A comparative analysis of pre-Silurian crustal building blocks of the northern and the southern Appalachian orogen

Abstract

The New York promontory serves as the divide between the northern and southern segments of the Appalachian orogen. Antiquated subdivisions, distinct for each segment, implied that they had lithotectonic histories that were independent of each other. Using new lithotectonic subdivisions we compare first order features of the pre-Silurian orogenic 'building blocks' in order to test the validity of the implication of independent lithotectonic histories for the two segments.Three lithotectonic divisions, termed here the Laurentian, Iapetan, and the peri-Gondwanan realms, characterize the entire orogen. The Laurentian realm, composed of native North American rocks, is remarkably uniform for the length of the orogen. It records the multistage Neoproterozoic-early Paleozoic rift-drift history of the Appalachian passive margin, formation of a Taconic Seaway, and the ultimate demise of both in the Middle Ordovician. The Iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the Iapetus Ocean, between Laurentia and Gondwana. In the northern segment, the realm is divisible on the basis of stratigraphy and faunal provinciality into peri-Laurentian and peri-Gondwanan tracts that were amalgamated in the Late Ordovician. South of New York, stratigraphic and faunal controls decrease markedly; rock associations are not inconsistent with those of the northern Appalachians, although second-order differences exist. Exposed exotic crustal blocks of the peri-Gondwanan realm include Ganderia, Avalonia, and Meguma in the north, and Carolinia in the south. Carolinia most closely resembles Ganderia, both in early evolution and Late Ordovician-Silurian docking to Laurentia.Our comparison indicates that, to a first order, the pre-Silurian Appalachian orogen developed uniformly, starting with complex rifting and a subsequent drift phase to form the Appalachian margin, followed by the consolidation of Iapetan components and ending with accretion of the peri-Gonwanan Ganderia and Carolinia. This deduction implies that any first-order differences between northern and southern segments post-date Late Ordovician consolidation of a large portion of the orogen. introductionThe Appalachian orogen (fig. 1) is the northeast-trending belt of Mesoproterozoic to Paleozoic rocks in eastern North America that was deformed during the Paleozoic (Rodgers, 1970). The structural grain of the orogen is remarkably consistent, defining a series of broad, harmonically curved promontories and embayments (fig. 1). These structural bends have been used to delineate segments of the orogen termed the southern (Alabama promontory-Virginia promontory), central (or middle, Virginia promontory-New York promontory), northern (New York promontory-St. Lawrence promontory), and Newfoundland Appalachians (north of St. Lawrence promontory) (Rodgers, 1949(Rodgers, , 1970 (fig. 1); however, commonly in modern parlance, the New York promontory, located near the center and at the narrowest part of the orogen, serves as the divide between two segments r...

Evidence of Late Ediacaran Hyperextension of the Laurentian Iapetan Margin in the Birchy Complex, Baie Verte Peninsula, Northwest Newfoundland: Implications for the Opening of Iapetus, Formation of Peri-Laurentian Microcontinents and Taconic – Grampian Orogenesis

“…Isotopic evidence of crustal contamination and zircon inheritance in the outboard peri-Laurentian terranes (e.g. Karabinos et al 1998;Whalen et al 1997;Zagorevski et al 2006;Brem et al 2007;Hibbard et al 2007;van Staal et al 2007;van Staal and Hatcher 2010;Allen et al 2010;Chew et al 2008Chew et al , 2010Skulski et al 2010, Zagorevski and indicate that these isolated blocks subsequently formed the basement to supra-subduction zone magmatism in the outboard arc complexes. Formation of H-blocks and/or large extensional allochthons during hyperextension would remove the necessity of having two discrete rifting events along the Humber margin, resulting in two coeval spreading centres Waldron and van Staal 2001;Burton and Southworth 2010).…”

“…The second break-up event, which led to the riftdrift transition on the Humber margin, between 540 and 530 Ma, was related to departure of a ribbon-shaped microcontinent, referred to as Dashwoods in the northern Appalachians Waldron and van Staal 2001;Allen et al 2010). The separation of Dashwoods led to opening of the relatively narrow Taconic seaway (Hibbard et al 2007;). …”

“…Karabinos et al 1998;Hibbard et al 2007;van Staal and Hatcher 2010;Allen et al 2010;Chew et al 2008Chew et al , 2010 initially could have formed, at least to a first approximation, as H-blocks or large extensional allochthons, analogous to the BrianHonnais crustal block in the Alps Mohn et al 2010). The crust of H-blocks is commonly thinned to less than 20 km and is generally associated with marked retardation of subsidence (Péron-Pinvidic and Manatschal 2010).…”

“…Hibbard et al 2007). Furthermore, the work of Jagoutz et al (2007) along the Iberian OCT has shown that formation of MORB along the protoridge in embryonic oceanic crust may be followed by widespread, delocalized extensional deformation and off-axis magmatism before true spreading occurs.…”

“…However, rift-related magmatism continued throughout the northern Appalachians for another 20 my (to ca. 550 Ma; Kumarapeli et al 1989;Bédard and Stevenson 1999;Hodych and Cox 2007;Burton and Southworth 2010, and references therein) and the thermal subsidence of Laurentia's (para)autochthonous rifted margin took place at least 40-50 my later (525-520 Ma; Bond et al 1984;Williams and Hiscott 1987;Waldron and van Staal 2001;Hibbard et al 2007). The apparent conflict between paleomagnetic and geological data posed a major conundrum and called into question models of late Neoproterozoic opening of Iapetus.…”