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

Abstract

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 (z t ) and bottom (z b ) of the magnetized layer. Synthetic tests reveal the increased accuracy of the estimated z b when values for z t and β are fixed prior to the inversion. We set z t to the thickness of sedimentary rocks overlying the magnetic bedrock and use various values of β to estimate z b . 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 z b 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 on the physical properties of the tectonic plate and will allow different tectonic models to be evaluated, allowing for a better understanding of the mechanical behavior of the Canadian Cordillera.

Mapping the Thermal Structure of Southern Africa From Curie Depth Estimates Based on Wavelet Analysis of Magnetic Data With Uncertainties

“…Various spectral methods have been proposed and applied to estimate depth to the bottom of the magnetized layer in the lithosphere (e.g., Tanaka et al., 1999; Audet & Gosselin, 2019; Bouligand et al., 2009; Chopping & Kennett, 2013; Gaudreau et al., 2019; Li et al., 2017; Maus et al., 1997; Salem et al., 2014). This depth will be identified with the Curie depth throughout the remainder of this paper.…”

The Upper Mantle Structure of Northwestern Canada From Teleseismic Body Wave Tomography

“…A variety of geophysical studies indicate that the NCC is underlain by a flat (Cook et al, 2004;Cook & Erdmer, 2005) and shallow Moho at 32 ±2-km depth (Tarayoun et al, 2017) and crust with a similarly shallow Curie depth (Gaudreau et al, 2019). The region has high heat flow (≥100 mW/m 2 ; Lewis et al, 2003), high temperatures at the Moho (∼900 • C) in contrast with ∼400 • C in the Canadian Shield (Audet et al, 2019;Lewis et al, 2003), and <20-km effective elastic thickness (Audet et al, 2007;Flück et al, 2003).…”

Seismic Evidence for Lithospheric Thinning and Heat in the northern Canadian Cordillera

“…Elastic thickness studies indicate a sharp decrease in lithospheric strength from east to west across the CDF (Audet et al, ; Flück et al, ). Curie depth estimates based on magnetic anomaly data imply much higher crustal temperatures in the NCC compared with the adjacent craton (Gaudreau, Audet, et al, ). Petrologic data also suggest that the depth to the adiabat is ~50–65 km below the NCC (Francis et al, ; Harder & Russell, ).…”