Magnetotelluric Soundings of Crustal Conductive Zones in Major Continental Rifts
2011; American Geophysical Union; Linguagem: Inglês
10.1029/sp014p0209
ISSN2771-6929
AutoresGeorge R. Jiracek, Mark E. Ander, H. Truman Holcombe,
Tópico(s)Geomagnetism and Paleomagnetism Studies
ResumoMagnetotelluric Soundings of Crustal Conductive Zones in Major Continental Rifts George R. Jiracek, George R. Jiracek Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131Search for more papers by this authorMark E. Ander, Mark E. Ander Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131Search for more papers by this authorH. Truman Holcombe, H. Truman Holcombe Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131Search for more papers by this author George R. Jiracek, George R. Jiracek Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131Search for more papers by this authorMark E. Ander, Mark E. Ander Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131Search for more papers by this authorH. Truman Holcombe, H. Truman Holcombe Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131Search for more papers by this author Book Editor(s):Robert E. Riecker, Robert E. RieckerSearch for more papers by this author First published: 01 January 1979 https://doi.org/10.1029/SP014p0209Citations: 15Book Series:Special Publications AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onEmailFacebookTwitterLinkedInRedditWechat Summary Following the successful seismic soundings across the Rio Grande rift in New Mexico by the Consortium for Continental Reflection Profiling (COCORP), eight magnetotelluric stations were occupied in the area in 1977. The MT stations were positioned so as to traverse a strong reflection seismic event which correlated with a magma chamber previously interpreted at about 20 km depth using microearthquake observations. Results from these MT stations within and transitional to the Rio Grande rift are compared to similar soundings from the Baikal rift in eastern Siberia, the East African rift, and the Rhine graben in Germany. The comparison consists of actual MT sounding data and geoelectric interpretations presented by several researchers. All interpretations made within the rifts themselves have a common characteristic that a zone of low resistivity (≤ 50 ohm-m) is modeled at depths less than 30 km. This conductive layer is not observed in the crust in adjacent regions. Laboratory measurements support the view that such a zone can result when significant amounts of pore fluids are present in an enhanced geothermal gradient giving rise to partial melting in the crystalline basement. Considering all effects, including hydrothermal alteration products, large resistivity decreases can be expected in a wet granitic crust at temperatures as low as 500°C. One-dimensional interpretation of the Rio Grande rift soundings does not show any discontinuity at 20 km depth. Instead, the depth to the conductive horizon appears to be near 10 km or less. Such an electrical interface may still be caused by a high temperature (∼900°C) magma at depth; a major electrical resistivity decrease requires only a small interconnected fraction of partial melt at a temperature as low as about 600°C and/or conductive hydrous mineral phases (e.g. amphibole) at still lower temperature (∼500°C). Rio Grande rift volcanics contain xenolithic inclusions inferred to originate from lower crustal depths (possibly below 10 km) which show a marked degree of interconnected amphibole. The Rio Grande rift results illustrate considerable variation within the rift zone and expose the need for three-dimensional interpretations. 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