Did Westward Subduction Cause Cretaceous-Tertiary Orogeny in by Robert S. Hildebrand

By Robert S. Hildebrand

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Extra info for Did Westward Subduction Cause Cretaceous-Tertiary Orogeny in the North American Cordillera? (Special Paper (Geological Society of America))

Sample text

This zone is often called the Columbia Embayment, and on diagrams the northern Sierra is typically connected to the Idaho batholith by a gently sweeping curved line as if there were an actual reentrant in the margin of North America (Dickinson, 2004). In those models the Idaho batholith was emplaced into the North American margin far to the east of the Sierra Nevada along a reentrant in the continental margin. This is unlikely, as both the Sierra Nevada and the Atlanta lobe of the Idaho batholith have similar terranes containing Tethyan faunas along their western margins and appear to be separated in right-lateral fashion across the Snake River Plain (Fig.

Other workers suggested that variations in flat-slab subduction, such as convergence velocity and dip beneath North America, generated the Laramide structures by increasing shear traction (Hamilton, 1988; Dickinson, 1981; Dickinson and Snyder, 1978; Bird, 1984). However, there was no arc magmatism of this age anywhere in the region, so a subduction model is generally nonviable. In the collisional model a solution is readily available because the two events are quite different in terms of kinematics in that the Cordilleran thrust belt involved rocks transferred to the upper, overriding plate and developed prior to slab failure, whereas the Laramide event involved rocks of lower plate North America and occurred during and/or just after slab failure.

Thus, not only can slab break-off account for the change from thin-skinned to thick-skinned deformation, but longitudinal variations in lithospheric thinning during break-off could produce the change from thick-skinned deformation without magmatism to the inverse situation: magmatism without thick-skinned deformation. This is exactly what is observed in western North America. The Sonora segment, south of the Colorado Plateau, contains another magmatic belt that is easily interpreted to represent slab break-off magmas (Fig.

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