Tectonic Development of Cordilleran Region: ABSTRACT

Abstract

The Cordilleran region of Western United States is a segment of the Cordilleran geosyncline. Its western and eastern borders are the Pacific Coast Ranges and the Wasatch or Teton line. The Cordilleran geosyncline differs from its cratonic neighboring regions, the Colorado Plateau and the Rocky Mountains, by (1) containing extreme thicknesses of sedimentary and volcanic rocks and by (2) having experienced true orogeny. Like its counterpart, the Appalachian geosyncline, it has dual facies expression. An inner (cratonward or miogeosynclinal) belt has thick sediments lithologically similar to cratonic rocks; an outer (seaward or eugeosynclinal) belt has very thick graywackes, volcanics, and other lithotopes. It is convenient to describe the tectonic development of the geosyncline in four stages, each of which exhibited a distinctive tectonic pattern. Stage I, comprising only the Cambrian period, was essentially simple failure of the continent margin by subsidence behind a progressively inward migrating hingeline. Sediment source was cratonic. Stage II was a complex interplay of orogeny, volcanism, and deposition with long duration. It End_Page 790------------------------------ opened in Ordovician time with development of a midgeosyncline hinge (the Manhattan line) which introduced the dual facies aspect. Dark, poorly sorted clastics in the outer belt were derived from tectonic island chains which rose within the belt itself (autocannibalism) while normal carbonates and clean sandstones accumulated in the inner belt. Folding and volcanism commenced in the Klamath Mountain area. Silurian and Devonian tectonic patterns resembled Ordovician. During the Carboniferous, intensified tectonic activity was expressed as folding and thrusting on the approximate site of the old Manhattan hingeline to produce an uplift (Manhattan geanticline of Eardley) which shed conglomerates into both belts. Folding, volcanism, and deposition of clastics continued in northern California. Further intensification of tectonic action occurred in the Permian, Triassic, and lower Jurassic periods. Profound subsidence of the outer belt accompanied deposition of autochthonous volcanics of regional extent and extreme thickness. A composite maximum of 70,000 feet (12 miles), centering in the California-Nevada area, dominated the entire outer belt. In addition, (1) strong tangential stresses produced local dynamometamorphism in the Sierra and Mohave areas of California in Permian time, (2) basic plutons were emplaced in central Oregon in Triassic time, and (3) ultrabasic intrusions of lower Jurassic age were injected in the Klamath area. These and earlier tectonic events indicate existence of a so-called Klamath tectogene in Washington, Oregon, and California. During the Mesozoi , midgeosyncline lands crowded the inner belt eastward, finally causing extinction. Stage III was climactic orogeny involving not only folding and thrusting but regional dynamometamorphism and large-scale batholithic intrusion. Beginning in upper Jurassic time, batholithic emplacement in the Klamath tectogene reached proportions unequalled in North America since the Precambrian. Repeated impulses of folding and thrusting marched eastward during late Jurassic and Cretaceous time to culminate as thrusts of major magnitude (Lewis, Bannock, Strawberry, Muddy) at the Teton-Wasatch line. Apparent post-orogenic relaxation of Stage IV caused collapse of the orogenic highland in the Great Basin. Extensive block faulting plus actual subsidence produced a Tertiary-Recent depositional basin (comparable with the Triassic of the Appalachian belt) which to-day exhibits internal sediment supply, and which in future may acquire a depositional history of some magnitude. Eventual broad, regional, epeirogenic uplift of the Cordilleran belt with associated erosional planation is predicted. End_of_Article - Last_Page 791------------