Amalgamation of Pangaea required a series of major plate reorganizations in Middle to Late Paleozoic time. Early Paleozoic closure of the Iapetus ocean required subduction of oceanic lithosphere, whereas subsequent continent-continent collision resulted in the termination of Iapetus subduction zones. The closure of Iapetus would seem to have necessitated transfer of some subduction processes to the paleo-Pacific realm in order to maintain a global balance between divergent and convergent plate margins. New and previously published field, petrographic, geochronologic, isotopic, and whole-rock chemical data for the Klamath-Sierra arc, California, establishes the age for initiation of subduction along the proto-Cordilleran margin. The Eocene-Recent Izu-Bonin-Marianas (IBM) arc serves as an actualistic model for evaluation of the Klamath-Sierra arc. In the Klamath Mtns, the Trinity ophiolite records initial extension and magmatism between 431-398 Ma. Trinity mafic rocks are consistent with partial melting of a depleted, residual MORB mantle source enriched by subduction-derived fluids and compare with Eocene forearc rocks formed during initiation of the IBM arc. A trench-proximal position is indicated by Devonian Yreka terrane melange and volcaniclastic turbidites deposited on Trinity basement. In the northern Sierra, the imbricate Shoo Fly complex (correlated with Trinity-Yreka rocks) is overlain by Middle to Late Devonian subduction-related metavolcanic rocks. Sierra basalts record a transition from early tholeiite to calc-alkaline volcanism reflecting Late Devonian (~368-378 Ma) stabilization and maturation of the volcanic arc, similar to the Oligocene to Recent IBM arc. Sedimentological and paleontological data place the Klamath-Sierra arc along the proto-Cordilleran margin of Laurentia. Development of the Klamath-Sierra subduction system followed ~200 million years of passive margin sedimentation and represented a major change in the proto-Cordilleran margin of Laurentia. The initiation of Klamath-Sierra subduction (431-398 Ma) was simultaneous with the Acadian, Caledonian and Scandian orogenies that recorded continental collisions resulting from closure of Iapetus. Thus, the Klamath-Sierra arc may document a major mid-Paleozoic plate reorganization in response to closure of the Iapetus Ocean and the early stages of the amalgamation of Pangaea.