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The Southern Appalachians
In contrast to the Appalachians
of Newfoundland, the Maritimes, and most of northern and western
New England, the Southern Appalachians suffered the effects of a collisional
event involving extensive crustal overthrusting of Gondwanaland over Laurentia
during the Alleghanian orogeny (Late Carboniferous/Permian).
This event is however also recognisable in south-eastern New England
in Eastern Connecticut and Massachusetts, where it involved westerly underthrusting
of an Avalonian terrane beneath the Acadian deformed rocks of the western
margin of the Iapetus ocean. Along the thrust zone metamorphism is of kyanite
grade (kyanite replacing sillimanite).
From West to East the Southern Appalachians passes from folded and thrust passive margin sediments of the Valley and Ridge into variably metamorphosed rift and passive margin rocks (Western Blue Ridge) as young as Mississippian, overthrust in places by Ordovician greenstones and felsic volcanic rocks. Further east, thrust units of the Eastern Blue Ridge are composite units composed of highly metamorphosed, structurally complex, deeper water sediments and ultramafic-mafic 'ophiolitic' rocks, as well as Grenville basement. The sediments may represent slope and rise deposits or even foreland basin deposits laid down in association with ophiolite obduction. Together with the rocks of the Inner Piedmont and the Charlotte belt they form the metamorphic core of the Southern Appalachians. Rocks east of the Inner Piedmont belonging to the Carolina terrane are relatively low metamorphic grade, and contain rare fossils of Atlantic/Avalonian provenance indicating that the terrane is exotic to North America. The Carolina terrane was likely accreted to North America during the Ordovician/Silurian. The most easterly terrane is the Suwannee terrane composed of fossiliferous Ordovician to Devonian sandstones and shales. The fossils have European/African affinities and it seems likely that the Suwanee originated as part of African Gondwanaland that collided with Laurentia in the Carboniferous/Permian, and that the Suwannee - Carolina terrane boundary marks the Alleghanian suture.
Lithotectonic map of the Southern Appalachians
Map of terranes composing the Southern Appalachians
Hatcher, R.D., et al. 1989. The Appalachian - Ouachita orogen in the United States, Volume F-2, Geology of North America, QE71.G48.
Hatcher, R.D., 1987. Tectonics of the southern and central Appalachian internides. Annual reviews of Earth and Planetary Sciences, 15, p. 337-362.
Horton, J.W., Drake, A.A., and Rankin, D.W., 1989. Tectonostratigraphic terranes and their Paleozoic boundaries in the central and southern Appalachians. Geol. Soc. America Special Paper 230, p. 213-245.
Hopson, J.L., Hatcher, R.D., and Stieve, A., 1989. Geology of the eastern Blue Ridge, Northeastern Georgia and the adjacent Carolinas. Georgia Geol. Soc. Guidebooks, 9, 3, p. 1-14.
Hibbard, J., 2000, Docking Carolina: Mid-Paleozoic accretion in the southern Appalachians. Geology, 28, p. 127-130
The following terrane descriptions are largely taken from Horton et al. (1989), supplemented by more recent research data.
Valley and Ridge
The most westerly deformed rocks of the Southern (Central) Appalachians are the folded and thrusted Paleozoic (Later Proterozoic to Permian) of the 'Valley and Ridge'. Cambrian fossils are of North American 'Pacific' (Ollenelus) type. Paleomagnetic studies indicate that the folding became progressively younger from east to west (from hinterland to foreland). Cleavage development in these rocks is Alleghanian in age. An example of mineralization in these rocks would be the East Tennessee Pb/Zn mine near Knoxville.
Valley and Ridge- Virginia Division of Mineral Resources
Western Blue Ridge/Talladega
Further east 'native terranes' of Laurentia are represented in the 'Blue Ridge' tectonic province by anticlinal Grenvillian basement massifs unconformably overlain by the basal Catoctin Volcanic Formation (a basal blue green basaltic unit that is present all along the length of the Appalachians, e.g. Tibbit Hill in Quebec); rift facies sedimentary sequences such as the Ocoee; and felsic volcanic complexes such as the Mount Rogers and Grandfather Mountain Formations. Late Proterozoic glacial deposits are also present at Mount Rogers. At the southwest end of the belt the western Blue Ridge is represented by rocks of the Talladegablock. The Talladega terrane is made up of a greenschist facies (Staurolite) Late Proterozoic Laurentian margin clastic sequence (Kahatchee Mountain Group = Great Smoky Group), followed by a a Paleozoic carbonate to clastic sequence (olistostromal deposits associated with the clastics), and fossiliferous Devonian cherts. The cherts are overthrust by the Ordovician Hillabee greenstone of arc-like character. Analogous terranes to the northeast in Maryland/Pennsylvania include the Hamburg and Westminster klippen. Sulphide mineralization occurs at Ducktown. The Talladega has been thrust over the unmetamorphosed rocks of the Valley and Ridge Province, and has itself been overthrust by higher grade rocks of the Eastern Blue Ridge/Jefferson Terrane to the east.
Ridge - Virginia Division of Mineral Resources
Virtual field trip to the Laurentian continental margin rocks of the Blue Ridge at Mount Rogers
Eastern Blue Ridge/Jefferson
The Eastern Blue Ridge/Jefferson terrane is formed of highly deformed pelitic and psammitic sediments of the Ashe, Lynchburg, Alligator Back and True Blue (Everona Limestone) sequences intercalated with ultramafic (Chunky Gal) and mafic rocks of controversial origin (rift?, oceanic?, arc?). (The basal quartzite of the True Blue group unconformably overlies mafic volcanic rocks.) It is the locus of Cu-Zn sulphide mineralization, e.g. Gossan Lead (North belt); Franklin (South belt). The Jefferson terrane has been thrust over the Laurentian margin as is evidenced by the presence of basement rocks in the Toxaway dome at the eastern edge of the belt. Kyanite-bearing leucosomes post-dating the dominant deformation fabric in granulite facies rocks, including ultramafic material, at Winding Stair Gap at the extreme southwest end of the North Carolina Eastern Blue Ridge, contain 470 Ma old zircons, indicating that the latest metamoprhism took place during the Taconic orogeny.
units of the Western and Eastern Blue Ridge and the Inner Piedmont just
east of the Brevard fault
Metamorphic zonation in the Western and Eastern Blue Ridge and the Inner Piedmont just east of the Brevard fault
Stair Gap controversy
Winding Stair Gap Field Trip Stop http://donahoe.geo.ua.edu/fieldtrips/windingst.html
Granulites at Winding Stair Gap, North Carolina; the thermal axis of Paleozoic metamorphism in the Southern Appalachians. Absher-B-Steven; McSween-Harry-Y Jr. Geological Society of America Bulletin. 96; 5, Pages 588-599. 1985.
Winding Stair Gap granulites; the thermal peak of Paleozoic metamorphism. Absher-B-Steven; McSween-Harry-Y Jr. Southeastern section of the Geological Society of America. Neathery-Thornton-L (editor) In the collection: Centennial field guide. 1986.; In the collection: (6.) 6; Pages 257-260. 1986.
Multiple granulite-facies events in the southern Appalachians, USA. McLellan, E., Linder, D., & Thomas, J., in Evolution of Metamorphic Belts, eds. Daly, J.S., Cliff, R.A., and Yardley, B.W., Geological Society Special Publication No. 43, pp. 309-314. 1989.
Moecher et al., 2000, GSA Abst., A-113
The Potomac terrane to the southeast of the Jefferson along the northeast part of the Southern Appalachian belt is composed of a stack of thrusts of melange units made up of ophiolite, arc and deep-water turbidite facies rocks. The thrust stack, along with the overlying Bel-Air (Balitmore) ophiolite sheets (489 Ma, Sinha et al. 1997) and Chopawamsic arc, was amalgamated and overlain by the Popes Head Formation prior to the intrusion of the early Ordovician 494+/14 Occoquan granite. The younger Ordovician Arvonia Slate - Quantico formation unconformably overlies the thrust stack. All these units have been thrust over the Laurentian margin represented by the Baltimore gneiss domes and the Goochland terrane east of the Chopawamsic. (Although possibly the Goochland terrane could be a Laurentian microcontinent that was located paleogeographically east of the Chopawamsic. The same explanation could be applied to the Pine Mountain terrane of Grenvillian rocks east of the Inner Piedmont terrane.)
Inner Piedmont - Smith River
The Inner Piedmont and Smith River terranes are either thrust over the Jefferson and Potomac terranes and underlie the Chopawamsic, or overlie all these units. The Inner Piedmont is a stack of thrust sheets containing schists, gneisses (some charnokitic), amphibolites, and sparse ultramafic bodies. Parts of the terrane are reminiscent of the Jefferson terrane, and the Inner Piedmont, Jefferson, and Potomac terranes could all be part of a superterrane. The northern boundary of the Inner Piedmont with the Eastern Blue Ridge/Jefferson is marked by the Brevard zone of intense shearing, and rocks of the Chauga belt to the south of the fault are of lower metamorphic grade. Both Ordovician and Devonian ages of metamorphism are recorded in zircons from Inner Piedmont granitic gneisses. Recent papers
- Virginia Division of Minera Resources
Tectonic units of the Western and Eastern Blue Ridge and the Inner Piedmont just east of the Brevard fault
Two possible sections along line A-A' across the Blue Ridge, based on seismic reflection and surface geologic data
Stratigraphic sequences in the Eastern Blue Ridge
The Juliette terrane contains abundant mafic and ultramafic fragments in a matrix of schistose metamudstones and metagreywacke. It is thought to be a melange complex.
The Uchee terrane is a migmatite complex of paragneiss, amphibolite, and granitoid gneiss, late Proterozic to early Cambrian in age.
Kings Mountain belt (western Central Piedmont)
The Kings Mountain belt consists mainly of quartzite, metaconglomerate, marble and mica schist. Volcanic rocks have similarities with volcanic rocks further east, but not with rocks of the Inner Piedmont.
Carolina including the Carolina Slate Belt (suprastructure) and the Charlotte Belt (infrastructure)
The Carolina terrane in the Southern Appalachians is a very extensive terrane of Late Proterozoic volcanic and volcanogenic sedimentary rocks. The terrane is characterized by the presence of Late Proterozoic Ediacarian faunas and Cambrian Atlantic province faunas. Evidence of Ordovician metamorphism suggests that the Carolina terrane was part of or located close to Laurentia during the Taconic orogeny. This is also indicated by paleomagnetic data. The Charlotte Belt is composed largely of Paleozoic intrusive rocks ranging in composition from gabbro to granite; the rocks of this belt have suffered amphibolite grade metamorphism. Eclogites are found in the Charlotte belt near Newberry just north of the Charlotte - Slate Belt boundary (Denis, A. J. et al., 2000, GSA Abst, p. A-234). Recent papers
Raleigh Belt/Goochland - Kiokee Belt
Located east of the Carolina slate Belt, the Raleigh and Kiokee belts have high grade gneisses in their cores, and were regionally metamorphosed during the Alleghanian. Granites of Alleghanian age intrude both belts.
Eastern Slate Belt
The Eastern Slate Belt consists mainly of greenschist facies metavolcanic rocks, metamorphosed plutons, and post-metamorphic intrusions similar to those in the Carolina Slate Belt.
The Suwannee terrane contains a sequence of fossiliferous Ordovician to Devonian sandstones and shales, unmetamorphosed and little deformed. The fossils are European or African (Atlantic) in character, and the sequence is similar to that of the Bove basin in Senegal and Guinea. The Suwannee is considered to be separated from the Carolina terrane by the Alleghanian suture, although it has also been suggested that the suture lies at the northern boundary of the Charleston terrane. Recent papers
Geological Map of North Carolina
Geological Map of South Carolina
Geological Map of Tennessee
Geological Map of Alabama
Map of Georgia I (http://home.att.net/~cochrans/ggmndx01.htm)
Geological Map of Georgia II (http://home.att.net/~cochrans/geomap01.htm)
Geological Map of Maryland
University of Alabama Field Guide - http://donahoe.geo.ua.edu/fieldtrips
for Appalachian geology - http://www.salemstate.edu/~lhanson/gls210/geomor_provinces/phpr_ah.htm
Isotopic data bearing on the crustal sources of rocks in the Blue Ridge and Piedmont
Fullagar et al., 1997. Nd and Sr isotopic characterisation of crystalline rocks from the southern Appalachian Piedmont and Blue Ridge, North and South Carolina. Gol. Soc. America Memoir 191.
geological map of the Blue Ridge and Piedmont
Nd Evolution diagram showing crystallization versus initial EpNd for North and South Carolina Blue Ridge, Inner Piedmont and central and eastern Piedmont rocks.
Schematic geological map of southeast New England - rocks of the Putnam-Nashoba zone (Avalonia) were underthrust beneath rocks of the Kearsarge - Central Maine synclinorium during the Alleghanian orogeny.
Gromet, P.L., 1989. Avalonian terranes and late Paleozoic tectonism in southeastern New England; constraints and problems. Geol. Soc. America Special Paper 230, p. 193-212.
Moecher, D.P., Cosca, M.A., and Hanson, G.N., 1997. Petrologic and 40Ar/39Ar geochronological constraints on the middle to late Paleozoic thermotectonic history of the southern Connecticut Valley zone, New England Appalachians. BGSA, 109, 2, 164-175.
Correlation with the Newfoundland and New England Appalachians
Discounting the effects of Alleghanian
thrusting, the following speculation is permissible as a terrane correlation:
|No correlative||Valley and Ridge Tectonic Province|
|Western Newfoundland foreland||Jefferson terrane|
|Fleur de Lys||Potomac terrane|
|Notre Dame Bay ophiolite/arc terrain||Bel Air/Chopawamsic ophiolitic arc terranes|
|Burlington Granodiorite||Occoquan pluton|
|Exploits||Inner Piedmont terrane|
|Davidsville/GRUB line||Juliette terrane|
|Gander Lake||Uchee or Charlotte terranes|
The Precordillera of Argentina
The presence in Cambrian rocks of the Precordillera of Argentina of trilobites, brachiopods and conodonts with Laurentian affinities suggests that the Precordillera rifted from the southern end of the Appalachians during the Cambrian. By Llanvirn-Llandeilo times, the Precordillera was attached to the Andean margin and became the site of Famatinian arc volcanism, and the habitat of a Gondwana margin Celtic fauna similar to that found in the Exploits zone of Newfoundland and the southern part of the British Caledonides. Other authors have suggested that the rifting event followed the closure of the Iapetus in the Ordovician, or during the Devonian.
Paleogeographic reconstruction showing supposed translation of the Pre-Cordillera terrane of Argentina from the Ouachita salient of the Laurentian margin
The Hercynian (Alleghanian) of Europe
The Hercynian system (Rheic ocean) of Western Europe extends westwards from its intersection with the Polish Caledonides through southern Germany, Central France, Brittany, Spain and Portugal, and possibly northwest Africa. Collision of the Gondwana realm with Avalonia took place from Late Devonian time to the Permian. The deformation front passes through South Wales and the South of Ireland but is not seen in Newfoundland. The leading edge of the deformation front may be present in New Brunswick and also in the western part (Boston, Rhode Island) of the New England Appalachians, beyond which however it becomes prominent as the western boundary of the Valley and Ridge Structural Province of the Southern Appalachians. The southern edge is located in the Montagne Noire region of the south of France, the Pyrennees, and eastern Central Spain. If the belt crosses the Mediterranean it would have to turn back on itself and pass through Morocco and south to Senegal. The collision of Gondwana with Avalonia/Laurentia completed the assembly of the Permo-Triassic Pangean supercontinent.
Late Paleozoic Hercynian-Variscan system of Europe - naphercyn1.jpg
Paleogeographic reconstructions for Laurentian, Avalonian, Baltic, and Gondwana during the Paleozoic - napmag1.jpg
p. A-113 Moecher, D.P., P-T conditions and timing of peak Taconian granulite facies metamorphism in the southern Blue Ridge: - post-kinematic kyanite bearing leucosomes in granulites at Winding Stair Gap (southern limit of the eastern Blue Ridge) have zircons with ages of 472.7 (cores) to 469.8 (rims), that is, Taconic. Pre-leucosome silimanite is oriented WNW, and cross-cut by kyanite.
p A-234 Bier, S.E., et al. Detailed geologic mapping of the Western and Central Inner Piedmont, north Carolina: - western IP is dominated by westward vergent NE -SW isoclinal folds and type-F thrust faults; central IP is dominated by complex fold interference patterns involving NW-SE isoclinal recumbent folds, and no major thrust faults ; large granitoid bodies, e.g. Henderson Gneiss, are involved in the deformation.
GSA Southeast Section, Abst. w. Prog, Charleston
p. A-9 Carrigan, C.W. et al. Grenville basement of the southern Appalacians, NC-GA - geochemistry and zircon ion microprobe geochronology: - zircon magmatic cores - 1200-1150 and rims 1050-900. Epsilon Nd values near CHUR at 1.2 Ga indicate that Grenville granitic magmas are composed of both juvenile and older crustal components. No pre-Grenville inherited cores found.
p. A-24 Hatcher, R.D. The southern Appalachians: a thrust dominated orogen: - Brevard fault was reactivated as an out-of-sequence brittle thrust during the Alleghanian, coeval with the emplacement of the Blue ridge - Piedmont thrust sheet. Brevard was also active during the Acadian as both a high temp dextral fault and a a lower Temp dextral fault. Clockwise rotation of Africa relative to Laurentia brought them into head on collision transforming a strike-dominated early Alleghanian orgen into a thrust-dominated orogen. Acadian thrusting propogated into the Blue Ridge at least to the Chattahoochee fault. Greenbrier, Hayesville- gossan Lead thrusts are Taconic and are fossil subduction zones and terrane boundaries. All Acadian IP and eastern Blue Ridge thrusts were gnerated under high T. Grenbrier mayhave originated as a thin skinned thrust.
p. A-60 McCarter, R. et al. Some observations on the petrology of the Stone Mountain North Carolina pluton: - Sonte Mountain is Devonian (390 Ma) instruded into the Alligator Back of the Eastern Blue Ridge (Gossen Lead Thrust Block).
p. A-61 McClellan, E.A. Ordovician age confirmed for the Hillabee Greenstone, Talladega belt, southernmost Appalachians: - rocks in the Talladega belt range from Carmbrian to early Mississippian; zircons from dacite of the Hillabee are 470+/-4; there are four ages of 490-500 and a 447 Ma lead loss age. Ages are older than those of K-bentonites preserved in eastern North America but contemporaneous with K-bentonites in the Argentine Precordillera.
p. A-62 Miller, B.V. et al. U-Pb ages form the Bakersville, North Carolina eclogite:- Taconian eclogite metamorphism followed by Acadian and Alleghanian cooling: - The Burnsville fault is the surface expression of the Taconian suture zone in the western North Carolina Blue Ridge Province southwest of the Grandfather Mountain window. Zircon from the eclogite is 461+/-4 interiot and 430 +/-15 rims. Titanite is 333.7+/-.4. Titanite from a partly amphibolitized eclogite has an age of 393.5+/-1.3.
p. A-65 Owens, B.E. et al. Late
Proterozoic plutons in the Goochland terrane, Virginia: Laurentian or Avalonian
connection? : - State Farm gneiss has zircons with an age of 1050 Ma. Lower
intercept discords are however at 600 Ma, the age of the Fine Creek Mills
A-type granite that intrudes the State Farm gneiss. Zircons from one sample
plot on a discordia line from 1070+/-15 to 629+/- 5. Lower intercept is
taken as the age of intrusion.
The Fine Creek may be associated with rifting of the Laurentian margin, rather than associated with the Slate Belt, although the age is different from the recognised rift pulses at 760-700 and 570 Ma.
p. A-73 Shervais, J.W. et al. Tectonic setting of the Newberry eclogite/granulite and its implications for the origin of the Carolina composite terrane: - The central Carolina terrane, Silverstreet subterrane, in western South Carolina comprises a 30 - 40 km wide zone of high grade gneiss that is distinct from greenschist facies 650-550 Ma metavolcanic rocks of the Carolina Slate Belt to the southeast and from amphibolite facies metavolcanic/plutonic rocks of the Charlotte belt to the northwest. The eclogites pre-date intrusion of the 415 Ma (early Devonian) Newberry granite and have an arc-type protolith. If the eclogites mark a suture then the Slate Belt - Charlotte is a composite terrane, with the Charlotte thrust under the Slate belt.
p. A-76 Steltenpohl, M.G. et al. The question of timing of metamorphism in the southernmost Appalachians revisited again: - White mica that forms the dominant cleavage in the Silurian-Early Devonian Lay Dam Fm of the Talladega belt has a cooling age of 40Ar/39Ar 327.6 Ma. The age is compatible with Middle Carboniferous metamorphism, which is also the cooling age of amphibolite facies rocks from the southern parts of the Alabama/Georgia Piedmont.
p. A-78 Trupe, C.H., et al. The Eastern-Western Blue Ridge contact in western North Carolina: evolving interpretations of a complex structure: - The eastern-western Blue Ridge contact is a pre-metamoprhic Taconian fault in southwestern North Carolina, an Acadian strike slip fault in western North Carolina southwest of the Grandfather Mountain window, and an Alleghanian thrust fault in northwest North Carolina north of the Grandfather window. Alleghanian thrust overprint strike-slip fabrics.
p. A-81 Waters, C.L. et al. The tectonothermal evolution of the Ashe metamorphic suite south of the Grandfather Mountain window, NC: implications for Paleozoic orogenic events in the eastern Blue Ridge: - zircons from sheared pegmatite within the dextral strike-slip Burnsville fault are 377.3+/-2.6. The shearing must therefore be younger than late Devonian (Acadian), which is an age of reactivation if the primary age of the thrust is Taconic.
p. A-83 Williams, S.T. et al. Detailed geologica mapping from the western to central inner Piedmont, North Carolina: - Inner Piedmont is the Acadian metamorphic core of the Southern Appalachians, comprised of a series of type-F thrusts. Preliminary Ordovician age for the Walker Top gneiss.
p. A-84 Wilson, J.R. et al. A digital map of the Central and Southern Appalachians: spatial and temporal distribution of plutons associated with collisional tectonics and their geochemical signatures: - firstname.lastname@example.org
1999, GSA, Abst. w. Prog, Denver
A-475 Duncan, J.G. Stratigraphic correlation of Paleozoic sequences from the Suwanee Terrane of Florida, Morocco, and the Avalon terrane of Newfoundland: Evidence for Gondwanan stratigraphic linkage: - North Florida basin is composed of Lower to Middle Paleozoic siliciclastic sediments. Best correlation is with the Moroccan Anto-Atlas. Succession includes Ordovician oolitic iron formation as also found in the Avalon terrane of Newfoundland. Silurian black shales overlie the Ordovician in Florida and Morocco. The Arenig Armorican quartzites of Cadomia also found in Florida.
A-429 Samson, S.D. et al. Provenance and paleogeography of Neoproterozoic Circum-Atlantic arc-terranes: contraints from U-Pb ages of detrital zircons: - Cadomia - 2.2-2.0 and late Archean; Carolina Terrane Cambrian Quartzite - 2.85-1.2, 44% between 2.2-2.0, no 1.2-1.0; Avalon - no 2.4-2.2, common 1.2-1.0; Meguma - no MesoProterozoic zircons. Cadomia and Meguma were proximal to North Africa, Avalon was proximal to Amazonia, and the Carolina terrane was nearer North Africa but "some detritus from 1.5-1.2 Ma provinces within Amazonia was transported far enough to be deposited in basins fringing the Carolina terrane."
Structural Provinces of North America.
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