2012 IRIS Workshop

Imaging the Base of a Tectonic Plate: Evidence for Melt and Volatiles at the Lithosphere-Asthenosphere Boundary

Nicholas Schmerr:

Cross-sectional portrayal of the origin for a high reflectivity Gutenberg discontinuity (G) observed by SS precursors at the lithosphere-asthenosphere boundary beneath the Pacific plate. The reflectivity of the G is enhanced by the stagnation of partial melt at the base of the lithosphere (blue lines). Another deeper discontinuity (blue dashed lines) predominantly occurs beneath major hotspots, mid-ocean ridges, and subduction zones and lies at the base of the upper mantle low-velocity zone.

Full-resolution graphics file in original format: 0006.png

The lithosphere-asthenosphere boundary (LAB) separates the upper thermal boundary layer of a rigid, conductively cooling plate from the underlying ductile, convecting mantle. Many different mechanisms have been proposed over the years to explain the origins of this transition, but the boundary still remains enigmatic. A regional, sharp seismic discontinuity near 30-120 km depth beneath the oceans, known as the Gutenberg discontinuity (G), is found to be roughly coincident with the LAB. If related to the LAB, this discontinuity enables direct seismic investigation of the change in properties from the lithosphere to the asthenosphere, though the character of the G beneath oceanic regions remains largely unmapped. New results use SS precursors to sample shallow discontinuity structure in the upper mantle across the entirety of the Pacific plate. The SS precursors reveal that a seismically sharp G interface is not present beneath the majority of the Pacific plate, and that a robust discontinuity with a weak age dependence only shows up beneath active surface volcanism and subduction zones. Several models have been proposed why G reflectivity is regionally variable and locally enhanced; including the presence of anisotropy, volatiles, temperature, and dynamical processes that produce melt, such as hot mantle upwellings, small-scale convection, and fluid release during subduction.

Acknoweldgements: Support came from a Carnegie Institution of Washington Department of Terrestrial Magnetism Postdoctoral Fellowship and from the NASA Postdoctoral Program. The facilities of the IRIS Data Management Center were used for access to the data required in this study.

For further reading: Schmerr, N., (2012), The Gutenberg Discontinuity: Melt at the Lithosphere-Asthenosphere Boundary, Science, 335 (6075), 1480-1483. doi: 10.1126/science.1215433

Keywords: lithosphere, asthenosphere, melt, volatiles

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