CRETACEOUS THERMOCHRONOLOGIC DISCONTINUITIES ALONG THE VERMONT-NEW HAMPSHIRE BORDER, NORUMBEGA FAULT SYSTEM, ME AND IN THE ADIRONDACK MOUNTAINS, NY: EVIDENCE FOR LATE MESOZOIC FAULTING AND DIFFERENTIAL EROSION

Apatite fission-track (AFT) ages from along the Vermont-New Hampshire border, across the Norumbega fault system in southern Maine, and from the Adirondacks in New York reveal significant time-temperature discontinuities in each of these regions. In the Connecticut River Valley, AFT ages of 79 and 83 Ma from east of Orford, NH contrast sharply with ages of 116 and 121 Ma near Strafford and West Hartford, VT, respectively. A Bellows Falls pluton sample to the south along the VT-NH border yielded an older AFT age of 106 Ma. These age patterns are consistent with 40Ar/39Ar age mineral age patterns previously published by Harrison et al. (1989). The Flying Point fault in the Casco Bay region of southern Maine is a major splay of the regionally extensive Norumbega fault system. Previously determined 40Ar/39Ar mineral ages revealed a significant thermochronological discontinuity across this fault that was suggestive of an episode of post-Paleozoic faulting, although a lower age limit for this faulting could not be determined. AFT age determinations for granites from west of the Flying Point fault range from 89 to 112 Ma while those to the east range from 140 to 159 Ma. The discontinuity in AFT ages suggests a significant episode of east-side down normal faulting along the Norumbega fault system in Late Cretaceous (or younger) time. Previously determined AFT ages from New York indicate a discontinuity between the Precambrian crystalline rocks in the High Peaks region (170-130 Ma) and those in heavily faulted southeastern Adirondack Mountain area (110-80 Ma). (U-Th)/He ages determined for Mt. Marcy (115 Ma), Whiteface Mt. (126 Ma) and Sawteeth Mt. (136 Ma) from the High Peaks yielded cooling rates ranging from 0.7°C/Myr to 2.4°C/Myr based on their previously published AFT ages and closure temperatures of 61°C for He retention and 100°C for fission-track retention in apatites. Assuming a geothermal gradient of 25°C/km, an unroofing rate ranging from 0.03 to 0.1 km/Myr was calculated. This data is consistent with a previously published range of 0.03 to 0.1 km/Myr for the southeastern Adirondack area. The agreement of these unroofing rates across two regions with significantly different AFT ages suggests the age discontinuity may be the result of differential erosion rather than Late Cretaceous reactivation older faults.