Mapping Curie Depth Across Western Canada From a Wavelet Analysis of Magnetic Anomaly Data

• Published in: Journal of geophysical research. Solid earth 2019-05, Vol.124 (5), p.4365-4385
• Main Authors: Gaudreau, É., Audet, P., Schneider, D.A.
• Format: Article
• Language: English
• Subjects: Anomalies; Bedrock; Computer simulation; Cratons; Curie depth; Curie temperature; Depth; Domains; Earth; Earth crust; Elevation; Estimates; Flow measurement; Fractal models; Geodynamics; Geographical variations; Geophysical studies; Geophysics; Geothermal gradient; Heat; Heat flow; Heat transfer; Heat transmission; Imaging techniques; Magnetic anomalies; Magnetic properties; Magnetization; Mapping; Mechanical properties; Monte Carlo simulation; Mountains; North American Cordillera; Physical properties; Plate tectonics; Plates (tectonics); Rock properties; Sedimentary rocks; Statistical methods; Tectonics; Tectonophysics; Temperature; Temperature differences; Temperature effects; Temperature gradients; Thickness; Topography (geology); Wavelet analysis; wavelet transform
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2018JB016726

In western Canada, geophysical studies infer an abrupt change in crustal temperatures between the Canadian Cordillera and the adjacent North American craton, with important implications for the tectonics and geodynamics of the area. We use a wavelet analysis of magnetic anomaly data in western Canada to map the depth to the bottom of the magnetic source, or Curie depth. This depth corresponds to the point at which crustal rocks reach their Curie temperature, thus providing an estimate of geothermal gradient. Our model is defined by a fractal distribution of magnetization characterized by the parameter β, as well as the depths to the top (zt) and bottom (zb) of the magnetized layer. Synthetic tests reveal the increased accuracy of the estimated zb when values for zt and β are fixed prior to the inversion. We set zt to the thickness of sedimentary rocks overlying the magnetic bedrock and use various values of β to estimate zb. We determine β a posteriori by comparing Monte Carlo simulations of predicted heat flow values (assuming a Curie temperature of 580 °C) with observed heat flow in various regions. Our results suggest a β value of 2.5 for the Canadian Cordillera and Slave craton, and 2 for the remaining North American craton. The Curie depths resolve geological domains and important structural features, with estimates for zb averaging 15 ± 1 km in the Cordillera, 32 ± 3 km in the Slave craton, and 34 ± 3 km in the North American craton to the south. Plain Language Summary Despite a relatively high elevation, geophysical imaging of the Earth beneath Canada's western mountains reveals a distinct 150 km thinning of the tectonic plates. The plates define the rigid outer shell of the Earth, and the thickness change is coincident with a topographic difference (higher) at the surface and a temperature difference (hotter) at depth. The relationship between the high topography with the thinner tectonic plate and hotter material at depth is not well understood. The objective of this research is to estimate temperatures of the Earth's crust below the surface in western Canada using the magnetic properties of rocks. At a certain temperature, rocks lose their magnetization and therefore do not produce magnetic anomalies. By analyzing magnetic anomaly data, we can infer the depth at which this temperature is achieved, which provides estimates of the thermal gradient in the crust. Combining these estimates with heat flow measurements provides more accurate constraints