ACCRETION OF NORTHERN APPALACHIAN ARC TERRANES

Compilation of numerous studies shows that western New England contains arc-related igneous rocks ranging in age from 496 to 428 Ma. Because arc terranes provide important constraints on along-strike correlations and the tectonic assembly of the Appalacians, it is critical to delineate separate arcs and to determine where they formed in Iapetus. For example, recent studies show that arc magmatism was widespread in the Appalachians and Caledonides between 480-470 Ma. Paleomagnetic and fossil evidence from Newfoundland, however, demonstrates that coeval arcs developed near Laurentia, near Gondwana, and within Iapetus (MacNiocaill and others, 1997); thus correlations based on age alone are tenuous. Karabinos et al. (1998) argued that the Shelburne Falls arc (SFA) formed on a rifted fragment of Laurentia and collided with the Laurentian mainland during the Taconic orogeny (470-455 Ma). They suggested that the younger Bronson Hill arc (BHA) (454-442 Ma, Tucker and Robinson, 1990) formed after a reversal in subduction polarity above a west-dipping subduction zone near Laurentia. In contrast, van Stall et al. (1998) correlated the BHA with the peri-Gondwanan Popelogan-Victoria arc, and suggested a Late Ordovician collision with Laurentia. A lack of paleomagnetic and fossil data in western New England makes it difficult to test these models, and geochemical studies have been inconclusive. The following observations suggest, but do not prove, that the BHA formed near Laurentia: (1) In western CT and MA arc-related rocks (453-428 Ma) intruded Grenville basement and previously accreted SFA rocks, indicating that a west-dipping subduction zone existed under Lauretia. (2) Volcanics rocks coeval with the SFA and BHA are separated by an unconformity in northern New Hampshire (Moench and Aleinikoff, 2003), (3) The Connecticut Valley trough, which separates the SFA and BHA, contains rift related volcanics that began to form soon after arc magmatism in the BHA ceased, consistent with a back-arc basin forming above a west-dipping subduction zone. To resolve the tectonic significance of the BHA, future studies should search for evidence of crustal contamination in arc-related igneous rocks and establish the provenance of sedimentary rocks intruded by arc plutons and interbedded with arc volcanics.