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Geological Society of America Memoirs

The Appalachian orogen: A brief summary

Robert D. Hatcher, Jr.

Geological Society of America Memoirs 2010;206;1-19 doi: 10.1130/2010.1206(01)

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Notes

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The Geological Society of America Memoir 206 2010

The Appalachian orogen: A brief summary

Robert D. Hatcher Jr. Department of Earth and Planetary Sciences and Science Alliance Center of Excellence, University of Tennessee, Knoxville, Tennessee 37996-1410, USA

ABSTRACT

The Appalachians are a orogen that formed in a complete Wilson cycle along the eastern Laurentian margin following the breakup of supercon- tinent and the coalescence of all of the continents to form supercontinent Pangea. The Appalachian Wilson cycle began by formation of a Neoproterozoic to early Paleozoic rifted margin and platform succession on the southeastern margin of . Three ultimately produced the mountain chain: the Ordovi- cian Taconic , which involved arc ; the Acadian–Neoacadian orog- eny, which involved north-to-south, transpressional, zippered, Late –early Mis sis sip pian colli sion of the Carolina superterrane in the southern-central Appala- chians and the Avalon-Gander superterrane in the New England Appalachians, and colli sion in the Maritime Appalachians and Newfoundland; and the Allegha- nian orogeny, which involved late Mississippian to collision of all previously formed Appalachian components with to form supercontinent Pangea . The Alleghanian also involved zippered, north-to-south, transpressional, then head-on collision. All orogenies were diachronous. Similar time-correlative orogenies affected western and central (Variscan events), eastern Europe and west- ern (Uralian events), and southern Britain and Ireland; only the Caledonide (Grampian–Finnmarkian ; Caledonian–Scandian) events affected the rest of Britain and the Scandinavian Caledonides. These different events, coupled with the irregular rifted margin of Laurentia, produced an orogen that contains numerous contrasts and nonthroughgoing elements, but it also contains elements, such as the platform margin and peri-Gondwanan elements, that are recognizable throughout the orogen.

INTRODUCTION and from there widen both to the north and south (Fig. 1). This narrowing attribute is not related to lack of exposure because of the The Appalachian Mountain chain extends from the conti- Coastal Plain overlap, but is a crustal property as well, as indicated nental margin off Newfoundland some 3000 km (2000 mi) south- by aeromagnetic and gravity data (see Hatcher et al., 2007a, their westward to the subsurface beneath the Coastal Plain of South Fig. 1B). Interestingly, the overall deformational patterns change Alabama and Georgia (Fig. 1). The chain was named by the Span- north and south of this narrow segment—the youngest deforma- ish in the 1500s for a Native American tribe, the Apalachis, who tion is to the south on the eastern and western exposed fl anks and lived far south of the exposed mountains in southern Georgia and the oldest is in the interior, whereas the northern segment contains northern Florida (Rodgers, 1970). The Appalachians reach their older deformation along the western margin and younger deforma- narrowest point in the area immediately west of New York City, tion along the exposed eastern margin (Hatcher and Odom, 1980).

Hatcher, R.D., Jr., 2010, The Appalachian orogen: A brief summary, in Tollo, R.P., Bartholomew, M.J., Hibbard, J.P., and Karabinos, P.M., eds., From Rodinia to Pangea: The Lithotectonic Record of the Appalachian Region: Geological Society of America Memoir 206, p. 1–19, doi: 10.1130/2010.1206(01). For permission to copy, contact [email protected]. ©2010 The Geological Society of America. All rights reserved.

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2 Hatcher

The Appalachians are an accretionary orogen (Fig. 1) that of Williams and Hatcher (1983); the Taconian clastic was constructed during the Paleozoic on the eastern margin of wedge; the Acadian–Neoacadian clastic wedge; and the peri– Laurentia via a complete, albeit complex, Wilson cycle. Its be- Gondwanan elements, consisting of the Carolina–Gander(?) ginnings followed the breakup of supercontinent Rodinia, and superterrane of the southern and central Appalachians, and the construction was completed with the formation of superconti- Avalon and Gander superterranes of the northern Appalachians. nent Pangea. It, and the other Paleozoic orogens of Europe, may More recently, van Staal et al. (1998) and Hibbard et al. (2006a, be unique among orogenic belts in that three major accretion- 2007a, 2007b) have attempted to track several major boundaries ary events were involved with their formation: the from Newfoundland at least to southern New England. The Taconian , Devonian–Mississippian Acadian–Neoacadian, and Baie Verte–Brompton line was recognized as a major boundary the Pennsylvanian–Permian Alleghanian orogenies, and their many years ago that separates rifted margin Laurentian sedi- equivalent events in Europe. The Silurian Salinic orogeny has mentary and volcanic rocks from distal Laurentian equivalents been recognized in the northern Appalachians (van Staal et al., (albeit diachronous, younging to the east). This boundary prob- 2004, 2008), and may also have affected part of the central Appa- ably extends southward throughout the orogen (Williams and lachians (Wilson, 2001; Sinha et al., 2010). Roughly coeval early, Hatcher, 1983). The Red Indian line separates the Dunnage mid-, and late Paleozoic events are similarly recognizable in the zone from the Gander superterrane (or Exploits subzone along Variscan orogen in Iberia and western-central Europe, and in the Dog Bay line) in Newfoundland (Williams, 1979) (Fig. 1). the Urals, but all of these events, like those affecting the Appala- Van Staal et al. (1998) and Hibbard et al. (2006a, 2007a, 2007b) chians, are diachronous (Shanmugam and Lash, 1982; Pushkov , extrapolated the Red Indian line as far south as southern New 1997; Matte, 2002; Martínez-Catalán et al., 2007). The Caledo- England, justifi ably, because Gander zone rocks have now been nides of Britain and Scandinavia record only two major accre- identifi ed isotopically in Connecticut (e.g., Aleinikoff et al., tionary events, the Late –Early Ordovician Grampian– 2007) despite the Red Indian line being covered by Silurian– Finnmarkian orogeny and the Late Ordovician–Silurian Devonian sediments throughout much of New Brunswick and Caledonian– (e.g., Andréasson et al., 1998; New England. The Dover fault has long been recognized as the Roberts et al., 2007). This history contrasts with the eoalpine, meso- boundary between the Gander and Avalon superterranes in New- alpine, and neoalpine events that affected the Alps, which were not foundland. It is recognized in southern New England among all major accretionary events (Trümpy, 1973; Laubscher, 1988). several Alleghanian faults that juxtapose Avalonian and Gan- While the Paleozoic Antler and late Paleozoic–early Mesozoic derian rocks in southern New England (Bothner and Hussey, 1999; Golconda orogenies were restricted to parts of the North Ameri- Goldstein and Hepburn, 1999). Farther south, the mid-Paleozoic can Cordillera, the mid-Mesozoic accretionary events preceded the Central Piedmont suture, which again was locally reactivated Late Cretaceous–Eocene Laramide orogeny, and the modern mar- during the Alleghanian, separates peri-Gondwanan of gin remains active (Burchfi el et al., 1992; Speed, 1994). the Carolina-Gander(?) superterrane from all Laurentian ter- The purpose of this paper is to present a brief summary of ranes (and Cat Square terrane) to the west (Figs. 1 and 2). These the developmental history of the Appalachian orogen supported large terrane boundaries have been either overprinted or partially by existing data. This is not intended to be an exhaustive review, reactivated by younger faults throughout the Appalachian oro- but more a discussion of the history in the context of our most gen; nevertheless, these boundaries remain sutures. up-to-date critical data sets. These data include stratigraphic, geochronologic (including detrital zircon, metamorphic, and Laurentian Margin pluton ages), geophysical (potential fi eld and seismic refl ection), The Laurentian margin formed with the breakup of super- and structural data. continent Rodinia at ~750 Ma on both fl anks of North America (Cawood and Nemchin, 2001; Cawood et al., 2001; Karlstrom APPALACHIAN COMPONENTS et al., 2001; Tollo et al., 2004a; Whitmeyer and Karlstrom, 2007). Rifting formed an irregular margin consisting of a series of Some components of the Appalachians have been recog- promontories (Alabama, New York, St. Lawrence) and embay- nized throughout the orogen, whereas others are restricted to ments (Tennessee, Pennsylvania, St. Lawrence, Newfoundland) smaller parts of the orogen. These include the Laurentian margin (Thomas, 2006), with the promontories receiving little or no and tectonostratigraphic terranes (and superterranes) that were sedi ment during the Neoproterozoic, and the embayments receiv- accreted at different times during the Paleozoic. ing sediment thicknesses ranging from a few to 15 km (Fig. 3). Failed rifting began ~735 Ma in the southern and central Appa- Throughgoing Elements lachians (Aleinikoff et al., 1995), and at about the same time in the northern Appalachians (Cawood et al., 2001), producing an The principal elements of the Appalachians that can be traced alkalic A-type plutonic-volcanic suite (e.g., Tollo et al., 2004b). throughout the orogen include the Laurentian Neoproterozoic– Successful rifting began ~565 Ma (Aleinikoff et al., 1995), with Early Cambrian margin and Cambrian–Ordovician platform, more deep-water distal facies of impure clastic sediments and and more internal equivalents mostly comprising the “Piedmont” volcanic rocks being deposited farther offshore, some on ocean Downloaded from memoirs.gsapubs.org on October 24, 2014

The Appalachian orogen 3

Figure 2. Block diagram in parts of Geor- gia and South Carolina depicting the suture possible geometry of the Late Devonian– Brindle KMSZ early Mississippian Central Piedmont Creek suture. This structure formed initially by thrust Inner Piedmont Whitmire reentrant southwest-directed A- of ter- sheet Piedmont ranes to the west beneath the overriding Carolina(–Gander?) superterrane, but SC Central sheet suitably oriented segments in the Caro- GA thrust linas and Georgia were locally re- Carolina superterrane activated by northwest-directed low- Abbeville suture angle thrusting. This interpretation is partially based on the location of the Piedmont magnetic anomaly (dashed black line east of the trace of the suture) that marks the suture, producing contrast- T Central ing signatures west and east of the A suture: from longer wavelength, low- frequency anomalies to the west to short- wave length , high-frequency anomalies to the east (Hatcher and Zietz, 1980). This contrast suggests that the dip of the suture becomes much steeper im- mediately southeast of the dashed line, and may have fl attened along a ductile-brittle transition deep in the crust. KMSZ—Kings Mountain shear zone; A—movement away from observer; T—movement toward observer.

crust, as the opened. The rift-to-drift transition oc- west (e.g., Rankin et al., 1989). From Late Cambrian to earli- curred along the entire margin during the Early Cambrian with est Middle Ordovician time, carbonate deposition transgressed development of the fi rst successful carbonate bank, indicating the across the entire margin into the central interior of North Amer- margin had formed and was facing the open Iapetus ocean (e.g., ica, extending westward as far as present-day New Mexico, and Williams and Hiscott, 1987; Hatcher et al., 2007a). northward into southern and eastern Canada as the younger part A relatively clean Lower Cambrian clastic sequence was of the Sauk sequence (Sloss, 1963, 1988). This formed the fi rst deposited on the Laurentian margin of Iapetus during the rift- extensive carbonate platform across interior Laurentia. Coeval to-drift transition, and it consists of upward-maturing alternating carbonate platform deposits accumulated in parts of northwest- sandstones and Chilhowee Group shales. The lowest unit usu- ern Africa (Morocco), and possibly elsewhere (Siberia, Estonia), ally consists of immature arkosic sandstone and greywacke, and followed by withdrawal of the seas from several continents, some interlayered basalt in northeastern Tennessee and south- forming the extensive Middle Ordovician unconformity, and western Virginia. Farther north, equivalent Lower Cambrian suggesting that this large-scale inundation of and simultaneous sequences (e.g., Cheshire Quartzite in Vermont) were deposited withdrawal from Laurentia, and parts of other continents, were on Neoproterozoic rifted margin succession sediments and vol- likely the products of lithospheric rather than local processes canics (Pinnacle and Underhill Formations). In many places along (Hatcher and Repetski, 2007). the margin, only the upper, cleaner sandstone and shale units Detrital zircon suites from the rifted margin succession were deposited on basement, particularly on the promontories. (Ocoee Supergroup, Ashe Formation and equivalents) across the The upper part of this sequence was probably deposited along southern Appalachian Blue Ridge and western Inner Piedmont the open ocean margin, because it locally becomes calcareous (Tugaloo terrane) are all dominated by ~1.1 Ga zircons, along shale and then grades upward into the fi rst successful carbonate with smaller components of 1.5–1.3 Ga zircons (Bream et al., bank assemblage. Both the clastic and the overlying carbonate 2004). Dahlonega gold belt detrital zircons, while also dominated succession (Shady–Tomstown–Dunham–Forteau Dolomite) are by 1.4–1.1 Ga components, contain 2.9–2.7 Ga, 2.2–1.9 Ga, and continuous throughout the Appalachian margin, but they thin 700–650 Ma zircons. These rocks have traditionally been consid- markedly onto the Alabama, New York, and St. Lawrence prom- ered a more distal facies of the Ocoee Supergroup (e.g., Hadley, ontories (Rodgers, 1970). 1970), but 2.2–1.9 Ga detrital zircons suggest that they may be Once the fi rst carbonate bank was established, the eastern more exotic, especially since much of the Grenvillian source ter- Laurentian margin continued to develop with pulses of progres- rane is Amazonian based on Pb isotopic signatures (Sinha and sively fi ner clastic sediment derived from the eroding Grenville McLelland, 1999; Loewy et al., 2003; Tohver et al., 2004), al- mountains. This is clearly refl ected in Cambrian facies, which though they could equally have been derived from the Lauren- are more carbonate-rich to the east and more clastic-rich to the tian Penokean orogen. Detrital zircon suites collected from the Downloaded from memoirs.gsapubs.org on October 24, 2014

4 Hatcher

Smith River allochthon in Virginia and North Carolina (Fig. 1) 1989; Thomas, 2001; Settles, 2002; Bream, 2003; McClellan reveal a Laurentian source (Carter et al., 2006; Merschat, 2009), et al., 2007). Similar detrital zircon suites have been collected by although a 532 Ma chemical age on monazite in this unit was Gray and Zeitler (1997), Cawood and Nemchin (2001), Cawood interpreted to indicate a peri-Gondwanan source of the alloch- et al. (2001), and Thomas et al. (2004) in different parts of the thon (Hibbard et al., 2003). In addition, the Dahlonega gold belt orogen, indicating the timing of rifting of Rodinia was very uni- contains 460 and 480 Ma volcanic arc complexes (Hopson et al., form along the entire orogen at ~750 Ma.

Oklahoma Southern aulocogen

Laurentia

MARATHON Rough Equator Creek EMBAYMENT EMBAYMENT Mississippi

OUACHITA PROMONTORY Valley

TEXAS PROMONTORY graben ALABAMA Rome

EMBAYMENT ST. LAWRENCE trough QUEBEC PROMONTORY Argentine transform Precordillera PENNSYLVANIA PROMONTORY PROMONTORY riftEMBAYMENT NEW YORK

VIRGINIA EMBAYMENTTENNESSEE

Figure 3. Possible late Neoproterozoic to Early Cambrian paleogeography Iapetus ocean showing possible relationships among Laurentia, the irregular Laurentian mar- gin, Gondwanan components, departure eri-G P ondwan of the Argentine Precordillera from an a Laurentia (Thomas and Astini, 1996), r and peri-Gondwanan arc terranes. (Lau- c s rentian margin confi guration in this and ( subsequent fi gures was modifi ed from Rio de A Thomas [2006]; Gondwanan components v La Plata a and equatorial position were modifi ed lo from Torsvik [2003].) The confi gura- n tions depicted in this and Figures 4, 6, ia n and 7 are consistent with available data. &

G

a

n

Amazonia d

e

r

i

a

n )

West Africa Downloaded from memoirs.gsapubs.org on October 24, 2014

The Appalachian orogen 5

The Tugaloo terrane in the southern Appalachians contin- orogeny forming the Taconic allochthons (Zen, 1967, 1972). ues northward into Virginia as the Milton terrane, then northward These allochthons have been recognized atop the platform from into the Chopawamsic, Potomac, Westminster, and Philadelphia southeastern New York to Newfoundland (Fig. 1). Ophiolites rest terranes. All of these terranes contain Ordovician arc plutons and on Taconic allochthons in the Bay of Islands region in western volcanic rocks and were part of the accretionary complexes ac- Newfoundland (Stevens, 1970; Williams, 1979). creted to Laurentia during the Ordovician–Early Silurian (Fig. 4). Carolina–Avalon–Gander superterranes. Coeval with the In New England, these assemblages form a narrow belt east of rifting of Rodinia, numerous peri-Gondwanan terranes de- the Whitcomb Summit thrust, are covered by Silurian–Devonian veloped in the intervening oceans separating Laurentia from metasedimentary rocks of the Connecticut Valley synclinorium, Gondwana (Fig. 3). All of these terranes developed proximal to and reappear in the Bronson Hill anticlinorium, but here they ap- Gondwana, formed composite superterranes, and were accreted pear to be intimately intermixed with possible Gander superter- to the Laurentian assemblage during the early to mid-Paleozoic rane rocks (Aleinikoff et al., 2007). Neoproterozoic, Cambrian, (e.g., Rast and Skehan, 1983; van Staal et al., 2007), with Bal- and Early to Middle Ordovician deep-water facies of Laurentian tica colliding with Laurentia during the Early to mid-Silurian provenance were thrust onto the platform during the Taconic (Torsvik et al., 1996). The peri-Gondwanan component located

Laurentia

Equator

Carbonate platform

Rifted margin Iapetus ocean

Taconian arcs Figure 4. Possible confi guration of Lau- rentia, Gondwana, peri-Gondwanan ter- ranes (“ribbon” microcontinents), Theic and Rheic oceans, and the soon-to-close Iapetus ocean during the early Middle Ordovician (late Arenig). A west-dipping Avalon– subduction zone may have existed along the entire Laurentian margin that was Carolina–Ganderia? subducted by the east-dipping subduc- Meguma tion during the main-phase Taconic Brunswick orogeny. (Charleston) ocean Theic

Gondwana Downloaded from memoirs.gsapubs.org on October 24, 2014

6 Hatcher in the southern and central Appalachians consists of the Carolina component of the superterrane. If not, it becomes a nonthrough- superterrane (or Carolinia) (Fig. 1), while in New England and going element. the Maritime Appalachians, the terms Avalon and Gander super- Most of the Carolina superterrane lies beneath the Atlantic terranes are used to describe these components. These superter- Coastal Plain and continental shelf (Fig. 1), so our knowledge of ranes also contain the basic components of most of the coeval this terrane is limited to surface exposure and a few drill holes. Panafrican terranes: dominance of 625–550 Ma volcanic arcs The aeromagnetic signature of the Carolina superterrane beneath and associated volcanogenic sediments, subarc plutonic com- the Coastal Plain cover, however, is similar to that of the exposed plexes, and rare basement massifs (except in the Mozambique component (see Hatcher et al., 2007a, their Figs. 1B and 1C), belt; see Kröner et al., 2007). Basement from which the arc ma- so its extent can be reasonably established with these data. The terials in the Carolina superterrane were derived appears to have aeromagnetic signature of the subsurface Brunswick (Charles- been evolved continental crust, based on inherited zircons at 2.5, ton) terrane contrasts with those of the peri-Gondwanan terranes, 2.1, and 1.0 Ga, and Sm-Nd whole-rock isotopic data (Samson, but it is cut by several faults that belong to the eastern Piedmont 1995; Ingle et al., 2003). The Greenhead Group in New Bruns- fault system, although the ~604 Ma concordant age date (of a wick is a Grenvillian basement assemblage that occurs beneath granite) determined by Mueller et al. (2005) suggests that at least Avalonian rocks; other small pre-Avalonian basement massifs part of this terrane is composed of peri-Gondwanan rocks. The occur in Cape Breton Island (Barr et al., 1998). northern extent of the subsurface Carolina superterrane beneath The Neoproterozoic component of the Carolina superterrane the Atlantic Coastal Plain in the central Appalachians is some- consists of a mafi c to felsic volcanic arc assemblage and associ- what speculative, but it is based on projecting similar character- ated volcaniclastic sedimentary rocks. Numerous plutons in the istic magnetic signatures northward from the exposed segments. range of 618–550 Ma were generated and intruded the underpin- If the interpretation of aeromagnetic data is correct, Carolina ning of the arc complex (Hibbard et al., 2002; Ingle et al., 2003). becomes very narrow in the central Appalachians but continues This assemblage was metamorphosed during the Cambrian, beneath the Coastal Plain from northern Virginia to offshore— because 530 Ma plutons cut upper amphibolite– to greenschist- then to onshore New England (Fig. 1). facies metamorphic rocks (Dennis and Wright, 1997; Barker et al., The Avalon superterrane in southeastern New England con- 1998). Neoproterozoic Ediacaran metazoan fossils have been sists of several assemblages of volcanic and volcaniclastic rocks identifi ed on bedding planes in low-grade felsic volcanic rocks that were intruded by Neoproterozoic plutons, and others as young near Durham, North Carolina (Cloud et al., 1976), and in the as Devonian; they are variously metamorphosed (Cameron and volcano-sedimentary sequence in the Albermarle area farther south Naylor, 1976). The thickest assemblage of low-grade to unmeta- in North Carolina (Hibbard et al., 2006a; Weaver et al., 2006). morphosed sedimentary rocks lies in the Boston basin , where a Unconformably overlying the arc complex in the Carolina sequence of clastic sediments occurs, including the Squantum til- superterrane is a sequence of Middle Cambrian and possibly lite. Most of the unmetamorphosed sedimentary rocks are thought Ordovician clastic sedimentary rocks. A Middle Cambrian Acado- to have a Neoproterozoic age (Rast and Skehan, 1983) and are Baltic (Paradoxides) fauna occurs in these sedimentary rocks in locally overlain by fossiliferous Lower and Middle Cambrian se- South Carolina (Secor et al., 1983), and Ordovician conodonts quences (Hoppin and Weymouth Formations, and Braintree Argil- have also been reported in another part of the sequence in North lite; Cameron and Naylor, 1976). Rast and Skehan suggested that Carolina (Koeppen et al., 1995). Hibbard et al. (2006b, 2006c), the less metamorphosed sedimentary rocks in southeastern New however, reported Ediacaran fossils from at least one of the same England rest on higher-grade basement, possibly equivalent to the localities, and their Neoproterozoic age is corroborated by U-Pb Greenhead Group in New Brunswick. radiometric ages. Rocks containing similar faunal assemblages Avalonian rocks in New Brunswick consist of a bimodal suite occur in Rhode Island, New Brunswick, and Newfoundland, but of volcanic rocks and sediments as young as Middle Cambrian that there are enough differences in both the faunas and stratigraphic are in fault contact with an older, higher-grade marble-quartzite sequences to suggest the Carolina superterrane is different enough sequence, the Greenhead Group (Rast and Skehan , 1983). These to warrant setting it apart from the other Avalonian terranes (Secor sequences are typical of the rocks of the Avalon superterrane, ex- et al., 1983; Samson, 1995; Hibbard et al., 2002, 2007b). cept for the presence of possible basement. High-grade components of the western Carolina superterrane The Avalon superterrane in Newfoundland consists of two in the southern Appalachians (excluding the more easterly Alle- separate and equivalent sequences: volcanic sequences with ghanian high-grade zones) were deformed and metamorphosed minor sedimentary rocks overlain by nonmarine and marine se- prior to ~530 Ma, following termination of arc magmatism quences that contain some volcanic rocks on the Avalon and Burin (Barker et al., 1998). This is principally the Charlotte terrane of Peninsulas. These are the primary reference Avalonian sequences Hibbard et al. (2002). If the subsurface Brunswick (Charleston) for points east and southwest (Williams, 1979; Rast and Skehan, terrane in South Carolina and Georgia has a peri-Gondwanan 1983) and are very similar to rocks on Cape Breton Island (Barr origin, as the one age date might suggest (Mueller et al., 2005), et al., 1998). The Conception Group contains an abundant Edia- and became part of the Carolina superterrane either during the caran fauna, particularly at Mistaken Point in southeasternmost 530 Ma event or possibly the Alleghanian, it is simply another Newfoundland. Downloaded from memoirs.gsapubs.org on October 24, 2014

The Appalachian orogen 7

The Gander superterrane lies between the Dover fault and because their accretionary histories are quite diachronous along Red Indian line in Newfoundland. It consists of an assemblage the length of the orogen (see Hibbard et al., 2007b). Moreover, of high-grade gneisses that were later intruded by mid-Paleozoic Gander and Avalon (and Carolina-Charlotte ) superterranes were plutons. Williams (1979) concluded that the Gander superterrane amalgamated before 530 Ma (Barker et al., 1998; Hibbard et al., rocks were deposited off Gondwana and were accreted to the arc 2007a, 2007b), well before their accretion to Laurentian compo-

complex during the Silurian (van Staal et al., 1998). Negative εNd nents during the mid-Paleozoic. values have been associated with Gander superterrane rocks in Newfoundland (D’Lemos and Holdsworth, 1995), whereas Ava- East Coast Magnetic and Gravity Anomalies

lonian rocks are associated with positive εNd values (Barr and The East Coast magnetic anomaly (Fig. 1) and correspond- Hegner, 1992; Murphy et al., 1995; Samson, 1995). Using these ing gravity anomaly (Committee for the Gravity Anomaly Map data, the Gander superterrane has been correlated southward by of North America, 1987) is another throughgoing element formed van Staal et al. (1998) with rocks in the Miramichi anticlinorium, along the continental margin of eastern North America during the and coastal Maine to the southwest, in Nova Scotia (Barr et al., breakup of Pangea. Both are linear high anomalies, and they are

1998), and New Brunswick. Similar negative εNd values suggest continuous features from offshore Georgia to off Nova Scotia. The that Ganderian assemblages may extend as far to the southwest magnetic anomaly has been interpreted as a series of moderately as Connecticut (Armstrong et al., 1992; Sevigny and Hanson, seaward-dipping, layered volcanic rocks that produce consistent 1993; Aleinikoff et al., 2007). The Charlotte terrane in the Caro- refl ectors in seismic-refl ection profi les (Grow et al., 1979; Austin lina superterrane consists of a high-grade assemblage of metavol- et al., 1990; Oh et al., 1995); a suite of mafi c dikes (Alsop and canic and plutonic rocks that were metamorphosed and accreted Talwani, 1984); an “edge effect” (Keen, 1969); and a deep-seated to the Carolina superterrane before ~530 Ma, long before being feature, probably a mafi c intrusive (Keller et al., 1954) along or accreted to Laurentian elements (Barker et al., 1998; Hibbard near the continent-ocean transition (Vogt, 1973; Vogt and Einwich, et al., 2002). This assemblage may be correlative with the Gander 1979). Nelson et al. (1985a, 1985b) and McBride and Nelson superterrane farther north, but insuffi cient isotopic data are cur- (1988) suggested the East Coast magnetic anomaly, and its con- rently available to test this hypothesis. tinuation across southern Alabama and Georgia, may represent the Sequences in the Avalon Peninsula in Newfoundland (Neo- Alleghanian suture. The source of the magnetic anomaly appears, proterozoic Harbor Main, Love Cove, and related groups), however, to be much more deep-seated, as suggested by the mod- Antigonish Highlands and Cape Breton Island in Nova Scotia eling of Keen (1969), Grow et al. (1979), and Alsop and Talwani (Neoproterozoic Fourchu and Georgeville Groups and Cambrian (1984), so its origin as a suite of seaward-dipping volcanic fl ows McDonalds Brook Group in the Antigonish Highlands), and Cale- is less likely. Alternatively, it could consist of a series of linear donia Highlands in New Brunswick (Neoproterozoic Colbrook gabbro plutons that are the product of decompression melting and Group) contain suites of arc affi nity (meta-) sedimentary, and were intruded near the base of the crust during the initial breakup mafi c and felsic volcanic and plutonic rocks, along with minor of Pangea. The magnetic anomaly also has been correlated with continental tholeiites in Nova Scotia (Keppie, 1995; McCutcheon anomalies that cross Florida and trace along the northern margin and McCloud, 1995; Williams et al., 1995). Because of the basic of the Gulf of Mexico, which have been suggested to be the root similarities of the rocks in each of the exposed Appalachian peri- zone of the main décollement in the Appalachians and Ouachitas Gondwanan assemblages (Carolina, Avalon, and Gander), they (Hall, 1990). The gravity anomaly could have been produced by have been traditionally assumed to be part of a single terrane (e.g., mafi c intrusions, but it is offset to the west of the magnetic anom- Williams and Hatcher, 1983). While the volcanogenic sequences aly; it has also been considered the product of an edge effect in the are broadly similar, details reveal that there are major dissimilari- transition from continental to (Grow et al., 1979). ties with respect to the sequences and their histories. Secor et al. Careful examination of the character of the East Coast Magnetic (1983) pointed out the differences in faunas between southern Anomaly (North American Magnetic Anomaly Working Group, and northern sequences. Hibbard et al. (2007b) compared both 2003) and comparison with the E-W–trending anomalies that sequences and isotopic data and concluded that, from several extend across Florida into the Gulf of Mexico indicate the East points of view, Carolina superterrane rocks have greater similari- Coast Magnetic Anomaly does not turn westward, but instead ter- ties to those of the Gander superterrane than to Avalon. Never- minates in the western Atlantic east of Florida. theless, there is little disagreement that all of these terranes had their origin during Neoproterozoic time close to Gondwana, with Nonthroughgoing Elements a bimodal volcanic-plutonic history beginning just after 700 Ma (Barr, 1993; Bevier et al., 1993; O’Brien et al., 1996; Coler et al., Numerous smaller terranes of limited extent exist in the 2000; Wortman et al., 2000). The Meguma terrane is the only peri- Appalachians, several of which have a Laurentian provenance. Gondwanan terrane that contains no primary volcanic or plutonic Hibbard et al. (2002) also recognized several smaller terranes components, but it is thought to be a backarc sequence that formed within the Carolina superterrane (see above), and Barr et al. off of the Gondwanan margin (Schenk, 1997). The Paleozoic his- (1998) identifi ed several smaller terranes in the Avalon superter- tories of these terranes diverge, however, as might be expected, rane on Cape Breton Island. Downloaded from memoirs.gsapubs.org on October 24, 2014

8 Hatcher

The Putnam–Nashoba terrane resides NW of Avalon in rocks that have a peri-Gondwanan affi nity, concluded to be de- southeastern New England and consists of high grade ortho- and posited off the Gondwanan margin (Schenk, 1997). Sequences paragneisses (O’Hara and Gromet, 1985). It is isolated between consisting of alternating thick, fi ne-grained, deep-water sand- the Clinton–Newbury and Bloody Bluff faults (Fig. 1). The Tatnic stone and shale units have been related to similar sequences in Hill Formation in this terrane contains Mesoproterozoic, Ordo- northwestern Africa. Schenk (1997) also concluded that an ex- vician, and Late Silurian (~425 Ma) detrital zircons, delimiting tensive area in the subsurface off Nova Scotia and Newfound- deposition and subsequent history of rocks of this terrane to be- land is underlain by Meguma terrane rocks. Aeromagnetic data tween 425 and 414 Ma, the ages of the oldest plutons cutting (e.g., North American Magnetic Anomaly Working Group, 2003) these terranes. The Berwick and Hebron Formations in the Merri- also support a confi guration similar to that suggested by Schenk. mack synclinorium to the west contain a similar detrital zircon Meguma likely was accreted to the outer parts of the already par- suite (Wintsch et al., 2007). Metamorphic ages of monazite and tially assembled Appalachian orogen in the early Alleghanian zircon of 407 Ma from both Putnam–Nashoba terrane and Mer- (Schenk, 1997). rimack synclinorium rocks further delimit the history of these In the southern Appalachians, the Talladega belt, Dahlonega terranes. Detrital zircons from the Kittery Formation, which gold belt, Cowrock, Cartoogechaye, and Kings Mountain belt lies in thrust contact above the Berwick Formation in the Merri- are nonthroughgoing terranes of Laurentian affi nity, although the mack synclinorium, consist predominantly of Mesoproterozoic Kings Mountain belt also has been considered part of Carolina zircons, one 650 Ma zircon, and a 485 Ma zircon, suggesting (Horton et al., 1989). The Cat Square terrane is similarly a non- a probable peri-Gondwanan source for this unit (Wintsch et al., throughgoing terrane, but it contains a Silurian-Devonian suite 2007). These detrital zircon suites are remarkably similar to those of metasedimentary rocks that could have begun their history as of the Cat Square terrane in the southern Appalachians, which is a southward continuation of the central New England Silurian- bounded by the Brindle Creek fault and Central Piedmont suture, Devonian assemblage (Dennis, 2007; Merschat and Hatcher, has a mixed Laurentian and peri-Gondwanan source, and was de- 2007). Both the Cat Square terrane and Merrimack synclino- posited between 430 and 407 Ma (Bream et al., 2004; Merschat rium and Putnam–Nashoba terrane rocks also have a dual source and Hatcher, 2007). affi nity, both Laurentian and peri-Gondwanan detrital zircons, The southern and central Appalachians also contain several along with abundant 430–425 Ma central Appalachians Salinic(?) smaller Laurentian terranes that occur only in this part of the event-derived zircons (Bream et al., 2004; Wintsch et al., 2007). Appa lachians. The Talladega belt in Alabama and western Geor- This strongly suggests that the Cat Square terrane is a southern gia consists of a group of siliciclastic rocks with ages that range continuation of the Merrimac synclinorium, and Cat Square ter- from Neoproterozoic to Mississippian (Tull et al., 1988, 1993; rane rocks were originally deposited in southern New England off Gastaldo, 1995; Tull, 2002). Reproducible Mississippian fossils of present-day New Jersey (Merschat and Hatcher, 2007) (Fig. 4). occur at the top of the sequence (Gastaldo, 1995), but the age of the lower parts of the sequence remains debatable (e.g., Higgins DISCUSSION: DEVELOPMENTAL HISTORY and Crawford, 2008). The Dahlonega gold belt occurs as a lower structural unit in the Georgia and North Carolina Blue Ridge, and The Appalachian orogen was built between the end of the it contains a suite of metamorphosed deep-water clastic sedimen- Middle Proterozoic Grenvillian Wilson cycle, during breakup of tary rocks, and several arc complexes; the arc complexes make the supercontinent Rodinia, and the end of the Appalachian Wil- up volumetrically only ~20% of the sequence (e.g., Hatcher and son cycle at the end of the Paleozoic. Passive-margin develop- Bream, 2009). This sequence has been correlated with the rifted ment followed rifting of Rodinia, which resulted in the greatest margin succession (Hadley, 1970), but, because of the presence inundation of the interior of Laurentia known in geologic time of volcanic rocks, the connection has never been clear. during the Late Cambrian and Early Ordovician (Fig. 2). Inte- Two other small terranes, Cowrock and Cartoogechaye, rior Laurentia, possibly along with parts of northwestern Africa occur in the southern Blue Ridge. Both are composed predomi- and other continents, was then uplifted by lithospheric processes, nantly of clastic sediments, with some mafi c-ultramafi c com- producing the widespread “post–Knox-Beekmantown” uncon- plexes. The Cartoogechaye terrane reached granulite-facies formity, recognizable from eastern Laurentia to as far west as pressures and temperatures ~460 Ma (Force, 1976; Eckert et al., Utah, and also in northwestern Africa (Morocco) and possibly in 1989; Moecher et al., 2004), the highest Taconian metamorphic Estonia and Siberia (Hatcher and Repetski, 2007). The eastern grades recorded in this part of the Appalachians. margin of Laurentia was again inundated by shallow seas at the The central Appalachians contain several small terranes— beginning of Middle Ordovician time, and passive margin depo- Westminster, Philadelphia, and others—but these were identifi ed sition resumed, but the new passive margin was short-lived, as by Faill (1997) as having originated in the Octararo sea during water depths increased and sediment began to arrive from the the Ordovician, so they are likely all related to the large suite of east. The had begun in the interior of the chain, distal terranes that formed in Iapetus off the Laurentian margin. and ophiolites began to load the outer platform in the northern The exotic Meguma terrane in Nova Scotia consists of a Appalachians (Stevens, 1970; Williams, 1979). In the southern sequence of Upper Cambrian to Lower Devonian sedimentary and central Appalachians, however, the foredeep began to de- Downloaded from memoirs.gsapubs.org on October 24, 2014

The Appalachian orogen 9

velop, but the sediment composition refl ects an eastern nonvol- 1975; Rowley et al., 1979) (Fig. 1). No Taconic allochthons canic, nonophiolitic source, which contrasts with the source of occur farther south, but several nappes were thrust into the Mar- sediment farther north that contains ophiolitic debris (Hiscott, tinsburg basin during the Middle to Late Ordovician and were 1984). Judging from clast compositions of Middle Ordovician originally interpreted as a Taconic allochthon, called the “Ham- conglomerate in the southern Appalachians that include all lith- burg klippe” (Stose, 1946; Lash and Drake, 1984; Drake et al., ol ogies of the early Middle Ordovician and older rifted margin 1989). Recent detailed paleontologic work, however, has re- and platform sequences, along with some basement clasts (Kell- vealed that the Cocalico Formation of variegated sandstone and berg and Grant, 1956), uplift of the platform east of the foredeep shale that originally was thought to comprise the Hamburg klippe occurred throughout the Middle and Late Ordovician without a contains olistoliths of older units, but was also thrust into the volcanic source, initially (incorrectly) suggesting that the uplift Martinsburg basin during the Middle to Late Ordovician (Ganis was a forebulge and the foredeep was a forebulge basin (Hatcher and Wise, 2008; Wise and Ganis, 2009). Wise and Ganis (2009) et al., 2004). Despite the source, the 3 km accumulated thickness also suggested this group of parautochthonous to allochthonous in the Sevier basin is too great for it to be considered a fore bulge rocks be called the “Hamburg complex.” This also was close to basin. Deposition in the central Appalachian foredeep began the time of accretion of several smaller more outboard terranes in the Middle Ordovician Darriwilian, and sediments reached in the central Appalachians, including the Westminster, Potomac, their greatest thickness (~5 km) in the Martinsburg basin during and Philadelphia terranes (Faill, 1997). the Late Ordovician. Middle Ordovician foreland basins are all Taconian deformation produced the foreland fold-and-thrust diachronous, becoming fi lled during the late Arenig (Floian) and belt from Vermont northward to Newfoundland (Rodgers, 1971; Llanvirn (Dapingian) in the southern and northern Appalachians, St. Julien and Hubert, 1975). Logan’s line thrust in Québec, and but continued to be fi lled during the Caradocian (Sandbian) in the the Champlain and Hinesburg thrusts farther south in Vermont, central Appalachians (Shanmugam and Lash, 1982). are the prominent faults in the Taconian foreland here. Devonian Taconian foreland deposition refl ects volcanic arc devel- rocks are deformed in the foreland farther south in the Hudson opment and closing of the Iapetus ocean and its margins. The Valley of New York, and Marshak (1986) suggested that this resolution of the nature of the arc systems varies markedly along thrust package formed during the . There are, the orogen because of the variability of the intensity of trans- however, no age constraints on the youngest possible timing of position and metamorphic grade. A complex arc system was deformation of these rocks, so these thrusts could easily have built off Newfoundland during the Ordovician that involved both formed during the Alleghanian orogeny and constitute the north- west- and east-dipping subduction zones, and the accretion of the eastern continuation of the southern-central Appalachian fore- Dashwoods block (van Staal et al., 2007) (Fig. 5). A parallel his- land fold-and-thrust belt. tory may have taken place farther south, but it is obscured by Taconian deformation becomes intense and polyphase into Taconian and later polydeformation, and medium- to high-grade the interior of the Appalachian orogen in New England and ap- metamorphism. The best-preserved ophiolites in the orogen were pears to be related to arc accretion between 496 and 428 Ma obducted onto the Laurentian margin in Newfoundland during (Stanley and Ratcliffe, 1985; Karabinos, 2006). A suite of ex- the Taconian event, along with accretion of several arc terranes ternal and more internal (Middle Proterozoic) basement massifs (e.g., Stevens, 1970). The only other recognizable ophiolites in (Hudson–Housatonic Highlands, Berkshire Mountains, Green the orogen occur in New Brunswick and Québec (St. Julien and Mountains, and Lincoln massif, and the internal Chester-Athens Hubert, 1975; van Staal et al., 1998, 2008), but these consist of dome) occurs inboard from the foreland from southeastern New incomplete sections. Ordovician blueschist has been recognized York to northern Vermont, and appears to represent rifted frag- in northern Vermont and southern Québec (Laird, 1988), and in ments of Grenvillian crust that were covered with Neoprotero- New Brunswick, but the New Brunswick occurrence appears zoic to Cambrian sedimentary and minor volcanic rocks, and to have a Late Ordovician age and emplacement related to the then thrust onto the Laurentian margin beneath the accreting arcs ~425 Ma Silurian Salinic orogeny (van Staal et al., 2008). Far- during the Taconian event (Stanley and Ratcliffe, 1985) (Fig. 4). ther south, either no ophiolites or blueschists are preserved on the The “Taconian suture” consists of a major fault and ter- Laurentian margin or they are located in the internides where they rane boundary that is traceable from one end of the orogen to are polydeformed and metamorphosed to amphibolite facies or the other. It is the Baie Verte–Brompton Line from Newfound- higher grade assemblages, thereby obscuring their origin (Robin- land to southern New England, consisting of faults of a variety son et al., 1998), although several mafi c-ultramafi c complexes of names: Whitcomb Summit, Cameron’s line, and several other in the southern Appalachians possess whole-rock and mineral faults in the central Appalachians, including the Martic line, then geochemical characteristics that suggest normal mid-ocean-ridge the Gossan Lead–Burnsville–Chattahoochee–Holland Mountain– basalt (N-MORB) to arc provenance and ophiolite emplacement Hollins line fault. The Hayesville fault forms the western bound- (Hatcher et al., 1984; Swanson et al., 2005). ary of two smaller terranes (Cartoogechaye and Cowrock) located A series of Taconic allochthons occurs from New York to the west of the Chattahoochee fault in the southern Appala- northward that transported deep-water, offshore facies onto the chians. These faults separate the rifted margin metasedimentary Laurentian margin (Zen, 1967, 1972; St. Julien and Hubert, and rift-related volcanic rocks (with the exception of the 471 Ma Downloaded from memoirs.gsapubs.org on October 24, 2014

10 Hatcher

NW Penobscot arc SE

GANDERIA A CAMBRIAN-TREMADOC (513-490 Ma)

Latitude: ~45°S obducted Popelogan-Victoria Penobscot arc arc

GANDERIA B ARENIG (478-475 Ma)

Latitude: ~20°S Latitude: ~35°S Latitude: ~43°S

Dunnage Active part of Summerford Remnant mélange Popelogan-Victoria arc Notre Dame seamount arc obducted Annieopsquotch arc Tetagouche-Exploits Penobscot arc accretionary tract backarc basin GANDERIA LAURENTIA

C Ridge subduction

Remnant of Ganderian crust LLANVIRN (465 Ma)

Latitude: ~30°S Latitude: ~38°S Summerford seamount Trench-fill Notre Dame deposits arc Red Indian Line Accreted Popelogan- Victoria arc

LAURENTIA

D Carmanville/Belledune mélange

CARADOC (455 Ma)

Figure 5. Example of the complexity of Late Cambrian to Late Ordovician arc geometries and convergence in the Newfoundland Appalachians where the orogen is less deformed and metamorphosed than farther south. Compare with McClellan et al. (2007, their Fig. 10). (Figure is from van Staal et al., 1998.) Downloaded from memoirs.gsapubs.org on October 24, 2014

The Appalachian orogen 11

arc-related Hillabee Greenstone in Alabama and western Geor- foreland began in the Middle Devonian (Chemung and Catskill gia; McClellan et al., 2007) from metasedimentary assemblages Groups) (Fig. 6). Deposition continued through most of the remain- contain ing abundant mafi c and ultramafi c rocks that could be con- der of the Devonian, but the clastic wedge became diachronous sidered dismembered ophiolites, but south of Québec the amount and younger as it propagated southwestward in the foreland, of deformation at high-grade metamorphic conditions prevents forming the extensive organic-rich shales (Ohio, Chattanooga) structural or stratigraphic characterization of the exact provenance that become coarser to the east (Ettensohn, 2004). Paralleling of these bodies. Despite this, numerous geochemical studies over foreland deposition, deformation and metamorphism occurred in the past 25 yr have consistently suggested that the mafi c and the interior of the orogen, reaching granulite-facies assemblages ultra mafi c rocks have a noncontinental origin. Detrital zircons, in southern New England and at least sillimanite II in the south- however, have proven that the metasedimentary rocks are distal ern Appalachians. This is the Acadian orogeny in New England, Laurentian sediments that were likely deposited in an oceanic set- New Brunswick, and Nova Scotia, and points south (Barr et al., ting, and all of these terranes have a Laurentian provenance. 1998; Robinson et al., 1998), and the Neoacadian orogeny from Taconian metamorphism reached upper-amphibolite-facies, New England southward. The Acadian orogeny began in the Late or higher, conditions in New England (Robinson et al., 1998) Devonian (~410 Ma), and the Neoacadian ended in the early and in the southern Appalachian Blue Ridge (Force, 1976; Mississippian (~345 Ma) (Osberg et al., 1989; Robinson et al., Eckert et al., 1989; Moecher et al., 2004). Timing of Taconian 1998; Merschat et al., 2005; Hatcher et al., 2007b; Merschat and metamorphism ranges from 460 to 455 Ma in the southern Hatcher, 2007). Appa lachians to 480–455 Ma in New England, to 495–455 Ma The Acadian orogeny was clearly the dominant event in the farther north (Robinson et al., 1998; van Staal et al., 2004; New England and adjacent Canadian Appalachians, producing Moecher et al., 2004). Timing of deformation and metamor- polyphase deformation and high-grade metamorphism in south- phism in the internides is roughly coeval with the timing of ern New England, and abundant plutonism (Robinson et al., foreland basin development. 1998). This event is likely related to collision of Avalon (and Gan- A few Ordovician (~460 Ma) granitoid plutons occur in the deria superterrane?) with Laurentian elements (Skehan and Rast, southern Blue Ridge (e.g., Hatcher et al., 2007b), but many more 1990). Wintsch et al. (2003) suggested that tectonic wedging occur in the Inner Piedmont (Miller et al., 2000; Bream, 2003; was involved with collisional emplacement of the Avalon ter- McClellan et al., 2007), and 458–470 Ma plutons and volcanic rane in southern New England, producing much of the complex rocks occur in the Milton–Chopawamsic–Potomac terrane (e.g., deformational effects in Connecticut. Much of this deformation Coler et al., 2000). These continue into the central Appalachian and metamorphism is now thought to be Alleghanian based on Piedmont of Virginia to Maryland, with the addition of several modern geochronologic data that demonstrate the involvement of Silurian (Salinian?) granitoid plutons (Wilson, 2001), and they Permian rocks in the deformation (Walsh et al., 2007). reappear in western New England as part of the Bronson Hill The Acadian and Neoacadian orogenies are the result of arc; most have an arc geochemistry (Stanley and Ratcliffe, 1985; the zippered north-to-south closing of the Rheic ocean as peri- Robinson et al., 1998). Gondwanan superterranes, Avalon and Carolina, and possibly Late Ordovician to Early Silurian molasse spread uncon- Gander, collided with Laurentia and the early Paleozoic terranes formably across most of the central and part of the southern accreted during the Taconian event (Rast and Skehan, 1983; Barr Appa lachians following the Taconic orogeny, and similar de- et al., 1998; Merschat et al., 2005; Hatcher and Merschat, 2006; posits formed in the north, truncating tilted Ordovician rocks in Hatcher et al., 2007a; Merschat and Hatcher, 2007). The Cat New Jersey and New York (the classic Taconic unconformity) Square remnant ocean accumulated detrital zircons from both (Rodgers , 1971). Early Silurian felsic volcanic rocks were ex- Laurentian and peri-Gondwanan sources, and zircons as young truded in central western Newfoundland, suggesting that rifting as 430 Ma (Bream, 2003), indicating deposition occurred during followed the orogeny (van Staal et al., 1998). There also is evi- the Silurian and Devonian (Merschat and Hatcher, 2007). From dence for compression in the northern and central Appalachians, the central Appalachians southward, Carolina subducted terranes which has been called the Salinic orogeny (van Staal et al., 1998, to the west beneath it after closing the Cat Square remnant ocean 2004; Aleinikoff et al., 2007). A suite of Late Ordovician to Early basin, producing anatectic melting by 407 Ma (Gatewood, 2007) Silurian granitoid plutons also intruded the Virginia to Maryland and wholesale migmatization of Cat Square and Tugaloo terrane central Appalachians (Wilson, 2001; Sinha et al., 2010). assemblages, and generating plutons in these terranes, as well as in the Carolina superterrane. Neoacadian plutons are not as Acadian and Neoacadian Orogenies abundant in the Carolina superterrane, but they are present in the Carolinas and Georgia as the Salisbury plutonic suite and associ- Clastic deposition began during the Late Ordovician(?) ated mafi c plutons in the Carolinas (Butler and Fullagar, 1978; and Silurian in internal New England and the Canadian Mari- McSween et al., 1991; Esawi, 2004), and the likely Devonian times (Robinson et al., 1998), and in the remnant Rheic ocean mafi c plutonic suite in central Georgia (e.g., Hooper and Hatcher, off the central Appalachians (Merschat and Hatcher, 2007). Par- 1989). In contrast, Acadian plutons are abundant, even dominant, allel depo si tion of clastic sediments on the central Appalachian in New England and the Canadian Maritimes. The reason for this Downloaded from memoirs.gsapubs.org on October 24, 2014

12 Hatcher

Laurentia

Borden

–Grainger

Figure 6. Possible relationships dur- ing Late Devonian to early Mississip- Chattanooga Bedford–Berea Catskill pian time (~360 Ma) among Laurentia, –Pocono–P PricePrice ocono already formed Taconic crust, and Central Carolina-Gander superterranes that collided transpressionally (southwest- New England–Maritimes (Merrimack syncl., etc.) directed) with the Laurentian–Taconian Talladega rifted marginTaconian Avalon–Ganderia assemblage and subducted these elements crust beneath them, producing high-grade metamorphism and uplift in southern Meguma New England and progressively subduct- Cat Square ing more of the Laurentian–Taconian Rheic terranes, together with sediments de- remnant posited in the remnant Rheic ocean, ocean Carolina–Ganderia? southwestward reaching burial depths of 18–20 km in a relatively short time ocean (1–4 m.y., depending on dip of the sub- duction zone). The result was a tectoni- cally forced, southwestward escaping, orogenic channel of partially melted Cat Square sediments and Laurentian Taconian crust (Hatcher and Merschat, Brunswick Theic 2006). Confi guration of diachronously (Charleston) prograding deltas on the platform (from Ettensohn, 2004) correlates directly with the northeast-to-southwest transpres- sionally zippered closing of the remnant Rheic ocean. (Figure is modifi ed from Merschat and Hatcher [2007].) era Gondwana Argentinecordill PrecordilleraPre

difference may be that fi nal emplacement of Avalon was by west- (Devo nian to ) granitoids and gabbros (Dallmeyer ward subduction beneath New England (Phinney, 1986; Robin- et al., 1986; McSween et al., 1991; Hibbard et al., 2002) probably son et al., 1998), but was eastward in the southern and central related to the middle to late Paleozoic docking of the Carolina Appalachians. Because of the transpressive nature of collision superterrane. The Smith River allochthon (Fig. 1) has been de- leaving only a remnant ocean, the amount of ocean crust that was scribed as a peri-Gondwanan terrane and outlier of the Carolina available to be subducted was minimal, thus limiting the ability to superterrane based on monazite chemical ages (Hibbard et al., generate suprasubduction-zone plutons. Once the limited amount 2003), but detrital zircon data (Carter et al., 2006; Merschat of ocean crust was subducted, continental crust began to be sub- and Hatcher, 2007; Merschat, 2009) do not support this conclu- ducted and the ability to generate plutons was shut off, analogous sion. The greenschist-grade and lower-grade central and eastern to the subduction of modern Australian continental crust beneath Carolina superterrane consists mostly of volcanic and volcani- Indonesia, shutting off arc volcanism (e.g., Hamilton, 1979). clastic rocks interrupted by the Alleghanian Kiokee–Raleigh The high-grade western Carolina superterrane contains a belt metamorphic core. Southeast of this metamorphic core are 360–350 Ma metamorphic overprint and numerous younger more low-grade volcanic and volcaniclastic rocks that have been Downloaded from memoirs.gsapubs.org on October 24, 2014

The Appalachian orogen 13 called different terranes by Hibbard et al. (2002), although the the continental margin (Thomas, 2006). Foreland deformation is Carolina superterrane was subdivided differently by Horton et al. Alleghanian, and timing of faulting in the domain of large strike- (1989). Some of the Hibbard et al. (2002) terranes vary only in slip faults in the interior from the Brevard fault eastward is pre- metamorphic grade and do not contrast in fundamental history or dominantly Alleghanian, with the exception of the Neoacadian stratigraphy. Central Piedmont suture, but it too was locally reactivated during Evidence supporting mid-Paleozoic accretion of Carolina the Alleghanian (Fig. 2). Farther north, the Alleghanian record superterrane consists of: (1) deposition of Cat Square terrane is confi ned to the interior of the chain, with several plutons and sedi ments that occurred subsequent to 430 Ma (Bream, 2003), extensive metamorphism in New England (including the large nearly coeval with deposition of the Silurian–Devonian sedi- Sebago batholith in Maine) (Wintsch and Sutter, 1986; Walsh ments in New England and the Canadian Maritimes; (2) par- et al., 2007), and several major dextral faults on the eastern side allel diachronous timing of metamorphism and deformation from southeastern New England to Nova Scotia, which re appear in the Tugaloo and Cat Square terranes in the southern Appa- in interior western Newfoundland (Fig. 1). The large, mostly lachian internides with diachronous, southward-younging clas- step over basins containing Carboniferous and Permian sediments tic wedge sedimentation in the foreland, beginning in the north are directly related to these faults (Bradley, 1982; Mosher, 1983). with the Devonian Catskill sediments, and ending in the south LeFort and Van der Voo (1981) and LeFort (1984) sug- with the latest Devonian–Mississippian Chattanooga Shale and gested that the Reguibat Promontory in West Africa collided with coarser equivalents to the east (e.g., Ettensohn, 2004; Merschat Laurentia here before collision of the main African continent and Hatcher, 2007); (3) both mafi c and felsic magmatism in in order to explain the narrow, strongly curved segment of the the overriding Carolina superterrane in North Carolina, South Pennsyl vania salient in the central Appalachians (Fig. 7). They Carolina, and Georgia (Butler and Fullagar, 1978; McSween concluded that collision of the promontory produced an escape et al., 1984; Hooper and Hatcher, 1989; Esawi, 2004); and (4) a scenario, where dextral faults facilitated southward es- mid-Paleozoic 360 Ma thermal overprint recorded in 40Ar/39Ar cape of crustal blocks and sinistral faults carried blocks north- plateau ages in the western Carolina superterrane (Dallmeyer et ward out of the collision zone. The movement sense of the array al., 1986). Moreover, criticism leveled at the conclusion based on of Alle ghanian faults south of the projected collision zone, in- abundant data of the mid-Paleozoic docking of Carolina super- cluding the Brevard, parts of the Central Piedmont suture, and all terrane (Hibbard et al., 2007a) is partly based on paleomagnetic of the eastern Piedmont fault system (Fig. 1), is clearly dextral, data that place Carolina superterrane at similar paleolatitude with but the movement sense of faults north of the collision zone, Laurentia in the Ordovician (Vick et al., 1987; Noel et al., 1988), including the Clinton-Newbury, Bloody Bluff, and all of those but these data do not indicate paleolongitude, permitting as much farther north, is also dextral (e.g., Bothner and Hussey, 1999; as 180° of uncertainty in the location of Carolina. Goldstein and Hepburn, 1999). Based on the ages of stratigraphic sequences and fault kinematics, I have proposed that the collision Alleghanian Orogeny involved both rotation and transpression, and that collision began at the northeastern end of the Appalachians and closed the Theic Clastic deposition coming from the interior of the orogen ocean southward like closing a zipper. In this scenario, Gond- began in the late Mississippian in the southern and central Appa- wana would have rotated into head-on collision with southeast- lachian foreland, and in basins in the interior of the orogen from ern Laurentia in Late Carboniferous to Permian time, producing New England to Newfoundland (Fig. 1). These interior basins are the Blue Ridge–Piedmont megathrust sheet that pushed foreland considered stepover rhomb grabens related to large dextral faults deformation in front of it from southern New York to Alabama (Cobequid–Chedabucto, Bellisle, Cabot), the initial stages of col- (e.g., Hatcher, 2002; Hatcher et al., 2007c). The zipper tectonics lision of Gondwana with Laurentia, and the closing of the Theic scenario fi ts more of the data related to the tectonostratigraphic ocean, which resulted in the fi nal assembly of Pangea (Bradley, and kinematic closing of the Theic ocean and fi nal assembly of 1982) (Fig. 7). Coeval with or slightly younger than deposition, Pangea than others. amphibolites-facies metamorphism, polyphase deformation, and The Alleghanian orogeny in Laurentia, and the equivalent plutonism are recorded in southern New England and in an an- Variscan components in Europe (including the Uralian orogeny; tiformal belt along the Coastal Plain overlap from Virginia to Pushkov, 1997), joined all existing continents to create super- Alabama, variously called the Goochland terrane–Raleigh belt, conti nent Pangea. This process took some 490 m.y. from the time Kiokee belt, and Pine Mountain terrane (Fig. 1). An Allegha- of initial rifting of Rodinia to the fi nal assembly of Pangea. nian thermal event is also recorded across much of the Tugaloo Appalachian history began with the Neoproterozoic breakup and Cat Square terranes in the south (Dennis and Wright, 1997; of supercontinent Rodinia, and the rifting and progressive sepa- Merschat et al., 2005), and is much more widespread across ration of southeastern Laurentia from Gondwana, opening the southern New England than was previously thought (Wintsch Iapetus ocean. Once rifting began, a continuous record of the tran- et al., 2003; Walsh et al., 2007). sition of rifted margin to stable platform depositional conditions The basic outline of the southern and central Appalachians is exists on the Laurentian margin. The Appalachian orogen may an Alleghanian footprint, inheriting the Neoproterozoic shape of be unique among orogenic belts in that three major orogenies Downloaded from memoirs.gsapubs.org on October 24, 2014

14 Hatcher

Late ABEarly Early Late Carboniferous Carboniferous Narragansett basin Laurentia LaurentiaLackawanna Clastic wedge Reguibat Clastic wedge Reguibat Promontory Promontory

Ouachitas Ouachitas

Gondwana Gondwana

Middle CDEarly Late Permian Carboniferous Laurentia Lackawanna Laurentia Blue Ridge-Piedmont Reguibat Alleghanian Reguibat megathrust sheet Promontory fold-thrust belt Promontory

Ouachitas Alleghanian Ouachitas Alleghanian suture suture

Gondwana Gondwana

Figure 7. Zipper closing of Theic ocean to form the Alleghanian orogen (continents are shown on Robinson projection; reconstruction modifi ed from that in Ziegler, 1990). Red lines and symbols indicate feature is active in the time interval shown. (A) Initial contact between Gondwana and Laurentia occurred in late Early Carboniferous (late Mississippian), producing initially sinistral faulting in New England followed immediately by dextral motion and pull-apart basins, then shedding of clastic sediments onto the continent, and Lackawanna-phase deformation. (B) South- ward movement and rotation of Gondwana with respect to Laurentia in early Late Carboniferous (early Pennsylvanian) produced dextral motion throughout orogen, waning of Lackawanna phase deformation, and greater dispersal of sediments onto the Laurentian foreland. (C) Continued clockwise rotation of Gondwana with respect to Laurentia during the Late Carboniferous closed the Theic ocean southward, bringing Gondwana into head-on collision with Laurentia, and producing the fi rst movement on the Blue Ridge–Piedmont megathrust sheet. (D) Early Permian head-on collision of Gondwana with Laurentia produced major transport on Blue Ridge–Piedmont megathrust sheet that drove foreland fold- thrust belt deformation (Valley and Ridge and Plateau) ahead of it.

affected the entire orogen during its development, one of which Catalán et al., 2002, 2007; Matte, 2002). Matte (2002) presented involved arc accretion (Taconian), while the other two involved an interesting comparison of the plate-tectonic processes that oc- large terrane (Acadian–Neoacadian) and continent-continent col- curred almost at the same time in all of these orogens, illustrating lision (Alleghanian) that completed the Paleozoic Wilson cycle, their parallel development. forming supercontinent Pangea. These three orogenies all pro- duced widespread penetrative deformation, metamorphism to ACKNOWLEDGMENTS at least amphibolite facies, volcanism, suites of felsic and mafi c plutons, and diachronous clastic wedges throughout the orogen This paper is an attempt to present a coherent, but not exhaus- (Taconian) or restricted to the southern and central Appalachian tive, data-based summary of Appalachian history from the foreland. These clastic wedges clearly track diachronous events breakup of Rodinia to the breakup of Pangea. It grew out of taking place deep inside the orogen. several decades of work in the southern Appalachians, a small Actually, the uniqueness of the Appalachian events is paral- amount of work in western New England (upstate New York leled by similar almost coeval events in the Variscan and Uralian and Vermont), and numerous fi eld trips throughout both the orogens. Similar Ordovician, mid-Paleozoic, and Carboniferous– United States and Canadian Appalachians. Support for this Permian events occurred in the Variscan of Iberia, in western- work has been provided by the National Science Foundation, central Europe, and in the Urals (Pushkov, 1997; Martínez the Department of Energy, the Nuclear Regulatory Commis- Downloaded from memoirs.gsapubs.org on October 24, 2014

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35° Tectonic Map of the Appalachians TN 30°N AL MS Robert D. Hatcher, Jr. 89°W 85° AL Alabama B (recess) Tennessee (salient) Tectonics and Structural Geology Research embayment Wiggins promontory TN terrane KY Department of Earth & Planetary Sciences Virginia W Alleghanian TB (recess) i HLF ? and Science Alliance Center of Excellence ? promontory 40° g ? ? ? ? DRW ? g TN K OH s) i BFF GA University of Tennessee–Knoxville n NC Sevier 80° (salient) s DGB MS embayment Chattahoochee clastic KY Pi Pennsylvania QuOn BrevardBre CR clastic vard wedge 2010 WV New York A CT VA Acadian and (recess)(reces FL AL AL TF TugaloTugalooo fault wedge promontory TFD MHTM GA HT Neoacadian 45° PPineine MMtn.tn. BF 75° DCFD WV PA Quebec wwindowindow CF fault Gossan & terraneterrane GMW Blue MD (salient)70° 50° Brindle clastic embayment S Modoc Mo Central terranterraneLeade Middle u Piedmont CreekC r e e k MartinsburgM 52° SuwanneeSuwannee Cat fault artinsburg wedge OrdovicianOrdovician w terrane fault 65° faultfault Square NWNW clastic Adirondacks clastic wedge a RF wedge Qu C KMB Ro Qu n a SMW r SRA Ridge 85° n GA SC o suture Canadian Tl l i C CHP BCF Bu e C na Hamburg e GH-SHF Milton WestminsterW thrust B EasterEasternn GH-SHF estminst nic Co terrane complexco TaconicTaco FL GA Chopawamsic terranete m Summit s rra er p r s n le Valley - u up - PotomacPotomac terrane e x GaspeGasp u e r t e r r a ne ML PA e 60° n NY allochthons VT QUE allochthonsalloc t terrane D.C. Ba nic Bronson NH synclinorium hthon s C Whitcomb ME u TaconicTaco Hill s LAB

w a Philadelphia Green Mtns Connecticut Boundary Mountains QUE r r r Ra o m anticlinoriumanticlinoriu e Hudson e i Goochland Ri MD HighlandsHighland NY QUE terrane S c li Raleigh– m n VA s Central k a MA Central Shield systesystem NJ terrane ME NB ( MD Ph Berkshires Ma MaineMaine Jx ( C s CLF P up DE NYC NH belt e h e CT ME r a rt i SC er - r NC PiedmonPiedmontrtane CBIS CNFCNF Merrimack Po G l t WDWD PNTPNT o e VA BBFB Bn n s BF anticlinorium d t faulfault NC Avalon ? LD LCF Norumbega w o African ? Pr Gander Miramichi a n RI n ) HVT fault Fr ? a n t ) FredericktonFrederic ? t e crus DTDT kton Anticosti e r r a Bo superterranesuperterr ? n e trough Isl. Rokelides St. J. (African) ? c ? AvalonAvalo ane BellisleBellisle t i fault basement ? E n e a ? (salient)

a a g n n embayment

St. Lucie m Newfoundland ? s ? meta. t o superterranesuperterra complex ? m NB 55° a Cobequid NFL a s t l ne C o y NS (recess) Meguma St. Lawrencepromontory 52° N Ct 80° 27° PEI ChedabuctoChedabuc Bay of Islands Long Range Mtns. Ocean Ha CB Line Meguma te Atlantic to VerteVer crust terrane Cabot fault Baie Abbreviations (SW to NE): zone fault Dunnage zone TB–Talladega belt Red IndianIndian Line 30° Gander HLF–Hollins line fault 0 100 Dog 70° Sy Exploits subzone DRW–Dog River window 75° 35° kilometers Bay BFF–Bartletts Ferry fault SRA–Smith River allochthon GanderGand zone Line 0 100 er zo Line DGB–Dahlonega gold belt BCF–Bowens Creek fault E a s t ne Subsurface contact (fault?) terrane GRUB TF–Towaliga fault CBIS–Chesapeake Bay impact structure miles C o a AvalonAvalon DCF–Dean Creek fault ML–Martic line fault s superterranesuperterra Subsurface fault t DoverDove fault ne MS–Murphy syncline CLF–Cameron’s line fault r

CR–Cowrock terrane LD–Lyme dome Exposed fault NFL

CT–Cartoogechaye terrane WD–Willamantic dome Burin PeninsulaPeninsula LCF–Lake Char fault m TFD–Tallulah Falls dome Exposed contact a MHT–Mars Hill terrane HVT–Putnam–Nashoba terrrane g BF–Burnsville fault CNF–Clinton–Newbury fault n e lon superterrane GMW–Grandfather Mountain window BBF–Bloody Bluff fault 65° t i c m a l y AvalonAva 40° a n o la St. J’s NW–Newton window DT–Dedham terrane insu KMB–Kings Mountain belt Acknowledgment: PeninsulaPen GH-SHF–Gold Hill-Silver Hill fault Electronic graphics assistance 60° CHP–Churchland pluton Geological Society of America SMW–Sauratown Mountains window by Andrew L. Wunderlich is very 50° RF–Ridgeway fault Memoir 206 much appreciated. 55°

Figure 1. Simplifi ed tectonic map of the Appalachians showing the distribution of major tectonic units, as well as the locations of major bound- gin assemblages. Medium blue from Alabama to Newfoundland is the Laurentian platform. Dark purple in the Valley and Ridge from Georgia to aries and the names of most major structures. An attempt was made to color this map so that as few tectonic units as possible are employed, and Newfoundland is the Middle Ordovician clastic wedge (labeled Sevier clastic wedge in the South). Lighter purple is the Valley and Ridge from to resolve tectonic units that are continuous as far as possible through the orogen. This will doubtlessly create some disagreements about correla- Tennessee to New York–Martinsburg clastic wedge (mostly Upper Ordovician). Tan in the western part of the orogen, and in the Alabama Inner tion of this or that tectonic unit. Subsurface boundaries between tectonic units are based primarily on aeromagnetic data (e.g., North American Piedmont, and Goochland terrane indicates Laurentian rifted margin succession. Light yellow in NE Georgia along the NW South Carolina state Magnetic Anomaly Working Group, 2003). Greens were employed for peri-Gondwanan terranes (Carolina, Avalon, Gander, and Meguma), and line is Tallulah Falls Quartzite. Brown is Mesozoic (–) basins (subsurface basins not shown). Cities from SW to NE: B—Birming- for separable ophiolites; muted colors were used for subsurface continuations of terranes. Orange indicates Carboniferous–Permian Alleghanian ham; A—Atlanta; Tl—Tallahassee; Jx—Jacksonville; S—Savannah; K—Knoxville; Co—Columbia; C—Charlotte; Ra—Raleigh; Ro—Roanoke; clastic assemblages, and yellow and light brown show Silurian–Devonian mostly clastic rocks in the northern Appalachians. Plutons were, for Ri—Richmond; D.C.—District of Columbia; Ba—Baltimore; Pi—Pittsburgh; Ph—Philadelphia; NYC—New York City; Pr—Providence; Bo— the most part, not separated on the map in an effort to show the primary kindred of tectonic units. Explanation of tectonic units not identifi ed on Boston; Ma—Manchester; Bu—Burlington; Mo—Montreal; Po—Portland; Bn—Bangor; Qu—Quebec City; St. J.—St. John; Fr—Frederickton; the map: Red in the southern and central Appalachians—Amazonian Grenville basement rocks. Deeper red in New England, and to the north— Ha—Halifax; Ct—Charlottetown; Sy—Sidney; CB—Corner Brook; St. J’s—St. John’s. This fi gure was modifi ed from van Staal et al. (1998, Grenvillian basement of Laurentian provenance (based on Pb isotopic studies; e.g., Sinha and McLelland, 1999; Loewy et al., 2003; Tohver et al., northern Appalachians), Hibbard et al. (2006a, northern Appalachians), Hatcher et al. (2007a, southern and central Appalachians), and numerous 2004). Shades of purple throughout the orogens indicate distal facies, largely diachronous (mostly younger) equivalents of Laurentian rifted mar- additional larger-scale maps. All other abbreviations represent state and Canadian province names.