Evolution of the Cordilleran Fold Belt
The geotectonic development of the Cordilleran fold belt in western North America during Phanerozoic time took place in three principal phases: (I) an initial orthogeosynclinal phase, (II) an orogenic–late geosynclinal phase, and (III) an orogenic-postgeosynclinal phase. These phases developed over a span of more than 500 m.y. and parallel comparable phases in the evolution of other major geosynclinal zones such as the Appalachian, Caledonian, and Andean. The orthogeosynclinal phase (I) resulted in accumulation of offshore–deep-water (eugeosynclinal) slope (transitional) and shelf (miogeosynclinal) facies along the western continental margin. This phase was terminated by the Antler orogeny (IIa) in Late Devonian through Middle Pennsylvanian time. Clastic debris from the Antler belt was shed westward during late Paleozoic time into flanking eugeosynclinal troughs and eastward into epicontinental troughs; deposition in the western trough was terminated by the Sonoma orogeny (IIb) of Late Permian and Early Triassic age. The orogenic–late geosynclinal phase culminated in Jurassic (Nevadan and Sevier) orogeny (IIc), which coincided with early stage development of plate tectonics along the western continental margin. Depositional events of the postgeosynclinal phase (III) during Mesozoic and Tertiary time were characterized by orogenic and postorogenic flysch and molasse sedimentation in troughs and basins along the continental margin. These events were accompanied by andesitic volcanism and were related to plate tectonics. Mesozoic and Tertiary continental margins in North America have been classified into two major types: Atlantic type, characterized by prograding clastic wedges, and Cordilleran type, by underthrusting along continental margins. A third type, orthogeosynclinal, which is characterized by long-continued sedimentation and volcanism of major Paleozoic geosynclines, is proposed here. Orogenic mechanisms for continental margins recently have been discussed principally in terms of ocean-floor spreading and plate tectonics. Orogenic activity within the interiors of continents requires other mechanisms; these may include subcrustal phase changes and thermal expansion related to loading and subsidence. It is even possible that such phase changes and thermal expansion may play a significant role in orogenic activity along continental margins.