IS THINNING OF THE LITHOSPHERE PREFERENTIAL DURING THE RIFTING PHASE WITHIN THE WOODLARK BASIN?

The western Woodlark Basin, Papua New Guinea, is an active transition zone between continental rifting and seafloor spreading. Seismic reflection, bathymetry, gravity, and magnetic data have been collected throughout the basin. Sparse sediment cover simplifies basement and fault structure imaging. Many faults are easily traced in multibeam bathymetry. The geophysical data have permitted characterization of the continent-ocean boundary, spreading history, and Euler poles. Previous workers have used this rich data set to calculate extension using fault heaves, subsidence, and Euler pole kinematics. A thorough evaluation of extension, including an estimation of the amount of extension accommodated by poly-phase and sub-resolution faulting, resulted in roughly equal amounts of extension from subsidence and brittle faults estimates. This has not been the case in many other studies (e.g. Davis and Kuznir, 2004). Extension estimates from Euler pole kinematics are significantly greater. A model in which the lower crust and mantle lithosphere preferentially extend during the rifting phase, and extend uniformly once seafloor spreading initiates, was proposed.

In contrast to previous studies, which have focused on the continental rifting to spreading transition in the Woodlark Basin, this study is located where seafloor spreading began at approximately 0.8 Ma. Propagating spreading centers, which are actively cutting into the continental margin, have created a complex continent-ocean boundary. In this setting, we cannot balance extension as estimated through subsidence, brittle faulting, and Euler pole kinematics. This supports the hypothesis that there is preferential extension of the lower crust and mantle lithosphere during continental rifting phase followed by uniform extension once seafloor spreading has initiated.