How continents break up: Insights from Papua New Guinea

The Woodlark Basin in the western Pacific forms a continuous system of acti ve continental rifting evolving to well-developed seafloor spreading. Thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic re flection data in the continental and oceanic parts of the basin. Magnetic i sochrons indicate a single Euler pole of opening for most of the basin hist ory and allow us to infer the opening kinematics along the rifted margins. In agreement with rigid plate tectonic models, continental rifting initiate d geologically synchronously (at similar to 6 Ma) along the length of the p rotomargins within a deforming plate boundary zone. Strain localization and seafloor spreading, however, developed in a time transgressive fashion fro m east to west within this zone of deformation. Spreading centers formed wi thin the theologically weaker protocontinental margins surrounded by strong er oceanic lithosphere in the Solomon and Coral Seas. The transition to spr eading occurred after a rather uniform degree of continental extension: 200 +/-40 km. Both early and late stage rifting involved high- and low-angle no rmal faults. We identify distinct styles in the transition from rifting to spreading which we refer to as nucleation, propagation, and stalling. These breakup styles impart varyingly concordant to discordant relationships bet ween the adjacent oceanic and continental rift structures. Continental tran sform margins which are or were juxtaposed against the ends of spreading ce nters show no evidence for thermal uplift or igneous underplating. The init ial spreading segments achieved much of their length at nucleation (within rift basins separated along strike by accommodation zones), with subsequent lengthening by spreading propagation into rifting continental crust. This early propagation, and the subsequent development of transform faults betwe en initially nontransform spreading segment offsets, produced rift and spre ading segmentation boundaries that are not simply correlated. The spreading centers nucleated approximately orthogonal in strike to the opening direct ion but, as the protomargins were oblique to this direction, nucleation jum ps occurred in order to maintain the new spreading centers within the proto margins. Thus stepwise spreading nucleation in order to remain within a the ologically weak zone, rather than rupturing of the lithosphere by stress co ncentration at the tip of a propagating ridge axis, is the dominant form of the rifting-to-spreading transition in the Woodlark Basin.