GEOPHYSICAL RESEARCH LETTERS, VOL. 15, NO. 1, PAGES 611-680, JULY 1988 , MAGNETOTELLURlC OBSERVATIONS ALONG THE LITHOPROBE SOUTHEASTERN CANADIAN CORDILLERAN TRANSECT AIM G. Jones 1 , Ron D. Kurtzt, Doug W. Oldenburg 2 , David E. Boerner 1 and Rob Ellis 2 Abstract. As part of the LITHOPROBE Southern Cordilleran Transect investigations, twenty-seven wide- band rnagnetotelluric (MT) responses were obtained along 150 km of the reflection seismic lines. The MT profile be- gins to the east on the Rocky Mountain Trench above au- tochthonous North American basement, traverses across the Purcell anticlinorium, the Kootenay Arc, the Nelson batholith, and ends on the east side of the Valhalla gneiss complex. Sites near Kootenay Lake respond to a major near- surface conductivity anomaly that is probably due to ei- ther graphitic schists or to an extension of the silver- bearing mineralization found further north. The short period «1 s) data from above the Nelson batholith in- dicate its thickness extent, whereas the long period (>1 s) data are highly complex and exhibit 3D regional fea- tures. Inversions of the data from the two sites on the Valhalla complex yield ID models with interfaces in elec- trical conductivity at depths that agree with an increase in the number of reflectors at 1=:::19 km, "the base of layered reflections" at 1=:::122 km, and the Moho at 1=:::135 km. EOCENE e':id aYENITE "ALIOe!N! ·U. CRET ACEOUS D ORTHOONl!ln MIO·CRETACEoul t ::: J GRANITE Introduction This letter presents the initial interpretation of a mag- netotelluric (MT) survey conducted across the eastern portion of the southern Canadian Cordillera. The survey is part of the multi disciplinary LITHOPROBE transect from the Rocky Mountain foreland thrust and fold belt to the compressional and extensional structures of the crys- talline interior of the orogen. Figure 1 illustrates the MT site locations, with the seismic lines and the gen- eralised geology from Cook et al. 1988), and shows that our profile goes om the Rocky Mountain Trench in the Rocky Mountain Belt to the east, to the Valhalla gneiss complex in the Omineca Crystalline Belt to the west. A cartoon sketch of the major seismic reflection results, based on Cook et al. (1988), is illustrated in Fig. 2, to- gether with the MT station locations. To the east, strat- ified rocks of the thrust and fold belt are detached from underlying autochthonous crystalline basement along a regional decollement at mid-crustal depths (10-25 km). These structures are interpreted to be truncated beneath JURA'SlC "; ?;"J ORANITE PRE "'O- JURA'"C MID-PROTEROZOtC TlA""N! o 20km LI __ ---J 1 o LOWER PALEOZOIC [lTIIJ U. PROTERO%OIC D Up,!" lII[] lOWER Fig. 1 Geology map of Southeastern British Columbia showing the locations of the LITHOPROBE seismic lines and the MT sites. Major Geological Features: VC: Valhalla Gneiss Complex; NB: Nelson Batholith; KA: Kootenay Arc; PA: Purcell Anticlinorium; RMT; Rocky Mountain Trench. Major faults: VSZ; Valkyr shear zone; SLF: Slocan Lake Fault; HLF: Hall Lake Fault; SMF: Saint Mary's Fault. Of major significance to our understanding of the cur- rent composition and state of the lower continental crust is that although for the Valhalla complex there are few reflections below 1=:::122 km the MT responses indicate that the zone is conducting. This is the only location known to the authors where the lower crust is seismically trans- parent but electrically conductive. 1 Geological Survey of Canada, Ontario 2 Department of Geophysics and Astronomy, University of British Columbia, Canada Copyright 1988 by the American Geophysical Union. Paper number 8L7327. 0094-8276/88/008L-7327$03.00 611 the Kootenay Arc by a major crustal-scale Eocene normal fault, the Slocan Lake fault (Parrish 1984), which delin- eates the eastern boundary of the Valhalla gneiss complex at this position. This fault may also define the eastern edge of the Canadian Cordilleran Regional conductor (see Gough 1986 and references therein). At the south-eastern end of seismic line 5, the upper crust is seismically trans- parent to about 4.0 s. Beneath this is a complex zone of reflections down to 1=:::1 8 s (1=:::122 km depth). Few reflections appear to exist in the depth range 22-35 km (8-12 s) and yet there are interpreted weak Moho reflections at 1=:::135 km at the western extremity of the line. Magnetotelluric Data and Preliminary Interpretation During late Summer 1987, remote reference MT mea- surements were made at the twenty-seven locations illus- trated in Fig. 1. In general, the data quality is very high