast by ~85 m (Fig. F7). The nature and orientation of the fault cannot be determined from the single existing MCS-reflection line.

A sedimentary megasequence onlaps the medium-amplitude, medium-frequency continuous reflection that marks the upper surface of the fault block at this site (Figs. F8, F9). Onlap is evident in both of the intersecting MCS-reflection profiles (Figs. F4, F6, F7, F8, F9) and suggests deposition of the megasequence by dominantly downslope processes rather than by pelagic sedimentation. A plausible source for the onlapping sediments is one or both of the bathymetric highs to the west and north-northwest. At Site 1184, the thickness of the sedimentary megasequence is 0.225 s TWT, thinning to the southwest and thickening to the northeast. Parallel to subparallel reflections are of low to medium amplitude, medium to high frequency, and low to medium continuity. In places, the reflection character is chaotic.

Reflection character within the fault block in the vicinity of Site 1184 differs from that of basaltic basement on the main plateau. Within the block, basement reflection character varies considerably, but some parallel-to-subparallel, high-frequency reflections of limited and variable continuity persist to depths as great as 1.0 s TWT (1625 m at a velocity of 3250 m/s) beneath the surface of the block. Coherent reflections within the fault block (Fig. F4) have an apparent dip of 8° in the direction of 32° (assuming a velocity of 3250 m/s). On the intersecting profile, coherent reflections within the fault block have an apparent dip of 5° in the direction of 302° (Fig. F6). We calculate the true dip of the reflections to be 9° in the direction of 0°.

Summary of Objectives

The main objectives at this site were to determine

  1. Compositions of rocks within the fault block, for comparison with those of lavas exposed in Malaita, Santa Isabel, and San Cristobal, and drilled at DSDP Site 289, ODP Sites 803 and 807, and the other Leg 192 sites;
  2. Ages of rocks within the fault block and of sedimentary rocks overlying it to help test the hypothesis that the eastern lobe was an important site of 90-Ma magmatism; alternatively, to ascertain whether this part of the plateau was affected by the later volcanic events recorded in Malaita (Tejada et al., 1996; Neal et al., 1997) and San Cristobal (Birkhold-VanDyke et al., 1996);
  3. Physical volcanology of rocks in the fault block and the nature of possible sedimentary interbeds, in order to deduce the eruptive environment (type of eruption and approximate water depths);
  4. Early subsidence history, as recorded in the rocks of the fault block and the overlying sedimentary succession; and
  5. Ages of sequence boundaries observed in the seismic record.

1Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
2Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 192IR-104

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