The Avalon Zone: A Pan‐African terrane in the Appalachian Orogen of Canada

Abstract

The Precambrian sequences of the Avalon Zone in Canada (southeastern margin of the Appalachian Orogen) are interpreted as a Pan‐African orogenic belt incorporated into the Appalachian Orogen during Palaeozoic times as its southeastern margin. The Precambrian evolution of the Avalon Zone was genetically unrelated to subsequent Palaeozoic evolution. The Avalon Zone shows marked similarities in age, tectonic history, and facies development to the Pan‐African belts adjacent to the West African Craton. Precambrian evolution of the zone began with circa 800 Ma rifting of a sialic gneissic basement and deposition of a Middle Proterozoic(?) carbonate‐clastic cover sequence. Early crustal rifting was associated with localized partial melting and metamorphism. Limited crustal separation led to the restricted development of circa 760 Ma oceanic volcanics. Continued rifting and subsequent closure of these narrow ocean basins led to the eruption of widespread subaerial volcanic suites, block faulting, granite plutonism, and local, late Proterozoic sedimentary basin formation. Precambrian evolution of the zone terminated with the Avalonian Orogeny (circa 650‐600 Ma), a deformational event, the affects of which are most evident locally along the northwestern margin of the zone. The controlling features of the Proterozoic evolution of the Avalon Zone are a series of linear intracratonic troughs and small ocean basins that formed during thinning and separation of the crust by ductile spreading, rupture, and delamination (cf. Martin and Porada 1977). The variation in degree of crustal separation led to subsequent variation in orogenesis during late Proterozoic compression. The zone marks the original westward limit of Pan‐African activity and displays no apparent genetic link with the Appalachian Orogen in Canada until Devonian times.

A paleogeographical review of the peri-Gondwanan realm of the Appalachian orogen1This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

“…Geological, lithostratigraphic, isotopic, faunal, and paleomagnetic data indicate that Avalonia records a Neoproterozoic tectonomagmatic history that predates opening of the Iapetus Ocean and originated along an active margin Gondwana (Williams and Hatcher 1983;O'Brien et al 1983;Nance et al 2002). However, previously proposed connections to West Africa (O'Brien et al 1983;Landing 1996;McNamara et al 2001;Thompson et al 2007) are inconsistent with available geological data.…”

“…However, previously proposed connections to West Africa (O'Brien et al 1983;Landing 1996;McNamara et al 2001;Thompson et al 2007) are inconsistent with available geological data.…”

“…In the classic synthesis of the northern Appalachian orogen, Williams (1979) defined the "Avalon zone" as including all exotic rocks that extend from eastern Newfoundland southwest to Maritime Canada into the eastern MassachusettsRhode Island area of New England. With the introduction of the terrane concept of Coney et al (1980), the Avalon zone was recognized as a far-travelled, fault-bounded block and renamed the "Avalon terrane" (Williams and Hatcher 1983;O'Brien et al 1983). Numerous researchers have since suggested separate terrane status for many of these areas and viewed Avalonia as an amalgamation of multiple terranes termed the "Avalonian plate" (Rast and Skehan 1983), "Avalon composite terrane" (Keppie 1985), "Avalon superterrane" (Gibbons 1990), "East Avalonia" (Pickering et al 1988), "West Avalonia" (Nance and Murphy 1994), and "Avalon sensu lato" (O'Brien et al 1996).…”

“…First recognized on the Avalon Peninsula in eastern Newfoundland in the mid 1960s (Williams 1964), these domains have been recognized as being far-traveled terranes that record a geologic and tectonic history that predates inception of the Iapetus Ocean and are therefore exotic to native Laurentian rocks elsewhere in the orogen (O'Brien et al 1983;Williams 1984). Along-strike comparison of these exotic sequences from Florida northward to Scotland (Fig.…”

Tectonic significance of Late Ordovician silicic magmatism, Avalon terrane, northern Antigonish Highlands, Nova Scotia 1 This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.2 Contribution to International Geological Correlation Programme (IGCP) Project 497.

“…650 and 480 Ma, Avalonia was one of several terranes (termed "peri-Gondwanan terranes," Nance and Thompson 1996) that was located along or adjacent to the northern margin of Gondwana (Fig. 1a;O'Brien et al 1983;Keppie 1985;van Staal et al 1998van Staal et al , 2009Murphy and Nance 2002;Cocks and Torsvik 2002;Nance et al 2010) although temporary seaways may have existed that allowed faunal endemism (Landing 1996(Landing , 2005. Reconstructions (Fig.…”