Appalachian salients and recesses: Late Precambrian continental breakup and the opening of the Iapetus Ocean

Abstract

Appalachian salients (convex toward the craton) and recesses (concave towaid the craton) are inherited from the initial breakup of a continental mass by the intersection of rift valleys radiating from triple junctions, beginning about 820 m.y. ago. The trace of axes of anticlinoria exposing crystalline rocks older than 1 b.y., here called the Blue-Green-Long axis after major Precambrian massifs, roughly coincides with the western margin of a series of late Precambrian water-filled graben. The rift system east of the axis probably did not open far enough to produce significant oceanic crust but, like the Triassic basins of a later opening cycle, was an initial stage in the opening of an ocean basin ultimately located still farther east. Although metamorphosed basalt is widely distributed along the Blue-Green-Long axis, known upper Precambrian or Eocambrian metamorphosed rhyolite is restricted to three salients: Mt. Rogers, Virginia' South Mountain, Pennsylvania; and the Sutton Mountains, Quebec, Canada. These rhyolites are peralkaline in affinity, are constituents of basalt-rhyolite suites, and are associated near Mt. Rogers and South Mountain with consanguineous aegirine granite. Suggestions of failed-arm troughs or aulacogens are found at all three salients. The basal Cambrian clastic sequence is thickest near the Mt. Rogers and South Mountain salients, Basal conglomerates are coarsest near Mt. Rogers. Geophysical anomalies suggest an area of buried mafic igneous rocks projecting northwest from the South Mountain salient. The Ottawa-Bonnech•re graben, the most convincing of the failed-arm troughs, strikes into the Sutton Mountains salient. The Cambrian carbonate bank in northwestern Vermont faces north into this graben, which also contains alkaline ring complexes of probable Cambrian age at its western end at Lake Nipissing, Ontario. Recesses near Roanoke, Virginia, and New York City are triple junctions whose failed arms have been carried away. An anomalously thin basal Cambrian clastic sequence between western Massachusetts and eastern Pennsylvania may reflect a structurally high area that coincides with the concave side of the New York recess. The Wilson cycle implies that fracture zones may be reactivated as zones of weakness in subsequent events. The incipient opening of the present Atlantic (as evidenced by the trends of the Triassic basins between North Carolina and Massachusetts)was essentially parallel to and nearly coincident with the incipient opening of the lapetus along the Blue-Green-Long axiS. The trend of the Monteregian intrusions extends from the Ottawa-Bonnech•re graben to the Sutton Mountains salient, suggesting reactivation of another zone of weakness. to avoid confusion with the early stages of opening of the present Atlantic.My concern with the problem started in 1962 during a detailed study of Upper Precambrian volcanic rocks at Mt. Rogers in the Blue Ridge of southwestern Virginia. I became curious as to why upper Precambrian metamorphosed basalt Copyright ¸ 1976 by the American Geophysi...

Satellite and surface geophysical expression of anomalous crustal structure in Kentucky and Tennessee

“…We also note the remarkable likeness between the gravity anomaly due to the Kentucky body and the Scranton Gravity High [31] both in dimensions and amplitude (Figure 7). The anomaly was interpreted [31] as the expression of a rift structure, but presumably it is related to the following Wilson cycle in which the Iapetus or Proto-Atlantic Ocean was formed during the late Precambrian and early Paleozoic [32]. Curiously, however, the Scranton structure nas no prominent associated magnetic anomaly.…”

Great Lakes tectonic zone in Marquette area, Michigan: Implications for Archean tectonics in north-central United States

“…In this interpretation, the Superior margin was a rifted continental margin. Two interpretations have been made for the origin of the Paleozoic continental margin: (1) rift segments offset by transform faults, as suggested by Thomas (1977Thomas ( , 1983, or (2) intersections between active rift arms at triple junctions (Rankin, 1976). Of these two suggestions, the rift-transform mechanism seems the more likely, with the Minnesota and Marenisco segments being the rifted segments ( fig.…”

Paleomagnetism of Late Hadrynian Fourchu gabbro, southern Cape Breton Island, Nova Scotia.

“…Geological arguments for a possible displacement of Avalon are based on 1) dissimilar pre-Late Paleozoic geologic records of the Avalon Zone and other zones of the northern Appalachians, 2) different Early Paleozoic faunas for Avalon and cratonic North America (WILSON, 1966;MCKERROW and ZIEGLER, 1972;WHITTINGTON and HUGHES, 1972;MCKERROW and COCKS, 1976;COCKS and FORTEY,1982;NEWMAN,1984), 3) actual fragments of probable Paleozoic crust between Avalon Zone and cratonic North America (ZEN and PALMER, 1981;WILLIAMS and HATCHER,1983;RAST and SKEHAN,1983;WILLIAMS, 1984). Precambrian rocks similar to the ones in the Avalon Zone occur throughout the Appalachian orogen in Newfoundland, Nova Scotia, New Brunswick, eastern Massachusetts, southeastern United States (Carolina State Belt) (SCHENK, 1972(SCHENK, , 1978RANKIN, 1976;WILLIAMS, 1979) and on the eastern side of the Atlantic Ocean (Wales, Brittany, Iberian Peninsula, Czechoslovakia, Turkey and northeast Africa).…”

“…The tectonic models include volcanic island arc (HUGHES and BRUCKNER, 1972), ensialic island arc (RAST et al, 1976), subduction and Basin and Range type rifting (PAPEZIK, 1972;SCHENK, 1978), continental extension and rifting (RANKIN, 1976), transitional tectonic regime (KING, 1982) resulting from continental margin orogenesis (RAST et al, 1976) and intraplate orogenesis (STRONG et al, 1978), Pan-African terrane predating the opening of an Early Paleozoic proto-Atlantic ocean distinct geological province or "suspect terrane" (WILLIAMS and HATCHER, 1982) moving independently from other Atlantic borderlands.…”