The Interrelationship of the Crust, the Upper Mantle, and Isostatic Gravity Anomalies in the United States

  1. Leon Knopoff,
  2. Charles L. Drake and
  3. Pembroke J. Hart
  1. George P. Woollard

Published Online: 18 MAR 2013

DOI: 10.1029/GM012p0312

The Crust and Upper Mantle of the Pacific Area

The Crust and Upper Mantle of the Pacific Area

How to Cite

Woollard, G. P. (2012) The Interrelationship of the Crust, the Upper Mantle, and Isostatic Gravity Anomalies in the United States, in The Crust and Upper Mantle of the Pacific Area (eds L. Knopoff, C. L. Drake and P. J. Hart), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM012p0312

Author Information

  1. Hawaii Institute of Geophysics, University of Hawaii, Honolulu, Hawaii

Publication History

  1. Published Online: 18 MAR 2013
  2. Published Print: 20 APR 2012

Book Series:

  1. Geophysical Monograph Series

Book Series Editors:

  1. Waldo E. Smith

ISBN Information

Print ISBN: 9780875900124

Online ISBN: 9781118663738

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Keywords:

  • Crust;
  • Density equivalents;
  • Granitic and basal layers;
  • Isostatic anomalies;
  • Isostatic gravity anomalies;
  • Maritime provinces and Newfoundland;
  • Seismic velocity values;
  • Upper mantle

Summary

The present paper differs from earlier studies by the writer, as well as those by other investigators on the same subject, in that the emphasis is on the relationships that exist between the crust and upper mantle and their combined effect on gravity relationships and surface elevation. Crustal cross sections are plotted across the United States to bring out relations between crustal structure, geologic provinces, and changes in upper mantle composition defined by variations in mantle velocity. It is found that there is a relationship between mean crustal velocity, crustal thickness, and mantle velocity. Areas of geologic uplift are characterized by mantle uplift, the absence of a basal high velocity layer in the crust, and decreases in mean crustal velocity and mantle velocity. Most basin areas are characterized by a thick crust and the development of a well defined high velocity basal crustal layer and abnormal mantle velocity values. Because of these abnormalities, there is no simple relation between crustal thickness and surface elevation. A correlation of this type can be established if the data are restricted to areas where the mantle has a velocity in the range 8.0 to 8.2 km/sec. This correlation suggests a freeboard-to-root ratio of 1:8.5 and a density contrast between the crust and mantle of about 0.343 g/cm3. The basin and range area is similar to areas of uplift in that the crust is thin and has a low velocity and the mantle velocity is low. Both types of areas are characterized by negative isostatic gravity anomalies. Basin areas are characterized by positive isostatic anomalies. The relations are opposite from those to be expected if there were a lack of isostatic equilibrium due to only abnormalities in crustal thickness and can be explained by the dominant gravity effect of the crustal mass over its compensation at depth. The relations also imply a transfer of mass between the crust and mantle which may be a fundamental mechanism for maintaining isostasy. It also appears that there is an abnormal relation between velocity and density in the basin and range province where the isostatic gravity anomalies are only slightly negative, the crust is of subnormal thickness for the surface elevation, and both the crust and mantle have subnormal velocity values. Either the crust has to have a markedly subnormal density, which appears unlikely in view of the gravity anomaly values, or the mantle an abnormally high density. It is probable that the apparent increase in density contrast between the crust and mantle in this region and the anomalous relation between the velocity of the mantle and its apparent density are related to a fundamental change in mantle composition induced by a change in thermal regime. This concept is presented as an alternative to thermal dilation, serpentinization of the mantle, or mixing of crustal and mantle material that have been advanced by others to explain the low mantle velocity values.