A COLLISIONAL ORIGIN FOR THE GREAT CORDILLERAN BATHOLITHS OF WESTERN NORTH AMERICA

The origin of Cordilleran-type batholiths, such as the Sierra Nevada, Salinia, Peninsular Ranges, Sonoran, Coast Plutonic Complex, and that of the Omineca belt, has been problematic because normal subduction flux doesn't appear to have sufficient energy to create the observed crustal melts nor the volume to create the requisite crustal thickening and uplift. Typical subduction of oceanic crust and its veneer of abyssal sediment beneath a continent – even when lasting 50-100 Myr – creates typical low-standing arcs such as Kamchatka, the Alaskan Peninsula, Central America, Japan, the Cascades, and the much-studied pre-collisional Jurassic arc of the US Cordillera. However, if Cretaceous-Tertiary orogeny of the North American Cordillera was caused by failed westerly-directed subduction of the segmented western margin of North America beneath an arc-bearing superterrane, then the origin of voluminous, 120-80 Ma magmatism that created the Cordilleran-type batholiths can be easily explained.

In the collisional model, uplift and erosion of the entire North American platform and the widespread deposition of intraplatformal gravels and conglomerates directly beneath the foredeep at about 124 Ma are readily interpreted to indicate passage of the marginal platform over the outer bulge to the trench. At this time the transitional leading edge of the continent and its overlying wedge of rise-slope sediments were entering the zone of dewatering beneath the overriding arc. The dewatering and melting(?) of the marginal sedimentary prism and subjacent crust generated a huge increase in the volume of magma rising into the crust. The increased flux of magma led to widespread melting, re-organization, and uplift of the upper-plate crust. Rollback of the subduction hinge caused magmatism to migrate eastward with time. As more of the North American craton entered the trench, convergence slowed because it became progressively more difficult to subduct the buoyant craton. The slowdown stalled asthenospheric circulation above the subducting slab, which led to the abrupt shutdown of arc magmatism at about 80 Ma. The opposed buoyancy forces caused necking of the cratonic crust, and ultimately, slab failure at about 75-70 Ma. Thus, the batholiths resulted from collisional orogeny, and not normal seafloor subduction as generally believed.