Although the Mesozoic-Cenozoic metallogeny of the western United States unquestionably resulted from convergence of the North American and Pacific Ocean plates, subduction alone does not adequately explain the distribution of the ore deposits in Laramide and post-Laramide time, when magmatism and mineralization extended irregularly eastward at least 1500 km from the continental margin. Stable isotopes (lead and initial 87Sr/86Sr) indicate derivation of some rock- and ore-forming elements from continental rather than oceanic lithosphere. A gross east-west metals zonation lacks the clearcut divisions evident to the north and south in the Cordillera. Lineaments, difficult or impossible to document unequivocally, seem nevertheless to have played some role in localizing mineral belts transverse to plate boundaries.
It is suggested that mantle convection in a reactivated ensialic back-arc, triggered at least in part by subduction on one or more Benioff Zones, became increasingly important both with time and with distance eastward from the Mesozoic trench. Small magma pockets were generated episodically by partial fusion of the continental lithosphere, itself probably in-homogeneous, where deep crustal flaws, many ancient but some perhaps newly created, relieved pressure and provided access to the upper crust.