COMPETING MODELS FOR THE ORIGIN OF THE SHELBURNE FALLS AND BRONSON HILL ARCS, NEW ENGLAND APPALACHIANS

Recent studies show that arc magmatism was widespread in the Appalachians and Caledonides between 480-470 Ma, coeval with the Shelburne Falls arc (SFA) in western New England. Paleomagnetic and fossil evidence from Newfoundland, however, demonstrates that coeval arcs developed near Laurentia, near Avalonia, and within Iapetus (Mac Niocaill and others, 1997); thus correlations based on age alone are tenuous. Karabinos and others (1998) argued that the SFA formed above an east-dipping subduction zone and collided with Laurentia during the Taconian orogeny (470-455 Ma). The younger Bronson Hill arc (BHA) (454-442 Ma, Tucker and Robinson, 1990) may have formed (1) above the same east-dipping subduction zone as the SFA (Ratcliffe and others, 1998), (2) above a new west-dipping subduction zone following the Taconian orogeny (Karabinos and others, 1998), or (3) far from Laurentia near the Avalonian margin (Van Staal and others, 1998). Because arc rocks in western New England range in age from 496 to 436 Ma, the first model requires subduction for 30 m.y. after initial collison of Laurentia and the SFA. It seems impossible to generate arc magmas for so long after closure of the ocean basin that separated Laurentia and the SFA. Furthermore, work in the Canadian Appalachians provides clear evidence for multiple Ordovician arcs as opposed to a single long-lived arc. A lack of paleomagnetic and fossil data make it difficult to rule out either the second or third model. The following observations favor the second model: (1) In western Conn. arc rocks coeval with the BHA intruded deformed and metamorphosed rocks correlated with the SFA. (2) In northern New Hampshire two sequences containing volcanics coeval with the SFA and BHA are separated by an unconformity (R.H. Moench, pers. com.), (3) The Connecticut Valley trough, which separates the SFA and BHA, contains rift volcanics that began to form soon after arc magmatism in the BHA ceased, consistent with both having formed above a west-dipping subduction zone.