Eastern Pacific Geosynclinal Belt

(Cordillera geosynclinal belt), a mobile region of the earth’s crust within the bounds of the eastern littoral of the Pacific Ocean from the Alaskan peninsula on the north to the island arc fringing the Scotia Sea on the south, including the North American Cordilleras and the Andes. On the east it is bounded by the North American (Canadian) platform and the Patagonian tableland, with a Baikal or Paleozoic foundation. The total length of the belt is approximately 20,000 km; its width (in the western USA) is approximately 1,500 km. Geosynclinal troughs began to be laid down in the late Precambrian period (in the Andes, during the Triassic). Plicate structures took shape toward the end of the Paleozoic period and in the Mesozoic and Cenozoic. Within the belt are various plicate regions and systems of differing ages: Paleozoic (the Puna system of Bolivia and Argentina), Mesozoic-Nevadan (the Rocky Mountain system and the Sierra Nevada), early Cenozoic-Laramie (the Mexican Sierra Madre range system), Middle Cenozoic-Andean (the Andes system), and late Cenozoic-Pacific (the system of North American Coast Ranges and the coastal cordilleras of South America). Plicate structures frame medial and internal massifs composed of Precambrian and Paleozoic rock.

Paleozoic structures are represented by large folds and reverse faults. Three separate structural elements can be distinguished in the region of Mesozoic (Nevadan) folding of the Rocky Mountains and the ranges of the Sierra Nevada: the Canadian marginal depression and the miogeosyncline and eugeosyncline of the Rocky Mountains. In the Upper Jurassic period, miogeosynclines and eugeosynclines were crumpled into folds and complicated by dislocations and reverse faults, and intrusion of granite batholiths occurred during the Lower Cretaceous period. South of the nevadites, in the eastern part of the California peninsula and in the Mexican Sierra Madre, there are traces of the Laramie (Upper Cretaceous-Paleocene) fold system, framing the Precambrian Mexican medial massif; on the south within its boundaries are the massifs of Oaxaca (Precambrian) and Honduras (Paleozoic). The region of Andean (Oligocene-Pliocene) folding is traced from the Isthmus of Panama to Tierra del Fuego.

A narrow band along the littoral of the Pacific Ocean is a modern geosynclinal region. It appears in southern Alaska, in the Coast Ranges of California and Oregon, and on the Isthmus of Panama. All these regions are characterized by volcanic activity, seismicity, and intensive vertical movements. The thickness of Neocene and Anthropogenic strata in depressions of the modern geosynclinal region—for example, in the flexures of the Cook Inlet, Lake Nicaragua, and in western Colombia—reaches several km.

Within the bounds of the eastern Pacific geosynclinal belt are rich deposits of gold (Alaska and California in the USA) and copper (Chile), as well as polymetallic deposits (Bolivia, Mexico, and the USA). Oil, coal, and other minerals are obtained from flexures and bowls of the belt (mainly in Alaska, California, Venezuela, Colombia, and Peru).