CRUSTAL CONTROLS ON THE LOCATION OF THE NORTHERN CORDILLERAN FOLD AND THRUST BELT

The northern Cordilleran fold and thrust belt in Canada and Alaska is marked by several significant changes in geometry. North and west from the craton-ward bulge in the central Yukon Territory, the fold and thrust belt reverses strike to the north-northeast in the Mackenzie Trough region, bulges northward in northeastern Alaska, trends west through central and western northern Alaska, and bends sharply to the northwest on the Lisburne Peninsula and Chukchi Sea. In a general sense, this inboard margin of the Cordilleran mobile belt marks the transition from thick- to thin-skinned structural deformation. Regional gravity and magnetic studies in northern Alaska help to explain the marked changes in the location of this structural transition. The North Slope deep-source magnetic high bounds the fold and thrust belt through central and western Alaska. This large magnetic feature is thought to reflect extensive mafic magmatism in an old (Devonian?) extensional domain. The North Slope deep-source magnetic high therefore is likely to define a strong crustal domain. Continental scale aeromagnetic compilations, such as the 2002 Magnetic Anomaly Map of North America (North American Magnetic Anomaly Group, 2002), display distinctive long-wavelength magnetic anomalies like the North Slope deep-source magnetic high. These broad magnetic highs correlate remarkably with the geometry of the northern Cordilleran fold and thrust belt as shown in the new Geologic Map of North America (Reed, Wheeler, and Tucholke, 2005). It appears that deep-source magnetic highs are an important indicator of strong crustal character and basement resistance to thick-skinned deformation. These basement buttress zones play an important role in the structural response of the northern Cordilleran continental margin to lateral tectonic forces.