Eastern Blue Ridge Research Articles

Slavery in the American Mountain South , and: The African-American Family in Slavery and Emancipation (review)

this novel enriches the indelible mountain epic created within all the works of his eloquent fictional landscape. Morgan's new book dramatizes the history the American Revolution at the Battle of Cowpens in a compelling and dramatic story of brave enemies and brave lovers. —Cecelia Conway Wilma Dunaway. Slavery in the American Mountain South. Cambridge and New York: Cambridge University Press, 2003. 352 pages. Hardcover. $70.00. Wilma Dunaway. The African-American Family in Slavery and Emancipation. Cambridge and New York: Cambridge University Press, 2003. 352 pages. Hardcover. $69.95. Back in what's rapidly becoming the "classic period" of Appalachian studies, we all learned and taught that Appalachia had few slaves, that slavery as a system was unimportant in the region's economy, and that racism in the mountains was less virulent than in the Deep South. Not so, says Wilma Dunaway, and she proceeds to prove it. In these two books, which reflect a single project divided into two topics, Dunaway establishes with rich documentation and incisive analysis that slavery was an integral part of antebellum Appalachian economy and life; that the condition of mountain slaves, employed, by and large, on small plantations or in non-agricultural work, was in general even worse than in the lowlands; that destructive influences on family life were especially rife in the mountains; that white attitudes toward persons of color could be just as racist as in Dixie and that the reasons for all this are rooted in Appalachia's broader economic relation to the outside world, as explicated by Immanuel Wallerstein's theories of core and periphery. These are two of the most important books on the region in many years, not only about antebellum Appalachia but about the nature of the region in general. Dunaway supports her thesis with a vast amount of quantitative and narrative evidence (which can be directly consulted on her web site). Here is one statement that shows both her quantitative leg work and her ability to draw telling conclusions from it: "On the one hand, a Lower South farm owner was twelve times more likely to run a large plantation than his Appalachian counterpart. On the other hand, Mountain slaveholders monopolized a much higher proportion of their communities' land and wealth than did Lower South planters." On the basis of facts like these, she greatly clarifies the whole racial/class structure of antebellum Appalachia—including the role of the bitter class divisions among white Appalachians in mutual reinforcement with the racial caste system. The result is magnificent and compelling, but as with any book worth reading, cautions are in order. For instance, Dunaway frequently mentions intra-regional distinctions, but in practice this important point tends to be overwhelmed by generalizations about "Appalachian slaves." We need to resist replacing one set of sweeping generalizations about the region with another. Speaking of generalizations, to give a small example of a large concern: Ever since her 1996 The First American Frontier, some of us have wondered why such a cutting-edge scholar should have taken over almost exactly John C. Campbell's 1921 geographical definition of Southern Appalachia. One reason, it would seem, is that this is a very broad one, perhaps too broad for many uses, but not too broad for Dunaway's agenda. In one passage she justifies her use of the Campbell 89 boundaries (not credited) with regard to her controversial inclusion of the Shenandoah Valley and eastern Blue Ridge slope (not clearly named) in a way that crosses the line between the contentious and the tendentious and makes one nervous about other, less material, aspects of her framework. In particular, this tactic calls to mind the frequently voiced criticism that she defines capitalism itself too broadly and subsistence agriculture much too narrowly. Were there no actual roots in the productive system(s) for a mentality and culture of resistance to capitalism by some mountain people? Dunaway likes to dismiss her critics on this and related points as middle-class romantics, but many of them have unimpeachably working-class roots and are just asking to have their own personal experience and community traditions acknowledged and respected, whatever new context these need to be placed in...

Prograde and retrograde history of eclogites from the Eastern Blue Ridge, North Carolina, USA

Abstract The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H2O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two‐pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg‐rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.

Ion microprobe age and geochemistry of southern appalachian basement, with implications for Proterozoic and Paleozoic reconstructions

Ion microprobe U–Pb analyses of zircons from basement units in the southern Appalachians, combined with supporting isotopic compositions and major and trace element geochemistry, have delineated a granitic magmatic pulse ∼1165–1150 Ma. The pulse is manifested by the Watauga River Gneiss (western Blue Ridge), Toxaway, Wiley, and Sutton Creek gneisses (eastern Blue Ridge), Pilot Mountain and Grassy Creek gneisses (Sauratown Mountains window), and possibly the Forbush gneiss (∼1140 Ma, Inner Piedmont or Sauratown Mountains window) and Cranberry-Mine Layered Gneiss (∼1190 Ma, western Blue Ridge). Additional samples analyzed include the Blowing Rock Gneiss (∼1080 Ma, Grandfather Mountain window), and the Carvers Gap Granulite Gneiss (∼1.8 Ga) and Cloudland Granulite Gneiss (detrital cores ∼1.2–1.8 Ga) from the Mars Hill terrane. Age data were evaluated by calculating concordia ages and concordia probability plots using 206 Pb*/ 238 U and 207 Pb*/ 206 Pb* data simultaneously. Rocks in the main magmatic pulse are granitic (63–72 wt.% SiO 2 ), but elevated in K and incompatible trace elements compared to typical subduction-related magmas, and initial Nd ratios cluster tightly near CHUR. Mars Hill terrane samples are distinct in age, geochemistry (poorer in K and incompatible elements), and isotopic compositions (ε Nd −7.6 and −5 at 1.0 Ga). Zircons from almost all samples have metamorphic rims that yield ages ∼1030 Ma, with the exception of the Blowing Rock Gneiss. Ages of Grenvillian magmatism and metamorphism are similar to reported ages from the northern Blue Ridge of Virginia, the Adirondack Highlands, the Central Metasedimentary Belt (Canadian Grenville Province), and the Llano uplift of Texas. This suggests the entire southeastern margin of Laurentia has a similar history ∼1.2–1.0 Ga. Although consistent with known ages in Laurentia, the presence of ∼1.8 Ga rocks and T DM ages commonly >1.6 Ga is inconsistent with the inferred 1.6 Ga margin of Laurentia. This suggests either that the 1.4–1.5 Ga mid-continent terrane separates older portions of Laurentia, that this region was exotic, or that it was a rifted fragment of Laurentia reattached during Grenville orogeny. Surprisingly few Paleozoic metamorphic zircon rims have been identified, but the few analyses obtained from samples in the eastern Blue Ridge yield late Acadian ages (∼350 Ma). The similarity of basement units across the southern Appalachians suggests a relationship between these provinces and that the Piedmont terrane is not exotic to Laurentia during the Appalachian orogenic cycle.

Evidence for Early Mesoproterozoic (and Older?) Crust in the Southern and Central Appalachians of North America

Limited evidence from Sm-Nd T DM model ages, U-Pb ages of xenocrystic zircon, and Pb isotopic data indicates the presence of Paleoproterozoic and Mesoproterozoic crust (2.0-1.3 Ga) in the southern and central Appalachian orogen. This apparently unexposed older crust must underlie much of the Blue Ridge, and it was recycled to produce most of the rocks of the Blue Ridge with ages ≤1.3 Ga. In the eastern Blue Ridge and in blocks to the southeast, there also is a significant juvenile Neoproterozoic source component. Going toward the southeast, the central and eastern Piedmont (Carolina terrane) appears to be underlain by progressively less source component older than 1.0 Ga. Late Proterozoic rocks of the Carolina terrane are derived largely from a juvenile source with a Nd isotopic composition that approaches that of depleted mantle.

Retrograde metamorphism of eclogite in the southern Appalachian Mountains, U.S.A.–A case involving seamount subduction?

AbstractThis work supports a growing body of evidence that the Ashe Metamorphic Suite (AMS) of the eastern Blue Ridge province in North Carolina has an ensimatic origin and is part of a subduction‐related accretionary mélange, marking the Taconic suture between the North American craton and the Inner Piedmont. In a palinspastic reconstruction, the thrust fault at the base of the AMS appears to have intercepted the greatest depths (i.e. highest‐P metamorphic rocks) beneath parts of the AMS now exposed adjacent to the Grandfather Mountain window. The greatest volume of mafic rock is found in these same areas. We suggest that the nascent, subduction‐related, basal thrust fault was deflected downward by an obstacle in the form of an isolated, mafic volcanic edifice on the oceanic crust–a sea mount.Pelitic and mafic rocks dominate the AMS. North of the Grandfather Mountain window, retrograded eclogite occurs in the amphibolite near the base of the AMS. Textures and mineralogy indicate that an original eclogite assemblage was subjected to the following sequence of parageneses:(a) Eclogite(I) facies: omphacite+garnet+quartz,(b) Eclogite(II) facies: omphacite+garnet+epidote+quartz,(c) Symplectic (diopside+plagioclase)+garnet+epidote+quartz,(d) Amphibolite facies: (diopside+plagioclase)+garnet+epidote+hornblende+quartz,(e) Amphibolite facies: plagioclase+garnet+epidote+hornblende+quartz. P–T conditions, estimated from geothermobarometry applied to relevant mineral compositions, are c.720 °C and c.16 kbar for (b) eclogite(II) facies; c.655 °C and c.8.5 kbar for (e) amphibolite facies.

Age and zircon inheritance of eastern Blue Ridge plutons, southwestern North Carolina and northeastern Georgia, with implications for magma history and evolution of the Southern Appalachian Orogen

High-resolution ion microprobe analysis of zircon has provided ages for previously undated plutons of the high-grade eastern Blue Ridge of northeastern Georgia and southwestern North Carolina. These data, together with backscattered electron imaging, reveal the presence of nearly ubiquitous inherited cores of highly variable age and magmatic rims that have experienced variable Pb loss, thus making interpretation of conventional U-Pb analyses very difficult. Ion probe rim analyses indicate that the plutons were emplaced during both the mid-Ordovician (Taconian orogeny; Whiteside pluton, 465 Ma; Persim- mon Creek Gneiss, 480 Ma) and mid-Devonian (Acadian orogeny; Rabun pluton, 375 Ma; Pink Beds pluton, 390 Ma; Looking Glass pluton, 380 Ma). A large trondhjemite dike north of Asheville is less confidently assigned an age of 415 Ma. Zircons from all intrusions have predominantly 1.0 to 1.25 Ga cores (Grenvillian). In addition, both Devonian and Ordovician plutons have smaller populations of Late Proterozoic-early Paleozoic (0.5-0.75 Ga), Middle Proterozoic (1.4 Ga), and Late Archean (2.6-2.9 Ga) cores. The ubiquitous, round cores and thick magmatic rims suggest significant resorption and then protracted growth within the melts. Zircon saturation tem- peratures based on whole-rock (melt) Zr concentrations are lower than ex- pected for magma generation (710°-760°C). Zirconium concentrations may not reflect saturation at maximum temperature, if melting was very rapid (10 5 yrs), or if zircon cores represent grains that were shielded from melt inside host grains for much of the magmatic history. The Late Proterozoic-early Paleozoic and Grenvillian inheritance is similar to documented ages of basement exposed in the eastern Blue Ridge. No expo- sures of 1.4 Ga rocks are known in the southern Appalachians, but detrital and inherited zircons of this age have been reported, and 1.4 Ga granites are widespread in the craton to the northwest. However, the combination of absence of Early Proterozoic zircon and presence of Late Archean zircon is inconsistent with the known distribution of basement rocks in southeastern North America. No Archean rocks or inherited zircons have been reported from the southern and central Appalachians; the nearest Archean exposures are 1000 km north, across a dominantly Early Proterozoic terrane. This suggests either that the Laurentian configuration of Archean basement was very complex or that the crust that underlay the plutons at the time of their emplacement included a far-travelled terrane (emplaced during Grenvillian or early Paleozoic orogeny?). Ages of magmatic and inherited zones of zircon from the plutons demon- strate that similar crust underlay the eastern Blue Ridge during both Taconian and Acadian orogenies, that there was no single episode of voluminous magma- tism, and that metamorphism and deformation began before 470 Ma and contin- ued after 370 Ma. These plutons do not constitute a significant convergence- related arc, though it is possible that they represent a displaced part of an arc that lies primarily to the east (in the Inner Piedmont?). Previous geochemical studies have demonstrated that the eastern Blue Ridge intrusions include both a very primitive component, either mafic magma or relatively young mafic source rock, and a component derived from more mature

Cryptic crustal events elucidated through zone imaging and ion microprobe studies of zircon, southern Appalachian Blue Ridge, North Carolina–Georgia

Compositional zoning reveals multistage growth histories and resorption events in zircon from a high-grade terrane in the eastern Blue Ridge of North Carolina and Georgia. These zoning patterns were used to guide high-resolution ion microprobe dating that places important constraints on the evolution of the southern Appalachian crust. Zircons from granulite facies metapelite have unzoned rims that yield concordant U-Pb ages of 495 ± 14 Ma. We interpret this as the time of rim growth, which occurred during peak metamorphism early in the protracted orogenic history of the region. Detrital cores, characterized by truncated euhedral zoning, are of Grenville age (1.04–1.26 Ga). Zircons from the Whiteside and Rabun plutons have well-defined, rounded, inherited cores and euhedral, oscillatory-zoned magmatic rims. Rims of Rabun zircons record magmatic crystallization at 374 ± 4 Ma, whereas Whiteside rims yield a 466 ± 10 Ma crystallization age. Cores from both plutons include 1.1–1.3 Ga and 2.6–2.7 Ga ages. These data indicate that there was no single, voluminous episode of plutonism in this area, that similar material underpinned the region at least from 370 to 470 Ma, and that previously unrecognized Archean basement or Archean basement–derived sedimentary rock was present in the southern Appalachians. Results of this study verify the value of combining zoning and ion microprobe studies: Using conventional U-Pb methods or ion microprobe dating without knowledge of zoning would have made interpreting the events recorded in these zircons and the ages that they yield difficult or impossible.

Igneous petrogenesis and tectonic setting of the Elkahatchee Quartz Diorite, Alabama Appalachians; implications for Penobscotian magmatism in the eastern Blue Ridge

Igneous petrogenesis and tectonic setting of the Elkahatchee Quartz Diorite, Alabama Appalachians; implications for Penobscotian magmatism in the eastern Blue Ridge

Crustal structures and the eastern extent of lower Paleozoic shelf strata within the central Appalachians: A seismic reflection interpretation

Reprocessing of line PR3 proprietary seismic reflection data has delineated Grenvillian, Paleozoic, and Mesozoic structures within the Appalachian foreland, Blue Ridge, and Piedmont of the central Appalachians in Virginia and West Virginia. The eastern portion of PR3 can be correlated along strike with the western portion of line I-64, reprocessed earlier at Virginia Tech. The combined seismic reflection data image the crust from the eastern Valley and Ridge, Blue Ridge, Piedmont, and Atlantic Coastal Plain provinces. Within the Piedmont, large (as much as 10 km wide) reflective structures imaged on both lines PR3 and I-64 are interpreted to be thrust sheets that might be composed of deformed Catoctin, Evington Group, and possibly younger metamorphosed rocks. A concealed extension of the Green Springs mafic mass intrudes a thrust sheet imaged along the PR3 profile. The Blue Ridge-Piedmont allochthon was transported north-west along the Blue Ridge thrust, which ramps upward ∼12 km east of the surface exposure of the Mountain Run Fault. Westward along line PR3, the Blue Ridge thrust maintains an undulating geometry; the maximum thickness of the Blue Ridge allochthon is interpreted to be ∼4.5 km. The Blue Ridge allochthon is generally acoustically transparent and overlies lower Paleozoic shelf strata. The maximum thickness of these strata is ∼8 km. Shelf strata are interpreted to extend in the subsurface 5 km east of the surface exposure of the Mountain Run Fault, the northeastward extension of the Brevard Fault Zone, where they are truncated by the Blue Ridge thrust at a depth of 10.5 km (3.5 s). Various folds and blind thrusts are imaged beneath the Appalachian foreland; however, the foreland has not experienced the same degree of deformation as observed in the eastern provinces. A basement uplift ∼45 km wide is imaged beneath the Valley and Ridge province and is interpreted as having formed prior to Late Cambrian time. Farther west, reflections imaged beneath the Glady Fork anticline in the Appalachian Plateau are interpreted as a positive flower structure associated with wrench fault tectonics. Relatively few deep (>9 km) crustal reflections are imaged along line PR3. The majority of reflections that do exist at these depths are observed beneath the Piedmont and eastern Blue Ridge. The high reflectivity associated with the Grenvillian basement in these areas might be the result of the Paleozoic orogenies and extension related to Late Proterozoic and Mesozoic rifting.

The Wayah granulite-facies metamorphic core, southwestern North Carolina: High-grade culmination of Taconic metamorphism in the southern Blue Ridge

Research Article| November 01, 1989 The Wayah granulite-facies metamorphic core, southwestern North Carolina: High-grade culmination of Taconic metamorphism in the southern Blue Ridge JAMES O. ECKERT, JR.; JAMES O. ECKERT, JR. 1Department of Geology, Texas A&M University, College Station, Texas 77843-3115 Search for other works by this author on: GSW Google Scholar ROBERT D. HATCHER, JR.; ROBERT D. HATCHER, JR. 2Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410 Search for other works by this author on: GSW Google Scholar DAVID W. MOHR DAVID W. MOHR 3Code 2110, Naval Coastal Systems Center, Building 110, Room 3C27, Panama City, Florida 32401-5000 Search for other works by this author on: GSW Google Scholar Author and Article Information JAMES O. ECKERT, JR. 1Department of Geology, Texas A&M University, College Station, Texas 77843-3115 ROBERT D. HATCHER, JR. 2Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410 DAVID W. MOHR 3Code 2110, Naval Coastal Systems Center, Building 110, Room 3C27, Panama City, Florida 32401-5000 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1989) 101 (11): 1434–1447. https://doi.org/10.1130/0016-7606(1989)101<1434:TWGFMC>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JAMES O. ECKERT, ROBERT D. HATCHER, DAVID W. MOHR; The Wayah granulite-facies metamorphic core, southwestern North Carolina: High-grade culmination of Taconic metamorphism in the southern Blue Ridge. GSA Bulletin 1989;; 101 (11): 1434–1447. doi: https://doi.org/10.1130/0016-7606(1989)101<1434:TWGFMC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Wayah granulite-facies metamorphic core, in the eastern Blue Ridge, southwestern North Carolina, contains the metamafic assemblage hornblende-orthopyroxene-clinopyroxene which defines the hornblende (lower) granulite facies. The metamorphic progression from the kyanite-almandine zone into the Wayah granulite core is continuous across the Hayesville fault, a major premetamorphic-peak tectonostratigraphic boundary. This metamorphic transition includes a distinct muscovite-absent (+alkali feldspar) second-sillimanite zone. Retrogression is limited. This entire range in metamorphic grade apparently was produced during a single, prograde, Paleozoic (likely Taconic) metamorphic event.The Hayesville fault crosses the study area mostly within the first-sillimanite zone, northwest of the Wayah granulite core. Both supracrustal sequences juxtaposed along this fault consist predominantly of metasedimentary rocks probably deposited during the Late Proterozoic. Although paragneiss pre-dominates, the southeastern terrane is distinguished by the presence of metamorphosed felsic, mafic, and ultramafic igneous rocks. No evidence supports the presence of Grenville basement within the study area.The second of four deformational events, synchronous with the metamorphic peak, created northeast-southwest- to east-west-trending, upright isoclinal folds which dominate the outcrop pattern. Macroscopic F2 folds deformed the Hayesville fault.Application of mineralogic thermometry and barometry yields conditions which range from 585 °C, 5.5 kb at the kyanite-sillimanite isograd to 842 °C, 9.8 kb in the hornblende granulite facies. Macroscopic distributions of migmatite and perthite, published experimental data, and barometric calculations suggest that T = ∼669 °C, P = ∼6.52 kb, and X(H2O) = ∼0.62 at the second-sillimanite isograd.Results indicate that the Wayah granulite-facies core constitutes the high-grade culmination of Paleozoic metamorphism in the southern Appalachian Blue Ridge. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Basement beneath the Blue Ridge and Inner Piedmont in northeastern Georgia and the Carolinas: A preserved, Late Proterozoic, rifted continental margin

Research Article| December 01, 1988 Basement beneath the Blue Ridge and Inner Piedmont in northeastern Georgia and the Carolinas: A preserved, Late Proterozoic, rifted continental margin PAUL D. FAVRET; PAUL D. FAVRET 1Department of Geology, University of South Carolina, Columbia, South Carolina 29208 Search for other works by this author on: GSW Google Scholar RICHARD T. WILLIAMS RICHARD T. WILLIAMS 2Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410 Search for other works by this author on: GSW Google Scholar Author and Article Information PAUL D. FAVRET 1Department of Geology, University of South Carolina, Columbia, South Carolina 29208 RICHARD T. WILLIAMS 2Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996-1410 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1988) 100 (12): 1999–2007. https://doi.org/10.1130/0016-7606(1988)100<1999:BBTBRA>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PAUL D. FAVRET, RICHARD T. WILLIAMS; Basement beneath the Blue Ridge and Inner Piedmont in northeastern Georgia and the Carolinas: A preserved, Late Proterozoic, rifted continental margin. GSA Bulletin 1988;; 100 (12): 1999–2007. doi: https://doi.org/10.1130/0016-7606(1988)100<1999:BBTBRA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Seismic reflection, magnetic, and gravity data from the Blue Ridge and Inner Piedmont of Georgia, North Carolina, and South Carolina indicate that southern Appalachian basement contains two sets of steeply dipping normal faults that bound rotated and tilted blocks. The predominant faults strike about N35°E, subparallel to the axis of the orogen, whereas the oblique set strikes about N10°W. Vertical offsets of as much as 1.3 km across individual N10°W-striking faults have been observed on seismic profiles. Regionally, the pattern is probably similar to normal faults in Precambrian rocks exposed in the Adiron-dacks or along other rifted margins. The initial age of faulting was Late Proterozoic, 570 m.y. or older, inferred from comparison of seismic reflectors in the eastern Blue Ridge and Inner Piedmont with stratigraphy along the Great Smoky thrust. Basement beneath the western portion of the Inner Piedmont in South Carolina and northeastern Georgia has been stable at least since the Middle Triassic and did not reactivate during the Mesozoic breakup of Pangea,based on the absence of crosscutting relationships between basement faults and the Brevard zone reflection.Basement structure related to rifting may partially explain the focal mechanisms of regional earthquakes. The oblique fault set is consistent with well-constrained north-south-striking, subvertical nodal planes obtained for many basement earthquakes in the Valley and Ridge of eastern Tennessee. Valley and Ridge earthquakes in southwestern Virginia are consistent with reactivation of the faults that strike parallel to the orogen. We suggest that the basement structures survived Phanerozoic orogeny because Alleghanian continental collision was interpreted in the southern Appalachians, so that convergence between Laurentia and Gondwana stopped about the time their respective lithospheres came into contact. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

New paleontologic evidence constraining the age and paleotectonic setting of the Talladega slate belt, southern Appalachians

The Talladega slate belt in the southern Appalachian orogen of Alabama and Georgia is a thick sequence of lower greenschist-facies metaclastic, metacarbonate, and metavolcanic rocks thrust above miogeoclinal rocks of the foreland and overthrust by higher grade metamorphic rocks of the eastern Blue Ridge terrane(s). The age assignments and tectonic affinities of this sequence have been highly controversial. Recent fossil discoveries in key stratigraphic units, however, combined with confirmed fossil occurrences in the Jemison Chert, have established a firm correlation with the Appalachian foreland, thus stratigraphically linking the Talladega belt with Laurentia. The lower predominantly clastic sequence, tectonically bounded at its base by the frontal Blue Ridge thrust system, grades upward into a 3.5-km-thick marble sequence. The basal carbonate unit (Jumbo Dolomite) contains Early Cambrian archaeocyathids, and these fossils, in addition to the stratigraphic position and carbonate lithofacies, establish correlation of this unit with the Lower Cambrian Shady Dolomite. The uppermost unit in the marble sequence (Gantts Quarry Formation) contains Early Ordovician (middle to late Canadian; = early to middle Arenigian) conodonts that confirm correlation of this unit with the Newala Limestone and Kingsport and Mascot Formations of the Appalachian foreland. The carbonate platform sequence is unconformably overlain by a thick clastic sequence that accumulated in a deep successor basin; the Lay Dam Formation is a marine fanlike deposit at the base of this sequence. Conodont molds from the top of the Lay Dam and fossils from the stratigraphically higher Jemison Chert indicate a Silurian to Early Devonian age for the Lay Dam Formation. Paleontologic data and the stratigraphic and structural setting indicate that the Talladega slate belt is the most distally preserved and relatively complete fragment of the Appalachian miogeocline; thus, the tectonic evolution of the Talladega belt is crucial to understanding the western margin of Iapetus. Linkage of the Talladega slate belt rocks with those of the western Blue Ridge to the northeast suggests that the latter once contained a thick Cambrian to Devonian cover sequence which subsequently has been mostly removed.

Generation, Diversification, and Emplacement of the Rockford Granite, Alabama Appalachians: Mineralogic, Petrologic, Isotopic (C &amp; O), and P--T Constraints

The Rockford Granite is a plutonic suite of synmetamorphic, two-mica granitoids that intrude graphite-bearing, amphibolite-facies metasediments in the eastern Blue Ridge of the Alabama Appalachians. Geochemical classification indicates peraluminous granite and trondhjemite with subordinate granodiorite. The trondhjemites are products of pervasive, infiltrative Na-metasomatism; whereas the granites and granodiorites sufficiently retain primary traits for petrogenetic modelling. A significant proportion of the Rockford Granite magmatic phases may represent refractory restite, expressed as centimeter-size biotite clots, normal oscillatory zoned plagioclase with a subidiomorphic zone-boundary core, and rounded, zoned xenocrystic zircons. Major and trace element modelling indicate that the Rockford Granite could be a product of 40% equilibrium partial melting of a metagraywacke source producing felsic, granitic melt plus a restite assemblage of plagioclase49-quartz31-biotite14-apatite5 5-zircon0 5. The limited geochemical heterogeneity of the Rockford Granite is attributed to progressive restite-melt separation, reflected by systematic collinearity of calculated source and restite compositions in bivariate diagrams. Whole rock oxygen and reduced carbon isotope analyses support the metagraywacke source model for the Rockford Granite and also record regional isotopic exchange with metamorphic fluids from the volumetrically-dominant metapelites. Combined whole-rock oxygen and carbon isotope determinations both constrain granitoid source rock characteristics and measure the extent of C-O-H metamorphic fluid-rock interaction. Quantitative geothermobarometric estimates for the Rockford Granite and surrounding metasediments indicate kyanite-sillimanite facies series regional metamorphism, with a Barrovian geothermal gradient of 30°C/km. The prograde P-T path is abruptly terminated by nearly isothermal decompression, interpreted as due to diapiric intrusion and associated thermal doming. Rockford Granite and migmatite samples fall in the P-T regime where the maximum overlap between muscovite dehydration and minimum melt conditions exist suggesting that the former controlled anatexis. The tectonic setting and depositional environment of the host metasedimentary protoliths are interpreted as associated with opening of the lapetus Ocean. Structurally lowermost are interlayered amphibolites and quartzites, representing synrift volcanism and sedimentation, overlain by thick passive margin pelitic and graywacke deposits. The Rockford Granite contains a Grenville-age (l·0–1·2 Ga) inherited, xenocrystic zircon component interpreted to record crustal recycling via weathering of a Grenville gneiss source to produce metagraywacke protoliths, followed by regional metamorphism and anatexis to yield the granitic magmas. Barrovian regional metamorphism, associated crustal shortening, and anatexis in an Acadian continental-oceanic subduction regime best represents the tectonomagmatic setting of Rockford Granite genesis.

Late Paleozoic tectonothermal evolution of the western Piedmont and eastern Blue Ridge, Georgia: Controls on the chronology of terrane accretion and transport in the southern Appalachian orogen

Hornblende within the Helen and Tallulah Falls nappes of the eastern Blue Ridge and portions of the western Piedmont (higher grade sequences of the Chauga- Walhalla complex and eastern portions of the Six-Mile nappe) record 322-333-m.y. 40 Ar/ 39 Ar plateau ages which date post metamorphic cooling through temperatures required for intracrystalline retention of argon. Hornblende within lower-grade portions of the Chauga-Walhalla complex and western portions of the Six-Mile nappe (Alto allochthon) records older plateau ages (322-362 m.y.). None of these release spectra suggests a super-posed metamorphic history, and the plateau ages are interpreted to date regionally diachronous cooling of the allochthon following a Late Devonian or earlier high-grade metamorphism. Muscovite within all structural units of the eastern Blue Ridge and western Piedmont records 302-318-m.y. 4O Ar/ 39 Ar plateau ages which date post meta-morphic cooling through lower closure temperatures (∼350 °C?). Biotite from these structural units yields 312-348-m.y. 40 Ar/ 39 Ar plateau ages. The geologic significance of the biotite ages is uncertain because of variable contamination with extraneous argon. The Alto allochthon appears to have originated within the western Piedmont following a Late Devonian or earlier high-grade metamorphism which may have developed during initial terrane accretion to North America. Diachronous postmetamorphic cooling of the allochthon likely occurred during its movement to higher crustal levels associated with emplacement over lower-grade rocks of the Chauga-Walhalla complex. Emplacement occurred prior to local development of retrogressive mylonitic fabrics related to tectonic activity within the Brevard fault zone and prior to regional cooling through muscovite closure temperatures.

Structural and stratigraphic setting of the Alto allochthon, northeast Georgia

The Alto allochthon is a postmetamorphic crystalline thrust sheet within the metamorphic core of the southern Appalachian orogen. The Toccoa Falls-Shorts Mill thrust fault separates structurally overlying migmatitic sillimanite- bearing Tallulah Falls Formation (?) rocks of the Alto allochthon from upper-greenschist/lower-amphibolite-facies rocks of the Chauga belt. Early D 1 -D 2 structures are truncated by the Toccoa Falls-Shorts Mill fault, suggesting emplacement of the allochthon subsequent to their formation. Later D 4 -D 5 structures appear to postdate thrusting, and the allochthon is preserved in an open F 4 synform. Emplacement of the allochthon was probably coincident with the formation of D 3 structures. Two distinct episodes of mylonitization have affected the rocks of the Alto allochthon. Early middle- to upper-greenschist-facies mylonites are locally developed along the Toccoa Falls-Shorts Mill fault. Along the northwestern margin,these early mylonites are overprinted by a more pervasive episode of lower temperature (lower-greenschist-facies) retrogressive mylonitization associated with deformation within the Brevard fault zone. Lithologic sequences within the Alto allochthon may be correlated with successions in both the eastern Blue Ridge (Tallulah Falls Formation) and the Inner Piedmont. Structural and stratigraphic data suggest that the allochthon is rooted to the southeast in the Inner Piedmont.

Tectonics of the Southern and Central Appalachian Internides

The Appalachian orogen was constructed along the ancient Precambrian continental margin of eastern North America by a series of compressional events that began in the Ordovician and episodically spanned much of the Paleozoic era. The processes of accretion of suspect and exotic terranes, together with terrane collision and ultimately completion of the Wilson cycle by continent-continent collision, resulted in construction of the Appalachian orogen (Figure 1). These paleogeographic elements are rep­ resented by the various lithotectonic units visible today in the southern and central Appalachians (Figure 2). These subdivisions represent the late Precambrian rifted margin and early Paleozoic platform that were formed directly upon North American Grenvillian basement (Cumberland/ Allegheny Plateau, Valley and Ridge, and western Blue Ridge), the offshore North American slope-rise assemblage (Hamburg klippe, southern Appalachian eastern Blue Ridge and Inner Piedmont, and higher thrust sheets of the Maryland-northern Virginia Piedmont), and the exotic Carolina, or Avalon, and Wilmington terranes. The Carolina ter­ rane contains the Alleghanian high-grade metamorphic core. These elements form the basis for this analysis and review of the present state of knowledge of the tectonic history of the southern and central Appalachian internides. The Appalachian orogen has been the subject of study by geologists at least since the early nineteenth century. Early fundamental concepts, such as the geosynclinal theory of mountain building (Dana 1873, Hall 1883)

Tectonic implications of new Appalachian Ultradeep Core Hole (ADCOH) seismic reflection data from the crystalline southern Appalachians

Summary. Some 180 km of new VIBROSEIS profiles have been acquired in the southern Appalachian Inner Piedmont, Brevard fault zone and eastern Blue Ridge as part of the ADCOH Project site investigation. These data are of the highest quality yet obtained in a crystalline terrane in the US, perhaps in the world, and reveal several conclusions that should have a direct bearing upon the world-wide nature of composite crystalline thrust sheets and their modes of interaction with the platform rocks beneath. Strong reflections previously interpreted as the base of the crystalline sheet are clearly part of the platform sedimentary (clastic rocks) sequence resting upon the autochthonous basement and early Palaeozoic rift basins. This reflection package and related transparent zones are clearly repeated beneath the crystalline sheet indicating a complex of thrusts repeating units within the platform succession. Reflectors (granitoid-amplibolite contacts) in the crystalline sheet in the Inner Piedmont represent recumbent folds of similar wavelengths and amplitudes to folds mappable on the surface. Duplexing of platform rocks beneath the crystalline sheet appears to have resulted in doming of the crystalline sheet. Similarly, duplex formation in the platform was probably controlled by both the thickness of the crystalline sheet and the rheological properties of the platform succession.

Geologic Investigations Associated with Appalachian Ultradeep Core Hole (ADCOH) Site Study: ABSTRACT

Geologic investigations related to the ADCOH site study are concentrated in several areas: (1) detailed geologic mapping of approximately 400 mi/sup 2/ of new area to add to the 3000 mi/sup 2/ of existing detailed geologic mapping, (2) description and interpretation of cores from four shallow boreholes ranging in depth from 500 ft to 1400 ft, and (3) structural interpretation of surface and reflection seismic data. The eastern Blue Ridge Mountains (BR) and inner Piedmont (IP) consist of an assemblage of late Precambrian rift-related sedimentary and mafic volcanic rocks. These were metamorphosed during the early Paleozoic and intruded by plutons of different ages. The Chauga belt (CB) consists of an assemblage of late Precambrian to early Paleozoic(.) greenschist-facies clastic, carbonate, and volcanic rocks separating the two higher grade terranes of the BR and IP. The Brevard fault zone (BZ) is localized along the western edge of the CB. The BZ is a complexly reactivated zone of faulting exhibiting an episodic movement history that may span much of the Paleozoic. One of its last movement episodes brought slices of platform carbonate (Knox.) from beneath the Blue Ridge-inner Piedmont thrust sheet to the surface along a brittle fault. Lithologic units of themore » BR (including the 1.1-Ga Wiley gneiss basement), CB, and IP were encountered in the four shallow drill holes. The basement-cover contact was crossed numerous times in the lower third of the BR hole, indicating isoclinal folding of this boundary. It is uncertain whether the basement-cover contact is a folded fault or an unconformity. New geologic mapping has revealed a major late shear zone near this basement-cover contact on the southeastern flank of the Tallulah Falls dome.« less

Regional Tectonic Implications of Seismic Reflection, Gravity, Structural, and Stratigraphic Data from New Appalachian Ultradeep Core Hole (ADCOH) Site Study: ABSTRACT

The Appalachian Ultradeep Core Hole (ADCOH) site study area is located in the eastern Blue Ridge Mountains and western Piedmont of the Carolinas and northeastern Georgia. The goal of the proposed project is an ultradeep core hole that would penetrate and prove the existence of the main detachment thrust of the Blue Ridge-Piedmont (BRP) thrust sheet. Intermediate objectives include study of higher level faults and rock units within the thrust sheet, as well as investigation of the Paleozoic platform cover and basement beneath the BRP thrust sheet. Site study data have revealed that the level of the main BRP detachment may have shallowed rapidly west of the Brevard fault zone, possibly bringing lower Paleozoic carbonate platform rocks to within 3-4 km of the surface near Hayesville, North Carolina, in a large duplex structure. If so, there are implications here and farther west for possible hydrocarbon accumulations. Duplex structure appears to be a dominant style of deformation both within and beneath the BRP thrust sheet. A recently discovered shear zone on the Tallulah Falls dome may represent an imbricate from one of the blind duplexes.

Stratigraphy and tectonics of the Virginia–North Carolina Blue Ridge: Evolution of a late Proterozoic–early Paleozoic hinge zone

Late Proterozoic (690-570 Ma) through Ordovician rocks of the Virginia—North Carolina Blue Ridge and Valley and Ridge record the evolution of an eastward-facing passive margin and its subsequent deformation during continent-arc collision (Taconic orogeny). Comparison of Blue Ridge stratigraphy with modern rifts and passive margins suggests that late Proterozoic rifting involved substantial crustal attenuation in the eastern Blue Ridge. The axial zone of the Blue Ridge from central Virginia southward is occupied by a major fault system (Hayesville-Fries-Rock-fish Valley fault). Contrasts between rocks deposited east and west of this fault suggest that it is a reactivated hinge zone, separating highly attenuated continental crust to the east from continental crust of more normal thickness to the west. During rifting, normal faulting and rapid subsidence of attenuating crust in the eastern Blue Ridge produced rift basins in which deep-water clastic and volcanic rocks accumulated (Lynchburg Group, Ashe Formation). Landward of the hinge zone in the western Blue Ridge, mainly terrestrial to shallow-water clastic and volcanic rocks were deposited in a series of subaerial rift basins (Swift Run/Catoctin, Grandfather Mountain, Mount Rogers Formations). The hinge zone not only controlled depositional patterns throughout the rift and passive-margin history, but it was also reactivated during the Taconic collision, when attenuated crust and its cover were thrust westward over the shelf edge. Turbidite-dominated rift sequences of the eastern Blue Ridge are unlike “typical” rift deposits, which are characterized by predominantly subaerial and shallow-water sedimentation. Deep-water rifts may be common on attenuated crust (as in the Bay of Biscay); however, they are poorly known from the stratigraphic record because of their susceptibility to destruction during orogeny.