Anisotropy beneath a highly extended continental rift

Abstract

We 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 westwardpropagating Woodlark Rift. This pattern differs from several other continental rifts that evince rift-strikeparallel 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.

Rifted continental margins: The case for depth-dependent extension

“…2 panel b) conceptual model. That the surrounding crust has subsided anomalously forming sag basins (Kusznir and Karner, 2007), which is consistent with lower crustal flow toward and into the core complexes (Fitz and Mann, 2013a), was also noted earlier.…”

Imaging continental breakup using teleseismic body waves: The Woodlark Rift, Papua New Guinea

“…The low velocities in the center of the rift lie directly along strike from the youngest spreading center at 151.5°E, and continue at least 250 km to 149°E. The amplitude of the anomaly suggests almost total lithospheric removal, implying mantle divergence and adiabatic upwelling of asthenosphere considerably ahead of the propagation of seafloor spreading (in agreement with spreading‐related anisotropic fabric [ Eilon et al ., ] and some other continental rifts [e.g., Rychert et al ., ]).…”

“…We may not formally reject the hypothesis that the shallow part of the model is also recording only temperature heterogeneity but other factors may also be important. Strong anisotropy in this region [ Eilon et al ., ] may bias estimates of [ Hacker and Abers , ] as might compositional heterogeneities associated with recent subduction (section 6.3). Moreover, several model nodes scatter toward large values of .…”

“…Eilon et al . [] established that this region contains strong anisotropy with N‐S fast geometry within the rift. Recent work [ Eilon et al ., ] indicates heterogeneous anisotropy throughout this region; the degree to which this influences S wave first‐arrival times will depend on polarization and is not straightforward to remove.…”

“…[] established that this region contains strong anisotropy with N‐S fast geometry within the rift. Recent work [ Eilon et al ., ] indicates heterogeneous anisotropy throughout this region; the degree to which this influences S wave first‐arrival times will depend on polarization and is not straightforward to remove. We separately measured S relative delay times on BHN and BHE channels and for this isotropic inversion used delay times of S waves on whichever component had more arrivals or higher mean values of cross correlation maxima—the preferred component is generally related to the S ‐wave polarization.…”