A COMPARATIVE ANALYSIS OF PRE-LATE ORDOVICIAN CRUSTAL 'BUILDING BLOCKS' OF THE NORTHERN AND THE SOUTHERN APPALACHIANS

The New York promontory serves as the divide between the northern and southern segments of the Appalachians. 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-Late Ordovician orogenic 'building blocks' in order to test the validity of the implication of independent lithotectonic histories for the two segments.

Three lithotectonic divisions characterize the entire orogen, including the Laurentian, Iapetan, and the peri-Gondwanan realms. The Laurentian realm, composed of native North American rocks, is remarkably uniform for the length of the orogen, recording the multistage Neoproterozoic-early Paleozoic rift-drift history of the Appalachian passive margin and its ultimate demise 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 here are not inconsistent with those of the northern Appalachians, although second order differences exist. 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. Southern equivalents of Avalonia and Meguma have yet to be recognized.

Our comparison indicates that, to a first order, the pre-Late Ordovician Appalachians 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.