Occam's Inversion and the North American Central Plains Electrical Anomaly

iterations excess structure was removed to reveal three discrete conductive zones of resistivity less than 1 Ωm at depths of 8–22 km in a relatively resistive background layer of 100–1000 Ωm. The inversion algorithm was modified to take advantage of the 1D structure of the shorter periods, which were inverted to obtain a model representative of surface sediments which extend to a depth of about 2 km. This surface structure was included in the long-period inversion by imposing a second penalty term in the regularized inversion, and the resulting model featured a broader, more complex conductive anomaly and a pronounced, westward-dipping fabric in the mid-to lower-crustal rocks. Graphitic rocks can account for the highly conductive parts of the models. In this paper we present the results of two-dimensional (2D), smooth inversions of the COPROD2 magnetotelluric (MT) data set (Jones and Savage, 1986; Jones, 1988) using the Occam’s inversion algorithm of Constable, Parker and Constable (1987), modified for 2D MT by deGrootHedlin and Constable (1990). The MT data were obtained over the North American Central Plains (NACP) and Thompson Belt electromagnetic anomalies in order to obtain a quantitative model of features previously identified in numerous geomagnetic depth sounding (GDS) surveys (see the summary by Jones and Craven, 1990). The GDS surveys indicated that the NACP anomaly is a large scale, linear feature extending from southeastern Wyoming northward to the exposed Canadian Shield in northern Saskatchewan, and from there extending eastward along the regional geological strike into the Hudson’s Bay basin. The alignment of zones of intense shearing and folding in the exposed Precambrian crust with both the northern and southern ends of the NACP anomaly led Camfield and Gough (1977) to suggest that the anomaly is associated with a Proterozoic plate boundary. Our aim here is to develop methods of interpretation of approximately 2D MT data sets using smooth inversion methods and to apply these techniques to determine the conductivity structure of the NACP anomaly. Although three-dimensionality is evident along the length of the conductivity anomaly in the GDS data (Alabi et al., 1975), as well as in MT observations recorded in two profiles to the north of the COPROD2 line (Jones and Craven, 1990), the NACP