Volcanism, Tectonism, and Plutonism in the Western United States

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Abstract
Through most of Phanerozoic time the western part of the United States has been the site of orogenic sedimentation and volcanism, both episodic but on the whole quasicontinuous. No record of regional peneplanation exists in the area. Basaltic and andesitic volcanics are represented in every system and in most series; locally they are associated with subsilicic or saturated minor intrusions, but in general no direct connection with plutons can be demonstrated. Beginning in the Ordovician and continuing practically to the present, siliceous lavas have been erupted widely and in huge bulk. The total volume of andesitic and more siliceous volcanics of Ordovician and younger age is probably fully as great as that of the basalts and may, indeed, be greater. Basalts have been commonly considered primary lavas, derived from the upper mantle, and their general independence of both orogeny and mafic plutons at depths accessible to erosion has long been recognized. Andesites and more siliceous lavas, on the other hand, have commonly been thought to be derived either by advanced differentiation or by syntexis. Their occurrence in great bulk seems to demand intracrustal reservoirs of comparable volume. And since the densities of such magmas are relatively low, we should expect to find plutons consanguineous with the siliceous volcanics of the greater fields in close association with them. True, we do find such plutons—generally small ones—unambiguously related to many Mesozoic and Cenozoic siliceous volcanic fields, but despite the deeper erosion to which older terranes have been subjected, not a single granitic pluton of Phanerozoic age demonstrably older than Early Triassic has yet been recognized between the Great Plains and the Pacific. Many older fields of siliceous volcanics are large and well exposed, and it seems most unlikely that consanguineous plutons would have escaped discovery, had they existed. Although granitic plutons have long been known from nonorogenic settings, the great majority are so closely associated with orogenic zones that to many geologists granitic masses equate with orogenies. The radiometric dating of an intrusive is commonly said to date an orogeny. As Daly pointed out long ago, however, the great batholiths of the west do not coincide areally with the belts of profound thrusting of Late Cretaceous and early Tertiary time. Nor do they conform in time. The radiometric dates of the Mesozoic and younger plutons rarely fit neatly into an idealized chronology of deformation—great intrusions following great deformations closely in time. It also seems that many orogenic belts active in Paleozoic and early Mesozoic time in the Cordillera have no associated granites. Structural and sedimentary evidence of Paleozoic and early Mesozoic orogenies is inescapable and widely distributed. Volcanism and tectonism have been more or less continuous at one place or another in the Cordillera throughout Phanerozoic time, but plutonism at a level accessible to later erosion has been essentially independent of both processes in place and catastrophic in time. As Lindgren noted 50 years ago, the batholiths of middle to early Late Cretaceous age in the western States are at least an order of magnitude larger than those of any other comparable time span. They are fully as bulky as those of all the rest of Phanerozoic time taken together—indeed they are probably several times as bulky. Areally, the greatest intrusions lie near or somewhat to the west of the average position of the boundary between the eugeosyncline and miogeosyncline during Phanerozoic time. Moore has pointed out the general restriction of quartz diorite and less siliceous intrusives to the west side of the belt of intrusives, with the more siliceous and potassic plutons to the east side. Anderson, Callaghan, and Moore have shown a generally similar distribution for the volcanic rocks—the more silicic and potassic to the east, the less silicic and more sodic to the west. These generalizations are broadly true, but a conspicuous exception is provided by a considerable province of silicic rocks in eastern Oregon, far to the west of the basaltic province of the Idaho plains. The major basaltic provinces, however, lie well to the west of Moore’s quartz diorite line. I tentatively suggest that the eugeosyncline developed in large part on an oceanic crust; in Well’s nomenclature, it is truly ensimatic. The miogeosyncline developed on a continental crust, and the critical junction between them was somehow the favored site for retention of plutons within the crust rather than their extravasation as volcanics on its surface. Perhaps this retention was because of a persistent tendency for the continent to spread over the oceanic crustal segment, as has been suggested for all continental margins by Gutenberg. Certainly there is a marked association of plutons with the continental margins along much of the Pacific borders. The syntectics on the west were less “contaminated” with continental crust than those to the east; this accounts for the chemical provinces among both plutonic and volcanic rocks. The concentration of plutonism in middle to Late Cretaceous time may mark the time when the ensimatic segment of the crust became welded to the continent—an event unique in Phanerozoic history. Orogenic movements, although also concentrated at the critical zone of the continental margin, were by no means restricted to that zone and are almost randomly associated with plutons. Present estimates of crustal thickness suggest that areas of late Cenozoic volcanics are also those of thin crust and low-velocity upper mantle. Perhaps the siliceous volcanics were erupted, rather than being retained in the crust as plutons, because of some mechanical properties of this combination of crustal and mantle character. Such a suggestion cannot be given much weight on so scanty evidence. It is nevertheless inescapable that orogeny and plutonism are far from synonyms in geologic history. Radiometric dates for plutons do not normally date orogenies in the Phanerozoic of the western United States. This crustal sample seems large enough to suggest that they do not in general.

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