Abstract The Cordilleran orogen lies on the western part of the North American continent and rims the northeastern Pacific Ocean Basin. That part of the orogen covered in this volume extends between the Mexican and Canadian borders, with some consideration of the geology on both sides of the border, and from the offshore continental borderlands of the Pacific eastward as far as the Black Hills of South Dakota and the mountains of west Texas. It ranges from 800 to 1, 600 km wide and is physiographically complex, consisting of high mountains, intervening lowlands, and plateaus that rise from a more gentle continental interior (Fig. 1). The physiography of the Cordillera largely reflects the younger underlying structure, and the present orogenic belt has limits coincident with its physiography.

Abstract Nearly continuous successions of late Proterozoic through Upper Devonian rocks are widespread in the western United States between the structural fronts of the Sevier and Sonoma orogens (Plates 2-1 to 2-6, 3-1 to 3-4). West of the Sonoma orogenic front in Washington, Oregon, northe Rn and southe Rn California, western Idaho, and western Nevada, rocks of this age are limited to many small areas of exposures, most of which are shown on Plate 3-5. East of the Sevier orogenic front, some large areas on the cratonic platform contain only partial stratigraphic records because of eithe R nondeposition or erosion. The western limit of mapped carbonate-shelf rocks can be obtained from only a few tectonic windows because those rocks disappear beneath thrust plates of western facies rocks of the same age or of younger and older rocks that moved eastward as much as 200 km or more during the Antler and later orogenies. Figure 1 shows Devonian and older paleotectonic features, major post-Devonian faults, and locations of the 12 generalized stratigraphic columns shown in Figure 2.

Abstract Upper Paleozoic rocks of the Cordillera form several contrasting belts that differ in their depositional and tectonic settings (Fig. 1). Cratonic and bordering continental shelf strata constitute the autochthonous portion of the Paleozoic Cordillera and provide an excellent, if only partial, sedimentologic record of late Paleozoic events affecting the western edge of the North American continent. Paleozoic rocks to the west are allochthonous with respect to the Paleozoic shelf and represent the preserved remnants of deep ocean basins and volcanic island arcs. Their less fossiliferous nature and greater degree of deformation make interpretation of their depositional setting and tectonic history more speculative. Paleozoic autochthonous rocks of the western U. S. Cordillera and adjacent regions were deposited in two distinct belts or provinces: the craton and cratonic basins province, and the miogeoclinal shelf province (Fig. 1). Paleozoic allochthonous rocks constitute four additional belts: the lower Paleozoic slope, rise, and ocean basin sequences of the Roberts Mountains allochthon; the late Paleozoic rocks of the Golconda allochthon; Paleozoic volcanic arc sequences exposed primarily in the western Sierra Nevada and the Klamath Mountains; and upper Paleozoic oceanic sequences tectonically included in accretionary subduction complexes. These six belts of rocks have general counterparts in the Canadian and Alaskan Cordillera to the north (Fig. 2) although the exact details of their depositional setting and subsequent tectonic history may differ along strike of the orogen.

Abstract The early Mesozoic evolution of the U. S. Cordillera differs greatly from its previous history of mainly miogeoclinal sedimentation with outboard marginal-basin-island-arc mobile zones. The Early Mississippian and Permian-Triassic thrust emplacement of eugeoclinal strata across the miogeocline signaled the initial propagation of subduction-related tectonism onto the sialic edge. Following these events, the sialic edge and the resulting accreted terranes became an active continental margin. The active margin history records not only eastward subduction of oceanic crust beneath North America, but also the formation, migration, and accretion of marginal basin and fringing island-arc systems along the continental margin. At the close of the Jurassic, the fringing arc-marginal basin system collapsed, resulting in a more direct interaction of major Pacific basin plates with hte Cordilleran margin. Such interactions are manifested by Andean and San Andreas types of marginal regimes which characterized the Cretaceous and Cenozoic. In this chapter we will discuss the tectonic evolution of the U. S. Cordilleran margin during the early phases of its active margin history (Triassic through Jurassic).

Abstract During the period lasting from about 150 to 80 Ma, two contemporaneous but geographically distinct tectonic regimes dominated the Cordillera (Fig. 1). In the eastern part, the largely thin-skinned Cordilleran fold and thrust belt and a coeval foreland basin developed as Proterozoic, Paleozoic, and lower Mesozoic strata were contracted and emplaced eastward onto the platformal cover of the North American craton. In the western Cordillera, subduction of oceanic crust along the continental margin gave rise to the magmatic arc, forearc-basin deposits, and subduction complex that are preserved respectively in the Sierran batholithic belt, Great Valley Group, and Franciscan Complex. Upper Jurassic, Lower Cretaceous, and lower Upper Cretaceous rocks elsewhere in the western Cordillera, including Washington and southwestern California, reside in large composite terranes that may have been displaced northward with respect to continental North America on the order of 1, 000 km or more since about 80 Ma. This hypothesis, supported chiefly by paleomagnetic data, is definitely controversial.

Abstract Late Cretaceous, Paleocene, and early Eocene time (ca. 80 to 50 Ma) was marked by major changes in the loci of magmatism, tectonism, metamorphism, and sedimentation in the western United States. Prior to this period, the Cordillera was segmented into an eastern fold and thrust belt with adjacent foreland basin, a central magmatic arc with flanking fore-arc and back-arc basins, and accretionary complexes and shifting terranes along the coast (Cowan and Bruhn, this volume).

Abstract Before the time of the Laramide orogeny, an active orogenic and magmatic system was more or less continuous along the continental margin of western North America and had long dominated the Cordileran geologic framework; Laramide events reflected a major break in that continuity and were unusual in several respects. Contractional Laramide orogenesis affected a very wide zone, with deformation and foreland uplift extending nearly to the middle of the continent. Laramide magmatism, too, although discontinuous along strike of the orogenic region, extended locally far eastward. These events may have occurred in response to rapid westward drift of the North American Plate and extreme flattening of the Farallon subduction zone (Dickinson and Snyder, 1978).

Abstract The Cordilleran orogen, with an evolution that has spanned the entire Phanerozoic, is one of the longest lived orogenic belts on the planet. A reason for its long history is that since at least Cambrian time the Re has been a large region underlain by oceanic lithosphere to the west of continental North America, and such continent-ocean lithospheric boundaries are tectonically unstable. Interaction between these two types of lithosphere resulted in intermittent tectonic activity during Paleozoic time, and nearly continuous tectonic activity during the Mesozoic and Cenozoic. Because Pacific oceanic plates are still present west of North America, the Cordilleran orogen continues to evolve. Plateboundary interactions along the western boundary of North America have included diverse types of convergent, divergent, and transform activity as well as combinations of these plateboundary interactions. Fortunately, the preservation of a large tract of Mesozoic and Cenozoic oceanic lithosphere west of North America enables the evolution of Pacific Basin plates to be reconstructed for Cenozoic time and, with less certainty, back to middle Mesozoic time (Engebretson and othe Rs, 1985; Stock and Molnar, 1988). Such reconstructions permit correlation between continental geology and lithospheric plate interactions to a degree uncommon in most post-Paleozoic orogens, and provide a testing ground for relating oceanic plate tectonics to continental deformation and evolution.

Abstract The widespread igneous rocks in the western United States have now been studied for more than a century; they provide primary geologic constraints on many types of ore deposits, key information on regional geologic history, and the basis for development of broad concepts applicable to global magmatism. Studies have been especially intense in the past few decades, fueled by societal needs such as volcanic hazard evaluations and the search for geothe Rmal and mineral resources. Also important have been an explosion of new techniques in geochronology and isotope geochemistry, increased availability and precision of mineral and rock chemical analyses, and detailed field structural studies of batholiths and stratigraphic studies of volcanic fields. Improved understanding of eruptive mechanisms has resulted from combined studies of historic volcanism and prehistoric deposits, and the rich interactions between studies of igneous activity and innovative plate tectonic concepts have led to new ideas on the origins of continental-margin magmatism.

Abstract Phanerozoic lithologic assemblages associated with specific plate-tectonic regimes are reasonably well understood, especially the siting of igneous and sedimentary rock suites. Less appreciated is the relationship between differential plate motions and subsolidus phase equilibria. Metamorphism is a complex but quantifiable function of the conditions attending recrystallization; in turn, the thermal-depth parameters of contrasting segments of the crust are related to individual plate-tectonic histories.

Abstract The nature of sedimentary assemblages is controlled by interactions of (a) provenance, which refers to source rocks as modified by the effects of weathering and varied relief; (b) mechanisms of erosion and sediment transport; (c) dispersal paths as a function of paleogeography; (d) depositional processes operative in different environments of sedimentation; and (e) diagenesis, which is influenced strongly by the thermal history and geochemical evolution of sedimentary basins. All these factors are dependent primarily upon tectonics and climate.

Abstract The U.S. Cordillera is typical of orogenic belts in its preservation of multiple episodes of extensional, strike-slip, and compressional deformation. Widespread, latest Proterozoic extension established an early Paleozoic passive margin ( Stewart; 1972; Bond and Kominz, 1984 ). Other events, probably of lesser overall magnitude and extent, include mid-Proterozoic rifting in the Pacific Northwest, resulting in the accumulation of the Belt Supergroup, late Paleozoic rifting along the continental margin arc ( Miller and others, 1984 ) and perhaps also within the craton and miogeocline ( Kluth, 1986 ), and Mesozoic extensional tectonism in the “hinterland” of the Mesozoic fold and thrust belt ( Allmendinger and Jordan, 1984 ).

Abstract Compressional tectonism within the Cordilleran orogen of the western United States has occurred sporadically, both temporally and spatially, since at least the end of the Devonian Period and has continued until the Present. The focus of this chapter, however, will be on the Mesozoic and early Cenozoic deformation, when the Cordillera experienced the widest and most diverse development of contractional structures. These structures are relatively completely described and illustrate a full range of structural styles. The provinces treated here are: the basement uplifts of the Rocky Mountain foreland, and the Cordilleran thrust belt and hinterland (as far west as the Sierra Nevada and Idaho Batholiths), including the southern Cordillera of Arizona and southeastern California (Fig. 1).

Abstract Strike-slip faulting is a major element in the tectonic development of the Cordilleran region. Since Mesozoic time, major strike-slip displacements have dominated the tectonic development of the Cordillera, and important Precambrian strike-slip systems may exist as well. Major displacements have been mostly parallel, or subparallel, to the western margin of North America and related to lateral movements between major crustal plates. The lateral movement is associated with either transform faulting, such as the present-day San Andreas system, or with oblique subduction that produced intra-arc or back-arc strike-slip faulting. Inland faults are related to distributed shear caused by the continental margin movements, to adjustments between regions of differential extension, or to shear within fold and thrust belts.

Abstract Metallogenic evolution of a region is related to evolving stratigraphic, structural, and magmatic processes through time, and all these are controlled by the region's tectonic evolution. This chapter considers the broad evolutionary litho- and tectonostratigraphic relations that control the distribution of major types of mineral deposits throughout the western United States. By this approach, ore deposits-in addition to their economic significance-become valuable guides to paleotectonic processes. The approach is intended to provide different insights from those based on detailed studies of individual ore deposits or types. Two maps (Figs. 1, 2) outline the distribution of deposits of widely differing ages and types within rocks belonging to ten successive time intervals, but in the discussion the latter are grouped into seven major successive and evolutionary tectonic intervals: >1.65 Ga, 1.65 Ga to 400 Ma, 400 to 208 Ma, 208 to 80 Ma, 80 to 40 Ma, 40 to 22 Ma, and 22 to 0 Ma. In combination, the two maps show the distribution of rocks of the ten time intervals; Figure 1 of the older basement rocks with the younger overlying ones of Figure 2 stripped away. The locations of major deposits or types of deposits discussed in the text are also plotted on the two figures, but the scale prevents plotting of all examples mentioned. Some deposits appear on both figures because metallogenic relations suggest that they have been formed by dual, overlapping processes of differing ages. The deposits plotted are discussed to illustrate key metallogenic relations, but the map

Abstract Ophiolites are “distinctive associations of mafic to ultramafic rocks . . . which when completely developed comprise from bottom to top: (1) an ultramafic complex, consisting of variable proportions of harzburgite, lherzolite, and dunite, usually with a metamorphic tectonite fabric (more or less serpentinized); (2) a gabbroic complex, ordinarily with cumulus textures commonly containing cumulus peridotites and pyroxenites and usually less deformed than the ultramafic complex; (3) a mafic sheeted dike complex; and (4) a mafic volcanic complex, commonly pillowed. Rocks commonly associated with ophiolites include: (1) an overlying sedimentary section, which typically includes ribbon chert, thin shale interbeds, and minor limestone; (2) podiform bodies of chromite generally associated with dunite; and (3) sodic felsic intrusive and extrusive rocks” (Anonymous, 1972). It is generally thought that ophiolites represent on-land fragments of oceanic crust and upper mantle (Dewey and Bird, 1971; Gass and others, 1973; Miyashiro, 1975; Coleman, 1977; Moores, 1982). The plate tectonic settings of ophiolite pedogenesis and continent edge emplacement are the subjects of major debate. Two general ophiolite types, Cordilleran and Tethyan, were differentiated in a major ophiolite review by Moores (1982). Cordilleran ophiolites were defined as complete to partial, or dismembered, sequences lacking continental-affinity structural basement; Tethyan ophiolites are more often complete crust-upper mantle sequences and commonly rest tectonically above continent edge basement. The contrast between Cordilleran and Tethyan type ophiolites is a useful starting point for this synthesis. Cordilleran ophiolites are considered here to be representative of the circum-Pacific orogenic style. The governing tectonic phenomenon in

Abstract Tectonic models for the evolution and growth of the North American Cordillera have become unprecedentedly mobilistic in the last 15 or 20 years. This is largely the result of paleomagnetic studies, which have themselves proliferated within the Cordillera at an even more unprecedented rate. For example, my first attempt to summarize paleomagnetic results for the western Cordillera ten years ago (Beck, 1976) involved only 17 studies. Now at least 88 entries are required (Tables 2-5, this paper), even though all data from north of southern British Columbia have been relinquished to other reviewers! At the present rate of growth, by the early 1990s we should have available networks of closely spaced, high-precision paleomagnetic studies for many crucial areas within the Cordillera, as well as adequate reconnaissance coverage of the entire range. Examples of the unparalleled usefulness of paleomagnetic data for regional tectonic analysis already exist (e.g., Wells and Coe, 1985; Luyendyk and others, 1985).