THERMOCHRONOLOGIC EVIDENCE FOR POST-EARLY CRETACEOUS DISPLACEMENT OF THE BRONSON HILL TERRANE AND HARTFORD BASIN, CONNECTICUT AND MASSACHUSETTS

Apatite fission-track (AFT) ages for 29 samples of Paleozoic metamorphic rocks from two transects, east-west and north-south, across the Connecticut Valley Synclinorium, and Bronson Hill, Merrimack and Central Maine Terranes of Massachusetts and Connecticut indicate that unroofing in these regions occurred from Late Jurassic through Early Cretaceous. Three new samples of Triassic-Jurassic arkoses from the Hartford basin in Connecticut yield AFT ages of 139-152 Ma suggesting a similar timing of exhumation to those from the crystalline rocks and are consistent with previously published AFT ages. 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 an eastward rotation of ~10° in each area. Zircon fission-track (ZFT) ages from the Hartford basin arkoses (167-238 Ma) and northern Bronson Hill (147-196 Ma) 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 postdating 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 occurred at times younger than the youngest AFT ages or < 100 Ma, Late Cretaceous.