Early Cretaceous Normal Faulting in Southern New England: Evidence from Apatite and Zircon Fission‐Track Ages

Abstract

New apatite and zircon fission‐track (AFT, ZFT) ages from Mesozoic sediments and adjacent crystalline rocks from southern New England reveal age gradients from Middle Jurassic to Early Cretaceous. These gradients reflect the rotation of crustal blocks after the setting of the youngest AFT ages (∼140 Ma). The AFT ages of 168–98 Ma for 32 samples of Paleozoic metamorphic rocks east and west and within the Hartford Basin of Massachusetts and Connecticut indicate that unroofing in these regions occurred from Late Jurassic through Early Cretaceous. In both the Hartford Basin and rocks from crystalline terranes east of the basin, AFT ages show a regional trend of increasing age to the east, suggesting a down‐to‐the‐east rotation of ∼10° in each area. The ZFT ages from the Hartford Basin arkoses (167–238 Ma) and northern Bronson Hill (147–196 Ma) rocks support this gradient of eastward increase in AFT age. A south‐to‐north gradient of decreasing AFT age (139–107 Ma) for the sedimentary rocks in the Hartford and Deerfield Basins of Connecticut and Massachusetts suggests a 1°‐hinged uplift to the north that postdates the youngest AFT age of ∼100 Ma. Final juxtaposition of AFT ages across the eastern Border fault between the Hartford Basin and Bronson Hill terrane in Massachusetts and Connecticut indicates displacement younger than the youngest AFT ages of ≤100 Ma (Late Cretaceous). Thus, the age of the graben structure of the Hartford Basin is Cretaceous, and this structure cannot be cited as evidence that these basins are Early Mesozoic “rift” basins.