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The Evolution of the Appalachians

The Appalachians resulted from a number of Paleozoic orogenies that occurred during

closure of the Iapetus Ocean and the collision of microcontinents and the African plate with the

North American plate (Pfiffner, 2006). In 1966, Tuzo Wilson came up with the Wilson cycle

which represents closing and opening of the Atlantic Ocean basin incorporated with the cyclicassembly and breakup of supercontinents based on his studies in the Appalachian Mountains.

Thomas, W.A. (2006) stated that there is a succession of two complete Wilson cycles in eastern

North America: closing of an ocean and assembly of the Rodinia supercontinent, breakup ofRodinia and opening of the Iapetus Ocean, closing of Iapetus and assembly of the Pangea

supercontinent, and breakup of Pangea and opening of the Atlantic Ocean (Figure 3).

Metamorphic and igneous rocks of the Grenville province, ranging in age from 1350 to1000 Ma, record closing of an ocean and assembly of the Rodinia supercontinent (Hoffman,

1991). Synrift sedimentary and igneous rocks, a post-rift unconformity, and early post-rift

sedimentary strata document the breakup of Rodinia, the opening of the Iapetus Ocean (Proto-

Atlantic Ocean) in Cambrian time (Thomas, 2006). The Appalachian orogen was built betweenthe end of the Middle Proterozoic Grenvillian Wilson cycle, during breakup of the

supercontinent Rodinia, and the end of the Appalachian Wilson cycle at the end of the Paleozoic.Passive-margin development followed rifting of Rodinia, which resulted in the greatest flooding

of the interior of the Laurentia known in geologic time during the Late Cambrian and EarlyOrdovician (Hatcher, 2010). The continental edge ran down the axis of the present day Blue

Ridge province, and Virginia lay south of the equator in a tropical climate (Fichter, 1999). Warm

tropical waters, plus an absence of sourcelands to supply sandstones and shales, led to carbonatedeposition many thousands of feet thick (Fichter, 1999).

Beginning about 450 Ma ago the tectonically quiet conditions changed dramatically from

that of shallow carbonate deposition, passive tectonic margin to a tectonically active foreland

basin setting with the Taconic orogeny. The effects of the orogeny extend along the entireeastern sea board from the Carolinas in the south through New England and the maritime

provinces of Canada in the north, including the Piedmont, Blue Ridge, Valley and Ridge, and thecraton (Fichter, 1999). By the end of the Ordovician, the collision of the northeast-southwesttrending volcanic island arc/microterrain complex with eastern Laurentia (Rodgers, 1971). The

first phases of collision pushed portions of the accretionary prism onto the Laurentian shelf

margin and due to the increased load, the region immediately west of the collision zone subsidedinto a foreland basin. Final phases of collision are recorded by the overthrust of the Laurentian

margin by the entire volcanic arc complex with subsequent subduction reversal from east to

west-dipping subduction (Rowley & Kidd, 1981).

The Acadian Orogeny started about 430 to 425 million years ago, when the Avalonianarc, moving northwest, and Baltica, moving west, impacted against the southward-moving Proto-

North America. Avalonia was a microcontinent made up of Late Precambrian to Early Paleozoic

rocks (Delany & Rast, 1983). The Acadian orogeny began with the latest Silurian to earliestDevonian collision of Avalonia with Laurentia during contraction and closure of the AcadianSeaway. Subduction of Avalonia beneath Laurentia led to the progressive migration of the

deformation front from the southeast to the northwest resulting in inversion of the Matapdia

Basin by the late Early to early Middle Devonian and termination of the Acadian orogeny in thenortheastern Appalachians (Fyffe, Johnson, and Van Staal, 2011). When the Avalonia terrane

was accreted onto North America, it created an area called a foredeep due to the strongly damped

harmonic wave created by the weight of the terrane on North America (Hamiltan-Smith, 1993).


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When the large mass was pushed on top of North America, the rocks responded viscously, and

cause a depression in front of the terrane (Hamiltan-Smith, 1993). An interesting set of sediments

resulted from the filling of the migrating foredeep called flysch and molasse (Hamiltan-Smith,1993). Flysch is a deep water sediment and thus indicates that the foredeep was inundated

(Hamiltan-Smith, 1993). As it filled with sediment it got progressively shallower and the next

phase of deposition consisted of the shallow water sediment, molasse (Hamiltan-Smith, 1993).The Alleghanian orogeny is the last major orogenic event to affect the Mid-Atlanticregion from the late Mississippian to the Permian time. The orogeny occured when northwest

Africa, a part of the supercontinent Gondwana collided with the eastern seaboard of North

America closing the Proto-Atlantic and Reich oceans (Fichter, 1999). In the Alleghanianorogeny, Africa (Gondwana) became a hinterland meaning it was thrust up and over the North

American edge (Thomas, 2006). About 306 million years ago, the supercontinent Pangea was

formed as multi-continental collisions continued. These continuing impacts created a vast

mountain range and deformed and metamorphosed rocks previously altered by earlier mountainbuilding episodes (Fichter, 1999). As a result, the late Paleozoic Alleghanian orogeny is

generally accepted to be an oblique collision between Gondwana and Laurentia in the late

Paleozoic (Rodgers, 1987; Figure 4).Pangaea began to break up about 200 million years ago. The North Atlantic Ocean

formed as North America separated from Africa, Europe and South America. As North America

pushed westward, the west coast entered a period of new tectonic activity. The breakup of

Pangaea led to increased regional differences in floras and faunas between continents. The riftingevent, once begun, proceeds rapidly, for by the early Jurassic ocean crust is beginning to form,

and the Atlantic ocean open (Fichter, 1999). Pangaea fragmented in stages. It first split along an

east-west line running through the present Mediterranean sea region forming a northerncontinent, Laurasia and a southern continent, Gondwana; then Laurasia and Gondwana each

split, in stages, east and west to open the Atlantic ocean (Fichter, 1999).

The southern Appalachians can comprise provinces of similar lithology and structure

parallel to the lenght of the mountains (Hatcher, 2010; Figure 1 & 2)). These provinces are fromnorthwest to southeast:

1-Appalachian Plateau: The province contains the westernmost deformed rocks in the

Appalachian orogen and is external, least deformed, part of a foreland fold and thrust belt. ThePlateau is underlain by unmetamorphosed sedimentary rocks that were deposited on the North

American Craton.

2-Valley and Ridge: The province comprises the most characteristic part of theAppalachian foreland fold and thrust belt. The Valley and Ridge is underlain by the same

sequence of unmetamorphosed sedimentary rocks that are found in the Appalachian Plateau.

These rocks have been folded into synclines and anticlines generally separated by thrust faults.

3-Blue Ridge: The province comprises thrust sheets that form the westernmost part of thecrystalline core in the Southern Appalachians. The western Blue Ridge rocks comprise the Ocoee

Supergroup and overlying rocks of a narrow sinuous belt known as Murphy Syncline. The

metasedimentary and metaigneous rocks in the southern part of the eastern Blue Ridge comprise

the Coweeta Group, Tallulah Falls Formation, and Precambrian basement gneisses that are likelyof Grenville affinity.

4-Piedmont: The province comprises the easternmost part of the crystalline core that is

exposed in the southern Appalachians. This province may consist of crustal fragments exotic toNorth America. The inner Piedmont comprises medium and high grade schists and gneisses that


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have been multiply deformed. All of the metamorphic rocks have been metamorphosed to

amphibolite facies.

Figure 1. Parts of the Appalachians

Figure 2. Parts of the Appalachians with a cross-section


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Figure 3. Fourteen cross-section to show the evolution of the Appalachian Mountains (Fichter,

1999).


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Figure 4. Zipper closing of Theic ocean to form the Alleghanian orogen (Hatcher, 2010).

References:

Pfiffner, O.A., 2006, Thick-skinned and thin-skinned styles of continental contraction: GeologicalSociety of America Special Papers, 414, p. 153-177, doi:10.1130/2006.2414(09).

Thomas, W. A., 2006, Tectonic inheritance at a continental margin [2005 Geological Society ofAmerica Presidential Address]: GSA Today, v. 16, no. 2, p. 4-11.

Hoffman, P.F., 1991, Did the Breakout of Laurentia turn Gondwanaland inside-out?: Science 252,p. 1409-1412.

Hatcher, R.D. Jr., 2010, The Appalachian orogen: A brief summary: GSA Memoir 206, p. 1-20. Fichter, L.S., 1999, The geological evolution of Virginia and the Mid-Atlantic region:

http://csmres.jmu.edu/geollab/vageol/vahist/index.html

Rodgers, J., 1971, The Taconic Orogeny: Geological Society of America Bulletin, v. 82, p. 1141-1178.

Rodgers, J., 1987, The Appalachian-Ouachita orogenic belt: Episodes, v. 10, p. 259-266. Rowley, D.B., and Kidd, W.S.F., 1981, Stratigraphic relationships and detrial composition of the

medial Ordovician flyisch of western New England; Implications for the tectonic evolution of theTaconic Orogeny: Journal of Geology, v. 89, p. 199-218.

Delany, F.M., and Rast, N., Profiles of orogenic belts: Geodynamics Series, v. 10, p. 309. Fyffe, L.R., Johnson, S.C., Van Staal, C.R., 2011, A review of Proterozoic to Early Paleozoic

lithotectonic terranes in the northeastern Appalachian orogen of New Brunswick, Canada, and

their tectonic evolution during Penobscot, Taconic, Salinic, and Acadian orogenesis: Atlantic

Geology, v. 47, p. 211-248.

Hamilton-Smith, T., 1993, Stratigraphic effects of the Acadian orogeny in the autochthonousAppalachian basin: GSA Special Paper 275, p, 153-164.
http://csmres.jmu.edu/geollab/vageol/vahist/index.htmlhttp://csmres.jmu.edu/geollab/vageol/vahist/index.htmlhttp://csmres.jmu.edu/geollab/vageol/vahist/index.html

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