Lehmann discontinuity

This article is about a discontinuity in Earth's mantle. For the discontinuity in the core, see Earth's inner core.

The Lehmann discontinuity is an abrupt increase of P-wave and S-wave velocities at the depth of 220 km (140 mi) in Earth's mantle, discovered by seismologist Inge Lehmann.^[2][3][4] The thickness is 220 km ^{[citation needed]}. It appears beneath continents, but not usually beneath oceans,^[5] and does not readily appear in globally averaged studies. Several explanations have been proposed: a lower limit to the pliable asthenosphere, a phase transition,^[6] and most plausibly, depth variation in the shear wave anisotropy.^[7]

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]

Notes

1. Figure patterned after Don L Anderson (2007). New Theory of the Earth (2nd ed.). Cambridge University Press. p. 102, Figure 8.6. ISBN 978-0-521-84959-3.; Original figure attributed to Grand and Helmberger (1984)
2. William Lowrie (1997). Fundamentals of geophysics. Cambridge University Press. p. 158. ISBN 0-521-46728-4.
3. Lehmann, I. (1936): P', Publications du Bureau Central Seismologique International, Série A, Travaux Scientifique, 14, 87–115.
4. Martina Kölbl-Ebert (December 2001). "Inge Lehmann's paper: " P' " (1936)" (PDF).
5. Lars Stixrude and Carolina Lithgow-Bertolloni (2005). "Mineralogy and elasticity of the oceanic upper mantle: Origin of the low-velocity zone". J. Geophys. Res. 110 (B3): B03204. Bibcode:2005JGRB..110.3204S. doi:10.1029/2004JB002965. hdl:2027.42/94924. 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.
6. Kent C. Condie (1997). Plate tectonics and crustal evolution (4th ed.). Butterworth-Heinemann. p. 123. ISBN 0-7506-3386-7.
7. MK Savage, KM Fischer CE Hall (2004). "Strain modelling, seismic anisotropy and coupling at strike-slip boundaries...". In John Gocott (ed.). Vertical coupling and decoupling in the lithosphere; Volume 227 of special publications. Geological Society. p. 14. ISBN 1-86239-159-9.

General references

• P. Caloi (1967). "The "20° Discontinuity"". In H. E. Landsberg, J. Van Mieghem (ed.). Advances in geophysics, Volume 12. Academic Press. p. 167 ff. ISBN 0-12-018812-0. – some historic background.

Further reading

Shun-ichirō Karato (2008). Deformation of earth materials: an introduction to the rheology of solid earth. Cambridge University Press. p. 318. ISBN 978-0-521-84404-8.

External links

• Inge Lehmann, UCLA
• Career highlights of Inge Lehmann from UCLA