Coincident conductive and reflective middle and lower crust in southern British Columbia

Abstract

SUMMARY Processing and interpretation of magnetotelluric data, recorded as part of the LITHOPROBE Southern Cordillera transect studies, across the boundary of the Intermontane and Omineca morphogeological belts reveals: (a) high electrical conductivity in the middle and lower parts of the crust everywhere, and (b) a depth dependency of geoelectric strike. The data have been modelled using two different inversion algorithms and different methods for correcting 'static shifts'. The two different approaches gave similar results: the depth to the top of a conductive layer decreases from 15-17km in the west across the Intermontane Belt to 8-10km across the transition to the Omineca Belt. The top of this conductive layer is closely coincident with a layer of increased seismic reflectivity as shown by reprocessing of collocated LITHOPROBE seismic-reflection data. The eastward shallowing is associated with an increase in heat flow such that the top of the conductive and reflective zones remains at 400-450 C. This coincidence suggests that the increased reflectivity and the high electrical conductivity observed in the middle crust may have a common cause, and that their presence is limited to where the present temperature exceeds a critical value. One explanation that meets these conditions is that both the conductivity and reflectivity are produced by a small amount of aqueous fluid porosity. We propose that fluids are trapped in the middle crust by a ductile shear zone, previously interpreted from the seismic sections as the Okanagan Valley Fault to the west of Okanagan lake. The geoelectrical strike varies from N25W for the first 5-10 km of the crust, to N20E for the middle/lower crust, and to N60E for the upper mantle. This variation indicates that the exotic terrane material is concentrated in the uppermost part of the crust and that the remainder of the crust is composed of ancestral North American rocks.