Regional-scale resistivity structure of the middle and lower crust and uppermost mantle beneath the southeastern Canadian Cordillera and insights into its causes

Abstract

SUMMARYSubduction zones are recognized as an important class of plate boundaries and are the location of a number of important geological processes. They are also important because of the mineral and geothermal energy resources formed by plate convergence. While subduction zones around the world have a number of common features, there are also significant differences among them. The Cascadia subduction zone in southern British Columbia is characterized by a relatively hot subducting plate, and a broad backarc region that is believed to exhibit a shallow, convecting asthenosphere. The magnetotelluric (MT) method is a useful tool to study subduction zones and backarc regions because measurements of subsurface resistivity are sensitive to the presence of fluids. A number of previous MT studies have taken place in this region, but they were limited to a 2-D approach to data analysis. As the MT method has developed, it has become clear that there is a significant advantage to using a 3-D approach to data analysis. This paper presents the first regional-scale 3-D resistivity model of the southern Canadian Cordillera and provides new insights into the lithospheric structure and the distribution of fluids. The southeastern Canadian Cordillera has high heat flow and numerous thermal springs, the locations of which are often controlled by faults. However, the deeper thermal structure and origin of the fluids are poorly understood. To develop an improved understanding of the structure of this area, MT data measured at 331 locations were used to create a 3-D model of subsurface electrical resistivity. This study is primarily focused on the Omineca and Foreland morphogeological belts in southeastern British Columbia, which are separated by the southern Rocky Mountain Trench. The resistivity model is presented to a depth of 100 km and a number of conductive features are observed in the crust and uppermost mantle of the southeastern Cordillera. The locations of these conductors broadly matched previously reported conductors, but the 3-D inversion revealed new details of their geometry. The previously reported Canadian Cordilleran Regional conductor was modelled as a number of discrete conductors in the depth range 15–55 km beneath the Omineca belt. Temperatures approximately in the range 400–700 °C are expected at depths of 15–26 km and saline aqueous fluids are likely the cause of the low resistivity. Temperatures approximately in the range 700–1300 °C are expected at depths of 26–55 km and small volumes of partial melt may explain the low resistivity. The Southern Alberta–British Columbia conductor, Red Deer conductor and Loverna conductor were imaged as a single connected conductor, whose low resistivity is likely caused by sulphide mineralization. A group of conductors was imaged near the southern Rocky Mountain Trench in the depth range 10–70 km and their low resistivity is likely caused by interconnected saline fluids and possibly interconnected graphite films. To understand if the distribution of thermal springs was correlated with the 3-D resistivity model, a statistical study was undertaken. This showed no clear correlation between crustal conductance and the distribution of thermal springs.