Downloaded from memoirs.gsapubs.org on October 24, 2014 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) Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions to subscribe to Geological Society of America Memoirs Permission request click www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA. Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. 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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 Paleozoic orogen that formed in a complete Wilson cycle along the eastern Laurentian margin following the breakup of supercon- tinent Rodinia 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 Laurentia. Three orogenies ultimately produced the mountain chain: the Ordovi- cian Taconic orogeny , which involved arc accretion; the Acadian–Neoacadian orog- eny, which involved north-to-south, transpressional, zippered, Late Devonian–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 Silurian colli sion in the Maritime Appalachians and Newfoundland; and the Allegha- nian orogeny, which involved late Mississippian to Permian collision of all previously formed Appalachian components with Gondwana 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 Europe (Variscan events), eastern Europe and west- ern Siberia (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. 1 Downloaded from memoirs.gsapubs.org on October 24, 2014 2 Hatcher The Appalachians are an accretionary orogen (Fig. 1) that terrane 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 Ordovician 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 Cambrian–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–Scandian orogeny (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 terranes 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