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 toward 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 per-alkaline 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-Bonnechere 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 Iapetus along the Blue-Green-Long axis. The trend of the Monteregian intrusions extends from the Ottawa-Bonnechere graben to the Sutton Mountains salient, suggesting reactivation of another zone of weakness.