REGIONAL MAGNETIC ANOMALIES INDICATE CRUSTAL CONTROLS ON THE LOCATION OF THE NORTHERN CORDILLERA FOLD AND THRUST BELT

The northern Cordilleran fold and thrust belt in Canada and Alaska lies at the boundary between the broad continental margin mobile belt and the stable North American craton. The fold and thrust belt is marked by several significant changes in geometry. Craton-ward excursions in the central Yukon Territory and northeastern Alaska are separated by margin-ward intervening zones. In a general sense, the broadly sinuous inboard margin of the Cordilleran mobile belt marks the strain transition from thick- to thin-skinned structural deformation and is related to a contrast in the overall strength of the crust. Regional magnetic anomaly patterns map deep thermal and compositional features that contribute to crustal strength variations. Our detailed analysis of one such feature, the North Slope deep-source magnetic high, helps to explain the geometry of the thrust front in northern Alaska. This large magnetic feature reflects extensive mafic magmatism in an old (Devonian?) extensional domain. The presence of massive amounts of mafic material in the lower crust implies geochemical depletion of the underlying upper mantle, which serves to strengthen it against thermal erosion by upper mantle convection. We infer that deep-source magnetic highs are an important indicator of strong lower crust and upper mantle that are resistant to thick-skinned deformation. These buttress zones play an important role in the structural response of the northern Cordilleran margin to lateral tectonic forces.