The continuum of active extensional processes, laterally varying from continental rifting to seafloor spreading, in the western Woodlark Basin-Papuan Peninsula region of Papua New Guinea (Fig. 1) provides the opportunity to investigate the mechanics of lithospheric extension and continental breakup. Seafloor spreading magnetic anomalies indicate that during the last 6 m.y. the formerly contiguous, eastward extensions of the Papuan Peninsula (the Woodlark and Pocklington Rises) were separated as a westward propagating spreading center opened the Woodlark Basin (Weissel et al., 1982; Taylor et al., 1995, unpubl. data; Goodliffe et al., 1997; Goodliffe, 1998). The present spreading tip is at 9.8°S, 151.7°E. Farther west, extension is accommodated by continental rifting, with associated full and half graben, metamorphic core complexes, and peralkaline rhyolitic volcanism (Smith and Simpson, 1972; Smith, 1976; Davies, 1980; Hegner and Smith, 1992; Davies and Warren, 1992; Hill et al., 1992, 1995; Baldwin et al., 1993; Hill and Baldwin, 1993; Stolz et al., 1993; Lister and Baldwin, 1993; Hill, 1994). Current rifting and spreading are confirmed by kinematic measurements using Global Positioning System (GPS) observations (Tregoning et al., 1998). Earthquake source parameters and seismic reflection data indicate that normal faulting is active at depths above 9 km in the region of incipient continental separation (Figs. 2, 3, 4, 5, 6; Abers, 1991; Taylor et al., 1995, 1996, unpubl. data; Mutter et al., 1996; Abers et al., 1997). A low-angle normal fault emerges along the northern flank of Moresby Seamount, a continental crustal block with greenschist metamorphic basement. Asymmetric basement fault blocks overlain by only minor ponded sediments characterize the margin to the south, whereas the margin to the north has a down-flexed pre-rift sedimentary basin and basement sequence unconformably beneath synrift sediments (Figs. 3, 4, and 5).

The primary objectives of Ocean Drilling Program (ODP) Leg 180 were to (1) characterize the composition and in situ properties (stress, permeability, temperature, pressure, physical properties, and fluid pressure) of an active low-angle normal fault zone to understand how such faults slip, (2) determine the sedimentology, magnetobiostratigraphy, physical properties, and vertical motion history of the northern margin, including the nature of the forearc basin (and basement?) sequence and hence the pre-rift history, and (3) determine the internal structure and composition of Moresby Seamount, including the nature of basement (rock type, P-T-t, structural fabric, and deformation history). These parameters will be used as input into regional models for the extensional deformation of continental lithosphere, particularly the mode, timing, and amount of extension prior to spreading initiation.

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