The Woodlark and Pocklington Rises, which are topped by reefs, shoals, and islands in the west, narrow and deepen eastward (Fig. F1). The rises separate the seafloor of the Woodlark Basin from that of the surrounding Solomon and Coral Seas, respectively. The rises merge westward with the Papuan Peninsula of New Guinea, where the central range is more than 3500 m above sea level (Figs. F1, F3). Other high topography is associated with the gneissic domes of the Suckling-Daymon massif (Davies, 1980) and of the D'Entrecasteaux Islands (Goodenough, Fergusson, Normanby; Davies and Warren, 1988) and, to a lesser extent, by volcanoes along the north coast of the Peninsula, such as Mounts Lamington and Victory that have erupted this century (Smith, 1981). The latter are part of a Quaternary calc-alkaline arc associated with southward subduction at the Trobriand Trough (Smith, 1981). The Trobriand Trough terminates at 153.5ºE, northeast of which the bathymetry and seismicity define a right-lateral transform fault along the northern margin of the Woodlark Rise (Fig. F3).
Because of the generally thin sediment cover, the pattern of faulting and the transition from block-faulted continental lithosphere of the Woodlark and Pocklington Rises to the rugged, seismically diffracting, oceanic lithosphere of the Woodlark Basin are readily apparent in swath bathymetry and seismic reflection data (Hill et al., 1984; Goodliffe et al., 1993, 1997; Taylor et al., 1995, in press). The continent-ocean boundary defined by Goodliffe (1998) and Taylor et al., (in press) is depicted in Figures F1, F3, F4, F5, F6, and F7. The extent of oceanic lithosphere decreases westward and terminates at 151.7ºE where it abuts Moresby Seamount. Further west, Milne and Goodenough Bays are full and half graben respectively, produced by continental rifting (Milsom and Smith, 1975; Mutter et al., 1996). Earthquake seismicity and focal mechanisms (Fig. F3) attest to (1) crustal tensional seismicity as far west as 148ºE (Weissel et al., 1982; Abers, 1991; Abers et al., 1997) and (2) rifting of the continental margins (to 153ºE) continuing after the onset of seafloor spreading (Taylor et al., 1995). The seismicity is characterized by normal and strike-slip events, with northerly T-axes. Several of the normal fault focal mechanisms permit slip on shallow-dipping (25º-35º) planes (Abers, 1991; Abers et al., 1997).
The neo-volcanic zone of the axis of seafloor spreading is defined by strong acoustic backscatter (Fig. F3). The spreading center sharply crosscuts the majority of the oceanic seafloor fabric. Goodliffe et al. (1997) showed that the crosscutting relationship is due to an extremely rapid reorientation of the spreading direction ~80 ka. There are many nontransform offsets of the spreading axis, as well as major transform offsets, such as Moresby and Simbo Transforms (Figs. F1, F3). Contrasting the morphology west vs. east of Moresby Transform (Martinez et al., 1996), the seafloor is smoother (100-200 m vs. 200-400 m relief), the spreading center has a rifted axial high in contrast to axial valleys, and the ridge flanks are systematically shallower by ~500 m since the onset of spreading. The lack of a linear region of strong acoustic backscatter east of Simbo Transform (Goodliffe, 1998), together with the northward progression of seafloor ages (see below), confirms the suggestion of Taylor and Exon (1987) and Crook and Taylor (1994) that the spreading center has been subducted there.
The magnetic data show that the formerly contiguous, eastward extensions of the Papuan Peninsula (the Woodlark and Pocklington Rises) were separated as the Woodlark spreading center propagated westward during the last 6 Ma (Weissel et al., 1982; Taylor and Exon, 1987; Taylor et al., 1995). Our magnetization solution is presented in Figure F4, and the seafloor spreading magnetic lineations inferred from it by Goodliffe (1998) and Taylor et al. (in press) are shown in Figure F5. We adopt the magnetic time scale of Cande and Kent (1995). The oldest magnetic chron identified is 3A.1 (5.89-6.14 Ma), which occurs in the easternmost basin immediately south of Chron 3.4. Goodliffe (1998) reconstructed the seafloor spreading history and Taylor et al. (in press) derived the following poles of opening: 2.437º/m.y. about 144ºE, 12ºS between 0.0 and 0.52 Ma and 4.234º/m.y. about 147ºE, 9.3ºS before 0.52 Ma. The data confirm and quantify the observation by Weissel et al. (1982) that the magnetic anomaly amplitudes are greater in the western basin than in the eastern basin. We find that east of Moresby Transform root-mean-square magnetic field and seafloor magnetization variations (Fig. F4) are 136 nT and 4.2 A·m-1 respectively, whereas west of Moresby Transform they are 273 nT and 6.6 A·m-1.
The free-air gravity field of the Woodlark Basin area (Fig. F6) broadly reflects the bathymetry, with a few notable exceptions associated with sedimentary basins. Presently active (e.g., Trobriand Trough) and former (e.g., Pocklington Trough) trenches (Davies et al., 1984) are marked by prominent gravity lows. Where the young lithosphere of the Woodlark Basin is being subducted beneath the New Georgia Group of the Solomon Islands, the islands are paralleled on the basin side by a 20-30 mGal relative high. Gravity lows with modest bathymetric expression are located in the New Georgia Basin area of the Solomon Islands and surrounding the D'Entrecasteaux Islands (Goodenough-Normanby). Of the latter lows, those to the south are associated with rifts such as Goodenough Basin. To the north, they reflect depocenters of the Miocene forearc basin. The depocenters are located south of the outer forearc basement high marked by the strong positive anomaly linking the Woodlark and Trobriand Islands. The eastern Woodlark Basin spreading centers are characterized by 10-20 mGal axial lows relative to flanking highs, whereas the western basin is characterized by a free-air gravity high along the spreading axis (except in the segment that approaches Moresby Seamount).
Bouguer gravity anomalies are shown in Figure F7. Removal of the topographic contribution to the gravity has significantly simplified the gravity anomalies, accounting for much of the short-wavelength variations seen in the free-air anomalies. Regionally, the oceanic lithosphere of the Solomon Sea and Woodlark and Coral Sea Basins is associated with Bouguer gravity highs, which is in contrast to the surrounding continental and arc lithosphere lows. Within the Woodlark Basin oceanic lithosphere there is a decreasing westward gradient in the Bouguer anomaly, as well as a 30-mGal step down westward across the Moresby Transform (Martinez et al., 1996). Relative lows paralleling the Woodlark-Trobriand residual high reflect the Trobriand Trough accretionary prism to the north and the Miocene forearc basin to the south.
Of the several MCS data sets available to us for the Woodlark Basin, we present in Figure F8 only those sections that were used to locate the sites drilled on ODP Leg 180. Some of these data have been described in Taylor et al. (1996, in press), and additional sections are presented in Mutter et al. (1996) and Abers et al. (1997). The MCS data image the structure and stratigraphy of
These data are interpreted in terms of drilling objectives and site locations in "Drilling Area," (in "Regional Setting" in the "Background and Regional Setting" chapter). Before Leg 180, we constructed an isopach map of the synrift sedimentary section in the vicinity of Sites 1109 and 1118 (Fig. F9) from the grid of all available MCS data. When account is taken of the regional subsidence and southward tilting of the angular unconformity, the data reveal a paleoerosional surface with a north-northwest-trending channel in the vicinity of Site 1109 and flanking isolated highs, such as those drilled at Site 1118.