Structural development of the Alabama Piedmont northwest of the Brevard Zone

Tull, James F.

doi:10.2475/ajs.278.4.442

Cited by 52 publications

(54 citation statements)

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“…Nappe stacking leads to disruption of pre-existing isothermal surfaces and tectonic burial of footwall rocks to deeper crustal levels followed by thermal relaxation and re-establishment of an equilibrium geothermal gradient (Oxburgh & Turcotte, 1974;England & Richardson, 1977;Crawford & Mark, 1982). Crustal thickening by thrusting in the southern Appalachians has been well established from structural and geophysical data (Tull, 1978;Cook, Albaugh, Brown, Kaufman, Oliver & Hatcher, 1979;Higgins, Atkins, Crawford, Crawford & Cook, 1984). Therefore, the compressional portion of the Wedowee Group f-Tpath ( Fig.…”

Section: Discussionmentioning

confidence: 97%

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Contact aureoles as constraints on regional P‐T trajectories: an example from the Northern Alabama Piedmont, USA

Gibson

Speer

1986

Journal Metamorphic Geology

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Contact aureoles of plutons emplaced into regionally metamorphosed terranes can provide indicators of physical conditions along a portion of regional metamorphic f-T trajectories, thereby allowing reconstruction of more complete f-T loops than would be otherwise possible. In the Northern Alabama Piedmont of the southern Appalachians, Wedowee Group metapelites preserve evidence for two regional metamorphic phases overprinted by contact metamorphism adjacent to the Blakes Ferry Trondhjemite. Textural evidence indicates that an early bt+st+grt assemblage was replaced by bt+chl+grt during the latter stages of regional metamorphism. Changes in AKFM topology, complex Fe-Mg-Ca garnet zoning, and the latestage appearance of epidote indicate that a sequence of continuous reactions (bt+st = grt+ ms followed by chl+ ms+Ca-grt+Ca-pl= bt+ (Fe+Mg)-grt+ep) occurred in response to increasing pressure arid resulted in the observed changes in mineral assemblage. PI-rns-bt-grt thermobarometry indicates conditions of 580 2 65"C, 8.5 _t 0.8 kbar for equilibration of grt+ bt+chl. Pluton emplacement, subsequent to penetrative deformation, caused textural annealing and mineral re-equilibration by the continuous reaction bt+(Fe+ Mg)-grt+ep = chl+ms+Ca-grt+Ca-pl within 50 m of the pluton. Conditions of 510 5 65°C. 5-7 kbar are inferred. A reconstructed P-T trajectory for this area is characterized by (1) early moderate-T, moderate-!' metamorphism; (2) an increase in P to approximately 8.5 kbar; and (3) decompression and slight cooling prior to pluton emplacement. The compressional phase of this path is interpreted to result from underthrusting of the Wedowee metasediments to mid-crustal levels during Palaeozoic crustal thickening. Late-stage decompression prior to intrusion records uplift of these rocks in response to movement on structurally lower thrusts. * Orogenic Studics Lahoratory

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Section: Discussionmentioning

confidence: 97%

“…Tectonic foliations recognized in the Blakes Ferry area may be directly correlated with S I , Sz, and S, as defined by Neathery & Reynolds (1975) and Tull (1978). In addition, the bt+st+ grt-and grt+bt+chl-bearing metamorphic assemblages correspond to Tull's MI and Mz.…”

Section: Regional Implicationsmentioning

confidence: 94%

Contact aureoles as constraints on regional P‐T trajectories: an example from the Northern Alabama Piedmont, USA

Gibson

Speer

1986

Journal Metamorphic Geology

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“…Blue Ridge rocks of the Talladega slate belt consist of Cambrian to early Mississippian clastic and carbonate rocks, which are overthrust from the southeast by the Hollins line fault (Butts, 1926;Tull, 1982;Tull et al, 1988;Gastaldo et al, 1993). The Hillabee and Hollins line faults thrust the Ordovician Hillabee Greenstone (dominantly mafi c but containing some felsic volcanic rocks) and overlying distal Laurentian clastic metasedimentary and metavolcanic rocks of the eastern Blue Ridge over all of the tectonic units to the west (Tull, 1978(Tull, , 1980(Tull, , 1982(Tull, , 1984(Tull, , 1995Tull et al, 2007;McClellan et al, 2005McClellan et al, , 2007. These faults may be a suture equivalent to the Allatoona-Hayesville-Soque River fault system farther northeast.…”

Section: Geologic Settingmentioning

confidence: 98%

Untitled

et al. 2013

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We dedicate this paper to the memory of coauthor Isidore (Izzy) Zietz, who died at age 93 while this paper was in review. After contributing to the early theoretical foundation for analysis of airborne magnetic surveys, Izzy became a leading advocate for aeromagnetic survey acquisition and interpretation throughout the U.S., combining surveys into regional, state, and national maps, and working with regional geologists in interpreting geophysical anomalies. Like many geologists, we have been energized by Izzy's contagious enthusiasm for using magnetic and gravity anomalies to delineate and characterize major tectonic features. Lessons from working with him on data from Alabama and elsewhere over the years will continue to infl uence our appreciation and understanding of aeromagnetic and gravity anomalies for interpreting the Earth's upper crust."Magnetics is never good, and gravity is even worse!" "…all of it can just be dashed." -Isidore Zietz ABSTRACT Aeromagnetic and gravity data sets obtained for Alabama (United States) have been digitally merged and fi ltered to enhance upper-crustal anomalies. Beneath the Appalachian Basin in northwestern Alabama, broad deep-crustal anomalies of the continental interior include the Grenville front and New York-Alabama lineament (dextral fault). Toward the east and south, high-angle discordance between the northeast-trending Appalachians and the east-west-trending wedge of overlapping Mesozoic and Cenozoic Gulf Coastal Plain sediments reveals how bedrock geophysical signatures progressively change with deeper burial. Highfrequency magnetic anomalies in the Appalachian deformed domain (ADD) correspond to amphibolites and mylonites outlining terranes, while broader, lower-amplitude domains include Paleozoic intrusive bodies and Grenville basement gneiss. Fundamental ADD structures (e.g., the Alexander City, Towaliga, and Goat Rock-Bartletts Ferry faults) can be traced southward beneath the Gulf Coastal Plain to the suture with Gondwanan crust of the Suwannee terrane. Within the ADD, there is clear magnetic distinction between Laurentian crust and the strongly linear, high-frequency magnetic highs of peri-Gondwanan (Carolina-Uchee) arc terranes. The contact (Central Piedmont suture) corresponds to surface exposures of the Bartletts Ferry fault. ADD magnetic and gravity signatures are truncated by the eastwest-trending Altamaha magnetic low associated with the Suwannee suture. Arcuate northeast-trending magnetic linears of the Suwannee terrane refl ect internal structure and Mesozoic failed-rift trends. Geophysical data can be used to make inferences on surface and subsurface geology and vice versa, which has applicability anywhere that bedrock is exposed or concealed beneath essentially non-magnetic sedimentary cover.

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“…STRATIGRAPHY-Major stratigraphic units within the Jefferson terrane from north to south include the Lynchburg Group as redefined by Rankin and others (1993a), a mafic volcanic unit east of Charlottesville, Virginia, which has been mapped as the "Catoctin Formation," Ashe Formation as redefined by Rankin and others (1993a), Alligator Back and Tallulah Falls Formations, Coweeta Group of Hatcher (1979), Richard Russell Formation, Helen Group, Sandy Springs Group (in part), New Georgia Group of Abrams and McConnell (1981), Wedowee Group as used by Tull (1978), Emuckfaw Formation of Neathery and Reynolds (1975), and Ashland Supergroup as revised by Tull (1978). The gneisses are quartzose, indicating a significant continental contribution.…”

Section: Na2e: Jefferson Terranementioning

confidence: 99%

“…REFERENCES- Abbott and Raymond (1984); Abrams and McConnell (1981); Absher and McSween (1985); Edelman and others (1988); Hatcher (1979); others (1984, 1988); Horton and others (1989a); Lacazette and Rast (1989); Lipin (1984); Neathery and Reynolds (1975); Odom and Fullagar (1973); Rankin and others (1989Rankin and others ( , 1991Rankin and others ( , 1993a; Shaw and Wasserburg (1984); Stow and others (1984); Tull (1978); Wehr (1985); Wones and Sinha (1988) NA2g: POTOMAC COMPOSITE TERRANE TERRANE BOUNDARIES-In Virginia, the Potomac composite terrane, as defined by Drake (1985a), has been thrust along the Mountain Run fault zone onto the True Blue formation (informal name) of Pavlides (1989). This map includes the True Blue in the Jefferson terrane, following the interpretation of Horton and others (1989a), although Pavlides (1989) regards the True Blue as part of Laurentia.…”

Section: Na2e: Jefferson Terranementioning

confidence: 99%