EVIDENCE FOR DIFFERENTIAL UNROOFING IN THE ADIRONDACK MOUNTAINS, NEW YORK STATE DETERMINED BY APATITE FISSION-TRACK THERMOCHRONOLOGY
Apatite fission-track ages of 168 to 83 Ma for
thirty-nine samples of Proterozoic crystalline rocks, three samples of Cambrian Potsdam
sandstone, and one Cretaceous lamprophyre dike from the Adirondack Mountains in New York
state indicate unroofing in this region occurred from Late Jurassic through Early
Cretaceous. Samples from the High Peaks section of the Adirondack massif yeilded the
oldest apatite-fission track ages (168-135 Ma) indicating that it was exhumed first.
Unroofing along the northern, northwestern, and southwestern margins of the Adirondacks
began slightly later as shown by younger apatite fission-track ages (146-114 Ma)
determined for these rocks. This delay in exhumation may have resulted from burial of the
peripheral regions by sediment shed from the High Peaks. Apatite fission-track ages for
samples from the southeastern Adirondacks are distinctly younger (112-83 Ma) than those
determined for the rest of the Adirondack region. These younger apatite fission-track ages
are from a sectionof the Adirondacks dissected by shear zones and
post-Ordovician
north-northeast trending normal faults. Differential unroofing may have been accommodated
by reactivation of the faults in a reverse sense of motion with maximum compressive
stress, s1,
oriented west-northwest. A change in the orientation of the post-Early Cretaceous
paleostress field is supported by a change in the trend of Cretaceous
lamprophyre dikes
from east-west to west-northwest (McHone 1978).
