Crustal Geophysics and Geochemistry Science Center

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Geophysics of Rio Grande Basins

Geophysical Expression and Hydrogeology of Faults

San Ysidro Fault model

The multi-disciplinary studies of faults and their impact on ground water are culminating in several important summary points, as follows.

  1. Unlike faults in crystalline rocks, faults in the poorly lithified basin sediments have fault-zone characteristics that make them likely partial barriers to ground-water flow. These characteristics include pervasive clay-rich cores, fault-localized cements, and a paucity of open fractures in damage zones. (See 2006 USGS Professional Paper 1719 by Minor and Hudson and Caine 2009 paper [PDF file, 7 MB]).
  2. Faults in basin sediments are commonly expressed in high-resolution aeromagnetic data and indicate there are many more shallowly buried faults than expected throughout the basin. Geophysical analysis and magnetic-property studies indicate the primary source of the fault-related aeromagnetic anomalies is tectonic juxtaposition of strata rather than an effect of chemical processes. Thus, aeromagnetic data provide information on the largest and most vertically extensive permeability contrasts that may be associated with aquifer compartmentalization. (See 2008 GSA Bulletin article by Hudson [PDF file, 3.8 MB], Grauch, and Minor; 2007 Geosphere article by Grauch and Hudson [ftp site contains pdf and compressed zip files, these files may not be 508 compliant])
  3. The surprising number of faults revealed by the aeromagnetic data and the likelihood that these faults have significant influence on groundwater flow imply that the effects of faults on the ground-water systems of alluvial basins is significant. More on these topics.

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