MAGNETOTELLRUIC MODELS OF THE SNAKE RIVER PLAIN, IDAHO

The USArray component of the Earthscope project consists of broadband seismic and long period magnetotelluric (MT) stations spaced throughout the US. The transportable array in 2008 collected data in Idaho and western Montana, and included the Snake River Plain of southern Idaho. Initial processing of the data indicates that the TM and TE impedance curves are relatively smooth between 10 and 10,000 seconds and possibly to 20,000 sec. A strike analysis based on principal axis impedance rotations and tipper directions indicates a wide distribution of directions for the tipper strike angles but within the Snake River Plain there is a rough east-west strike direction. A wide distribution of strike directions implies a 3-D electrical conductivity environment that requires 3-D modeling, but Wannamaker (1984) has shown that 2-D models can be obtained from TM mode data. A series of 2-D models were constructed perpendicular and along the Snake River Plain's axis using smoothed TM mode apparent resistivities and phases. Several 2-D inversions were constructed using data rotated to impedance and tipper strike directions, and various starting models were used to determine what features were required by the data and the depth sensitivity of the data. Final models were constructed which shows low resistivity values beneath the Snake River Plain, however these low values are shifted north of the surface expression of the Plain. The low resistivity region is approximately 70-90 km wide and can be imaged to approximately 150-200 km below the surface. The resistivity values are mainly below 10 ohm-m which implies that there is a significant amount of partial melt. This agrees with broadband seismic studies which show that there is NW-dipping low velocity zone beneath the Snake River Plain. An axial model along 43.5 degrees north, indicates that the low resistivity values basically end at approximately 114 degrees west which agrees with regional gravity anomalies where the high amplitude gravity anomaly over the Snake Plain becomes lower in amplitude and discontinuous at 114 west. The MT results when combined with the broadband seismic and regional gravity anomalies as an additional constraint to the plume tail model of Shervais and Hanan that the plume tail track is controlled by pre-existing lithospheric structure.