HOW DO OROGENIES END? AN EXAMPLE FROM THE TACONIC OROGENY IN THE NORTHERN APPALACHIANS

A viable tectonic model must not only explain the architecture of an orogenic belt in terms of plate interactions, but also account for how the orogeny ended. The Taconic orogeny is commonly attributed to collision of Laurentia with an arc that formed above an east-dipping subduction zone. The end of the Taconic orogeny must be related to a reconfiguration of plate boundaries that transferred convergence from the orogen to a new subduction zone. Accurately determining the timespan of the Taconic orogney in western New England and Quebec is critical to this problem. Currently the orogeny is constrained by the oldest metamorphic cooling ages of Laurentian rocks (ca. 470 Ma; Laird et al., 1984, Castonguay et al; 2001) and deposition of the youngest syn-orogenic flysch in the Taconic thrust belt (ca. 452 Ma, Bradley, 1989, using the timescale of Tucker and McKerrow, 1995). Karabinos et al. (1998) argued that the Shelburne Falls arc (485 to 470 Ma) collided with Laurentia rather than the Bronson Hill arc (454 to 442 Ma; Tucker and Robinson, 1990). Because a continent-arc collision stops the continuous supply of oceanic lithosphere to the subduction zone, Karabinos et al. (1998) also hypothesized that the Bronson Hill arc formed above a new west-dipping subduction zone near the Laurentian margin, after a flip in subduction polarity. Although it is possbile that the Bronson Hill arc formed above an east-dipping subduction zone far from the Laurentian margin (e.g. Van Staal et al., 1998), the spatial overlap of older and younger arc-related rocks in Connecticut and New Hampshire suggests that the Bronson Hill arc formed near Laurentia. Because the Bronson Hill arc began to form at approximately the same time that the Taconic orogeny ended, it is likely that initiation of the new west-dipping subduction zone was instrumental in bringing the orogeny to a close. Prevalent 40Ar/39Ar cooling ages in western New England in the range of 450 to 440 Ma probably record orogenic collapse and rapid exhumation of rocks in the Taconic thrust belt following polarity reversal, analogous to the present situation in Taiwan.