Mesozoic aseismic ridges on the Farallon Plate and southward migration of shallow subduction during the Laramide Orogeny

Abstract

We propose that an aseismic ridge of Late Jurassic and Early Cretaceous age on the Farallon plate was subducted beneath the Unites States Cordillera during the Laramide orogeny. The relative buoyancy of the aseismic ridge caused shallow subduction which resulted in a magmatic lull 70–40 Ma in the existing near‐trench magmatic arc and Laramide uplift and faulting of crystalline basement 1000–1500 km inland from the trench. The timing and latitudinal limits of the subduction of the aseismic ridge are similar to the timing and latitudinal limits of both the Laramide orogeny and the magmatic lull in the western Cordillera. Because the north–northeast trending aseismic ridge was moving northeast relative to the North American plate, the point of collision of the aseismic ridge with North America, and therefore the locus of shallow subduction, migrated southward with time, causing the well‐documented southward migration of the magmatic lull.