Anisotropy beneath a highly extended continental rift

Abstract

AbstractWe have employed shear wave splitting techniques to image anisotropy beneath the D'Entrecasteaux Islands, in southeastern Papua New Guinea. Our results provide a detailed picture of the extending continent that lies immediately ahead of a propagating mid‐ocean ridge tip; we image the transition from continental to oceanic extension. A dense shear wave splitting data set from a 2010 to 2011 passive‐source seismic deployment is analyzed using single and multichannel methods. Splitting delay times of 1–1.5 s are observed and fast axes of anisotropy trending N‐S, parallel to rifting direction, predominate the results. This trend is linked to lattice‐preferred orientation of olivine, primarily in the shallow convecting mantle, driven by up to 200 km of N‐S continental extension ahead of the westward‐propagating Woodlark Rift. This pattern differs from several other continental rifts that evince rift‐strike‐parallel fast axes and is evident despite the complex recent tectonic history. We contend that across most of this rift, the unusually high rate and magnitude of extension has been sufficient to produce a regime change to a mid‐ocean‐ridge‐like mantle fabric. Stations in the south of our array show more complex splitting that might be related to melt or to complex inherited structure at the edge of the extended region.