Effective elastic thickness Te of the lithosphere in western Canada

Abstract

A stochastic relationship between topography and Bouguer gravity is used to calculate high‐resolution variations in effective elastic thickness, Te, of the lithosphere in western Canada. The topography‐gravity coherence is calculated using a two‐dimensional, maximum‐entropy‐based spectral estimator. This method allows for smaller data windows and provides Te determinations with higher spatial resolution than standard Fourier spectral estimators. Our analysis shows significant variations in Te in western Canada. Te increases from ∼20–40 km in the weak, young portions of the Cordillera to 100 km and greater in the strong, old Canadian Shield. Te estimates are in good agreement with lithospheric temperatures calculated from surface heat flow and radioactive heat generation data. Our calculated Te distribution also shows strong correlation with other thermally related geophysical parameters, such as lithospheric age, regional heat flow, seismicity, seismic properties, and the stress field. Consequently, we infer that lithospheric temperatures exert a primary control on large‐scale variations in Te. Collectively, the correlations readily explain why the Craton continues to be stable and undeformed, whereas the Cordillera has continued to be deformed through the Cenozoic. An exception is the Wopmay Orogen, which includes the easternmost part of the northern Cordillera. There Te is ∼90 km, although the surface heat flow is ∼90 mW/m2. We infer that the high heat flow in this region is caused primarily by very high radioactive heat generation in the upper crust and that deep lithospheric temperatures are moderately low as expected from its age and long‐term geological stability.