Lehmann discontinuity

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

  1. ↑ 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)
  2. ↑ The thickness is 220 km. William Lowrie (1997). Fundamentals of geophysics. Cambridge University Press. p. 158. ISBN 0-521-46728-4.
  3. ↑ 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.
  4. ↑ Kent C. Condie (1997). Plate tectonics and crustal evolution (4th ed.). Butterworth-Heinemann. p. 123. ISBN 0-7506-3386-7.
  5. ↑ 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.
  6. ↑ 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


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