During Leg 180 we drilled a transect of sites just ahead of the spreading tip: Sites 1109,1115, and 1118 on the down-flexed northern margin; Sites 1108 and 1110–1113 into the rift basin sediments above the low-angle normal fault zone; and Sites 1114, 1116, and 1117 on the footwall fault block, Moresby Seamount —Site 1114 near the crest, 1116 on the southern flank, and 1117 into the upper fault face (Figs 3, 4, 5, 6, 7, 8; Table 1).

The drill sites are located within a grid of multichannel seismic (MCS) lines and multibeam bathymetry (Figs. 7, 8). The northern margin sites (1109, 1115, and 1118), cored to 802, 803, and 927 meters below seafloor (mbsf), respectively, penetrated the synrift Pliocene–Pleistocene cover sequence and into pre-rift sections: dolerites at Sites 1109 and 1118, and Miocene forearc clastics at Site 1115. Penetration into greenschist metadiabase beneath the 286-m-thick Pliocene–Pleistocene section and 6-m-thick fault breccia at Site 1114 extended to 407 mbsf. Basement was not reached beneath the 159 m of coarse rift clastics at Site 1116. The planned triple casing reentry hole at Site 1108 was aborted because of hydrocarbon concerns after drilling Hole 1108B to 485 mbsf. The presence of higher molecular weight hydrocarbons at depth (Site 1108), and the extent of talus proximal to Moresby Seamount where the fault is shallow (Sites 1110–1113, which penetrated 25–174 mbsf), precluded use of the available technology to meet our primary objective. At Site 1117, however, the ~100-m-thick fault zone of gouge, mylonite, and breccia, was cored through (meta)gabbro. Sites 1109, 1114, 1115, and 1118 were all logged with triple combo geophysics and Formation MicroScanner (FMS) sonic tool strings, and vertical seismic profiles were obtained at Sites 1118 (complete), 1109 and 1115 (partial), making this one of the most successful ODP logging legs to date.

The primary results are presented first by site and then by theme. In the following sectionthe site summaries are arranged in geographic order, progressing from north to south along the transect.


To 180 Site Summary for Site 1115

To 180 Table of Contents