Magnetotelluric Soundings of Crustal Conductive Zones in Major Continental Rifts
- Robert E. Riecker
Published Online: 21 MAR 2013
Copyright © 1979 by the American Geophysical Union.
Rio Grande Rift: Tectonics and Magmatism
How to Cite
Jiracek, G. R., Ander, M. E. and Truman Holcombe, H. (1979) Magnetotelluric Soundings of Crustal Conductive Zones in Major Continental Rifts, in Rio Grande Rift: Tectonics and Magmatism (ed R. E. Riecker), American Geophysical Union, Washington, D. C.. doi: 10.1029/SP014p0209
- Published Online: 21 MAR 2013
- Published Print: 1 JAN 1979
Print ISBN: 9780875902142
Online ISBN: 9781118664988
- Crustal conductive zones;
- Geoelectric models;
- Geomagnetic depth soundings (GDS);
- Magnetotelluric soundings;
- Rio Grande rift
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. The lack of three-dimensional considerations in any of the world-wide MT studies casts some doubt on the absolute values of resistivity, depth, and thickness of the calculated crustal conductive zone.