Upper Crustal Structure of Alabama from Regional Magnetic and Gravity Data: Using Geology to Interpret Geophysics, and Vice Versa

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Upper crustal structure of Alabama from regional magnetic and gravity data: Using geology to interpret geophysics, and vice versa

Mark G. Steltenpohl1, J. Wright Horton, Jr.2, Robert D. Hatcher, Jr.3, Isidore Zietz4, David L. Daniels4, and Michael W. Higgins5 1Department of Geology and Geography, Auburn University, Petrie Hall, Auburn, Alabama 36849, USA 2U.S. Geological Survey, 926A National Center, Reston, Virginia 20192, USA 3Department of Earth and Planetary Sciences and Science Alliance Center of Excellence, University of Tennessee, Knoxville, Tennessee 37996-1410, USA 4U.S. Geological Survey, 954 National Center, Reston, Virginia 20192, USA 5The Geologic Mapping Institute, 1752 Timber Bluff Drive, Clayton, Georgia 30525-6011, USA

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 apprecia- tion 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 Gondwanan crust of the Suwannee terrane. and all other crustal blocks in the subsurface of Within the ADD, there is clear magnetic dis- Alabama are truncated along the boundary with Aeromagnetic and gravity data sets tinction between Laurentian crust and the the Suwannee terrane, a huge mass of Gond- obtained for Alabama (United States) have strongly linear, high-frequency magnetic wanan crust sutured to the Laurentian margin been digitally merged and ﬁ ltered to enhance highs of peri-Gondwanan (Carolina-Uchee) and left orphaned here as the modern Gulf of upper-crustal anomalies. Beneath the Appa- arc terranes. The contact (Central Piedmont Mexico (Early Jurassic) and Atlantic oceans lachian Basin in northwestern Alabama, suture) corresponds to surface exposures of (Early Triassic) began to form (Applin, 1951; broad deep-crustal anomalies of the conti- the Bartletts Ferry fault. ADD magnetic and Barnett, 1975; Neathery and Thomas, 1975; nental interior include the Grenville front gravity signatures are truncated by the east- Pojeta et al., 1976; Chowns and Williams, and New York–Alabama lineament (dextral west–trending Altamaha magnetic low asso- 1983; Horton et al., 1984, 1989, 1991; Guthrie fault). Toward the east and south, high-angle ciated with the Suwannee suture. Arcuate and Raymond, 1992). The discovery in the late discordance between the northeast-trending northeast-trending magnetic linears of the 1950s of Lower Ordovician through Devonian Appalachians and the east-west–trending Suwannee terrane reﬂ ect internal structure sedimentary rocks containing Gondwanan faunal wedge of overlapping Mesozoic and Ceno- and Mesozoic failed-rift trends. Geophysical assemblages from exploration wells penetrating zoic Gulf Coastal Plain sediments reveals data can be used to make inferences on sur- the pre-Jurassic basement (here meaning either how bedrock geophysical signatures pro- face and subsurface geology and vice versa, or both Gondwanan and crystalline Appalachian gressively change with deeper burial. High- which has applicability anywhere that bed- basement) beneath Gulf Coastal Plain sedimen- frequency magnetic anomalies in the Appa- rock is exposed or concealed beneath essen- tary rocks in south Alabama (Applin, 1951) was lachian deformed domain (ADD) correspond tially non-magnetic sedimentary cover. a critical piece of the puzzle that Wilson needed to amphibolites and mylonites outlining to help solidify his thoughts on cycles of ocean terranes, while broader, lower-amplitude INTRODUCTION basins opening and closing and continents domains include Paleozoic intrusive bodies colliding. and Grenville basement gneiss. Fundamen- Alabama (United States) contains several Thick accumulations of younger sedimentary tal ADD structures (e.g., the Alexander City, major crustal-lithospheric boundaries that rock coupled with deep, sub-tropical weathering Towaliga, and Goat Rock–Bartletts Ferry played an important role in J. Tuzo Wilson’s and saprolitization of crystalline Appalachian faults) can be traced southward beneath (1966) formulation of what later would become rocks in Alabama leave us, however, with a the Gulf Coastal Plain to the suture with known as the Wilson cycle. The Appalachians fragmented understanding of parts of this geo-

Geosphere; August 2013; v. 9; no. 4; p. 1044–1064; doi:10.1130/GES00703.1; 5 ﬁ gures; 1 table; 1 supplemental ﬁ le. Received 17 March 2011 ♦ Revision received 28 August 2012 ♦ Accepted 5 March 2013 ♦ Published online 16 July 2013

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logical history. Eighty percent of the crystalline play their interactions with one another, provid- also the Ouachita foreland basin, and perhaps rocks in the state are concealed beneath Paleo- ing new insights into the tectonic evolution of is best characterized as a foreland basin that zoic and younger sedimentary cover, with 60% southeastern North America. formed adjacent to the syntaxis of both orogenic of that cover comprising Mesozoic and younger belts. Regional dip of this part of the continen- sediments of the Gulf Coastal Plain. The high- GEOLOGIC SETTING tal platform is southwest toward the Ouachitas angle discordance between strike of the north- and subparallel to Appalachian strike. Inter- east-trending Appalachians and the generally Two complete Wilson cycles and the begin- rupting the continuity within the foreland are east-west–trending Coastal Plain onlap is one of ning of a third are recorded in the lithosphere several anticlines (Sequatchie and Birmingham; the more prominent features on the geological of southeastern North America (Hatcher, 1978, Fig. 2) cored by Ordovician and younger rocks map of North America (Reed et al., 2005), and 1987, 2004, 2010; Thomas, 2006) (Fig. 1). that constitute the western limits of thin-skinned nowhere is this difference more pronounced than The fi nal phase of assembly of the supercon- Alleghanian deformation in Alabama (Rodgers, in Alabama (Fig. 1). This southward-thickening tinent Rodinia was the closing of ocean basins 1950). The Appalachian foreland fold-thrust wedge of Coastal Plain sediments provides the and collision of eastern Laurentia with pre- belt consists of a series of major folds and large opportunity to evaluate how remotely sensed Gondwanan continents during the ca. 1 Ga imbricate thrust sheets that have a northeast- geophysical signatures from surface exposures Grenville event (Hoffman, 1991). Breakup of southwest trend and become continuous with progressively change with deeper burial. Rodinia involved separation of Laurentia from other structures to the northeast in Georgia. The objectives of this paper are to use digi- West Gondwana and opening of the Iapetus ocean These areas are underlain by platform Cambrian tal maps of aeromagnetic and gravity data to at roughly 570–535 Ma (Odom and Fullagar, to Ordovician carbonate and siliclastic rocks better understand the upper-crustal surface and 1984; Aleinikoff et al., 1995). Subduction in unconformably beneath Middle Ordovician subsurface structure of Alabama with the goal Iapetus, 470–455 Ma, accreted the Taconian and Silurian carbonate and siliciclastic rocks, of incorporating its broader signifi cance for the volcanic arc system to southeast Laurentia which are themselves unconformably overlain tectonic evolution of southeastern North Amer- (Hatcher and Odom, 1980). An ocean remained by Devonian and Mississippian carbonate and ica. Aeromagnetic and gravity data that have off eastern Laurentia that closed obliquely in siliciclastic rocks, and fi nally by the Pennsyl- been obtained for parts of Alabama over several the middle to late Paleozoic with zippered col- vanian clastic sedimentary rocks of the Allegha- decades have been merged and digitally fi ltered lision of a collage of peri-Gondwanan arcs with nian clastic wedge (Thomas, 1988, 1991, 1995; to enhance anomalies. Aeromagnetic anomalies Laurentia (West, 1998; Wortman et al., 1998; Pashin, 1994, 2004). record upper-crustal structure down to the Curie Hatcher et al., 1999; Bream et al., 2000, 2004; Southeast of the Appalachian foreland are the isotherm (25–30 km) for induced magnetiza- Hibbard et al., 2002, 2007; Merschat et al., remnants of the Blue Ridge geologic province tion of the buried rocks. Temperatures beneath 2005; Hatcher and Merschat, 2006; Steltenpohl extending southwestward from Pennsylvania to these depths exceed the blocking temperatures et al., 2006, 2008). At ca. 330 Ma, thermal activ- Alabama. The Talladega fault (Fig. 2) separates of magnetic minerals such that remanent mag- ity marked the beginning of the Alleghanian the Appalachian foreland from the Blue Ridge. netization is absent. Gravity data reveal upper- orogeny and construction of Pangaea (see Secor Blue Ridge rocks of the Talladega slate belt con- crustal and deeper structures. New digital map et al., 1986, and Hatcher, 1987). Alleghanian sist of Cambrian to early Mississippian clastic images combine contours, shaded relief, and collision was oblique north-to-south, produc- and carbonate rocks, which are overthrust from color to accentuate regional patterns of highs ing dextral strike-slip blocks, followed by late the southeast by the Hollins line fault (Butts, and lows, gradients, and lineaments simultane- Pennsylvanian–Permian collision producing 1926; Tull, 1982; Tull et al., 1988; Gastaldo ously. Combined aeromagnetic and gravity data, the Blue Ridge–Piedmont megathrust sheet and et al., 1993). The Hillabee and Hollins line and to a lesser extent limited radioactivity data, foreland fold-thrust belt (Clarke, 1952; Bentley faults thrust the Ordovician Hillabee Greenstone provide much more useful information on this and Neathery, 1970; Cook et al., 1979). Finally, (dominantly mafi c but containing some felsic crust than does their separate analysis. Inter- Mesozoic breakup of Pangaea led to the opening volcanic rocks) and overlying distal Laurentian pretation of the crustal-lithospheric structure of the Atlantic Ocean (Thomas, 2006; Huerta clastic metasedimentary and metavolcanic rocks of Alabama cannot be made without consider- and Harry, 2012). of the eastern Blue Ridge over all of the tectonic ation of adjacent regions (Fig. 1), although the Key elements that help to defi ne these Wilson units to the west (Tull, 1978, 1980, 1982, 1984, geologic interpretation is based largely on geo- cycles are found in the surface and subsurface 1995; Tull et al., 2007; McClellan et al., 2005, physical maps of Alabama. In addition, owing to geology of Alabama, which consists of compo- 2007). These faults may be a suture equivalent the hydrocarbon wealth of the Gulf Coast, wells nents of the Interior Low Plateaus (otherwise to the Allatoona–Hayesville–Soque River fault have been drilled that sporadically penetrate the known as the Highland Rim, along the south- system farther northeast. The Goodwater-Enita- pre-Mesozoic basement beneath the Coastal ernmost fl ank of the Nashville dome) and the chopco fault (Neathery and Reynolds, 1973; Plain sedimentary rocks (Neathery and Thomas, Appalachian Plateaus, Valley and Ridge, and Tull, 1978; McConnell and Costello, 1980; 1975; Guthrie and Raymond, 1992), helping to Piedmont physiographic provinces (Sapp and Raymond et al., 1988; Tull and Holm, 2005) has defi ne the nature of the crust in the subsurface. Emplaincourt, 1975) (Fig. 2). Each of these cut the frontal edge of the eastern Blue Ridge Our analysis paints a much clearer picture geologic provinces is truncated at the surface thrust sheet, leaving two “marooned” structural of key tectonic elements, such as the Grenville in western and south-central Alabama by the salients that are partitioned by the Millerville “front,” the New York–Alabama (NY-AL) lin- Cretaceous to Holocene sedimentary rocks of reentrant and contain correlative sequences of eament, the Suwannee terrane, and the Suwan- the Gulf Coastal Plain. Surface rocks in north- migmatitic Ashland Supergroup (i.e., northern nee suture, that are cryptic because they are not ernmost Alabama consist of Ordovician to Mis- salient is Mad Indian above Poe Bridge Moun- exposed due to their burial beneath younger sedi- sissippian and Pennsylvanian carbonate and tain Groups, and southern salient is Hatchet ments and/or Appalachian thrust sheets. More siliciclastic rocks of the Appalachian foreland. Creek above Higgins Ferry Groups; Fig. 2). importantly, these and Appalachian deformed The southwestern segment of the Appalachian Directly southeast of the Goodwater-Enita- domain (ADD) elements now are imaged to dis- basin (i.e., the Black Warrior basin; Fig. 2) is chopco fault, Paleozoic granitic plutons, such as

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LAURENTIAN Sub-Coastal Plain Platform PERI-GONDWANAN Mesozoic rift deposits Margin (WBR) Charleston terrane Distal slope & rise (EBR) Spring Hope terrane Inner Piedmont (supect) Roanoke Rapids terrane Grenville basement Hatteras terrane Modoc, Box Ankle & GONDWANAN Goat Rock shear zones Suwannee terrane PERI-GONDWANAN WVA Infrastructural terranes KY 84° Savannah River SRA VA Milton belt Uchee terrane 37° Suprastructural terranes Milledgeville TN SMW GMW Mesozoic rift basin CST suture

86° ont Piedm NC 35° 35° SC AL GA 80° NC entral N C SC A CST CR 82° Brevard zone 34°

TS PMW DRW Coastal Plain South Georgia basin

s cynBruwik (Altamaha) nomal A

A′ FL

Suwannee terrane

0 100 km Gulf of Mexico FL B GONDWANANPERI- LAURENTIAN GONDWANAN

GULF COASTAL PLAIN

PM Hollins line fault Goodwater- Enitachopco fault Alexander City fault W Brevard fault zone Sequatchie anticline Helena fault fault Talladega

synform ′ A Tallassee A TA T A

A T T A SUWANNEE ga e Su TERRANE Towali n Bartletts Fewannee suture fault zo Goa t Rock fault rry/ NORTH AMERICAN (GRENVILLE) CONTINENTAL CRUST zone

Figure 1. (A) Tectonic map illustrating Alabama’s position within the southern Appalachians with section line A–A′ (modifi ed from Hatcher, 2004; Horton et al., 1984, 1989; Hibbard et al., 2002, 2006; Steltenpohl, 2005); Alabama is partially outlined in red. (B) Simplifi ed cross section A–A′ (modifi ed from W.A. Thomas and coworkers as depicted in Thomas [1989], Thomas et al. [1989], Hatcher et al. [1990], and Stelten- pohl [2005]). Abbreviations: CR—Cartersville reentrant; CST—Cat Square terrane; DRW—Dog River window; EBR—eastern Blue Ridge; GMW—Grandfather Mountain Window; PMW—Pine Mountain window; SMW—Sauratown Mountains Window; SRA—Smith River allochthon; TS—Tallassee synform; WBR—western Blue Ridge. Cross Section: A—Away; T—toward; no vertical exaggeration.

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35° 85° N Fold-Thrust belt Talladega fault

Hollins line fault Talladega slate belt SA Goodwater-Enitochopco fault

CDB EasternAlexander Blue Ridge City fault BA HR Brevard zone BWB CDB

Stonewall line fault MR Inner Piedmont Towaliga fault

Tallasseenform Eg sy Bartletts Ferry/ Zg JG Pine MountainGoat Rockwindow fault Kg Uchee terrane Coastal Plain onlap Eg = Elkahatchee Quartz Diorite Kg = Kowaliga Gneiss Zg = Zana Granite SA = Sequatchie anticline BA = Birmingham anticline HR = Hightower reentrant MR = Millerville reentrant BWB = Black Warrior basin CDB = Coosa Deformed belt JG = Jacksons Gap

31° 86°

Figure 2. Geologic map of surface exposures in Alabama (simpliﬁ ed from Szabo et al., 1988) illustrating rock units, rock packages, and faults discussed in the text. Inset map of physiographic provinces is from Sapp and Emplaincourt (1975) and Raymond et al. (1988). A digital geological map of Alabama can be found at http:// www.ogb.state.al.us/gsa/gis_data.aspx (Geological Survey of Alabama and State Oil and Gas Board, 2012).

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the major Elkahatchee Quartz Diorite batholith tics, quartz arenites, and conglomerate (Jack- the Central Piedmont suture (Hatcher and Zietz, and smaller bodies of Rockford, Bluff Springs, sons Gap Group) (Bentley and Neathery, 1970; 1980; Steltenpohl et al., 2008, 2010a). and Almond granites, occur in the eastern Blue Steltenpohl et al., 2005a). The Inner Piedmont The Gulf Coastal Plain occupies ~60% of the Ridge (Russell, 1978; Drummond et al., 1997; of Alabama has been subdivided into the Dade- exposed surface geology in Alabama. Coastal Tull et al., 2009; Schwartz et al., 2011). Timing ville and Opelika Complexes (Bentley and Neat- Plain sediments consist of Cretaceous, Tertiary, of pluton emplacement in the Alabama eastern hery, 1970; Neathery and Tull, 1975; Osborne and Holocene sediments overlain by Quater- Blue Ridge is only beginning to be understood et al., 1988). The Dadeville Complex is pre- nary stream deposits (as shown or described (Russell, 1978; Stowell et al., 1996; Steltenpohl dominantly a meta-igneous and metavolcanic in Osborne et al., 1988; Raymond et al., 1988; et al., 2005b; Tull et al., 2009; Schwartz et al., complex whereas the underlying Opelika Com- Mancini et al., 1989). Coastal Plain sediments 2011). The Elkahatchee Quartz Diorite, a pre- plex mainly comprises metasedimentary rocks include semiconsolidated sand, marl, limestone, metamorphic batholith (Fig. 2; Tull, 1978; Alli- intruded by Ordovician plutons (i.e., Bottle and clay units traceable across Alabama. Trends son, 1992; Drummond et al., 1994, 1997), was Granite–Farmville Metagranite; Bentley and of Coastal Plain units are east-west along the previously reported to be ca. 490 Ma based on Neathery, 1970; Neathery and Tull, 1975; Ray- Alabama-Georgia border, but become more U-Pb isotopic dating of multi-grain aliquots of mond et al., 1988; Osborne et al., 1988; Stelten- northwest in the Mississippi embayment seg- zircons (Russell, 1978). Recent reconnaissance pohl et al., 1990, 2005a; Grimes et al., 1997). ment in western Alabama (Szabo et al., 1988). SHRIMP-RG U-Pb dating of zircons from two The contact between these two complexes is the The subsurface geology beneath the south samples assigned to the Elkahatchee, however, synmetamorphic Stonewall line fault (Bentley Alabama Gulf Coastal Plain contains well-con- suggest igneous crystallization ages between ca. and Neathery, 1970; Grimes, 1993; Steltenpohl solidated Triassic, Jurassic, and Cretaceous rock 388 and 370 Ma (Tull et al., 2009; P.M. Mueller, et al., 1990). units that consist of sandstone, shale, evaporite, 2010, personal commun.). Other eastern Blue The Pine Mountain window occurs in some and carbonate of the rift-to-drift sequence that Ridge intrusions northwest of the Brevard zone of the more southern exposures of the Piedmont accompanied the opening of the Gulf of Mexico. in Alabama are lumped broadly into the Rock- in Alabama, and is an east-plunging antiform These include the Eagle Mills continental rift- ford, Bluff Springs, and Almond suites of granite exposing Grenvillian basement and platformal facies clastics (Triassic) and the eastern extent (Deininger et al., 1973; Deininger, 1975; Rus- metasedimentary cover rocks (Galpin, 1915; of the Louann Salt, the Buckner Anhydrite, and sell, 1978; Defant, 1980; Defant and Ragland, Adams, 1933; Crickmay, 1933, 1952; Clarke, the Smackover Limestone (Jurassic) overlain 1981; Defant et al., 1987; Drummond, 1986; 1952; Bentley and Neathery, 1970; Sears et al., by Lower Cretaceous and younger sequences Osborne et al., 1988; Drummond et al., 1997), 1981; McBride et al., 2005; Steltenpohl et al., (Mancini et al., 1989; Salvador, 1991). from which Schwartz et al. (2011) recently 2010a). It is framed by the Towaliga fault that Drill cores penetrating the pre-Mesozoic reported U-Pb zircon dates of ca. 365 Ma, ca. separates the window from the Inner Piedmont basement reveal the distinctly Gondwanan rocks 377 Ma, and ca. 350–330 Ma, respectively. to the northwest, the Box Ankle fault that closes of the Suwannee terrane (Applin, 1951; King, The Alexander City fault separates the the east end of the window and is truncated by 1961). In Alabama, the Suwannee terrane con- Wedowee Group from the Emuckfaw Group the Towaliga fault, and the Dean Creek and tains felsic volcanic rocks intruded by grano- (Muangnoicharoen, 1975; Neathery and Bartletts Ferry–Goat Rock faults that separate diorite that are overlain by Lower Ordovician Reynolds, 1975). The Emuckfaw Group com- the Pine Mountain window from the Uchee through Devonian sedimentary rocks containing prises mostly metagraywacke and schist that terrane (peri-Gondwanan; Steltenpohl et al., Gondwanan faunal assemblages (Applin, 1951; is intruded by Paleozoic granitoid plutons 2008) to the southeast (Bentley and Neathery, Barnett, 1975; Neathery and Thomas, 1975; assigned to the Kowaliga batholith or the suite 1970; Sears et al., 1981; Hooper et al., 1997). Pojeta et al., 1976; Chowns and Williams, 1983; of sill-like Zana granites (Fig. 2; Russell, 1978; The Pine Mountain basement-cover units are Guthrie and Raymond, 1992; Mueller et al., Stoddard, 1983; Bieler and Deininger, 1987; considered to be an outboard remnant of the 1994, 1996). Grimes et al., 1997; Drummond et al., 1997; ancient, subducted Laurentian margin (Clarke, Steltenpohl, 2005). Russell (1978) reported 1952; Odom et al., 1973; Schamel et al., 1980; DATA SOURCES AND PROCESSING whole-rock Rb-Sr “errorchron” ages for the Sears et al., 1981; Higgins et al., 1988; Hooper METHODOLOGY Kowaliga and Zana granites within the Silurian and Hatcher, 1988; Mueller et al., 2005; Stelten- and Devonian Periods, respectively, whereas pohl et al., 2005b, 2010a). The Geologic Map of Alabama (Szabo et al., U-Pb analysis of zircons (multi-grain aliquots) The Uchee terrane is the most outboard 1988) is a product of many decades of geologic gave Middle Ordovician ages. Most workers Appalachian terrane exposed in Alabama and mapping (Fig. 2 is a small-scale derivative). The interpret the eastern Blue Ridge of Alabama to comprises dioritic gneiss, amphibolite, and vari- aeromagnetic anomaly map of Alabama (Fig. 3) be part of the Neoproterozoic slope-rise facies ous metasedimentary and metavolcanic rocks is a composite of aeromagnetic surveys (see the of the distal Laurentian margin (Drummond (Bentley and Neathery, 1970; Hanley, 1983, Supplemental File1) fl own at different altitudes, et al., 1994, 1997; Steltenpohl, 2005; McClellan 1987; Steltenpohl, 2005). Because the Uchee fl ight-line separations, and fl ight-line directions et al., 2007; Tull et al., 2007). is sandwiched between Laurentian continental between 1972 and 1981, and compiled digitally. Separating the eastern Blue Ridge from the basement of the Pine Mountain terrane beneath An aeromagnetic map of the exposed crystalline Inner Piedmont is the Brevard fault zone (Jonas, and Gondwanan crust of the Suwannee terrane Appalachians in central-eastern Alabama was 1932; King, 1955; Hatcher, 1978, 2001; Higgins above, it occupies a critical tectonic position. made to aid geologists attempting to map in the et al., 1988). The Brevard fault zone changes U-Pb isotopic dating has demonstrated that the highly saprolitized units (Neathery et al., 1976, character at Jacksons Gap, Alabama (Fig. 2), Uchee contains 640–620 Ma zircons, indicating from a fault zone dominated by medium-grade an exotic peri-Gondwanan (Hibbard et al., 2002, 1Supplemental File. Zipped fi le containing 6 maps and explanatory text. If you are viewing the PDF of phyllonite and mylonite derived from fi ne- 2007) or Gondwanaland origin (Steltenpohl this paper or reading it offl ine, please visit http://dx.doi grained siliclastic rocks and ortho gneisses, to et al., 2006, 2008; Mueller et al., 2010), and the .org/10.1130/GES00703.S1 or the full-text article on mylonitic rocks derived from coarse siliciclas- Bartletts Ferry–Goat Rock fault zone here is www.gsapubs.org to view the Supplemental File.

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Figure 3. Major crustal features annotated on a composite magnetic anomaly map of Alabama. Thicker black lines, and dashed ones, are our geologic interpretations of regional aeromagnetic and gravity anomaly gradients (see text). Red lines are surface faults from the Alabama state geologic map (Szabo et al., 1988) and are dashed where we interpret their projection beneath the Coastal Plain sedimentary rocks. Num- bers and lines are described in the text; data sources are in the Supplemental File (see footnote 1). White line is the Gulf Coastal Plain onlap.

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1977a). Data were collected along north-south DELINEATION AND GEOLOGIC using composite aeromagnetic anomaly maps fl ight lines at 1 mi spacing and at an elevation of INTERPRETATION OF MAGNETIC of the eastern U.S. (King and Zietz, 1978; 500 ft (150 m) above ground. The fi rst aeromag- AND GRAVITY ANOMALIES Steltenpohl et al., 2010b). It is a magnetic gradi- netic map of the entire state of Alabama (Wilson ent bounding crustal blocks 2 and 4, and thus and Zietz, 2002) was compiled using available Figures 3 and 4 show the aeromagnetic and separates two different types of crust with the data from the Alabama Geological Survey, the gravity maps of Alabama, respectively, with our southeastern block characterized by relatively U.S. Department of Energy’s National Ura- geologic interpretations of regional anomaly smoother and higher magnetic and lower grav- nium Resource Evaluation (NURE) Program, gradients. Our interpretations were developed ity signatures. and the U.S. Geological Survey (USGS) and fi rst using the original individual magnetic 4. Large crustal block characterized by published at 1:1,000,000 and 1:500,000 scales surveys because some features were smoothed indistinct magnetic and gravity character and with 100 nanotesla (nT) and 20 nT contours. when digitized at 1:500,000 scale; these data northeast-trending structural grain, bordered to The Wilson and Zietz (2002) aeromagnetic map sources are provided in the Supplemental File the northwest by feature 3 and southeast by fea- was compiled by hand by connecting appropri- (see footnote 1). For this reason, some bound- tures 5, 6, and 8. It is generally characterized by ate contours across survey boundaries. Because aries may not appear to lie precisely upon the northeast-trending magnetic highs with 1200– the Neathery et al. (1976, 1977a) survey was steepest gradients in Figure 3. Figure 5 illus- 1520 nT relief and gravity lows ranging from contoured by hand (analog) methods, the map trates how features exposed at the surface relate 0 to –50 mGal. This block may also include the produced by Wilson and Zietz (2002) has more to our interpretations of the aeromagnetic fea- area of circular feature 6, described below. detail and character than our computer-gener- tures. Where geophysics provides the primary 4A. Northeastern part of domain 4, hav- ated map. The NURE data were collected from evidence for a buried feature, the interpreted ing relatively high, 880–1600 nT relief. The 400 ft (120 m) above ground along east-west line (black) depicted in Figure 3 is generally Sequatchie anticline and other Appalachian fl ight lines of 3 mi (4.8 km) and 6 mi (9.6 km) sketched along the steepest gradient taken from foreland structures and rocks overlie the area of spacing; magnetic surveying was not the pri- the data source maps in the Supplemental File feature 4. mary objective in the design of the NURE sur- (see footnote 1); black lines are dashed where 4B. Southwestern part of domain 4, distin- veys, and the wide spacing limits resolution in the gradients are relatively shallow and also guished by magnetic lows (as much as 300 nT). the digitized aeromagnetic map. The USGS where they separate geophysical domains. The 4C. Distinct N15°E-trending magnetic low data were collected along east-west fl ight lines position of a magnetic gradient can be affected within domain 4. at 1 mi (1.6 km) spacing, 1000 ft (300 m) above by factors such as varying depths to the contact 5. Magnetic gradient bounding crustal ground. Digital processing of the aeromagnetic of a source body, dip of the contact, magnetic- domains 4 (relatively high) and 9 (relatively data followed an identical methodology to that mineral distributions, and data quality. The low). Near the triple point between the Talla- of the Georgia aeromagnetic map (Daniels, evaluation of such factors and modeling of dega–St. Clair–Calhoun County lines (Figs. 2 2001). The digital data set not only increases subsurface features, however, are beyond the and 4), this lineament loses defi nition as indi- the accuracy of the compilation, but can also be scope of this regional study. Interpretations of cated by the dashed line. Magnetic gradient 5 easily manipulated to generate maps of a variety aeromagnetic anomaly gradients that are associ- loosely follows the southeast boundary of of scales, projections, enhancements, and com- ated spatially with shallow geologic features are Carboniferous-cored synclines and the Coosa binations, and is available for digital analytical constrained by knowledge of the local surface deformed belt (Fig. 2; Thomas and Drahovzal, tools favorable for interpretation of the data. geology. In a given geologic formation, group, 1974; Thomas, 2007), suggesting a relationship The Bouguer gravity anomaly map of Ala- sequence, belt, or terrane, only a small fraction with upper-level detachments in the thrust belt. bama (Fig. 4) was compiled from databases kept of the lithologic components may be suffi ciently 6. Broad circular feature defi ned by a mag- by the National Geophysical Data Center (Dater magnetic to produce anomalies on the aeromagnetic high anomaly and coincident with a gravity et al., 1999). The map was contoured at an inter- netic map, and these anomalies represent an low. Magnetic gradients are relatively fl at in the val of 5 milligals (mGal). average of a volume of rock. Thus, while inter- center (1400–1520 nT) and steeper on the mar- Neathery et al. (1976, 1977b) produced a total- preted boundaries may differ from observed gins (1400–1000 nT; ~25 mGal). This feature count radioactivity map of the exposed crystal- geologic contacts, the trends are commonly corresponds to part of a regional Appalachian line Appalachians in east-central Alabama. Total similar. The following numbers are keyed to gravity low, and it is one of the most prominent count gamma-ray anomalies measure gamma numbers identifying geophysical anomalies and features on the map. It appears to truncate short- rays produced in the upper one-half meter of patterns attributed to crustal features in Figures wavelength anomalies and thus is attributed to earth material, which in the Alabama Piedmont 3, 4, and 5. The list below constitutes an attempt a shallow-crustal feature, possibly a large bul- is mainly soil and saprolite derived from under- to describe each feature, indicating what char- bous pluton of unknown age. The northwestern lying bedrock. Radiometric anomalies in the acteristics separate it from adjacent crustal fea- margin of this feature is not clearly defi ned and Alabama Piedmont refl ect near-surface Pied- tures. Table 1 provides an abbreviated guide to is dashed. mont bedrock geology. However, radiometric aid in relating these features to Figures 3, 4, and 7. Magnetic gradient bounding crustal anomalies in the Coastal Plain province refl ect 5 and an interpretation of each feature. domains 6 (relatively high) and 9 (rela- surface Coastal Plain sediments that cover and 1. Domains characterized by correlative mag- tively low). are independent of the crystalline bedrock units netic highs and gravity highs, interpreted to be 8. Localized short-wavelength magnetic high delineated by the aero magnetic surveys. During buried basement mafi c-rock bodies. interpreted as a possible mafi c enclave within the our analysis of the aeromagnetic and gravity data, 2. Crustal block having north-northeast mag- pluton of domain 6, or perhaps a segment of we consulted the aeroradio activity map, where netic grain, constituting country rock to mafi c the same thrust sheet containing mafi c rock coverage was available, and found it to corre- bodies of domain 1. of nearby domain 17, described below. spond closely to the surface geology depicted 3. Southern extension of the NY-AL linea- 9. Crustal domain characterized by relative on the state geologic map (Szabo et al., 1988). ment based on the originally defi ned features magnetic (up to 1700 nT) and gravity highs.

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85° 35° 1 N 2 1 1 1 2 4A 3 1 1 9 5 2 1 4A 10 16 19B mGal 16 17 9 18 19A 22 4B 17B 4B 23A 19 17C 5 17 24 10 9 16 22 4C 18 16A 21 17 17A 24 25 25A 25B 9B 17 20 23A 22 23B 31 23A 17B 33 32A 34A 6 8 24A 24B 23B 34 17D 21 32C 32B 26 27 34 30 7 29 12 17B 31A 4B 28 34 35 35 36 9 32D 35 34 11 9A 13 14 5 9 9A 12 13A 13B 14 15 15 15 86° 31°

Figure 4. Major crustal features from Figure 3 superposed on a Bouguer gravity anomaly map of Alabama.

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1 35° 85°

1 N 1 1 2 4A 2 3 1 1 9 5 1 4A 10 16 19B 16 17 9 18 19A 22 4B 17B 23A 19 17C 5 17 24 10 9 16 22 18 16C 21 17A 24 25 17 25A 25B 4B 9B 17 20 23A 22 23B 31 23A 33 17B 32A 34A 6 7 24A 24B 34 17D 21 23B 32C 32B 26 27 34 30 29 12 35 17B 31A 28 34 35 36 9 32D 35 34 11 9A 13 14 8 9 9A 12 13A 13B 14 15 15 15

31° 86°

Figure 5. Annotated features from Figure 3 layered on the simpliﬁ ed geologic map of Alabama presented in Figure 2. Figure also serves to illustrate county locations relative to the aeromagnetic interpretations to aid in our discussion.

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TABLE 1. GEOPHYSICAL FEATURES AND PATTERNS ATTRIBUTED TO UPPER CRUSTAL SOURCES AS NUMBERED IN FIGURES 3, 4, AND 5 Geophysical feature and distinguishing characteristics based Feature (type)* on combined magnetic and gravity anomalies Upper-crustal source interpretation 1 (A) Domains characterized by correlative magnetic highs and gravity highs. Buried basement maﬁ c-rock bodies. 2 (A) Domain distinguished by north-northeast magnetic grain. Country rock to maﬁ c bodies of domain 1.

3 (L) Magnetic gradient bounding domains 2 (northwest) and 4 (southeast). Domain 4 has Southern extension of the New York–Alabama lineament, relatively smoother and higher magnetic signatures and lower gravity signatures; it thus based on that originally deﬁ ned using composite magnetic separates two different types of crust. anomaly maps of the eastern U.S. (King and Zietz, 1978; Steltenpohl et al., 2010b).

4 (A) Area of indistinct magnetic and gravity character and NE-trending grain, bordered on Large crustal block characterized by indistinct magnetic the northwest by feature 3 and on the southeast by features 5, 6, and 8. Generally and gravity character and NE-trending structural grain. characterized by NE-trending magnetic highs with 1200–1520 nT relief and gravity lows The Sequatchie anticline and other Appalachian foreland ranging from 0 to -50 mGal. May also include the area of circular feature 6, described structures and rocks overlie the area of feature 4. below. 4A. Northeastern part of domain 4 having relatively high, 880–1600 nT relief. 4B. Southwestern part of domain 4, distinguished by magnetic lows (as much as 300 nT). 4C. Distinct N15°E-trending magnetic low within domain 4.

5 (L) Magnetic gradient bounding crustal domains 4 (relatively high) and 9 (relatively low). Near the triple point between the Talladega–St. Clair–Calhoun County lines (Figs. 2 and 4), this lineament loses deﬁ nition as indicated by the dashed line.

6 (A) Broad circular feature defi ned by a magnetic-high anomaly and coincident gravity low. Possibly a large bulbous pluton of unknown age. Magnetic gradients are relatively fl at in the center (1400–1520 nT) and steeper on the margins (1400–1000 nT; ~25 mGal). This feature corresponds to part of regional Appalachian gravity low, and it is one of the most prominent features on the map. It appears to truncate short-wavelength anomalies and thus is attributed to a shallow- crustal feature. The northwestern margin of this feature is not clearly defi ned and is dashed. 7 (L) Magnetic gradient bounding crustal domains 6 (relatively high) and 9 (relatively low).

8 (A) Localized short-wavelength magnetic high. Possible maﬁ c enclave within the pluton of domain 6, or perhaps a segment of the same thrust sheet containing maﬁ c rock of nearby domain 17, described below.

9 (A) Crustal domain characterized by relative magnetic (up to 1700 nT) and gravity highs.

9A (A) 9A. Circular magnetic and gravity highs (up to 1600 nT relief) mostly within crustal block 9. Interpreted to be maﬁ c plutons.

9B (L) Northwest-trending lineament that appears to offset domains 9 and 17. Left-slip shear zone that offsets domains 9 and 17.

10 (A) Domains with small, short-wavelength, relatively high magnetic anomalies (up to 900 nT Corresponds to ferruginous sandstones of the Kahatchee relief) within domain 9 that, although muted in Figure 3, were clearly deﬁ ned on the more Mountain Group (Talladega slate belt) and Weisner detailed 1:500,000-scale aeromagnetic map of Alabama (Wilson and Zietz, 2002). Formation (Appalachian foreland), both of which correlate with the Chilhowee Group in Tennessee, Georgia, and North Carolina. The Kahatchee Mountain Group produces a stronger anomaly than the Weisner Formation likely because the former is the low-grade (chlorite zone) metamorphosed equivalent of the latter and magnetite is a product of chlorite- grade metamorphism. The Talladega thrust marks the boundary between the Appalachian foreland and Talladega slate belt in Alabama but does not have much expression in the aeromagnetic data.

11 (L) Regional gravity gradient (dashed line to emphasize the lack of an associated magnetic signature), generally low to the north and high to the south with an easterly trend distinct from the normal Appalachian gravity gradient trend.

12 (L) E-W–trending linear truncation of all NE-trending magnetic and gravity anomalies to the Interpreted as the northern boundary of the Suwannee-Wiggins north that correspond to Appalachian and older structures. suture zone (Neathery and Thomas, 1975; Neathery et al., 1977a; Horton et al., 1984).

13 (A) Prominent magnetic-low (13B) domain and gradient (13A) on its north side as distinguished The suture zone is overprinted by Mesozoic extensional faults, on a more detailed, 1:250,000-scale magnetic map (Neathery et al., 1977a). and the Altamaha magnetic-low anomaly may or may not 13A. Magnetic gradient on the north side of the Altamaha magnetic-low anomaly (Higgins correspond to a deep part of the early Mesozoic South and Zietz, 1983). Georgia Basin. Early Mesozoic sediments of the South 13B. Altamaha magnetic-low anomaly (Higgins and Zietz, 1983). Georgia Basin are crudely delineated by drilling and produce a muted, ﬂ at magnetic signature. Mesozoic basalt and diabase coincide with local magnetic highs that interrupt the magnetic-low anomaly.

14 (L) Southern boundary of the Altamaha magnetic low. Possible southern boundary of a rift basin (early Mesozoic?) formed across the earlier suture zone.

15 (A) Large domain including parallel arcuate, ~N45°E trends on the magnetic anomaly map of Suwannee terrane (undivided). Circular magnetic-high southern Alabama. Contains circular magnetic-high anomalies coincident with circular anomalies coincident with circular gravity highs are here gravity highs. interpreted as maﬁ c plutons within the Suwannee terrane. (continued)

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TABLE 1. GEOPHYSICAL FEATURES AND PATTERNS ATTRIBUTED TO UPPER CRUSTAL SOURCES AS NUMBERED IN FIGURES 3, 4, AND 5 (continued) Geophysical feature and distinguishing characteristics based Feature (type)* on combined magnetic and gravity anomalies Upper-crustal source interpretation 16 (L) Northwestern limit of short-wavelength magnetic highs (up to 1300 nT). Hollins line fault and the northwestern limit of eastern Blue Ridge amphibolites that coincide with short-wavelength magnetic highs (up to 1300 nT). The Hillabee Greenstone, mostly a massive greenstone, appears to be non-magnetic. The Hollins line fault is aeromagnetically distinct where it truncates amphibolites of the eastern Blue Ridge but has little expression elsewhere. 16A (L) Right-slip offset of feature 16 and magnetic linear anomalies in domain 17. Transcurrent right-slip shear zone that offsets both the Hollins line fault and magnetic linear anomalies of domain 17; here it is interpreted as a transcurrent shear zone because it appears to cut the overlying thrust sheet.

17 (A) Domain of short-wavelength, high-magnetic (up to 1600 nT) signatures. Zone of amphibolite-rich Hatchet Creek (Mitchell Dam 17A, 17B. Sub-areas lacking high-frequency magnetic-high anomalies. Amphibolite) and Poe Bridge Mountain (Ketchapedrakee 17C Sub-area generally having higher-frequency magnetic anomalies. Amphibolite) Group metasedimentary and metavolcanic 17D. Sub-area characterized by weakly elliptical, N-NE–trending, low- to moderate- rocks (eastern Blue Ridge). wavelength magnetic highs (-200 to -260 nT). The magnetic signature of this domain is 17A. Corresponds to amphibolite-poor Wedowee and Higgins different from the much-broader-wavelength character of domain 21 to the east and the Ferry Group metasedimentary and metavolcanic rocks. less-elliptical and lower-magnitude magnetic contours of domain 17B to the west. 17B. Corresponds to amphibolite-poor Mad Indian Group metasedimentary and metavolcanic rocks. 17C. Wedowee Group metasedimentary and metavolcanic rocks (mostly undifferentiated phyllite, schist, and gneiss), containing intermittent amphibolite (Beaver Dam Amphibolite) generally having higher-frequency anomalies, injected by Devonian to Carboniferous granitoid bodies (Almond Trondhjemite and Bluff Springs Granite) with lower- frequency anomalies. 17D. Crustal domain (not exposed) interpreted as possible non-magnetic Wedowee Group metasedimentary rocks. 18 (L) Boundary between domains 17 and 17A and between domains 17 and 17B distinguished by contrasting magnetic signatures.

19 (L) Goodwater-Enitachopco fault, where previously mapped (Osborne et al., 1988), has essentially no magnetic expression and juxtaposes metasedimentary rocks that cannot be distinguished by magnetic signatures. 19A. Questionable segment of the Goodwater-Enitachopco fault as previously mapped (Osborne et al., 1988) crosses apparent structural grain suggested by numerous small short-wavelength anomalies on both the magnetic map and radioactivity map. 19B. Relatively sharp, linear magnetic anomaly that may be a shear zone or fault, perhaps an alternate extension of the Goodwater-Enitachopco fault (needs to be ﬁ eld checked). 20 (A) Long-wavelength (low frequency) magnetic pattern resembles that of the nearby Wedowee Group metamorphic rocks and Rockford Granite; the Elkahatchee pluton (domain 21) directly to the east. latter is suggested to be a Devonian to early Mississippian intrusion. 21 (A) Long-wavelength (low frequency) magnetic pattern; one of the most distinctive large Elkahatchee pluton; Paleozoic quartz diorite batholith. domains on the magnetic-anomaly map. Extension of the Elkahatchee pluton beneath the Coastal Plain is clearly delineated by its characteristic magnetic pattern, which shows it to be one of the largest and most extensive bodies of plutonic rock in the southeastern United States. 22 (L) Linear zone of NE-trending, short-wavelength, magnetic anomalies along distinctive SE Alexander City fault zone. Magnetic character varies along border of domain 21. strike and includes SE margin of Elkahatchee pluton, and local NE-trending lineament strands. The characteristic magnetic signature of the Elkahatchee pluton combines with the linear set of distinct, short-wavelength strands associated with the Alexander City fault zone to make the latter the most distinct and continuous anomaly on the map, appearing to continue southwestward to the suture zone where it is truncated.

23 (A) Relatively featureless magnetic domain of broad, ﬂ at, long-wavelength anomalies (similar Magnetically featureless terrane between major faults, to domain 21). comprising domains 23A and 23B. Sub-domain 23A has an indistinct magnetic pattern except in areas marginal to major faults 23A. Corresponds mostly to Emuckfaw Formation (eastern (e.g., Brevard and Towaliga faults), where linear, moderate-frequency anomalies occur. Blue Ridge) and Auburn Gneiss and Loachapoka Schist Sub-domain 23B has a magnetic signature similar to that of sub-domain 23A, although with (Opelika Complex, Inner Piedmont) metasedimentary rocks. generally even lower magnitudes and lacking the moderate-frequency anomalies. A relatively thin package of metasedimentary rocks, the Jacksons Gap Group, crops out along the structurally upper parts (i.e., east) of this domain. 23B. Corresponds to Paleozoic granitic plutons of the eastern Blue Ridge (Kowaliga Gneiss and Zana Granite) and Opelika Complex of the Inner Piedmont (Bottle Granite and Farmville Metagranite). (continued)

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TABLE 1. GEOPHYSICAL FEATURES AND PATTERNS ATTRIBUTED TO UPPER CRUSTAL SOURCES AS NUMBERED IN FIGURES 3, 4, AND 5 (continued) Geophysical feature and distinguishing characteristics based Feature (type)* on combined magnetic and gravity anomalies Upper-crustal source interpretation 24 (L) Linear zone of high-frequency, N55°E-trending, linear magnetic anomalies, which changes The Brevard fault zone; this N55°E-trending mylonite zone character just northeast of Jacksons Gap (see Fig. 2 for location) and to the south is changes character south of Jacksons Gap (see Fig. 2 for marked by truncation of ENE–trending magnetic anomalies. location) corresponding to an apparent lack of the late brittle Alleghanian overprint. From there southwestward, the older ductile Brevard fault zone swings around the Inner Piedmont due to late folding related to the formation of the Tallassee synform (Fig. 2).

24A, 24B (A) 24A and 24B: Arcuate-shaped “islands” of higher-frequency anomalies. 24A and 24B. May be faults involving imbrication of the Emuckfaw Formation.

25 (A) Domain containing both high-frequency, curved and linear magnetic highs and broad low- Inner Piedmont (Dadeville Complex). High-frequency, curved frequency anomalies. and linear magnetic highs represent amphibolites of Dadeville Complex (Ropes Creek Amphibolite) as well as local ultramaﬁ c bodies. Broad low-frequency anomalies are felsic metaplutonic and metavolcanic units (Camp Hill Gneiss, Rock Mills Granite Gneiss, Chattasofka Creek Gneiss, and Waverly Gneiss). Overall pattern is consistent with the Dadeville Complex as a gently northeast-plunging, recumbent sheath fold, similar to those mapped in the Inner Piedmont of the Carolinas (Merschat et al., 2005; Hatcher and Merschat, 2006).

25A and 25B (L) 25A. No correlation with magnetic anomaly map. 25A. Stonewall line fault zone, which separates mainly 25B. Magnetic lineament. meta-igneous rocks of the Dadeville Complex from mainly metasedimentary units of the Opelika Complex (Inner Piedmont). Although there is no correlation of the Stonewall line with the magnetic anomaly map, the fault trace is distinct from linear trends on the radioactivity map (not shown; Neathery et al., 1977b). 25B. Magnetic lineament internal to Opelika Complex that may separate Farmville Metagranite plutonic bodies from Auburn Gneiss metasedimentary rocks.

26 (A) Domain of high-frequency magnetic anomalies (similar to domain 25) with a substantial Emuckfaw Formation metasedimentary rocks and Zana area of broader, low-frequency positive magnetic anomalies. Granite. The magnetic character is similar to that of domain 25 (Inner Piedmont, Dadeville Complex). Area of broader, low-frequency positive magnetic anomalies may represent metasedimentary rocks or metaplutons as in domain 25; high-frequency parts resemble amphibolite-rich areas of Dadeville Complex in domain 25. Although the Emuckfaw Formation is known to locally contain minor, thin amphibolite layers (Bentley and Neathery, 1970; Raymond et al., 1988), the aeromagnetic signature of domain 26 suggests that this area may contain more-substantial mafi c and perhaps even ultramafi c material that is not depicted on the state geologic map (Osborne et al., 1988). The northernmost parts of domain 26 are exposed north of the Coastal Plain onlap, and fi eld checking in that area is needed to examine the source of this distinctive magnetic character.

27 (L) E-NE–trending linear anomalies. Interpreted to mark an unnamed splay of the Towaliga fault.

28 (A) Magnetic signature is similar to domain 26. Block (beneath coastal plain) between two splays of the Towaliga fault (i.e., features 27 and 29).

29 (L) E-NE–trending zone of linear magnetic anomalies. E-NE–trending linear strands marking another splay of the Towaliga fault.

30 (A) Domain having similar magnetic and gravity signature as domain 32A. Crustal block between the main strand of the Towaliga fault (feature 31) and splay 29 that has a similar magnetic and gravity signature as domain 32A.

31 (L) Main Towaliga fault, which is younger than and has excised plastic mylonites and ultramylonites of the Towaliga fault zone. The main Towaliga fault marks the boundary between the Opelika Complex (Inner Piedmont) and the Pine Mountain basement-cover massif.

31A (L) Another splay of the Towaliga fault. (continued)

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TABLE 1. GEOPHYSICAL FEATURES AND PATTERNS ATTRIBUTED TO UPPER CRUSTAL SOURCES AS NUMBERED IN FIGURES 3, 4, AND 5 (continued) Geophysical feature and distinguishing characteristics based Feature (type)* on combined magnetic and gravity anomalies Upper-crustal source interpretation 32 (A) 32A. Area of moderate-frequency, elliptical magnetic highs. Pine Mountain window. 32B. Area of low-frequency, broad, fl at, only weakly elongate magnetic lows. 32A. Pine Mountain Group cover sequence, Halawaka Schist, 32C. Magnetically high area of high-frequency anomalies. and mylonites and ultramylonites. 32D. Area of broad, fl at magnetic anomalies. 32B. Mesoproterozoic granitic gneiss basement rocks of Pine Mountain window. The magnetic signature is grossly similar to that of the Paleozoic granitoids of the eastern Blue Ridge, domains 21 and 23B). Some Halawaka Schist occurs within this domain. 32C. High-frequency magnetic-high domain beneath the Coastal Plain that is similar to Pine Mountain Group cover of domain 32A. 32D. Broad fl at anomalies buried deeply beneath the Coastal Plain that are similar to domain 32B, but with domain 32C in between them, interpreted as possible Mesoproterozoic basement of the Pine Mountain window.

33 (L) Linear zone characterized by contrasting broad ﬂ at magnetic lows to the northwest and the Bartletts Ferry fault zone, delineated on the magnetic map by strong, high-frequency magnetic highs to the southeast (domain 35 below). contrasting broad ﬂ at magnetic lows of the Pine Mountain basement granites to the northwest and the strong, high- frequency magnetic highs of mylonites and mylonitized amphibolites and dioritic gneisses of the Uchee terrane to the southeast (domain 35, below). The Uchee terrane has a peri-Gondwanan or Gondwanan origin, requiring that the northwest limit of Bartletts Ferry fault zone must also be the Central Piedmont suture or a later structure that has overprinted the suture. The dashed red line is our projection of the Bartletts Ferry fault zone beneath the Coastal Plain.

34 (L) Sharp, linear magnetic gradients. Strands of mylonite and ultramylonite within the Bartletts Ferry–Goat Rock fault zones that interlace the Uchee terrane. The numerous fault strands south of the Bartletts Ferry fault zone deﬁ ne patterns typical of a strike-slip duplex (Steltenpohl, 1988).

34A (L) Goat Rock fault zone based on the geologic map. Note that the magnetic signature is not remarkably different from the strands of mylonite that otherwise lace the Uchee terrane.

35 (A) Uchee terrane (peri-Gondwanan) rocks serving as country rock protoliths sheared by the Bartletts Ferry–Goat Rock system of fault zones.

36 (A) E-NE–trending domain of magnetic and gravity highs (amplitudes up to 1400 nT and ~+30 E-NE–trending terrane of magnetic and gravity highs lying mGal, respectively) lying south of domain 35 (Uchee terrane) and south of domains 12 south of the Uchee terrane and south of the suture (domains and 13. 12 and 13). These anomalies likely correspond to mafi c and ultramafi c complexes. They appear in and adjacent to the suture zone and thus could refl ect remnant oceanic lithosphere from between the peri-Gondwanan arcs (Uchee terrane) and Gondwanan crust of the Suwannee terrane. Alternatively, they may represent substantial accumulations of basaltic lava associated with the Mesozoic South Georgia Basin. *Type: L—linear feature or gradient; A—area.

9A. Circular magnetic and gravity highs Tennessee, Georgia, and North Carolina (Tull, 12. East-west–trending linear truncation of all interpreted to be maﬁ c plutons (up to 1600 nT 1982; Tull and Guthrie, 1985). The Kahatchee northeast-trending magnetic and gravity anoma- relief) mostly within crustal block 9. Mountain Group produces a stronger anomaly lies to the north that correspond to Appalachian 9B. Northwest-trending lineament that than the Weisner Formation likely because the and older structures, interpreted as the northern resembles a left-slip shear zone that offsets former is the low-grade (chlorite zone) metamor- boundary of the Suwannee-Wiggins suture zone domains 9 and 17. phosed equivalent of the latter and magnetite is (Neathery and Thomas, 1975; Neathery et al., 10. Domains with small, short-wavelength, a product of chlorite-grade metamorphism. The 1977a; Horton et al., 1984). relatively high-magnetic anomalies (up to 900 nT Talladega fault marks the boundary between the 13. Suture zone overprinted by Mesozoic relief) within domain 9 that, although muted in Appalachian foreland and Talladega slate belt in extensional faults. The distinction between 13A Figure 3, were clearly deﬁ ned on the more Alabama but does not have much expression and 13B is based on a more detailed 1:250,000- detailed 1:500,000-scale aeromagnetic map of in the aeromagnetic or gravity data. scale magnetic map (Neathery et al., 1977a). Alabama (Wilson and Zietz, 2002). These corre- 11. Regional gravity gradient (dashed line to 13A. Magnetic gradient on the north side spond to ferruginous sandstones of the Kahatchee emphasize the lack of an associated magnetic of the Altamaha magnetic low (Higgins and Mountain Group (Talladega slate belt) and Weis- signature), generally low to the north and high Zietz, 1983). ner Formation (Appalachian foreland), both of to the south with an easterly trend distinct from 13B. Altamaha magnetic low, which may or which correlate with the Chilhowee Group in the normal Appalachian gravity gradient trend. may not correspond to a deep part of the early

1056 Geosphere, August 2013

Mesozoic South Georgia Basin. Early Meso- lite-poor Mad Indian Group metasedimentary and most extensive bodies of plutonic rock in zoic sediments of the South Georgia Basin are and metavolcanic rocks. the southeastern United States. crudely delineated by drilling and produce a 17C. Wedowee Group metasedimentary and 22. Alexander City fault zone. Its magnetic muted, fl at magnetic signature. Mesozoic basalt metavolcanic rocks (mostly undifferentiated character varies along strike and includes the and diabase coincide with local magnetic highs phyllite, schist, and gneiss; Bentley and Neathery, southeast margin of Elkahatchee pluton, and that interrupt the magnetic low anomaly (Guthrie 1970; Osborne et al., 1988), containing intermit- local northeast-trending lineament strands. The and Raymond, 1992). tent amphibolite (Beaver Dam Amphibo lite, characteristic magnetic signature of the Elka- 14. Southern boundary of the Altamaha mag- generally having higher-frequency anomalies), hatchee pluton combines with the linear set of netic low. This is possibly the southern bound- injected by Devonian (Russell, 1978; Schwartz distinct, short-wavelength strands associated ary of a rift basin (early Mesozoic?) formed et al., 2011) granitoid bodies (Almond Trond- with the Alexander City fault zone to make the across the earlier suture zone. hjemite and Bluff Springs Granite with lower- latter the most distinct and continuous anomaly 15. Suwannee terrane (undivided), which frequency anomalies; Bentley and Neathery, on the map, appearing to continue southwest- includes parallel arcuate, ~N45°E trends on the 1970; Osborne et al., 1988). ward to the suture zone where it is truncated. magnetic anomaly map of southern Alabama. 17D. Domain that is not exposed but char- 23. Magnetically featureless (broad, fl at, long- Circular magnetic high anomalies coincident acterized by weakly elliptical, north-northeast– wavelength, like feature 21) terrane between with circular gravity highs are here interpreted trending, low- to moderate-wavelength mag- major faults, comprising domains 23A and 23B. as mafi c plutons within the Suwannee terrane netic highs (–200 to –260 nT). The magnetic 23A. Mostly indistinct magnetic domain and/or basaltic lavas within the South Georgia signature of this domain is different from the except in areas marginal to major faults (e.g., Basin. The circular magnetic high in Covington much-broader-wavelength character of domain Brevard and Towaliga faults) where linear, and Escambia Counties (see Fig. 2 for county 21 to the east and the less-elliptical and lower- moderate-frequency anomalies occur; this locations) corresponds with several wells pen- magnitude magnetic contours of domain 17B to domain corresponds mostly to Emuckfaw etrating amygdaloidal basalt and interbedded the west. This domain is interpreted as possible Group (eastern Blue Ridge) and Auburn Gneiss mudstones below the Jurassic Smackover sec- non-magnetic Wedowee Group metasedimen- and Loachapoka Schist (Opelika Complex, tion (Guthrie and Raymond, 1992). Whereas tary rocks. Inner Piedmont) metasedimentary rocks. A rela- relief on the basement could be the cause of 18. Boundary between domains 17 and 17A tively thin package of metasedimentary rocks, some magnetic anomalies, no rationale was and between domains 17 and 17B, distinguished the Jacksons Gap Group (Bentley and Neathery, found to identify such anomalies. Euler mag- by contrasting magnetic signatures. 1970; Neathery and Tull, 1975; Raymond et al., netic source analysis of the magnetic map gives 19. Goodwater-Enitachopco fault, which has 1988; Osborne et al., 1988), crops out along highly variable depths depending on the struc- essentially no magnetic expression and juxta- the structurally upper parts (i.e., east) of this tural index used. Such analysis is beyond the poses metasedimentary rocks that cannot be dis- domain. scope of this study. tinguished by magnetic signatures. 23B. Magnetic signature like domain 23A 16. Hollins line fault (Neathery and Reynolds, 19A. Questionable segment of the Good- although with generally even lower magnitudes 1975; Tull, 1978) and the northwestern limit water-Enitachopco fault as previously mapped and lacking the moderate-frequency anomalies. of eastern Blue Ridge amphibolites (short- (Osborne et al., 1988). It crosses apparent struc- This domain corresponds to Paleozoic granitic wavelength magnetic highs up to 1300 nT). The tural grain suggested by numerous small short- plutons of the eastern Blue Ridge (Kowaliga Hillabee Greenstone, mostly a massive green- wavelength anomalies on both the magnetic Gneiss and Zana Granite) and the Opelika Com- stone, appears to be non-magnetic. The Hollins map and radioactivity map. plex (Bottle Granite and Farmville Metagranite; line is aeromagnetically distinct where it trun- 19B. Relatively sharp linear magnetic anom- Bentley and Neathery, 1970; Neathery and Tull, cates amphibolites of the eastern Blue Ridge but aly that may be a shear zone or fault, perhaps 1975; Osborne et al., 1988; Steltenpohl et al., has little expression elsewhere. an alternate extension of the Goodwater-Enita- 1990, 2005a, 2005b). 16A. Transcurrent right-slip shear zone that chopco fault (this needs to be fi eld checked). 24. Brevard fault zone, characterized by high- offsets magnetic linear anomalies of domain 17 20. Wedowee Group metamorphic rocks frequency, linear, N55°E-trending magnetic and, apparently, merges with or cuts the Hol- and Rockford Granite; the latter is suggested anomalies; the magnetic signature changes char- lins line fault beneath the Coastal Plain cover; to be a Devonian to early Mississippian intru- acter just northeast of Jacksons Gap (see Fig. 2 here it is interpreted as a transcurrent shear sion based on scattered Rb-Sr and U-Pb zircon for location), where to the south it is marked by zone because it appears to cut the overlying results and fi eld relations (Russell, 1978; Drum- truncation of east-northeast–trending magnetic thrust sheet. mond, 1986; Drummond et al., 1997). The long- anomalies within the Inner Piedmont (Dadeville 17. Zone of amphibolite-rich Hatchet Creek wavelength (low-frequency) magnetic pattern Complex, feature 25, below; see Fig. 2 for loca- (Mitchell Dam Amphibolite) and Poe Bridge resembles that of the nearby older Elkahatchee tion). This change in character corresponds to Mountain (Ketchapedrakee Amphibolite) Group pluton (feature 21) directly to the east. an apparent loss of the late brittle Alleghanian metasedimentary and metavolcanic rocks (east- 21. Elkahatchee pluton. This Paleozoic overprint at Horseshoe Bend. From there southern Blue Ridge) with short-wavelength, high- quartz diorite batholith (Gault, 1945; Bentley westward, the older ductile Brevard fault zone magnetic (short-wavelength magnetic highs up and Neathery, 1970; Russell, 1978; Drummond swings around the Inner Piedmont due to late to 1600 nT) signatures. et al., 1994, 1997) is characterized by a long- folding related to the formation of the Tallassee 17A. Sub-area lacking high-frequency mag- wavelength (low-frequency) magnetic pattern synform (Fig. 2; Bentley and Neathery, 1970; netic-high anomalies corresponding to amphibo- that makes it one of the most distinctive units Steltenpohl et al., 2005a). lite-poor Wedowee and Higgins Ferry Group on the magnetic anomaly map. Extension of the 24A and 24B. Arcuate-shaped “islands” of metasedimentary and metavolcanic rocks. Elkahatchee pluton beneath the Coastal Plain is higher-frequency anomalies that may indicate 17B. Sub-area lacking high-frequency mag- clearly delineated by its characteristic magnetic imbricate faults involving imbrication of the netic-high anomalies corresponding to amphibo- pattern, which shows it to be one of the largest Emuckfaw Group.

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25. Inner Piedmont (Dadeville Complex). 27 and 29). The magnetic signature is similar to not remarkably different from the strands of High-frequency, curved and linear magnetic domain 26. mylonite that otherwise lace the Uchee terrane. highs represent amphibolites of Dadeville 29. East-northeast–trending linear strands 35. Uchee terrane (peri-Gondwanan) rocks Complex (Ropes Creek Amphibolite; Bentley marking another splay of the Towaliga fault. serving as country rock protoliths sheared and Neathery, 1970; Osborne et al., 1988) as 30. Block of rock between the main strand of by the Bartletts Ferry–Goat Rock system of well as local ultramafi c bodies; broad low- the Towaliga fault (feature 31) and splay 29 that fault zones. frequency anomalies are felsic metaplutonic has a similar magnetic and gravity signature as 36. East-northeast–trending terrane of mag- and metavolcanic units (Camp Hill Gneiss, feature 32A. netic and gravity highs (amplitudes up to Rock Mills Granite Gneiss, Chattasofka Creek 31. Main Towaliga fault, which is younger 1400 nT and ~+30 mGal, respectively) lying Gneiss, and Waverly Gneiss; Bentley and Neat- than and has excised plastic mylonites and south of domain 35 (Uchee terrane) and south hery, 1970; Osborne et al., 1988; Raymond ultra mylonites of the Towaliga fault zone (Sears of the suture (domains 12 and 13). These anom- et al., 1988; Neilson and Bittner, 1990; Neilson et al., 1981; Steltenpohl, 1992). The main Towa- alies likely correspond to mafi c and ultramafi c et al., 1997; Steltenpohl et al., 1990; Drummond liga fault marks the boundary between the Ope- complexes. They appear in and adjacent to the et al., 1997). The overall pattern is consistent lika Complex (Inner Piedmont) and the Pine suture zone and thus could refl ect remnant oce- with the Dadeville Complex as a gently north- Mountain basement-cover massif (Sears et al., anic lithosphere from between the peri-Gond- east-plunging, recumbent sheath fold, similar 1981; Steltenpohl, 1988). wanan arcs (Uchee terrane) and Gondwanan to those mapped in the Inner Piedmont of the 31A. Another splay of the Towaliga fault. crust of the Suwannee terrane. Alternatively, Carolinas (Merschat et al., 2005; Hatcher and 32. Pine Mountain window. they may represent substantial accumulations Merschat, 2006). 32A. Pine Mountain Group cover sequence, of basaltic lava associated with the Mesozoic 25A. Stonewall line fault zone (red line traced Halawaka Schist, and mylonites and ultra- South Georgia Basin. from Alabama state geologic map). This fault mylonites, characterized by moderate-fre- zone separates mainly meta-igneous rocks of quency, elliptical magnetic highs. DISCUSSION AND DIRECTIONS FOR the Dadeville Complex from mainly metasedi- 32B. Mesoproterozoic granitic gneiss base- FUTURE STUDIES mentary units of the Opelika Complex (Inner ment rocks of Pine Mountain window, char- Piedmont; Steltenpohl et al., 1990). Although acterized by low-frequency, broad, fl at, only Based on decades of surface geological stud- there is no correlation of the Stonewall line with weakly elongate magnetic lows (a magnetic ies, correlations of buried rocks with other- the magnetic anomaly map, the fault trace is dis- signature grossly similar to that of the Paleozoic wise known features outside of Alabama, and tinct from linear trends on the radioactivity map granitoids of the eastern Blue Ridge, domains drill-core data, the crust underlying Alabama (not shown; Neathery et al., 1977b). 21 and 23B). Some Halawaka Schist occurs is recognized as belonging to three fundamen- 25B. Magnetic lineament internal to Opelika within this domain. tal types: 1) Laurentian, 2) peri-Gondwanan, Complex that may separate Farmville Meta- 32C. High-frequency magnetic-high domain and 3) Gondwanan (Neathery and Thomas, granite plutonic bodies from Auburn Gneiss beneath the Coastal Plain that is similar to Pine 1975; Hatcher, 1978, 1987, 2010; Tull, 1982; metasedimentary rocks. Mountain Group cover of domain 32A. Chowns and Williams, 1983; Horton et al., 26. Domain of high-frequency magnetic 32D. Broad fl at anomalies buried deeply 1984; Tull, 1978, 1980, 1982, 1984; Thomas, anomalies corresponding to Emuckfaw Forma- beneath the Coastal Plain that are similar to 1989; Thomas et al., 1989; Guthrie and Ray- tion metasedimentary rocks and Zana Gran- domain 32B, but with domain 32C in between mond, 1992; Drummond et al., 1994, 1997; ite. The magnetic character is similar to that of them, interpreted as possible Mesoproterozoic Steltenpohl, 2005; McClellan et al., 2005, 2007; domain 25 (Inner Piedmont, Dadeville Com- basement of the Pine Mountain window. Steltenpohl et al., 2008, 2010a). Our results plex). There is a substantial area of broader, low- 33. Bartletts Ferry fault zone (Bentley and from comparing regional geophysical data with frequency positive magnetic anomalies that may Neathery, 1970). This feature is delineated on the surface and subsurface geology indicate represent metasedimentary rocks or metaplutons the magnetic map by contrasting broad fl at clear demarcation of Laurentian crust in roughly as in domain 25; high-frequency parts resemble magnetic lows of the Pine Mountain basement the northwestern half of Alabama, and peri- amphibolite-rich areas of the Dadeville Complex granites to the northwest and the strong, high- Gondwanan or Gondwanan crust underlying the in domain 25. Although the Emuckfaw Forma- frequency magnetic highs of mylonites and southeastern quarter. The remaining east-cen- tion is known to locally contain minor, thin mylonitized amphibolites and dioritic gneisses tral, triangle-shaped area underlain by Piedmont amphibolite layers (Bentley and Neathery, 1970; of the Uchee terrane to the southeast (domain rocks is distinct and contains the Appalachian Raymond et al., 1988), the aeromagnetic signa- 35, below). The Uchee terrane has a peri-Gond- rocks and structures responsible for the colli- ture of domain 26 suggests that this area contains wanan or Gondwanan origin, requiring that the sion and suturing of these fundamental crustal more-substantial mafi c and perhaps even ultra- northwest limit of Bartletts Ferry fault zone blocks, which we refer to as the Appalachian mafi c material that is not depicted on the state must also be the Central Piedmont suture or a deformed domain (ADD). geologic map (Osborne et al., 1988). The north- later structure that has overprinted the suture The Grenville front (Ciesielski, 1991), as ernmost parts of domain 26 are exposed north (Steltenpohl et al., 2008). defi ned on the regional magnetic or gravity of the Coastal Plain onlap, and fi eld checking in 34. Sharp magnetic gradients that are strands anomaly maps and by samples recovered from that area is needed to examine the source of this of mylonite and ultramylonite within the Bartletts drilling outside of Alabama (Society of Explo- distinctive magnetic character. Ferry–Goat Rock fault zones that interlace the ration Geophysicists, 1982), and how, or if, it 27. East-northeast–trending linear anomalies Uchee terrane. The numerous fault strands south is expressed in the subsurface of Alabama have marking an unnamed splay of the Towaliga fault of the Bartletts Ferry fault zone defi ne patterns been the subject of debate (see Steltenpohl et al., (Sears et al., 1981; Steltenpohl, 1992). typical of a strike-slip duplex (Steltenpohl, 1988). 2010b, and references therein). It is inferred to 28. Block (beneath coastal plain) between 34A. Goat Rock fault zone based on the geo- connect northward with the Grenville front of two splays of the Towaliga fault (i.e., features logic map. Note that the magnetic signature is Ontario, Canada, and the adjacent northern U.S.

1058 Geosphere, August 2013

(including the southeast-dipping COCORP seis- Traversing toward the southeast, the fi rst, weak and rheologies of the fault rocks was needed. mic refl ector interpreted as the Grenville front in but fi ne-scale Appalachian magnetic signatures Our alternative extension of the Goodwater- western Ohio; Pratt et al., 1989) and southward appear with the Hollins line fault (feature 16 in Enitachopco fault in the area of the Hightower with the Grenville front directly north of the Figs. 3, 4, and 5). The macro-scale ductile right- reentrant suggested in Figure 3 (feature 19B) Llano uplift in Texas (Mosher, 1993; Mosher slip offset (i.e., feature 16A) of linear aeromag- does not follow any geological contact shown et al., 2004). netic signatures of domains 8 and 17 apparently on the state geologic map. The segment of the Steltenpohl et al. (2010b) presented a geologi- merges with or cuts the Hollins Line fault beneath Goodwater-Enitachopco fault shown on the cal and geophysical analysis of the NY-AL linea- the Coastal Plain cover (Figs. 3, 4, and 5). This state geologic map in this area (i.e., the most ment for the segment in Tennessee, Georgia, and could explain the macro-scale ductile thinning of northeastern segment of feature 16 in Fig. 3) Alabama, and suggested that it is a continent- the Hillabee Greenstone depicted in this general is questionable because it crosses multiple scale right-slip fault that has displaced the Gren- area on the Alabama state geologic map (Osborne short-wavelength magnetic and radiometric ville front and other related structures. In their et al., 1988). Other faults depicted as thrusts in lineaments. Mies (1991) reported detailed map interpretation, the N15°E-trending magnetic low the foreland on the state geologic map, as well as observations from the Hightower area that labeled 4C in Figure 3 roughly corresponds to the lithologic contacts within both the Talladega slate appear most compatible with the geophysics. severed and displaced Grenville front; the sub- belt and eastern Blue Ridge, similarly swing from Field investigations are needed to resolve incon- parallel low directly to the east, combined fea- northeast strike to northwest strike mimicking the sistencies between the geophysical anomalies tures 9 and 17B, was similarly interpreted to be strike of this hypothetical transcurrent shear zone and existing geologic maps showing the fault, the southern continuation of the Amish anomaly, (Figs. 2 and 5). The Shady Grove fault within the as well as possible extensions of the fault. a belt of relatively non-magnetic metasedimen- Millerville reentrant, as depicted in Neathery and The aeromagnetic character of the Elka- tary rocks bored in West Virginia and thought to Reynolds (1975, their plate 5), has a similar ori- hatchee Quartz Diorite (Fig. 2) and its imme- represent an intra-Grenville suture zone (Culotta entation, geometry, and sense of offset as linear diate country rocks (feature 21 in Figs. 3 and et al., 1990; King et al., 1998). The NY-AL lin- feature 16A. Further, the position of lineament 4) can be confi dently traced to the Suwannee- eament is clearly expressed in Alabama (linea- 16A fi ts within the somewhat regular, ~70–80 km Wiggins suture, together with the Alexander ment 3 in Fig. 3) with its characteristically sharp wavelength spacing of major orogenic reentrants City fault zone that borders it to the southeast demarcation between mottled aeromagnetic in the southernmost Appalachian Blue Ridge, (feature 22, Figs. 3 and 4). The Alexander City textures of the Mid-Continent granite-rhyolite that is, from southwest to northeast, at feature fault zone presents the most intense and con- province to the northwest (domains 1 and 2) 16A, Millerville, Hightower, and Cartersville tinuous high-amplitude/short-wavelength shear and N15°E-trending linear pattern (domains (Figs. 1, 2, and 5). The origin of these reentrants zone anomaly on the entire map (Fig. 3). This 4B, 9, and 17) to the southeast (Steltenpohl remains unresolved. is surprising because the Alexander City fault et al., 2010b) as well as its gravity signature The Goodwater-Enitachopco fault (Fig. 2; zone is restricted to the eastern Blue Ridge, and (Zietz, 1982). Neathery and Reynolds, 1975) is an unusual and it is generally not considered a “fundamental” Both the aeromagnetic and gravity data for the somewhat perplexing structure and our interpre- southern Appalachian fault zone. The geom- Alabama Piedmont contain higher-amplitude and tation of the aeromagnetic data in this area pro- etry and kinematics of the Alexander City fault shorter-wavelength, northeast-southwest–linear, vides more questions than answers. In surface zone has not been examined in detail and move- Appalachian trends that are sharply truncated at exposures, it is an important post–peak-meta- ment sense has been suggested as reverse slip the Suwannee-Wiggins suture. To the northwest, morphic shear zone that has excised the Hollins (Bentley and Neathery, 1970; Neathery and geophysical trends of the ADD gradually dimin- line thrust to form isolated structural salients Reynolds, 1973; Osborne et al., 1988), right slip ish in strength in accordance with disappearing of the Higgins Ferry and Mad Indian Groups (Guthrie, 1995), and normal slip (Drummond, Appalachian deformation observed in rocks at in the eastern Blue Ridge (Tull et al., 1985; 1986; Drummond et al. 1994, 1997). Our obser- the surface. This northwestward broadening of Mies, 1991; Tull, 1995). Farther northeast into vations are that it dips steeply to the southeast aeromagnetic and gravity anomaly wavelengths Georgia most geologic maps depict the fault to and exhibits oblique-normal but predominantly also is attributed to some dampening effects of be the Allatoona fault (Hurst, 1973; Tull, 1978; dextral shear, and it is cut by later (Mesozoic?) the great thickness, up to 23,000 ft (6,900 m) Hatcher et al., 1990; Tull and Holm, 2005). The normal-slip faults. Perhaps the steep attitude of thick estimated in parts of the Appalachian fore- Goodwater-Enitachopco/Allatoona fault has the zone contributes to the sharp aeromagnetic land (Thomas, 1982; Neathery and Thomas, been interpreted as a southeast-dipping reverse lineament but more work needs to be done on 1983), and largely subhorizontal Paleozoic cover. (thrust) fault (Neathery and Tull, 1975; Pick- this potentially important feature. Short-wavelength linear aeromagnetic trends of ering, 1976; Tull, 1978, 1984; McConnell and One of the more prominent aeromagnetic the ADD correspond to moderately to steeply Costello, 1980, 1982; Crawford and Cressler, features of the ADD is the anomalous, curving dipping amphibolites (including the Ropes 1982; McConnell and Abrams, 1984; Osborne “hook” shape of domain 23B (Fig. 3), which has Creek Amphibolite and various amphibolites in et al., 1988; McClellan et al., 2005; Tull and important implications for delimiting the geom- the eastern Blue Ridge) and strands of similarly Holm, 2005). Tull et al. (1985), on the other etry of the most southern surface exposures of oriented mylonite zones. Broader-wavelength hand, interpreted the Goodwater-Enitachopco the Appalachians. Domain 23B corresponds to domains correspond to granitic gneisses, espe- to be a normal fault, reasoning that a thrust partially exposed units that frame the closure cially large Paleozoic intrusives in the eastern fault cutting the basal eastern Blue Ridge thrust of the regional, gently northeast-plunging Tal- Blue Ridge (i.e., Elkahatchee, Kowaliga, and (i.e., the Hollins line) should emplace Talladega lassee synform (Fig. 2; Bentley and Neathery, Zana) and Grenville basement gneisses in the slate belt rocks in the hanging wall. Steltenpohl 1970; Keefer, 1992; Sterling, 2006; White, Pine Mountain window. The Talladega fault (2005) reported oblique normal-and-right-slip 2007). Major Appalachian fault zones, the Bre- emplaced crystalline Blue Ridge rocks upon the movement indicators in several exposures of the vard fault zone and the Towaliga fault, mark foreland metasedimentary strata, but it is trans- Goodwater-Enitachopco fault and noted that a the west and east limbs, respectively, and tra- parent in the magnetic and gravity maps. systematic analysis of the geometry, kinematics, ditionally, units in the core have been assigned

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to the Inner Piedmont (Bentley and Neathery, There is a clear aeromagnetic distinction lina state line to ~80 km along the Alabama- 1970; Raymond et al., 1988; Steltenpohl et al., between the broad, ﬂ at magnetic lows of the Georgia state line, and all Appalachian com- 1990). The hinge area of the Tallassee synform Laurentian crust in the Pine Mountain basement ponents appear to be completely excised only is covered by Coastal Plain sediments, making (domains 32A–32D) and the strongly linear, 100 km farther southwest along the Suwannee- it impossible to conﬁ dently connect the two high-frequency magnetic highs of mylonitized Wiggins suture. We interpret the 285 Ma age limbs by surface mapping alone (Keefer, 1992; amphibolites and dioritic gneisses of the peri- established for the Goat Rock–Bartletts Ferry Steltenpohl, 2005; White, 2007). Quartzites Gondwanan Uchee terrane to the southeast fault zone (Steltenpohl et al., 1992) to, there- of the Opelika Complex exposed on the east (domain 35; Figs. 1–5). Surface exposures of fore, place a maximum on the age of this suture. limb of the synform are similar to quartzites of the Bartletts Ferry fault zone correspond to the Jacksons Gap Group along the west limb this geophysical break (domains 33–34A) that CONCLUSIONS (Fig. 2; Steltenpohl et al., 2005a). Along both is interpreted as the southern continuation of limbs, the quartzites are structurally interleaved the Central Piedmont suture (Fig. 1) between The map of digitally processed aeromagnetic with garnet-kyanite schist, graphite schist, and the ancient Laurentian margin and the peri- data from Alabama (Fig. 3) paints a clearer pic- felsic gneiss. The Jacksons Gap Group on the Gondwanan arc terranes (Hibbard et al., 2002, ture of the anomalies caused by rocks exposed west limb is in fault contact with the overly- 2007; Steltenpohl et al., 2006, 2008). The high- at the surface and those buried beneath younger ing Dadeville Complex (Inner Piedmont) and frequency magnetic highs corresponding to sedimentary cover and Appalachian thrust the underlying Emuckfaw Group of the eastern the peri-Gondwanan rocks appear to continue sheets. Combining the aeromagnetic map with Blue Ridge. Along the east limb, meta-arkosic southward to the Suwannee-Wiggins terrane gravity data (Fig. 4) and, to a lesser extent, schist and gneiss of the Opelika Complex (Inner suture (Steltenpohl et al., 2010a). limited radioactivity data (not shown) provides Piedmont) are lithologically similar to those of The aeromagnetic and gravity signatures for more comprehensive information on the crust the Emuckfaw Group, and both packages are crust of the Laurentian margin and the ADD underlying Alabama than does their separate intruded by abundant and similar-appearing are sharply truncated by lineament 12 along analysis. granitoid plutons (Steltenpohl et al., 2005a). the northern margin of the east-west–trend- In terms of Wilson cycles recorded in the crust Retrogressive, greenschist-facies, right-slip, ing Altamaha magnetic low that is associated and lithosphere of Alabama, this work has elu- mylonites in the Brevard fault zone formed with the Suwannee-Wiggins suture (Higgins cidated several buried tectonic elements that are

during the second deformational event (D2), and Zietz, 1983). Aeromagnetic anomalies of not exposed. The Grenville front records the ﬁ nal and they do not accompany the Jacksons Gap the Gondwanan Suwannee terrane show dis- phase of assembly of the supercontinent Rodinia, Group around the hinge of the Tallassee syn- tinct, parallel, arcuate, ~N45°E trends on Fig- and we interpret it to be related to the N15°E- form (Steltenpohl et al., 2005a; Sterling, 2006). ure 3. These trends may reﬂ ect either lithologic trending magnetic low in northwest Alabama

Near Jacksons Gap (Fig. 2), D2 shear zones structure within the Suwannee terrane or trends (feature 4C in Fig. 3) that is coupled with another and fabrics appear to splay southwestward out resulting from failed rifting and extension of low to the east (feature 9 and associated lows) from the Jacksons Gap Group lithologic pack- the terrane during early Mesozoic time. Along considered to be the southern continuation of the age merging with the Alexander City fault. Peak the southern boundary of the ADD, there is a Amish anomaly, an intra-Grenville suture zone. amphibolite-facies (kyanite zone) metamorphic gradual, though distinct, west-to-east decrease The NY-AL lineament is clearly expressed with and annealed mylonitic fabrics formed during in the angle of discordance such that south of its characteristically sharp demarcation between

the ﬁ rst deformational event (D1), and they are domain 35 (Fig. 3) the high-frequency magnetic mottled aeromagnetic textures of the Mid-Con- exposed in the hinge area and along the east limb highs of the peri-Gondwanan rocks parallel the tinent granite-rhyolite province to the northwest (Grimes, 1993; Grimes et al., 1993). The earlier- Altamaha anomaly. Given the high concentra- and the distinct N15°E-trending lineaments to

formed D1 structures of the Brevard fault zone tion of major mylonite zones exposed directly the southeast. As described in Steltenpohl et al. on the west limb might, therefore, reemerge as along the Coastal Plain onlap in this area, and (2010b), the NY-AL lineament appears to be a the Stonewall line shear zone on the east limb. their correspondence to these high-frequency right-slip fault that has displaced the Grenville The aeromagnetic signature of domain 23B magnetic highs, we interpret this asymptotic “front” and the Amish anomalies, which explains thus is compatible with these surface relations, approach of linear magnetic anomalies to the the earlier diffi culty with recognizing Grenville supporting the suggestion of Steltenpohl et al. Suwannee-Wiggins suture to refl ect progres- structures south of Tennessee. Right-slip move- (2005a) that the Opelika Complex should be sive mylonitization and rotation into the suture ment along the NY-AL lineament likely initiated reassigned to the eastern Blue Ridge, and that zone. This interpretation is supported by aero- during the fi nal stages of assembly of Rodinia, the Dadeville Complex is a very thin allochtho- magnetic maps of the southeastern U.S. (Zietz and it might have been reactivated during the nous “scoop-shaped” klippe emplaced during and Gilbert, 1980; Steltenpohl et al., 2010b) that rifting of Rodinia, during Appalachian dextral an early stage of development of the Brevard indicate an intense, interlaced system of mag- shearing, and/or under the active stress fi eld of fault zone. Future fi eld studies in this area will netic highs (mylonite zones?) beneath Atlantic the eastern U.S. (Steltenpohl et al., 2010b). help to further constrain the tectonostratigraphy Coastal Plain deposits that projects into this part With regard to Appalachian consolidation and structural confi guration of the southernmost of Alabama. Aeromagnetic lineaments tracing of Pangaea, aeromagnetic lineaments defi ned Brevard fault zone, and mapping in Georgia is this anastomosing network of mylonite zones by exposed Piedmont units and their structures needed to delimit the extent and signifi cance of clearly end at the Suwannee-Wiggins suture can be traced with remarkable clarity for large this Blue Ridge window beneath the Dadeville where lineament 33 (i.e., our projected trace for distances (>145 km) and depths (>6000 m) Complex that has been referred to as the Ope- the Bartletts Ferry fault zone) is truncated by beneath the Coastal Plain cover, to their trun- lika belt (Hibbard et al., 2006) and the Dog lineament 12 roughly 80 km south of the onlap cation by the Suwannee-Wiggins suture, as if River window (Fig. 1; Higgins and Crawford, boundary (Fig. 3). The aggregate width of peri- the Coastal Plain sediments were transparent. 2007; Hatcher, 2010; Steltenpohl et al., 2010a, Gondwanan terranes narrows drastically from Despite the correlations of geology with our 2011; Higgins et al., 2011). ~450 km along the South Carolina–North Caro- magnetic map, there is little correlation between

1060 Geosphere, August 2013

gravity features in the exposed portions of the ACKNOWLEDGMENTS Crawford, T.J., and Cressler, C.W., 1982, Talladega “Series,” crystalline Appalachians and mapped geol- Great Smoky fault, and Emerson fault: Relationships in We greatly appreciate careful reviews of an ear- the Cartersville area, Georgia, in Bearce, D.N., Black, ogy. One explanation for this might be that the lier draft by Tony Neathery and Steve Snyder, which W.W., Kish, S.A., and Tull, J.F., eds., Tectonic Stud- exposed crystalline southern Appalachians are resulted in signifi cant improvements. We also thank ies in the Talladega and Carolina Slate Belts, Southern Appalachian Orogen: Geological Society of America allochthonous, and reside in the thin, northwest- three anonymous reviewers, reviewer Jack Pashin, and Special Paper 191, p. 31–34. directed Blue Ridge–Piedmont megathrust the Geosphere editors for their thoughtful comments Crickmay, G.W., 1933, The occurrence of mylonites in the sheet (Hatcher and Zietz, 1980). The high-fre- that greatly improved our manuscript. crystalline rocks of Georgia: American Journal of Sci- ence, v. 26, p. 161–177, doi:10.2475/ajs.s5-26.152.161. quency magnetic highs and sheer linear extent REFERENCES CITED Crickmay, G.W., 1952, Geology of the crystalline rocks of the Alexander City fault suggests that it is a of Georgia: Georgia Geological Survey Bulletin 46, fundamental Appalachian shear zone of unde- Adams, G.I., 1933, General geology of the crystalline rocks p. 32–36. of Alabama: The Journal of Geology, v. 41, p. 159– Culotta, R.C., Pratt, T., and Oliver, J., 1990, A tale of two termined signifi cance. 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