Plate motions and continental extension at the rifting to spreading transition in Woodlark Basin, Papua New Guinea: Can oceanic plate kinematics be extended into continental rifts?

Woodlark Basin provides a unique opportunity to compare brittle extension, subsidence, and extension predicted from long-term plate motions at the rifting to spreading transition of a non-volcanic margin. Detailed analysis of reflection seismic data near the current rifting to spreading transition yields 111 km (± 23 km) of brittle extension. Subsidence along the same transect predicts 115 km (± 47 km) of extension. Long term plate motions, derived from seafloor spreading and transform azimuths in the eastern portion of Woodlark Basin, predict ∼ 220 km of extension in this area. These results imply no large discrepancy between brittle extension and extension predicted from subsidence, contrary to observations at ancient rifted margins. However, there remains a pronounced two-fold discrepancy between post-breakup plate motions and extension during rifting. This implies that continental extension rates do not match seafloor-spreading rates, and requires that the extension within the rift can only be modeled using Euler pole kinematics after the initiation of seafloor spreading. This is hypothesized to result from a decoupling of the lithosphere, in which the upper crust moves more slowly than the mantle lithosphere. Pre-breakup plate motions are accommodated primarily by extension of the mantle lithosphere as a result of rollback of the Solomon Sea plate. After the initiation of seafloor spreading, spreading rates are controlled by the motion of the mantle lithosphere and the upper crust moves at the same rate as the rest of the lithosphere.