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The Appalachian mountain system of eastern North America comprises a complex of deformed rocks which has stimulated geologic thinking since the area was first settled in the 17th century. Little attention was paid to it in the early days of European settlement, but in the early nineteenth century interest grew rapidly, leading to the creation of numerous state surveys and research that has continued unabated to this day. The earliest tectonic concepts were understandably rather primitive and mostly concerned the simpler foreland portion of the orogen. Upheavals, explosions, and other vertical motions were the main ingredients of the theory of the Rogers brothers and the geosyncline of Hall and Dana. Awareness of the importance of horizontal movements increased as continued detailed mapping revealed the presence of large thrust faults, culminating in the middle of this century with the elucidation of a décollement tectonics that pervades the entire Appalachian foreland.

The Appalachian crystallines were originally considered to be part of the crust, and they formed a large part of Appalachia, Dana’s geanticline that supplied sediment to the adjoining geosyncline. Studies early in the 1900s began to reveal the complexities of this terrane and by the middle of this century, the crystallines had been divided into a number of distinctive belts. With the application of the plate tectonics scheme in the 1970s, geologists realized that these belts probably are micro-continents, either broken off from the Laurentian craton, or having come from other plates. With the accession of deep seismic data, the crystallines are no longer thought of as being rooted in the crust, but are believed to have undergone an allochthony at least as extensive as the foreland’s.

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