The Middle American channel connecting the primeval Atlantic and Pacific oceans was subjected to forces in Upper Jurassic time that folded the sea bed into a series of parallel ridges striking SE. to NW. The westward and most tenuous of the ridges was ruptured by extrusive material that grew from the channel floor and emerged to form a chain of volcanic islands, the Western Archipelago. Erosion of the islands and deposition to the northeast provided the sediments of the Nicoya complex now exposed along the west coast of Costa Rica and Panama.

Volcanic eruption and continuing erosion throughout the Cretaceous supplied sediment to the shallowing Channel area. Deposition during this period was mainly from the Archipelago although some material was derived from the northern nuclear Central American mass. By the end of Cretaceous most of the denuded islands had foundered and the Western Archipelago had disappeared.

Diastrophism accompanying the Laramide revolution rejuvenated and further upfolded one of the interior ridges. The Guanarivas Island emerged in northern Costa Rica and southern Nicaragua. Volcanoes on Guanarivas were the north end of a chain that continued as volcanic islands southward and eastward through Panama. Eastern Panama, belonging to the Choco borderland, which had been emergent throughout the Cretaceous, began to founder in lower Eocene and was submerged by the beginning of the middle Eocene. Volcanic detritus and submarine laval flows are predominant in the accumulating Eocene sediments of the channel.

Guanarivas Island and the volcanic islands had disappeared by lower Oligocene time which was an epoch of comparative quiescence. Renewed activity in the middle Oligocene resulted in the growth of the Talamanca ridge and the appearance of islands in southern Costa Rica and northeastern Panama.

Continued growth through early Miocene culminated in development of the West Talamanca fault and total emergence of the ridge by the end of the middle Miocene. The faulted upthrust block was tilted eastward, creating compressive forces that fractured the eastern front of the high area and initiated folding on the Atlantic foreland of southern Costa Rica and northeastern Panama. The Miocene diastrophism was accompanied by the growth of volcanoes on the ridge in Panama.

Total emergence of a narrow strip of land, bordered by the Pacific Ocean and the Nicaraguan depression opening to the Caribbean, resulted in the first uninterrupted connection of South America with nuclear Central America in Pliocene time. During the Pliocene, strike-slip faulting on the west side of the new Isthmus extended from Nicaragua to Panama bringing up the Jurassic Nicoya complex that is now exposed as the core of peninsulas from Santa Elena to Azuero. In what may have been the same adjustment that caused the faulting a new chain of volcanoes appeared along the Pacific coast from Nicaragua to the northern edge of the Talamanca ridge.

By Quaternary time the Talamanca ridge had become stabilized and adjusted, the Nicaraguan depression was filled in leaving only Lakes Nicaragua and Managua and the San Juan River to mark its former course, and the Isthmus had assumed the shape we know today.

This relatively simple tectonic history provokes questions concerning forces and crustal behavior, validity of fixed mobile continental theories, isthmian links, volcanic island arcs, and continental front folding that cannot be answered today. The scope of the problems are indicated, however, and direction of investigation indicated that may occupy geologists for many generations.

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