The Cordilleran orogenic belt of western North America is the product of sedimentational, magmatic, and tectonic processes along a continental margin initially formed by rifting in late Precambrian time. Deformation in the orogen is attributable to diverse interactions of the North American and various oceanic plates across that rifted margin. Phanerozoic deformation in the western United States largely has been due to convergent-plate interactions. As a consequence, thrust faulting is the most characteristic expression of pre-middle Tertiary Cordilleran orogenesis. Episodes of plate convergence may have been accompanied

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by important transcurrent components, but evidence for such components in the western United States is much less pronounced than in Canadian and Alaskan parts of the orogen.

Paleozoic, Mesozoic, and early Tertiary thrust faults in the southwestern United States display a distinct pattern of bilateral symmetry. In western parts of the orogen, thrust plates are characterized by a westward displacement relative to lower plate rocks and a westward decrease in age. Thrust plates in central and eastern parts of the orogen are, with few exceptions, east-directed and show an eastward decrease in age. Cordilleran thrust faults fall into three geographic and temporal groups, each of which can be characterized by a particular mode of plate interaction.

East-directed thrust plates can be divided into two distinct groups--those in central parts of the orogen formed during the Antler (Devonian-Mississippian) and Sonoma (Permian-Triassic) orogenies, and those of younger age, generally in areas to the east. The Antler and Sonoma orogenies represent progressive stages in the closure of a marginal basin located between the continent and an offshore Klamath-Sierran island arc which developed in Ordovician time. Episodic closure of the basin occurred during times of accelerated plate convergence in the arc region. Antler and Sonoma allochthons consist of sedimentary and volcanic rocks from the marginal basin and slices of their oceanic basement displaced eastward across the continental shelf. Closure of the marginal sea was accompanied by su duction, probably eastward, of the bulk of the oceanic crust on which the basin fill had been deposited. Complete closure resulted in accretion of the island arc to the western margin of the continent.

East-directed thrust plates of post-Sonoma age are intracontinental, having developed within the North American plate, east of the Andean-type Mesozoic-early Tertiary igneous complex. Subduction-related magmatism produced a thermally controlled zone of high-crustal ductility along the western leading edge of the American plate. Eastward intraplate yielding by thrust faulting was localized largely across the eastern boundary of this ductile zone as cooler, more rigid parts of the plate moved westward into and beneath it. The geometry of yielding also was influenced by stratigraphic anisotropy in thick sedimentary accumulations (Belt Supergroup and Cordilleran miogeosyncline). Eastward migration of thrusting occurred in response to an eastward shifting of plutonism and the zone of high- uctility contrast.

Thrust faults in the western part of the orogen are products of eastward subduction of oceanic lithosphere, initially beneath the Paleozoic Klamath-Sierran arc but also beneath the continental margin after Triassic accretion of the arc to the continent. The westward shifting of these thrust faults from Devonian through early Tertiary time reflects westward shifting of subductive activity by growth of melange wedges and accretion of oceanic and island arc rock assemblages to the continental margin.

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