Contrasting Types of Lower Crust

  1. Robert F. Mereu,
  2. Stephan Mueller and
  3. David M. Fountain
  1. Scott B. Smithson

Published Online: 9 APR 2013

DOI: 10.1029/GM051p0053

Properties and Processes of Earth's Lower Crust

Properties and Processes of Earth's Lower Crust

How to Cite

Smithson, S. B. (1989) Contrasting Types of Lower Crust, in Properties and Processes of Earth's Lower Crust (eds R. F. Mereu, S. Mueller and D. M. Fountain), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM051p0053

Author Information

  1. Program For Crustal Studies, Dept. of Geology and Geophysics University of Wyoming, Laramie

Publication History

  1. Published Online: 9 APR 2013
  2. Published Print: 1 JAN 1989

ISBN Information

Print ISBN: 9780875904566

Online ISBN: 9781118666388

SEARCH

Keywords:

  • Earth—Crust—Congresses;
  • Geophysics—Congresses

Summary

The deep crust of the Basin and Range of NE Nevada and the Minnesota Archean represent strongly contrasting tectonic features. the lower crust in the Basin and Range of NE Nevada is horizontally layered, probably as a result of extensional plastic flow during the Cenozoic, representing a major metamorphic event generating granulites in the deep crust. the lower crust is compositionally layered and predominantly mafic underlain by a more mafic 3-km zone of cumulate or residual material related to magmatic history. the Moho is a sharp compositional boundary marked by strong interlayering of mantle and crustal rocks to produce unusually high reflectivity, and the crustal rocks may be partially molten to explain the reflectivity. About 50% of the crust is mafic material, and probably a maximum of 25% of the crust represents mafic additions and underplating during Cenozoic extension. In the Minnesota-early Archean crust, reflections become weaker and less continuous in the lower crust, and absent in the lowermost crust and Moho zone. Here most of the deep crust seems to have been thickened by stacking of nappes, which are “floating” in small bodies of later anatectic granite. the lowermost Archean crust is a distinctly different tectonic pattern possibly formed by subhorizontal compressional shearing that may have erased larger scale structures or by later gabbroic underplating that thickened the early Archean crust. the Moho in Minnesota must be strikingly different from the Basin and Range. It is either gradational or a relatively small step-function change in chemical composition such as garnet pyroxene granulite to peridotite; it is probably not layered unless the velocity changes across the layering are very small. If it is gradational, then it is probably formed by the gabbro-eclogite phase change, and the gabbroic zone could have been underplated.