[unav_all] Webinar: Three-dimensional inversion of EarthScope magnetotelluric data: crustal and mantle electrical conductivity of the NW US

Andy Frassetto andyf at iris.edu
Mon Nov 19 12:25:26 MST 2012

The next IRIS webinar will present "Three-dimensional inversion of 
EarthScope magnetotelluric data: crustal and mantle electrical 
conductivity of the NW US" on November 27, 2012 from 3-4 pm ET (8-9 pm UTC).

To attend, register by entering your name and email here: 
https://www2.gotomeeting.com/register/817110738. You will then receive a 
confirmation email containing information about joining the webinar. The 
webinar will be recorded and available for viewing afterward. Following 
the presentation there will be the opportunity to direct questions to 
the speaker. More information on IRIS research webinars, including links 
to previously recorded presentations and related materials, may be found 
here: http://www.iris.edu/hq/webinar/

Please contact Andy Frassetto (andyf at iris.edu) with any additional 

Presenter: Gary Egbert, Oregon State University

Summary: "Long period (10-20,000s) magnetotelluric (MT) data are being 
acquired in a series of temporary arrays deployed across the continental 
USA through the EMScope component of EarthScope. The MT data are highly 
sensitive to fluids and melt, and thus provide a valuable complement to 
other observational components of EarthScope. In this presentation we 
will review basics of the MT method, and then discuss 3D inversion and 
interpretation of EMScope data from the Northwestern US, acquired in 
2006-2011. For the inversion we use full impedances and vertical field 
TFs from 325 sites on a quasi-regular grid (nominal ~70km spacing of the 
seismic TA) covering a rectangular area from NW Washington to NW 
Colorado. The inverse solutions reveal extensive areas of high 
conductivity in the lower crust and uppermost mantle beneath the 
extensional Basin and Range, High Lava Plains, and Snake River Plain 
provinces, as well as beneath the Cascade volcanic arc. These high 
conductivities can only be explained by partial melt and/or magmatic or 
subduction related saline fluids. Stable Proterozoic lithosphere in the 
northeastern part of the domain is generally much more resistive, with 
the thickest resistive sections coinciding with the Wyoming and Medicine 
Hat Cratons. Oceanic lithosphere of the subducting Juan de Fuca Plate is 
clearly imaged as a zone of very high resistivity beneath the Coast 
Ranges. Other prominent resistive zones in the NW part of the domain may 
represent relict oceanic lithosphere: the accreted “Siletzia” terrane 
beneath the Coast ranges and Columbia Embayment, and a deep vertical 
resistive feature just to the east—the seismically fast “slab curtain” 
beneath Eastern Idaho that others have interpreted to be stranded 
Farallon lithosphere. Aesthenospheric conductivities are generally 
consistent with laboratory results for moderately hydrated (~200-300 
ppm) olivine, with a potential temperature of ~1300C. Higher 
aesthenospheric conductivities occur east of the Rocky Mountain front, 
where greater hydration or higher temperatures are required, and in the 
back arc, where broad fingers of high conductivity rise to very shallow 
depths. The most prominent of these features occurs in Washington State, 
where anomalously high conductivities dip to the S-E, suggesting 
aesthenospheric flow around the slab curtain. There are also some areas 
of reduced aesthenospheric conductivity (beneath the slab curtain, and 
south of the Yellowstone hot spot), suggesting lower temperatures and/or 
depletion of volatiles in these areas."

System Requirements
PC-based attendees
Required: Windows® 7, Vista, XP or 2003 Server
Macintosh®-based attendees
Required: Mac OS® X 10.5 or newer

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