Review of Evidence for Velocity Inversions in the Continental Crust

  1. John G. Heacock
  1. M. Heacock,
  2. S. Muller and
  3. B. J. Mitchell

Published Online: 15 MAR 2013

DOI: 10.1029/GM014p0011

The Structure and Physical Properties of the Earth's Crust

The Structure and Physical Properties of the Earth's Crust

How to Cite

Heacock, M., Muller, S. and Mitchell, B. J. (1971) Review of Evidence for Velocity Inversions in the Continental Crust, in The Structure and Physical Properties of the Earth's Crust (ed J. G. Heacock), American Geophysical Union, Washington D. C.. doi: 10.1029/GM014p0011

Author Information

  1. Geosciences Division, University Of Texas At Dallas, Dallas, Texas 75230

Publication History

  1. Published Online: 15 MAR 2013
  2. Published Print: 1 JAN 1971

ISBN Information

Print ISBN: 9780875900148

Online ISBN: 9781118664049



  • Continental crust;
  • Delta-West profile;
  • Hydration and dehydration;
  • Molasse basin;
  • Refraction arrival Pc;
  • Rhine graben rift system;
  • Sialic low-velocity zone;
  • Sub-basement echoes


Observations of sub-basement echoes at near-normal incidence and in the undercritical distance range, in combination with recordings of strong later arrivals in seismic refraction experiments, furnish evidence for velocity reversals in the continental crust. These lowvelocity layers may significantly alter the location and strength of wave amplitude maximums near the critical points associated with higher-velocity refractors lying just below the proposed inversions. The velocity contrast at the fairly abrupt intervening interfaces can be cited in support of reported observations of sub-basement reflections. The sialic low-velocity zone, postulated to have a lower boundary at depths between approximately 8 and 15 km, has been most often interpreted in areas that have been the site of major tectonic adjustments at some time in their geologic history. This low-velocity region might be associated with semi-continuous acidic intrusions having lower velocities and densities than the surrounding country rocks. Although not universal in extent, many of the world's continents have produced data compatible with this feature. It is significant that most of the shallow earthquakes in continental regions have depths similar to those postulated for the sialic low-velocity zone. A second low-velocity region, which maybe continuous over large portions of North America and Eurasia, has been proposed for depths of the order of 20 km. This feature has been associated with a low-resistivity zone in several areas for which suitable data are available. More basic layers at greater depths were found to be continuous in Oklahoma, even though faults with large surface displacements extend through the shallower layers. The degree to which this deeper inversion is developed may depend on the temperature and pressure at corresponding depths in the crust, and it is possible that enhanced crustal seismicity may be associated with a well developed relatively deep crustal low-velocity zone.