Permo-Triassic diastrophism in the western Cordilleran region

Abstract

Essentially continuous orogeny and epeirogeny in portions of generally subsiding orthogeosynclinal belts is further confirmed by study of latest Permian and Triassic in the Cordilleran region. Apparent absence of Lower and Middle Triassic in most of British Columbia and Alaska was recognized early, but was not interpreted in terms of important tectonism. Evidence of orogeny has been known in Nevada and central British Columbia. In the latter area, Upper Triassic unconformably overlies late Permian that was intruded by ultrabasic rocks. The same relationships exist in central Oregon except that there important acidic and intermediate plutons also exist. Wide absence of recognized Lower and Middle Triassic, presence of pre-late Triassic intrusions, angular unconformities, and character of coarse clastic material suggest that the latest Permian-earlier Triassic interval was one of complex diastrophism, plutonism, volcanism, and elevation of lands in much of the western Cordilleran belt. Fossiliferous early and medial Triassic marine sediments are nearly confined to the miogeosyncline northward from California and Nevada, but during late Triassic there was widespread transgression in the eugeosyncline with deposition of much limestone, other sediments, and local volcanic rocks. Relationships of acidic to ultrabasic plutons, deformation, and metamorphism during most orogenic episodes in the Cordilleran region were complex. Late Jurassic ("Nevadan") age was long assumed for all the ultrabasic bodies until pre-Jurassic ones were discovered in central Oregon. Most of these, along with many more acidic ones, are now known to be pre-late Triassic as are some of those in British Columbia, but others in British Columbia, SE. Alaska, and California are apparently Jurassic and Cretaceous. Most of the ultrabasic rocks in Washington are not adequately dated. Folding, batholithic emplacement and regional metamorphism are all quite variable in age along, as well as across, mobile belts. Ultrabasic bodies are equally variable and, therefore, do not necessarily mark any single or first stage of geosynclinal foundering. Mobile belt histories appear so complex when studied in detail that even famous named orogenies such as Nevadan and Laramian become somewhat indistinct one from another. Therefore, most attempts at naming and correlation of still smaller-scale orogenic pulses have questionable merit and perpetuation of the over-simplified textbook dogma of a generalized "geosynclinal cycle" is unwarranted.