Anne F. Sheehan
Office: ESCI 440A
Web: Sheehan Research Group
Seismology, Geophysics, Earth Structure
Current Research: Magnetotellurics: Using lightning and the solar wind to determine subsurface electrical properties beneath Colorado and New Mexico
In collaboration with Paul Bedrosian at the U.S. Geological Survey in Denver and CIRES graduate student Daniel Feucht, a magnetotellurics experiment is underway to determine deep electrical properties of the crust and upper mantle beneath Colorado and New Mexico. Magnetotellurics is an electromagnetic- sensing method that makes use of electrical currents in the earth that are induced by Earth’s varying magnetic field. The method uses that information to determine Earth’s subsurface electrical properties. The natural fluctuations in Earth’s magnetic field are typically caused by the solar wind and lightning strikes. Electrical conductivity is a unique physical property influenced by factors hard to evaluate other ways, such as very small amounts of fluids (including melts). Magnetotellurics provides information complementary to seismic tomography for deep Earth imaging; has applications to geothermal exploration; and is very sensitive to the presence of subsurface fluids, including melt and aqueous fluids. Our 2012–2014 magnetotelluric experiment across the Rio Grande Rift in Colorado and New Mexico will provide constraints on the thermal and rheological state of the lithosphere beneath this region of intracontinental extension. The Rio Grande Rift formed between 35 million and 29 million years ago when the crust began to spread apart, triggering volcanism in the region. The Rio Grande Rift extends hundreds of miles from Colorado’s central Rocky Mountains south into Mexico. In collaboration with CIRES Fellow Steve Nerem and using a CIRES Innovative Research Program award to start the project, we have been monitoring since 2006 the active tectonics of the Rio Grande Rift by using Global Positioning System (GPS) geodesy. The GPS results indicate that the rift is still active but that the strain rates are very low. The magnetotellurics experiment provides imaging that will help us understand the rifting process. Important questions about continental rifting remain unresolved, including the role of magmatism, volatiles, and inherited lithospheric structure in the initiation and development of rifting. Recent seismic-imaging studies show thinned crust and low seismic wave speeds in the upper mantle beneath the Rio Grande Rift. New geodetic work confirms the low strainrate environment of the region and shows surprisingly broad and uniform deformation over an area far wider than the rift’s physiographic expression. Electrical conductivity models from the magnetotelluric experiment will provide information complementary to seismic studies, and will be combined with seismic models to assess the relative contribution of thermal and compositional heterogeneity to the rifting process. A comparison of results from northern and central rift segments will be used to assess extent and hypothesized northward propagation of the rift into Colorado.
PDF files of Sheehan publications can be found on the Sheehan Research Group site.
Sheehan is a CIRES Professor.