A Geophysical Characterization of the Orogenic Rifting to Sealoor Spreading Transition in Western Woodlark Basin, Papua New Guinea

The Woodlark Basin, forming as the mountains of Papua New Guinea collapse, is a classic example of the transition from mountain building to orogenic rifting, and then to seafloor spreading, capturing the complete orogenic cycle. Within the basin itself we find the only place on Earth where the moment of spreading center initiation has been captured. Found adjacent to Moresby Seamount and focused on a shallow angle normal fault (~30°), the current rifting-to-seafloor spreading transition has been thoroughly imaged geophysically. Ring dikes and high-acoustic backscatter on the seafloor are coincident with a high amplitude positive magnetization anomaly, and multichannel seismic lines image intrusions that represent a nascent spreading center, offsetting the two sides of the shallow-angle fault. This study presents a systematic geophysical characterization of the transition zone through an interpretation of bathymetry, magnetic, gravity, and seismic data. Through analysis of the shape of the shallow angle fault and the volume of the material accreted at the margin we answer a number of questions related to the rifting-to-seafloor spreading transition, namely: 1) what is the stress state at the transition zone; 2) how does the transfer from strain accommodation by faulting to strain accommodation by accretion takes place; 3) what is the stratigraphic response to the transition; and 4) how do the intrusives interact with the syn-rift sediments. We present a model that describes in detail the process by which a nascent seafloor spreading center nucleates in a rift basin as orogenic rifting draws to a close.