TheAlleghanian orogeny orAppalachian orogeny is one of the geological mountain-forming events that formed theAppalachian Mountains andAllegheny Mountains. The term and spellingAlleghany orogeny was originally proposed by H.P. Woodward in 1957.
The Alleghanianorogeny occurred approximately 325 million to 260 million years ago[1] over at least five deformation events[2] in theCarboniferous toPermian period. The orogeny was caused by Africa's collision with North America. At the time, these continents did not exist in their current forms: North America was part of theEuramerica super-continent, while Africa was part ofGondwana. This collision formed the super-continentPangaea, which contained all major continental land masses. The collision provoked the orogeny: it exerted massive stress on what is today theEastern Seaboard of North America, forming a wide and high mountain chain.[3] Evidence for the Alleghanian orogeny stretches for many hundreds of kilometres on the surface fromAlabama toNew Jersey and can be traced further subsurface to the southwest. In the north, the Alleghanian deformation extends northeast toNewfoundland. Subsequent erosion wore down the mountain chain and spread sediments both to the east and to the west.
The immense region involved in the continental collision, the vast temporal length of the orogeny, and the thickness of the pile of sediments and igneous rocks known to have been involved are evidence that at the peak of the mountain-building process, the Appalachians likely once reached elevations similar to those of theAlps and theRocky Mountains before they were eroded.[4][5]
As the continents collided, the rock material trapped in-between was crushed and forced upward. With nowhere to go, rocks along the eastern margin of the North American continent were shoved far inland (the same occurred in the opposite direction along the margin of the African continent, forming theAtlas Mountains ofMorocco and the westernSahara).[6] Close to the boundary between the colliding plates, tectonic stresses contributed to the metamorphism of the rock (i.e. the transformation ofigneous andsedimentary rock intometamorphic rock).
The sedimentary rock in the easternAppalachian Basin region was squeezed into greatfolds that ran perpendicular to the direction of forces. The greatest amount of deformation associated with the Alleghanian orogeny occurred in the Southern Appalachians (North Carolina,Tennessee,Virginia, andWest Virginia). In that region, a series of greatfaults developed in addition to the folds. As the two continents collided, large belts of rock bounded bythrust faults piled one on top of another, shortening the crust along the eastern edge of North America in the North Carolina and Tennessee region by as much as 300 kilometres (200 mi). The relative amount of deformation gradually diminishes northward. The fold belt extends northward throughPennsylvania and gradually fades in the vicinity of theNew York border. TheKittatinny Mountains in northwestern New Jersey mark the northeasternmost extension of the high ridges of theValley and Ridge Province. The influence of Alleghanian deformation on the regions east of the Valley and Ridge Province must have been even more intense; however, there is little evidence preserved. Rocks ofMississippian,Pennsylvanian, andPermian age are missing along the Eastern Seaboard.[6]
The mountains formed by the Alleghanian orogeny were once rugged and high[7][8] during theMesozoic and latePaleozoic but in our time are eroded into only a small remnant: the heavily eroded hills of thePiedmont.Sediments that were carried eastward formed thecoastal plain and part of thecontinental shelf. Thus, the coastal plain and Piedmont are largely the byproducts of erosion that took place from 150+ million years ago to the present. Sediments that were carried westward formed theAllegheny andCumberland plateaux. Although people living in those respective areas sometimes refer to them as mountains, they are more accuratelyuplifted and eroded plateaux.[9]
A portion of the Alleghanian mountain system departed with Africa when Pangaea broke up and theAtlantic Ocean began to form. Today, this forms theAnti-Atlas mountains of Morocco. The Anti-Atlas have been geologically uplifted in relatively recent times and are today much more rugged than their Alleghanian relatives.
This article incorporatespublic domain material fromGeology of Manassas National Battlefield Park.National Park Service. This article incorporatespublic domain material fromNYC Regional Province: Valley and Ridge Province.United States Geological Survey.