COMPOSITE ARC TERRAINS IN THE NEW ENGLAND APPALACHIANS

Linear belts of arc-related rocks are commonly used as markers for identifying terrain boundaries in orogens. However, their apparent simplicity may belie a more complicated architecture. For example, the controversy whether the Shelburne Falls arc (SFA) and the Bronson Hill arc (BHA) represent two separate arcs or one long-lived arc was based, in part, on a false premise that they are simple structures. More detailed analysis and new isotopic data indicate that they are both composite arcs, and that these two belts preserve parts of three separate arcs that formed in different subduction zones.

The SFA in CT, MA, and VT is dominated by 485-470 Ma plutons that formed above an east-dipping subduction zone prior to the Taconic orogeny (Karabinos et al., 1998). In CT younger arc-related rocks (453-436 Ma, Sevigny and Hanson, 1995) intruded the SFA. The BHA in NH, MA, and CT is dominated by 460-440 Ma plutons and volcanics (Tucker and Robinson, 1990; Aleinikoff et al, 2007) but in northern NH rocks coeval with the SFA are also exposed (Moench and Aleinikoff, 2003).

Karabinos et al. (1998) argued that the BHA formed above a west-dipping, post-Taconic subduction zone. Karabinos et al., (2003) suggested that younger rocks in the SFA in CT and older rocks in the BHA in NH reflect spatial overlap of two temporally distinct peri-Laurentian arcs, although Hibbard et al. (2007) posited a Ganderian affinity for the BHA. New evidence indicates that the BHA in CT is fundamentally different from contiguous rocks to the north. Aleinikoff et al. (2007) used isotopic geochemistry to argue persuasively for a Ganderian affinity for BHA rocks in CT. In contrast, Dorais et al. (2006) presented compelling data in favor of a Laurentian affinity for the BHA in NH. Thus, the best available age and isotopic data indicate that the BHA is a composite of the SFA, the BHA, and a Ganderian arc. The Red Indian line of van Staal (2003) is east of the BHA in NH and west of the BHA in CT. The juxtaposition of the three arcs into a linear belt reflects a reversal in subduction polarity near the Laurentian margin, strike-slip faulting, and/or underthrusting of the Ganderian margin below Laurentia, as suggested by the deeper level of exposure in CT. Tectonic events in the Salinic, Acadian, and Alleghenian may all be possible explanations for the complexity of the BHA.