EVIDENCE FOR WIDESPREAD EARLY CRETACEOUS UNROOFING IN THE SOUTHEASTERN ADIRONDACKS, CHAMPLAIN VALLEY AND VERMONT BASED ON APATITE FISSION-TRACK AGES

The region of Early to Late Cretaceous unroofing has been extended from the southeastern Adirondack Mountains in New York state eastward through the Lake Champlain Valley and across Vermont to the Connecticut River Valley based on 43 new apatite fission-track (AFT) ages. An existing trend of 80-110 Ma AFT ages determined for samples of Proterozoic crystalline rocks and Potsdam sandstone from the southeastern Adirondack Mountains continues throughout central eastern New York into the heavily normal faulted areas surrounding Lake George (89-105 Ma), Ticonderoga (89 Ma), Sacandaga Lake (97-110 Ma) and the McGregor fault system (107 Ma). AFT ages ranging from 90-120 Ma extend across the Champlain Valley into western Vermont and remain in the 110-120 Ma range eastward to the Connecticut River Valley.

Proterozoic metamorphic samples from the core of the Green Mountains in Vermont yield AFT ages ranging from 113 Ma in the north (Ripton) to 103 Ma in the south (Bennington). In eastern New York state, Early Cretaceous AFT ages continue south in the Rensselaer Graywacke increasing from 116 Ma east of Troy to 142 Ma near Nassau.

A significant increase in AFT age to 170 Ma occurs in the Precambrian gneisses at Day Mountain in western Massachusetts and continues for Paleozoic metamorphic samples (~150 Ma) from closer to the western margin of the Hartford basin in central Massachusetts. The AFT age discontinuity between southern Vermont (~100 Ma) and northwestern Massachusetts (~170 Ma) suggests differential unroofing during the Late Jurassic and Early Cretaceous in this region.

U-Th/He analyses for three Proterozoic crystalline samples from the southeastern and northwestern Adirondacks yield cooling ages within error of and slightly younger than the AFT ages determined for those samples, 90-138 Ma (He) and 83-138 Ma (AFT) suggesting uplift was rapid and continuous in these regions during the Early and Late Cretaceous.