The primary objective at this site is to determine the internal structure and composition of Moresby Seamount, including the nature of the basement (rock type, P-T-t, structural fabric, and deformation history). By comparison with the stratigraphy and basement at the other sites, this will constrain the offset on the inferred low-angle normal fault. If Moresby Seamount is not a core complex, it should comprise sedimentary rocks above upper crustal rocks (presumably prerift metamorphic basement) without evidence of intense ductile deformation and rapid decompression.
Site ACE-8a
This proposed site is situated on the southern edge of the graben north of Moresby Seamount and will be drilled though the hanging wall, across the fault zone, and into the footwall of the inferred low-angle fault (Figs. 3, 4, 5). The expected section at Site ACE-8a includes up to 900 m of Quaternary hemipelagic sediments (only 0.7 s TWT, but are apparently fast-interval velocities averaging 2.6 km/s) overlying 250-300 m of fault zone and metamorphic basement rocks.
The primary objectives at this site are to determine the (1) sedimentology, biostratigraphy, structural fabric, and vertical motion history in the graben; (2) in situ stress, temperature, physical properties, and fluid pressures in and around the fault zone; and (3) nature of the basement (rock type, P-T-t, structural fabric, deformation history) in and below the fault zone.
Site ACE-9a
Proposed Site ACE-9a is north of a major south-dipping normal-fault system that is antithetic to the low-angle fault dipping north from Moresby Seamount (Fig. 4). The site is positioned to cross two angular unconformities (Fig. 4). Beneath the lower angular unconformity, a stratified sequence, interpreted to be prerift forearc basin sediments, dips north. The expected sequence at Site ACE-9a includes 780 m of Holocene to Pliocene hemipelagic, synrift sediments unconformably overlying 220 m of consolidated Miocene forearc basin sediments.
The primary objectives at this site are to determine the (1) sedimentology, biostratigraphy, and vertical motion history of the synrift sediments on the northern margin and (2) nature of the forearc basin sequence and, hence, the prerift history.
Alternate Sites ACE-1c and ACE-7b
These are alternate sites located on the northern margin (Figs. 3, 4, 5). The stratal geometry, including two angular unconformities, requires that at least two sites be drilled to best characterize this region. Site ACE-9a is a compromise site between these two.
Proposed Site ACE-1c is just north of a major south-dipping normal-fault system that is antithetic to the low-angle fault dipping north from Moresby Seamount (Fig. 4). It is also located above north-dipping reflectors at the base of the interpreted prerift forearc basin sequence. The expected sequence at Site ACE-1c includes 950 m of Quaternary hemipelagic, synrift sediments unconformably overlying 50 m of Paleogene volcanic or metamorphic basement.
Proposed Site ACE-7b is 15 km farther north on the margin and is positioned to cross two angular unconformities ((Fig. 4). Beneath the lower angular unconformity, a stratified sequence dips north. The expected section at Site ACE-7b includes 640 m of Pliocene to Quaternary hemipelagic sediments unconformably overlying 110 m of consolidated Miocene forearc basin sediments.
The primary objectives at these sites are to determine the (1) sedimentology, biostratigraphy, and vertical motion history of the northern margin; (2) nature of the forearc basin and basement sequence and, hence, the prerift history; and (3) in situ stress orientation, permeability, temperature, physical properties, and fluid composition in the basement for comparison with the same parameters in the active low-angle fault zone.