LEG 180 DRILLING RESULTS

An extensive description of Leg 180 drilling results can be found in the Leg 180 Initial Reports volume (Taylor, Huchon, Klaus, et al., 1999) and in this volume. Eleven sites were drilled on both the footwall and hanging wall of the active low-angle normal fault (Fig. F2). Rocks recovered from the northern hanging wall sites revealed the presence of a complete mid-Pliocene to Holocene rift-basin sedimentary sequence overlying a unit of basalt and diabase rocks at Sites 1109 and 1118 and unconformably overlying an inferred Miocene forearc basin conglomerate at Site 1115 (Robertson et al., in press). Drilling of the southern footwall recovered lesser amounts of rift subsidence-related sediments overlying diabase rocks at Site 1114 and diabase grading to gabbro at Site 1117. The first few meters of coring at Site 1117 also recovered fault gouge interpreted to be related to recent normal faulting (Taylor, Huchon, Klaus, et al., 1999).

Most igneous rocks recovered from the footwall (Sites 1114 and 1117) and at the base of the hanging wall rift-basin strata (Sites 1109 and 1118) were cored as a mafic sequence consisting of a thin layer of basaltic clasts overlying several meters of diabase, which increased in grain size to gabbro at deeper levels (Site 1117). The rocks were variably altered and brecciated, with greenschist-grade metamorphism, hydrothermal alteration, brecciation, and deformation decreasing with depth away from the fault surface in the footwall sites. These rocks were cored from the footwall (i.e., Sites 1114 and 1117 on the Moresby Seamount) directly beneath the fault and from within a conglomerate below middle- to late-Pliocene rift-basin strata in the hanging wall (Site 1118) and as far north as the southern Trobriand Platform (Site 1109). Diabase clasts from Sites 1109 and 1118 above the low-angle normal fault are rounded and have "onion skin" oxidation rinds consistent with subaerial fluvial deposition (Robertson et al., in press). Formation MicroScanner (FMS) log imaging and rounded diabase clasts recovered from the hanging wall at Site 1118 indicate the presence of a diabase boulder conglomerate with clast sizes ranging from 5 cm to ~2-3 m (Taylor, Huchon, Klaus, et al., 1999).

Igneous and metamorphic rocks from Site 1114 were cored at the top of the Moresby Seamount below 286 m of Pliocene-Pleistocene claystone, siltstone, and sandstone and consist of diabase and lower greenschist facies metadiabase. Sixty-seven meters of rock was recovered; the majority was brecciated and variably hydrothermally altered and metamorphosed. Hydrothermal alteration is inferred by the presence of secondary quartz and epidote veins, both containing disseminated pyrite. Original diabase mineralogy includes equal amounts of plagioclase and clinopyroxene with accessory magnetite. Secondary phases include quartz, calcite, epidote, and chlorite. Rocks at Site 1114 are the most altered and brecciated among the sites containing igneous and metamorphic rocks.

Quartz gabbro was cored from greater structural depth within the Moresby Seamount (relative to Site 1114) and directly beneath the low-angle normal fault zone at Site 1117. The first 1.5 m recovered from Hole 1117A consisted of light green fault-gouge material containing talc, chlorite, calcite, ankerite, and three forms of serpentine: chrysotile, lizardite, and antigorite. Quartz gabbro cored below the fault zone is the most likely protolith for the gouge material, as talc, chlorite, and serpentine are common alteration products of mafic and ultramafic rocks. Mylonitic epidote and chlorite-rich rocks were cored directly beneath the gouge, giving way to diabase breccia and undeformed quartz gabbro with depth. Bands of sheared epidote- and chlorite-rich rocks exist below the fault-gouge material. They are interpreted to be related to the fault zone directly above, but the timing and depth at which shearing occurred is unknown. Undeformed gabbro at Site 1117 has a similar modal mineralogy to diabase from Site 1114 and has patchy hydrothermal alteration with secondary mineralization similar to that of diabase from Site 1114.

Basalt and diabase were cored at Sites 1109 and 1118 at 740 and 860 mbsf, respectively, below a mid-Pliocene to Holocene sequence of sediments and sedimentary rocks interpreted to have formed during rifting and subsidence (Taylor, Huchon, Klaus, et al., 1999). The uppermost part of the mafic unit is a calcite-cemented basaltic conglomerate containing 10-cm-diameter clasts, grading with depth to a diabase conglomerate. Although the mafic rocks recovered comprise a conglomerate unit in the uppermost parts of each site, it is not certain that the lowermost parts of the units are in place or part of the same conglomerate unit. Large intact cores recovered deeper within the units represent either large boulders within a conglomerate or in situ mafic rocks. However, FMS imaging strongly suggests that the lowermost portion of the mafic rocks cored at Site 1118 are indeed part of a large boulder conglomerate (Taylor, Huchon, Klaus, et al., 1999). Diabase from Sites 1109 and 1118 has modal mineralogy similar to samples from Sites 1114 and 1117, but the intensity of hydrothermal alteration varies from site to site. Site 1109 diabase is the least altered of the four sites and contains equal amounts of mostly pristine plagioclase and clinopyroxene with both ophitic and granular textures. Site 1118 is extensively hydrothermally altered and contains saussuritized plagioclase and veins of quartz, calcite, pyrite, and chlorite.

A larger diversity of rock types, including a microgranite and hydrothermally altered felsic volcanics, mica schists, carbonate schists, and highly sheared mafic rocks, was cored at the top of the hanging wall sedimentary strata close to the Moresby Seamount at Sites 1108 and 1110-1113. Robertson et al. (in press) interpreted these clasts as detritus resulting from Pleistocene initiation and movement of the low-angle normal fault, depositing larger, angular clasts on top of deep-water sediments that were deposited during regional rift-related subsidence. However, longitudinal transport and deposition of at least some of these clasts from nearby sources (e.g., the D'Entrecasteaux Islands to the west) cannot be ruled out, because lithologies other than diabase and gabbro were not cored in place in the vicinity of the Moresby Seamount.