Tectonics of the northern Canadian Cordillera imaged using modern magnetotelluric analysis

Magnetotelluric data from LITHOPROBE SNORCLE Corridor 2 in western Canada crossing the Rocky Mountain Trench-Tintina Fault between 56 degrees and 60 degrees N have been analyzed using modern MT methods to provide new insights into the tectonics of the North American Cordillera and Ancestral North America. Resistivity images of the crust were obtained using two independent sets of unconstrained 2-D inversions and constrained 2-D inversions incorporating seismic constraints. Structure to the west of the Tintina Fault includes uniform, moderate, mid to lower crustal resistivity in a seismically-defined westward-tapering wedge of Ancestral North American rocks. To the east, between the Tintina Fault and the Liard line, a transfer zone in asymmetric Neoproterozoic rifting, the wedge includes a major synformal conductor that parallels seismic reflections. The large lateral changes in resistivity can be explained by transition between different blocks of asymmetric rifting. The synformal conductor is attributed to graphitic rocks from the Aida Formation of the Mesoproterozoic Muskwa Assemblage that were re-distributed in a sag basin on the lower plate margin in the Blackwater Block. Spatially-complex upper crustal resistivity in the Cordillera can be interpreted in terms of lithological variations and alteration processes. An east-dipping conductor in Stikinia is attributed to east-dipping Jurassic and relatively high conductivity beneath the Eskay rift may be explained by crustal fertilization. A conductor in the Cache Creek terrane is attributed to listwanite alteration of ultramafic bodies on the King Salmon Fault, and high conductivity in the Slide Mountain terrane within the Sylvester Allochthon to localized carbonaceous alteration. A series of conductors above North American Basement is attributed to deformation and metamorphic processes associated with Mesozoic thrust faulting. To the west of the Tintina Fault the source material for these conductors is likely from the Stikinia terrane, and to the east it is likely from the Muskwa Assemblage.