Type:
Research Report, Student Research
Link:
Geothermal exploration and tectonic studies in southwestern Canada using the magnetotelluric method
Author:
Cedar Hanneson
Citation:
Hanneson, C. (2023). Geothermal exploration and tectonic studies in southwestern Canada using the magnetotelluric method. [Doctoral dissertation, University of Alberta]. ERA.
Abstract:
Subduction zones are an important class of plate boundaries and are the location of several important geological processes. Significant mineral and geothermal energy resources are also formed by plate convergence. The magnetotelluric (MT) method is a useful tool to study subduction zones, volcanoes, backarc regions, and geothermal systems because measurements of subsurface resistivity are sensitive to the presence of fluids such as brine and partial melt. In this thesis, the following three applications of MT exploration in the Cascadia subduction zone and surrounding areas in southwestern Canada are described.
1) The Mount Meager volcanic complex in southwestern British Columbia (BC) is an active volcano and one of Canada’s most promising geothermal prospects. MT data collected at Mount Meager were inverted to create an electrical resistivity model to a depth of 25 km. The model is characterized by high resistivity (> 100 Ωm) in the upper 6–7 km, implying relatively dry, unaltered rock. Within this resistive layer, localized conductors observed in the upper 2 km correspond to clay-rich layers that may act as caprocks and trap geothermal fluids below. Beneath the resistive upper crust, there is a major conductor in the depth range 5–15 km below sea level with an average resistivity of ∼3 Ωm. Laboratory experiments of melt resistivity and petrological data from erupted volcanic rocks were used to interpret the resistivity of this feature. The inferred magma body was estimated to have a minimum volume of ∼2×10^12 m^3 comprising 18–32% dacitic-to-trachydacitic melt with 6–8 wt.% H2O at a temperature of 800–900 °C. The model also shows low resistivity fluid pathways from the northern part of the magma body, that rise toward Mount Meager and nearby fumaroles. Along with other geophysical and geological models produced by the Garibaldi Geothermal Energy Project, this resistivity model will reduce the exploration risk associated with geothermal energy development.
2) Geothermal exploration with MT is widely used in focused studies such as that described above at Mount Meager. However, MT also has the potential to determine regional-scale controls on the distribution and location of geothermal resources. MT data measured at 331 locations were used to create the first regional-scale 3-D resistivity model of the southern Canadian Cordillera. A number of conductive features were observed in the crust and uppermost mantle of the southeastern Cordillera. 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 in the range 400–700 °C are inferred at depths of 15–26 km and saline aqueous fluids are likely the cause of the low resistivity. Temperatures 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 (SABC), 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.
3) MT data were used in southwestern Canada to study changes in lithospheric thickness. Previous geophysical studies have shown that the lithosphere-asthenosphere boundary (LAB) is at a depth of 60–70 km beneath the Cascadia backarc, increasing to more than 150 km beneath the craton in Alberta. MT can also image the depth of the LAB and the model derived in the previous study was used to map the depth of the LAB. A subvertical backarc-craton lithosphere step (BCLS) occurs between 117 °W and 119 °W in southern BC. The vertical projection of the BCLS at the surface of the Earth is located between the Kootenay Arc and a line marking granitic rocks with an initial 87Sr/86Sr ratio of 0.706. The cratonic lithosphere thickens to the north, from 150 km in southern Alberta to 250–300 km in central Alberta. The transition from a subhorizontal LAB in southern Alberta to a northward dipping LAB occurs below the SABC.
The results of these studies could aid further geothermal exploration and tectonic studies in southwestern Canada.
Keywords:
Geothermal, Geophysics, Cascadia, Resistivity, Volcanology, Tectonics