The rifting and breakup of continental lithosphere has produced conjugate passive margins with a rich diversity of structural, magmatic, and sedimentary architectures. Yet only a handful of modern examples exist where these processes are active and so may be examined in situ. During Ocean Drilling Program (ODP) Leg 180 we investigated one of these examples in the western Woodlark Basin of Papua New Guinea (Fig. F1). This is an area of low- and high-angle normal faulting, metamorphic core complex formation, and seafloor spreading that has been propagating westward into rifting lithosphere since 6 Ma (e.g., see review in Taylor, 1999). The Papuan continent being rifted is a young orogen with inherent complexities such as ophiolite emplacement in the Paleogene and Neogene arc magmatism.
We summarize the results of studies associated with the transect of holes drilled during Leg 180 along ~151°35´E from 9°11´ to 9°52´S (Fig. F2). The drilling transect was focused on the center of active deformation (Moresby rift) in front of the spreading tip and on the adjacent rift margins: Sites 1114 and 1116 on Moresby Seamount to the south and Sites 1109, 1115, and 1118 on the Woodlark Rise to the north. We had planned a cased reentry hole to intersect at ~900 meters below sea level (mbsl) the ~27°N-dipping Moresby fault in order to characterize the in situ properties of an active low-angle normal fault. Shipboard concerns about the presence of trace hydrocarbons encountered at Site 1108 and impenetrable drilling conditions in talus aprons encountered at Sites 1110-1113 thwarted those plans. The Moresby fault was penetrated, but only at Site 1117, where it cropped out at 1163 mbsl.
We review new dating, geochemistry, and petrofabric studies that permit an evaluation of the origin and deformation history of the basement. Analyses of the cores and logs, in the context of a regional grid of seismic profiles, provides a high-resolution record of margin subsidence, sedimentation, and paleoenvironment. Detailed investigations of the core physical properties, structures, and pore fluids, together with log interpretation, have produced an understanding of the postdepositional changes, from diagenesis to deformation.
The results from Leg 180 document the upper crustal processes occurring in this region of continental rifting and breakup. Together with estimates of strain rate, thermal regime, and crustal structure, they also have implications for lower crustal flow and the mechanisms of crustal thinning. After some discussion of the regional context, our synthesis below proceeds from the basement to the depositional history, diagenesis-geochemistry-microbiology, and then to the deformation. Each section is a mini-synthesis, and therefore we do not have a separate conclusion section.