The tectonic evolution of the western United States
Authors: GILLULY JAMES 
Published: October 2009

The Pre-Cambrian history of the western United States is fragmentary, and the radio-metric dates so far measured give no obvious clues to ancient orogenic belts or to systematic continental accretion.

From Cambrian until Devonian time the western part of the area of the present Cordillera was geosynclinal, the eastern part the site of intermittent shelf seas. The geosyncline was compound, the western part eugeosynclinal, the eastern miogeosynclinal.

In Early Mississippian time the Antler orogenic belt rose near the axis of the geosyncline through Nevada and Idaho in an alpine mountain chain at least 500 miles long. The former geosyncline was split into two, the western segment remaining eugeosynclinal, the eastern miogeosynclinal. Notable orogenies took place in the western area in Pennsylvanian, Permian, Triassic, and Jurassic time, across Colorado in Pennsylvanian to Early Permian time, and across southern Arizona in Triassic time. At the end of the Jurassic, after the Nevadan orogeny, the western geosyncline was displaced still farther to the west, the eastern one was drained, and a new one formed on the site of the former craton, in the area of the present eastern Cordillera. Over a million cubic miles of largely clastic sediment accumulated in this new geosyncline.

In mid-Cretaceous time orogenic pulses began on the western side of the Cretaceous geosyncline and gradually extended eastward until by the end of the Eocene the entire belt was involved. On the west side of the trough great thrusts were formed from Arizona and Nevada to Montana and beyond into Canada; toward the east, uplift of anticlinal areas and depression of synclinal ones occurred, shortening the surficial crust only slightly but creating structural relief as great as eight or ten miles.

The western trough has frequently been much disturbed and broken up by orogenic episodes since the beginning of the Cretaceous. Some structures formed by the ‘Mid-Pleistocene’ Pasadenan orogeny are still highly active.

The Colorado Plateau became established as a structural unit by mid-Eocene time but its uplift was delayed until Pliocene and Quaternary time. It may owe its elevation to an iso-static response to the transfer of crustal material by currents in the mantle.

The Basin Range structural province is dominated by normal faulting and extends far beyond the region of internal drainage. The distension of the crustal segment indicated by the normal faults may also be attributed to differential drag by currents in the mantle.

Transcurrent (strike-slip) faults are important only in California, Nevada, and Idaho. The great San Andreas fault in California (whose movement may have begun in Cretaceous time and is still continuing) seems to demand drag of crustal segments by differential movements in the mantle as the immediate cause.

Volcanic activity has occurred in the western part of the area during every period of the Phanerozoic. The Cambrian and Ordovician extrusive rocks are chiefly andesitic or basaltic, but siliceous lavas became abundant during the Silurian and have since been conspicuous. The greatest volcanic field (Permian-Lower Triassic) was submarine; eruptions extended from southern California to Alaska. The extrusion of the flood-basalts of the north-west began in Eocene time, when the greatest outpouring took place. The Columbia River basalts are Miocene and early Pliocene; basaltic flows continued in the Snake River region until Pleistocene or even Recent time. Most of the Tertiary volcanic rocks of the Great Basin are siliceous; the total bulk of the rhyolitic welded tuffs and lavas of this area probably equals that of the Columbia River flood-basalts.

Plutonic activity has been dominated by the great batholiths of the Peninsular Range, the Sierra Nevada, and Idaho, with their satellitic intrusions. Most of the radiometric dates are Cretaceous, but many small intrusions are Triassic, Jurassic, and Eocene. The youngest large granitic intrusions are of Miocene age.

In Cretaceous time plutons probably a thousand times larger than those of all the rest of the Phanerozoic were emplaced. The records of sedimentation, tectonics, and surface volcanism are uniformitarian, but plutonic activity has been catastrophic—completely lacking during the Palaeozoic and since trivial except during the mid-Cretaceous and Miocene. Plutonic activity is evidently not a necessary accompaniment of normal orogenic processes.

The great plutons have arisen on the site of a persistently eugeosynclinal crustal segment. Their volume demands supply either from huge volumes of differentiating mantle or by remelting of large volumes of sial; either mechanism requires differential movement of crust and mantle on a vast scale.

The abrupt boundary of the continent against the ocean basin and the complete independence of the large strike-slip faults of the two domains also suggest relative movement of continental crust and mantle. Either the continent is drifting westward over the ocean floor, or the ocean floor (with the sialic segments that formerly lay off-shore) is moving eastward under the continent. The local mechanics would be the same in either case, though the wider mechanisms would be quite different. It is probable that the continent as a whole is moving away from a widening Atlantic.