CONSTRAINTS ON THE NATURE OF THE G-BOUNDARY IN SUBDUCTING OCEANIC LITHOSPHERE, BASED ON HIGH FREQUENCY (14 HZ) SEISMIC REFLECTION DATA

Abstract:

In a recent study, we interpreted ~ 100-km-deep seismic reflections to be the base of the Pacific oceanic plate as it subducts beneath southern North Island, New Zealand*. The top of the plate is at a depth of ~ 25 km, and thus the total thickness of the subducted plate is ~ 75 km and significantly thinner than that predicted by plate cooling models for 120 Ma lithosphere. It is, however, consistent with observations of the depth to the G-discontinuity beneath the Pacific plate, which has a weak correlation with age. Recent work shows that the G-discontinuity corresponds to a change in radial anisotropy, believed to be linked to the compositional differences as the plate is created at a mid-ocean ridge. Our study imaged not one, but two, parallel, and distinct reflectors at the 100 km depth, defining a channel ~10 km thick, that lies parallel to the top of the plate (dipping ~15°). The frequency of our seismic reflections sets an important limit on the “sharpness” criterion for the boundary that generates the reflected energy. Namely, the transition zone for the acoustic impedance contact can be no thicker than about half a wavelength, or ~300 m. We argue that this is too abrupt a transition to be due to a change in radial anisotropy, and instead requires a change in phase from rock, to rock with melt and/or water. Moreover the simplest explanation of an ~10 km thick channel with sharp acoustic boundaries at top and bottom is a weak zone with a significant melt fraction (~2%) in a zone of enhanced mantle shear, although other interpretations ( e.g. elastically accommodated grain-boundary sliding) are possible. If we are imaging the G-boundary, then it is both sharp and likely to be actively accommodating some relative plate motion, as the plate slides over the underlying mantle.

* Stern T.A. et al (2015). A seismic reflection image for the base of a tectonic plate, Nature, 518, pp. 85-88, doi10.1038/Nature14146.