Reflections from the Subcrustal Lithosphere
- Muawia Barazangi and
- Larry Brown
Published Online: 15 MAR 2013
Copyright 1986 by the American Geophysical Union.
Reflection Seismology: The Continental Crust
How to Cite
Fuchs, K. (1986) Reflections from the Subcrustal Lithosphere, in Reflection Seismology: The Continental Crust (eds M. Barazangi and L. Brown), American Geophysical Union, Washington, D. C.. doi: 10.1029/GD014p0067
- Published Online: 15 MAR 2013
- Published Print: 1 JAN 1986
Print ISBN: 9780875905143
Online ISBN: 9781118670118
- Seismic reflection method—Congresses
Today there is a wide consensus that near-vertical seismic reflections out of the crust and from the crust-mantle boundary are not generated by isolated first-order discontinuities but rather by laminated zones which increase the reflectivity by constructive interference of multiple internal reflections in certain frequency bands. Although at present there are only a few observations of near-vertical reflections from the subcrustal lithosphere, in this region of the upper mantle the conditions appear to be favourable for the generation of such reflections. Probing of the upper mantle on long-range seismic profiles revealed properties of the subcrustal lithosphere which make it very likely that the same reflection mechanism could be effective here. The observation of unexpectedly high P-wave velocities of up to 8.5–8.6 km/s as shallow as 10–30 km below the crust-mantle boundary in layered zones embedded in regions of “normal” upper mantle material of around 8.0–8.2 km/s indicates that velocity contrasts in the upper mantle may become as large as at the crust-mantle boundary. A second property of the upper mantle recently discovered is an azimuth-dependent velocity distribution. This anisotropy starting at the Mono reaches into the upper mantle to a depth of about 100 km or even deeper. It may be generated by a preferred orientation of olivine in a more or less horizontal flow pattern. The same flow in a shear stress field orienting the crystals will also transform lateral heterogeneities with non-horizontal boundaries into flat horizontal layers in which the velocities depend mainly on depth. Such a shear-flow flattening is most likely the mechanism which produces laminated zones forming the reflectors within the crystalline earth.
From these properties of the subcrustal lithosphere there is good reason to expect that reflections from the upper mantle exist and can be found if a search is made for them as systematically and thoroughly as has been done during the past decade in the crust. In some cases there is already observational evidence for the existence of upper-mantle reflections.