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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...

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...