Continental rifting in the Woodlark Basin, Papua New Guinea: A comparison of different estimates of extension at the rifting-spreading transition.

The Woodlark Basin is one of few places where it is possible to investigate an active transition from continental rifting to seafloor spreading. The Papuan Peninsula began N-S extension at 8.4 Ma, followed by seafloor spreading at 6 Ma. To date, seafloor spreading has propagated west 500 km. In the proximity of the modern rifting to spreading transition the northern margin has subsided 2-3 km with minor brittle faulting. The southern margin has subsided a similar amount but is characterized by large faults. Previous work shows that the observed continental extension is half the amount resolved by seafloor-spreading kinematics. It has been proposed that this discrepancy is due to mid-crustal decoupling, where the mantle lithosphere and lower crust are detached.

The N-S profile across the current rifting to spreading transition is a natural laboratory for extensional environments. The work herein presented is a continuation of prior studies, but incorporates a new approach to extensional modeling, specifically the use of the Move software package. The profile presented includes ODP Leg 180 wells. Structural and stratigraphic interpretations originate from nearby seismic lines. Biostratigraphy and paleomagnetism data are the basis for age-depth relationships. Interpreted sedimentary packages permit backstripping and decompaction models that assume Airy Isostasy. Extension is estimated through the restoration of fault heaves and back rotation of fault blocks. From previous studies we know the width of the Papuan Peninsula to be 320 km in the vicinity of the profile presented. Furthermore, those studies estimate 220 km of extension across the margin based on Euler pole kinematics. This gives an original margin width of about 100 km, and Beta greater than 3. We present herein an extension estimate based on 2-D kinematic modeling, and contrast this with prior extension estimates of 111-115 km.