Considerable progress in the mapping and geological understanding of Alaska has been made in recent years through the efforts of the United States Geological Survey, petroleum companies and academic institutions. Sufficient reliable data now seem to be available to attempt, very tentatively, a synthesis of its geological history in the light of modern tectonic theories.

Distribution of major tectonic elements

A. Southern Alaska

East central Alaska is divided fundamentally along the line of the Tintina Trough (Fig. 1), a feature which lies on approximately the same general trend as the homologous Rocky Mountain Trench of western Canada. It separates unmetamorphosed Palaeozoic and early Mesozoic miogeosynclinal rocks to the north from a series of eugeosynclinal belts to the south, and may have marked the edge of the continental platform in early Palaeozoic times. The Tintina Trough cannot be traced with certainty for more than 300 kms westwards into Alaska, and the continental margin then appears to have degenerated into a narrow welt along the Ruby Geanticline.

The Ruby Geanticline, which consists of rocks as yet poorly dated, also bounds the eugeosynclinal belts to the south: but, interposed between it and the miogeosyncline, there is now the Koyukuk Geosyncline, a middle Mesozoic feature with associated ophiolites, that was compressed during the Cretaceous. This possibly represented a deep ocean basin during the Palaeozoic and the Ruby Geanticline an island arc, analogous perhaps to the modern Bering Sea and Aleutian Arc.

The various tectonic belts of southern Alaska seem to represent a history of

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