Structure and Tectonic History of Alaska1
The major trends of the Cordilleran backbone of North America can be traced through the conterminous United States and Canada into Alaska, where they form a distinctive arcuate pattern bending sharply near the border and flaring out to the west. The origin and pattern of the dominant tectonic elements of Alaska can be traced back to geanticlines and geosynclines that developed from Middle Jurassic to early Tertiary time. The Brooks Range geanticline forms Alaska’s northernmost mountain barrier, a counterpart of the Rocky Mountain system. The range is thrust north over the edge of the west-trending Colville geosyncline. Subsurface data indicate that this geosyncline deepens beneath the Arctic foothills and coastal plain to at least 20,000 feet and then rises to within 2,500 feet of the surface near Point Barrow.
South of the Brooks Range and extending to the Alaska Range is an irregular array of low mountains, uplands, and flat lowlands, the Alaska counterpart of the intermontane area between the Rocky Mountain system and the Pacific Mountain system of the conterminous United States. Although several tectonic elements have been traced, some trends are oblique or normal to the major Mesozoic and Cenozoic arcuate pattern.
The Pacific Mountain system is extended into Alaska by two major mountain chains divided by a line of depressions that bend in a great arc around the North Pacific Ocean. These mountains consist primarily of tightly folded graywacke, argillite, conglomerate, and basaltic and andesitic lava flows and tuffs.
The greatest igneous activity has been in the Pacific Mountain system, and includes a belt of granitic batholiths and a chain of recently active volcanoes. Though numerous, granitic intrusive masses between the Alaska and Brooks Ranges are small, except a few in the southeastern part of the Yukon-Tanana upland. Relatively few igneous rocks are known within and north of the Brooks Range.
The Paleozoic and much of the Mesozoic tectonic history of Alaska involving a broad geosynclinal tract that lay between the Pacific Ocean to the south and west and a stable platform region to the north and east, can be considered conveniently in three major stages—Cambrian through Silurian, Middle Devonian through Permian, and Triassic through Early Cretaceous. The mobile belt encroached northward at the expense of the stable region, until by mid-Mesozoic time eugeosynclinal conditions extended over most of Alaska. In mid-Jurassic time in southern Alaska, and in Early Cretaceous time in central and northern Alaska, Alaska became differentiated into positive and negative tracts of erosion and deposition that were maintained through the rest of the Mesozoic Era. Basins of known Tertiary marine deposition are limited to the northern and southern borders of Alaska. Inland, Tertiary basins were sites of accumulation of continental sediments.
Figures & Tables
Backbone of the Americas: Tectonic History from Pole to Pole
The first article in this book is the address that introduced the technical program of the 46th Annual Meeting of the AAPG. The organization and presentation of this symposium volume was developed in an orderly geographic continuity. Modern concepts of structural form and the sequence of tectonic events are carefully reported all along the mountainous western margins of the American continents. The relationship of this structural knowledge to the accumulation of oil and gas is constantly emphasized in the 26 papers contained herein.