LATE JURASSIC TO EARLY CRETACEOUS EXHUMATION IN EASTERN MASSACHUSETTS AND CONNECTICUT BASED ON APATITE FISSION TRACK AGE GRADIENTS AND DISCONTINUITIES

New apatite fission-track (AFT) ages for 14 samples from eastern Massachusetts reveal both gradients and discontinuities adjacent to and across faults that suggest reactivation of Paleozoic faults, and tilting of Paleozoic thermo-tectonic terranes. In a SE traverse across the Central Maine terrane, AFT ages range from ~144 to ~106 Ma from Gardner to Sterling (NW of the Wekepeke Fault, (WpF)) indicating a ~40 m.y. age gradient, and middle Cretaceous or younger exhumation. These ages contrast with Late Jurassic ages of ~160 Ma in the adjacent Merrimack terrane, revealing a > 50 m.y. age discontinuity across the WpF. In a second SE traverse across part of the Avalon terrane AFT ages range from Late Triassic to Late Jurassic from Southville to Milford, indicating a ~50 m.y. age gradient, and Early Cretaceous or younger exhumation. These ages contrast with preliminary data from the Nashoba terrane, with cooling ages of ~160 Ma, revealing another age discontinuity, here of > 40 m.y. across the Bloody Bluff fault zone, again indicating km-scale Jurassic reactivation. All of these data contrast with the Late Jurassic AFT ages (~145 Ma) of four samples from near Plainfield and Danielson, Connecticut. These sample locations cross the Lake Char fault (a possible extension of the Bloody Bluff fault) but show no evidence for any age discontinuity across this fault.

Exhumation without tilting would not produce the two age gradients identified here. On the contrary, the gradients can be most readily explained by rotation of these blocks with a down to the northwest, and up to the southeast sense. This younging to the SE gradient is similar to that found in the Newark basin (Steckler et al. 1993), but opposite to that found in the Hartford basin (Roden-Tice and Wintsch, 2001). All of these gradients can be understood in the context of a set of synthetic listric normal faults with only the eastern border fault of the Hartford basin antithetic. They are probably joined by a major detachment fault that extends farther east, into the Gulf of Maine. Thus some Paleozoic normal faults were reactivated, and Jurassic and Cretaceous extension transmitted west of the eventual position of Atlantic opening by movement of this major basal detachment fault.