title: A seismic reflection image for the base of a tectonic plate.
abstract: Plate tectonics successfully describes the surface of Earth as a mosaic of moving lithospheric plates. But it is not clear what happens at the base of the plates, the lithosphere-asthenosphere boundary (LAB). The LAB has been well imaged with converted teleseismic waves, whose 10-40-kilometre wavelength controls the structural resolution. Here we use explosion-generated seismic waves (of about 0.5-kilometre wavelength) to form a high-resolution image for the base of an oceanic plate that is subducting beneath North Island, New Zealand. Our 80-kilometre-wide image is based on P-wave reflections and shows an approximately 15° dipping, abrupt, seismic wave-speed transition (less than 1 kilometre thick) at a depth of about 100 kilometres. The boundary is parallel to the top of the plate and seismic attributes indicate a P-wave speed decrease of at least 8 ± 3 per cent across it. A parallel reflection event approximately 10 kilometres deeper shows that the decrease in P-wave speed is confined to a channel at the base of the plate, which we interpret as a sheared zone of ponded partial melts or volatiles. This is independent, high-resolution evidence for a low-viscosity channel at the LAB that decouples plates from mantle flow beneath, and allows plate tectonics to work.
authors: Institute of Geophysics, Victoria University, Salamanca Road, Wellington 6140, New Zealand.
authors: T A
authors: TA
authors: Stern
authors: Institute of Geological and Nuclear Sciences, 1 Fairway Drive, Lower Hutt 5010, New Zealand.
authors: S A
authors: SA
authors: Henrys
authors: Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, California 90210, USA.
authors: D
authors: D
authors: Okaya
authors: Seismological Observatory, University of Nevada, 1664 North Virginia Street, Reno, Nevada 90210, USA.
authors: J N
authors: JN
authors: Louie
authors: Institute of Geophysics, Victoria University, Salamanca Road, Wellington 6140, New Zealand.
authors: M K
authors: MK
authors: Savage
authors: Institute of Geophysics, Victoria University, Salamanca Road, Wellington 6140, New Zealand.
authors: S
authors: S
authors: Lamb
authors: Earthquake Research Institute, Tokyo University, 1-1-1 Yoyoi, Tokyo 113-0032, Japan.
authors: H
authors: H
authors: Sato
authors: 1] Institute of Geophysics, Victoria University, Salamanca Road, Wellington 6140, New Zealand [2] Institute of Geological and Nuclear Sciences, 1 Fairway Drive, Lower Hutt 5010, New Zealand.
authors: R
authors: R
authors: Sutherland
authors: Earthquake Research Institute, Tokyo University, 1-1-1 Yoyoi, Tokyo 113-0032, Japan.
authors: T
authors: T
authors: Iwasaki
references: Science. 2009 Apr 24;324(5926):499-502
references: 19390042
references: Nature. 2013 Mar 21;495(7441):356-9
references: 23518564