Abstract

The Laramide orogeny has generally been attributed to a shift from normal-angle to shallow-angle subduction beneath the western margin of the North American plate. In addition to important mechanical effects, this shift may have had important thermal effects on the lithosphere in the western Cordillera. Before the Laramide, geothermal gradients in the western Cordillera were probably normal to high because of the presence below of a hot asthenospheric wedge. With the shift to shallow subduction, the wedge was probably expelled and replaced by a cold subducting slab that extended just below the Cordilleran lithosphere. This would shift the western Cordillera into a cold, forearc-like thermal setting dominated by the refrigeration effects of the subducting slab. Thermochronologic data from the Sierra Nevada, Great Basin, Mojave Desert, western Arizona, and perhaps the Colorado Plateau record latest Cretaceous-early Tertiary cooling that may be evidence of such refrigeration. If regional refrigeration occurred, it would have several important implications: (1) metamorphism in the western Cordillera would have waned with the start of the Laramide orogeny; (2) the crust in the western Cordillera would have strengthened and become much more resistant to deformation (e.g., gravitational collapse and extension of thickened crust in the Sevier hinterland would have been impeded); and (3) latest Cretaceous-early Tertiary isotopic cooling ages may not be valid indicators of a period of major regional uplift and unroofng.

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