Abstract

During the latest Cretaceous-early Paleogene Laramide orogeny, the lithosphere beneath the southernmost Sierra Nevada batholith and the adjacent Mojave Desert region batholith was sheared off and displaced deeper into the mantle. The lithosphere beneath the greater Sierra Nevada batholith to the north was left intact until mid-Miocene time, when fragments of it were entrained as volcanic xenoliths. The Laramide slab was evidently segmented into a shallow flat segment to the south and a deeper segment to the north. Shearing off of the upper mantle to the south was followed by the tectonic underplating of schists derived from Franciscan subduction complex and possibly forearc basin protoliths. The overlying batholithic crust was deformed, deeply denuded, and tectonically breached westward across its forearc region while the schists were underplated. Westward breachment resulted from a combination of west-directed thrusting and extensional collapse. The westernmost breached rocks were tectonically removed by a combination of trench linked transform and subduction erosion processes. Subsequent tectonic erosion by the Neogene San Andreas transform has left the Salinia and the San Gabriel terranes as dispersed residual fragments of the westward breached arc segment.

The shallow slab segment appears to have been ∼500 km in width, measured along the plate edge. To the south, the slab descent appears to have remained deep beneath the Peninsular Ranges batholith. Classic Laramide structures of the craton are concentrated in a corridor that corresponds to the shallow slab segment as defined by plate edge relations and the corresponding trajectory of Farallon–North American relative plate motions when viewed on a pre-Neogene palinspastic base. The plate interior is suggested to have been deformed first by end loading as the shallow slab segment initially descended beneath the plate edge, and then by greater basal traction components as the shallow segment progressed beneath the cratonic region. The subcontinental mantle lithosphere beneath the cratonic deformation zone remained intact through Laramide time.

Similar segmented slab topologies have been resolved beneath the modern Andean orogen where the shallow segments are correlated with the subduction of aseismic ridges. A number of researchers have suggested that the Laramide orogeny arose from the subduction of a counterpart of the Hess-Shatsky large igneous province of the northwest Pacific basin. Fragments of rock assemblages that are correlative to the Hess-Shatsky province that were accreted to the Franciscan complex in Laramide time support this view. Recognition of the resulting shallow slab segment and its trajectory beneath North America explains the geographic focusing of Laramide deformation, relative to the rest of the Cordilleran orogen, and the relationships between plate edge and plate interior deformational regimes.

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