Lehmann discontinuity
This article is about a discontinuity in Earth's mantle. For the discontinuity in the core, see inner core.
Velocity of seismic S-waves in the Earth near the surface in three tectonic provinces: TNA = Tectonic North America SNA = Shield North America and ATL = North Atlantic.[1]
The Lehmann discontinuity is an abrupt increase of P-wave and S-wave velocities at the depth of 220±30 km, discovered by seismologist Inge Lehmann.[2] It appears beneath continents, but not usually beneath oceans,[3] and does not readily appear in globally averaged studies. Several explanations have been proposed: a lower limit to the pliable asthenosphere, a phase transition,[4] and most plausibly, depth variation in the shear wave anisotropy.[5] Further discussion of the Lehmann discontinuity can be found in the book Deformation of Earth Materials by Shun-ichirō Karato.[6]
Notes
- ↑ Figure patterned after Don L Anderson (2007). New Theory of the Earth (2nd ed.). Cambridge University Press. p. 102, Figure 8.6. ISBN 0-521-84959-4.; Original figure attributed to Grand and Helmberger (1984)
- ↑ The thickness is 220 km. William Lowrie (1997). Fundamentals of geophysics. Cambridge University Press. p. 158. ISBN 0-521-46728-4.
- ↑ Lars Stixrude and Carolina Lithgow-Bertolloni (2005). "Mineralogy and elasticity of the oceanic upper mantle: Origin of the low-velocity zone" (PDF). J Geophys. Res. 110: B03204. doi:10.1029/2004JB002965.
The first possible explanation is that the Lehmann is not a global feature...the Lehmann is more prevalent under continents and may be absent under all or most of the oceans.
- ↑ Kent C. Condie (1997). Plate tectonics and crustal evolution (4th ed.). Butterworth-Heinemann. p. 123. ISBN 0-7506-3386-7.
- ↑ MK Savage, KM Fischer CE Hall (2004). "Strain modelling, seismic anisotropy and coupling at strike-slip boundaries...". In John Gocott. Vertical coupling and decoupling in the lithosphere; Volume 227 of special publications. Geological Society. p. 14. ISBN 1-86239-159-9.
- ↑ Shun-ichirō Karato (2008). Deformation of earth materials: an introduction to the rheology of solid earth. Cambridge University Press. p. 318. ISBN 0-521-84404-5.
General references
- P Caloi (1967). "The "20° Discontinuity"". In H. E. Landsberg, J. Van Mieghem. Advances in geophysics, Volume 12. Academic Press. p. 167 ff. ISBN 0-12-018812-0. – some historic background.
External links
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