Idea Transcript
Appalachian Mountains Advanced Tectonics James S. Aber Tectonic overview The Appalachian Mountains represent a series of Paleozoic orogenies along the eastern margin of North America. The Appalachians extend from Alabama through Maine in the United States, and continue across the southeastern provinces of Canada to Newfoundland. More than 10,000 m of Paleozoic sedimentary and volcanic strata are contained in the exposed portions of the Appalachian Mountains, about 10 times the thickness of equivalent strata in the mid-continent region. The Appalachian Mountains are divided classically into four main provinces, which are best developed from New York southward. Piedmont – Stretching from Alabama to New York, the Piedmont is a plateau of moderate elevation (150-300 m) that forms the eastern portion of the mountain system. It consists of varied crystalline metamorphic and igneous rocks of Paleozoic age. Many of the rocks began as marine sediments and volanic deposits—typical oceanic crust—that were deformed and metamorphosed mostly to the greenschist facies of chlorite and biotite schists and slate. Numerous granite intrusions form domes within the Piedmont, and narrow belts of serpentine are common. All Paleozoic and older rocks of the Piedmont Province are now thought to be completely allochthonous thrust masses. Blue Ridge – Upthrust Proterozoic basement rocks form a relatively narrow ridge separating the Piedmont from the Valley and Ridge provinces. The boundary between the Piedmont and Blue Ridge is marked by the Brevard Fault zone in the southern Appalachians. This fault zone contains remnants of unmetamorphosed sedimentary rocks derived from the deep décollement beneath the Piedmont Province. Like the Piedmont, the Blue Ridge is considered to be allochthonous.
Appalachian Trail sign. The trail extends along the Blue Ridge and other high ridges of the Appalachian Mountains from Georgia to Maine. Great Smoky Mountain National Park.
Valley and Ridge – Paleozoic sedimentary rocks are thrust and folded into large anticlines and synclines. This deformation took place at low temperature over shallow décollements (shale, salt) within the sedimentary sequence. Metamorphic and igneous rocks are conspicuously absent. Differential erosion of the thrust and folded structures has led to the distinctive valley-and-ridge topography for which this province is famous. Deformation diminishes westward in a transition into the Appalachian Plateau. Appalachian Plateau – The plateau is underlain by gently folded to little-deformed Paleozoic sedimentary strata. The amplitude of folding and thickness of strata decrease toward the continental interior. Deep stream entrenchment has created a rugged topography, which was the main geographic barrier to westward expansion of the early United States. The Appalachian Mountains have somewhat different expression in New England and southeastern Canada. Neither the Blue Ridge nor Valley and Ridge provinces are developed. Most of the mountains consist of metamorphic rocks with numerous igneous intrusions. Logan's Line, a deep fault, is a major structural break between the Appalachian Mountains and the Canadian Shield. Southern New York is the zone of transition between typical structures of the northern and southern Appalachians, which have quite different tectonic histories.
Manned-space photograph looking toward the northwest over New York City, western Long Island and vicinity. This autumn views depicts fall foliage colors. Long Island and Staten Island represent parts of the Atlantic Coastal Plain, whereas most of the mainland is part of the Piedmont Province of the Appalachian Mountains. Obtained from NASA Johnson Space Center.
Manned-space photograph looking toward the northwest. This color-infrared photograph emphasizes active vegetation in red and pink colors. New York City is the blue zone at bottom center. Most of this scene portrays Piedmont rocks, and a major ridge of the Appalachian Mountains runs across the upper left portion of this view. Compare with the image above. Obtained from NASA Johnson Space Center.
The tectonic history of the Appalachian Mountains is divided traditionally into three main orogenic phases during the Paleozoic. Each phase begins with accumulation of thick marine sediments and volcanic deposits, culminates with deformation and uplift of mountains, and finally ends with tectonic quiessence. A particular consequence of orogeny is production of sediment in great volumes as uplifted mountains undergo rapid erosion. Thus, each phase is marked by buildup of a so-called "delta" filling shallow seas on the continental side of the orogeny. These deltas are actually clastic fans that include sediments deposited in many environments—terrestrial, coastal, near-shore, and off-shore settings. Much of what is known about the timing, location, and magnitude of mountain uplift is deciphered from the record of sediment accumulation. Taconic Orogeny – First important tectonic activity took place during the Ordovician and Silurian (Macdonald et al. 2014). Thrusting and folding were mainly in the northern portion, namely western New England and eastern New York. Uplifted mountains shed sediment to the west, and the "Queenstown delta" built up near Albany, New York. Acadian Orogeny – Major orogeny of the northern Appalachians occurred in the Devonian, centered in New England and southern New York. Strong folding, thrusting, metamorphism and granite intrusion took place. Sediments accumulated in the "Catskill delta" across southern New York and northern Pennsylvania. Allegheny Orogeny – Major orogeny of the southern Appalachians culminated in the Pennsylvanian. Thrusting, folding, metamorphism and intrusion happened from Pennsylvania southward to Alabama. A clastic "delta" spread over western Pennsylvania, West Virginia, Kentucky and Tennessee. All three orogenies are interpreted in terms of multiple collisions during the piecemeal closing of the Iapetus Ocean between North America (Laurentia), Europe and Africa (Gondwana). Sea-floor spreading, subduction-zone volcanism, and accretionary wedges were all part of a long, drawn-out sequence of events. Both continental and oceanic terranes were involved with collisions at different times and places. The Taconic Orogeny represented Laurentian and Gondwanan terrane collisions with North America (Macdonald et al. 2014). The Acadian Orogeny took place between North America and Europe, and is contemporaneous with Caledonian Orogeny of the British Isles, Greenland, and Scandinavia. Finally the Allegheny Orogeny involved the collision between Africa and the North American-European continent. The ultimate result was joining of several large land masses to create Pangaea by the end of the Paleozoic.
Appalachian images Manhattan Schist, Central Park, New York City, NY. Proterozoic crystalline metamorphic rock underlies the island of Manhattan and supports New York's tall buildings. This view shows glacial smoothing and grooves on the rock surface.
Complex folding is evident in metamorphic rocks of the Hudson Highlands, north of New York City. Exposures along I-84, eastern New York.
Thick, lower Devonian limestone marks the northern margin of the Appalachian Plateau near Oneida, in upstate New York. The rocks are little deformed. The Appalachian Plateau of south-central New York is underlain by thick middle and upper Devonian strata. These interbedded sandstone, siltstone, and shale layers were deposited in a marine environment as part of the so-called "Catskill delta" during the Acadian Orogeny. This view illustrates little-deformed, gray-colored beds exposed at Montour Falls, New York. Devonian clastic marine strata of the "Catskill delta" extend into western New York and Pennsylvania. A thick sequence is displayed in the walls of Genesse Gorge, near Rochester, western New York.
Streams have entrenched deep valleys in little-deformed strata of the Appalachian Plateau. This false-color Landsat image highlights stream erosion near Renova, Pennsylvania. Obtained from NASA Goddard Space Flight Center.
Landsat image of the Appalachian Plateau in West Virginia. The Ohio River valley loops through the scene. Another "dry" valley represents the abandonned preglacial Teays Valley. Obtained from NASA Goddard Space Flight Center.
Thick, coal-bearing, Pennsylvanian sandstone forms Cumberland Falls, eastern Kentucky. This is the traditional western limit of the Appalachian Mountains.
Lower Mississippian strata exposed in syncline of Sideling Hill, western Maryland. The syncline supports a ridge in the Valley and Ridge Province.
Proterozoic, low-grade metamorphic rock of Blue Ridge, seen here at Newfound Gap, Great Smoky Mountains National Park.
Mafic schist represents ancient seafloor crust now thrust into the Piedmont of North Carolina. Exposure along I-26, south of Hendersonville, North Carolina.
Closeup view of folded biotite schist--part of an ophiolite sequence. Hendersonville, North Carolina.
Gneiss and schist exposed in the spillway of Lake Murray, near Columbia, South Carolina. Rocks of this type underlie large portions of the Piedmont Province.
Amphibolite schist dike in gneiss. The dike formed as a mafic intrusion, and later the rocks were deformed and metamorphosed. Lake Murray spillway, near Columbia, South Carolina.
Stone Mountain as seen from tour boat, near Atlanta, Georgia. Stone Mountain is a "granite" dome within the Piedmont Province. Its geomorphic prominence is a result of erosion of surrounding softer rocks.
Sky lift at top of Stone Mountain, Georgia.
Closeup view of Stone Mountain granite with hornblende phenocryst and garnets. The garnets represent a metamorphic phase after the granite had cooled and solidified.
Continental rifting Pangaea remained a stable supercontinent during the Permian and Triassic. However, signs of breakup had begun to appear already in the Triassic. A series of rift basins developed in the Piedmont Province. These fault-bounded basins are filled with volcanic rocks and continental sediments that include vertebrate fossils. Triassic basins resembled the modern East African and Rio Grande rift systems, but failed to progress to complete continental rifting. Actually rifting and seafloor spreading began in the Jurassic and accelerated during the Cretaceous as the Atlantic Ocean openned up in several stages. With continued seafloor spreading, the margins of the continent began to subside slowly and coastal plain sediments accumulated. For the most part, these Cretaceous and Cenozoic sediments are clastic deposits derived from the eroding Appalachian Mountains, but carbonate sediments formed in southern Florida.
Middle Eocene sandstone exposed in a roadcut on the margin of a terrace in the coastal plain, near Columbia, South Carolina. These sediments are consolidated only slightly.
Closeup view of Middle Eocene sandstone showing coarse quartz sand and kaolin pebbles. Strong weathering has reduced original granitic pebbles to clay. Near Columbia, South Carolina.
Canal-cut exposure of the Miami Oolite, a late Pleistocene limestone that underlies the city of Miami, Florida. The oolite shoals formed during a high stand of sea level (+ 4-8 m) about 125,000 years ago.
Quarry exposure of the Key Largo Limestone, a coraline limestone of a late Pleistocene reef complex. Windley Key, Florida Keys. The coral reef formed during a high stand of sea level (+ 4-8 m) about 125,000 years ago.
Surficial exposure of "rock reef" in the Florida Everglades, southern Flordia. The carbonate shoals formed during a high stand of sea level (+ 4-8 m) about 125,000 years ago.
Reference Macdonald, F.A., Ryan-Davis, J., Coish, R.A., Crowley, J.L. and Karabinos, P. 2014. A newly identified Gondwanan terrane in the northern Appalachian Mountains: Implications for the Taconic Orogeny and closure of the Iapetus Ocean. Geology 42/6, p. 539-542.
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