Wide-angle seismic reflections reveal a lithosphere-asthenosphere boundary zone in the subducting Pacific Plate, New Zealand.

Abstract

New wide-angle seismic reflection data from offshore New Zealand show that the lithosphere-asthenosphere boundary (LAB) is more structured than previously thought. Three distinct layers are interpreted within a 10- to 12-km-thick LAB zone beginning at a depth of ≈70 km: a 3 (±1)–km-thick layer at the bottom of the lithosphere with a P-wave (VP) azimuthal anisotropy of 14 to 17% and fast azimuth subparallel to the direction of absolute plate motion and a 9 (±2)–km-thick, low VP channel with a P-wave–to–S-wave velocity ratio (VP/VS) of >2.8 in the upper 7 km of the channel and 1.8 to 2.6 in the lower 2 km of the channel. The high VP/VS ratios indicate that this channel may contain 3 to 20% partial melt that facilitates decoupling of the lithosphere from the asthenosphere and reduces resistance for plate motion. Furthermore, the strong azimuthal anisotropy above the low-velocity layer suggests localization of strain due to melt accumulation.