A seismic reflection image for the base of a tectonic plate
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Authors
Stern, T. A.
Henrys, Stuart A.
Okaya, D.
Louie, John N.
Savage, Martha K.
Lamb, S.
Sato, H.
Sutherland, R.
Iwasaki, Toshia
Issue Date
2015-02-05
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Geophysics , Seismology , Solid Earth sciences
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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.
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In Copyright