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 toward 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 composed chiefly of Paleozoic limestone, shale, quartzite, slate and schist in faulted folds, and giant plates and nappes, overturned and thrust north over the edge of the west-trending Colville geosyncline. Subsurface da a 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 through this area, some trends are oblique or normal to the major Mesozoic and Cenozoic arcuate pattern. The eastern part of this area is underlain by metamorphic rocks of Precambrian and early Paleozoic age, including what are probably the oldest rocks in Alaska. West interior Alaska is underlain chiefly by folded and faulted marine and continental sedimentary rocks of Cretaceous age and volcanic rocks of Mesozoic age. Bedrock of the Seward Peninsula is mainly chist, gneiss, marble, and metavolcanic rocks cut by granitic intrusive masses. Older structural trends in the metamorphic rocks are chiefly northward on which a younger eastward-trending pattern has been superimposed.

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 great thicknesses of tightly folded graywacke, argillite, conglomerate, and basaltic and andesitic lava flows and tuffs.

The greatest igneous activity has been in the Pacific

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Mountain system, and includes an almost continuous belt of granitic batholiths and a chain of active and 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 on the south and west and a stable platform region on 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 Tertia y marine deposition are limited to the northern and southern borders of Alaska. Inland, Tertiary basins were sites of accumulation of continental sediments.

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