PRELIMINARY MAGNETOTELLURIC INVESTIGATION OF ELECTRICAL RESISITIVTY VARIATIONS ACROSS THE CASCADE ARC AND COLUMBIA RIVER PLATEAU BASALTS, WASHINGTON AND IDAHO

The recent long period magnetotelluric (MT) experiment in the Pacific Northwest is a vital part of the USArray component of the Earthscope project. MT stations were collected in Washington and western Idaho at an approximately 70 km spacing that roughly corresponds to the station spacing of the USArray seismic experiment. Initial processing of the data indicates that the TM and TE impedance curves that are relatively smooth between 10 and 10,000 seconds. A strike analysis based on principal axis impedance rotations and tipper directions indicates a wide distribution of directions for the tipper strike angles. This 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 2-D model was constructed from the Olympic Mountains to Idaho (~ Lewiston) 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 to determine what features were required by the data and the depth sensitivity of the data. A final model was constructed which shows low resistivity values within the Olympic Mountains accretionary complex with higher values of the subducting Juan de Fuca plate below it. The Cascades volcanic arc is seen by low resistivity values in the lower crust with these values being roughly connected to low resistivity values in the upper mantle within the backarc region of the CRPB, and whose location corresponds to high heat flow values. The crustal region under the CRPB is seen to be conductive in the middle crust and probably corresponds to Tertiary sediments and a more resistive suite of rocks underneath them. The presence of these high resistivity regions agrees with higher density and velocity models obtained from gravity and high frequency seismic analyses (Moschetti et al., 2007). Lastly, the Precambrian region of eastern Washington/western Idaho contains two regions of high resistivity values, the metamorphosed Belt Basin sediments extend to approximately 117 degrees E while a higher resistivity region extends into the upper mantle and to the area west of the Belt Basin. This more western location agrees with interpretations of gravity and magnetic data (Mickus, 2002) of the Precambrian plate boundary location under the CRPB.