LATE MESOZOIC EXHUMATION OF THE NORTHERN APPALACHIAN OROGEN: INFORMATION FROM APATITE FISSION TRACK AGES

Patterns of previously determined apatite fission-track (AFT) ages from northern New England have revealed distinct age discontinuities across major orogen-parallel fault systems and suggest these faults likely accommodated significant (over a km) vertical movement in Cretaceous time. The data presented below represent early results from a comprehensive FT study in New England aimed at evaluating the role of these orogen parallel fault systems in the post-orogenic exhumation history of the region.

In Maine, AFT ages have been determined along several sampling traverses across the regionally extensive Norumbega fault system. Previously published AFT ages from the Casco Bay region revealed a sharp age discontinuity across this structure with significantly older ages being found on the southeastern side of the fault system. New AFT ages determined along the strike of the Norumbega fault system both northeast and southwest of the Casco Bay area reveal similar age patterns (111-133 Ma on the NW side versus 126-167 on the SE side) although there is some overlap in places and the age discontinuity does not appear to be as dramatic. This may suggest that Late Mesozoic movement associated with the Norumbega fault system varies along its length or it may be focused on other structures. Ongoing research will aid in evaluating the role of the Norumbega fault system in the Late Mesozoic exhumation of the region.

Across Vermont, AFT ages range from ~100-120 Ma from west to east and increase to ~140 Ma in south central Vermont near the Massachusetts border. These AFT ages suggest that Early Cretaceous unroofing occurred throughout Vermont. A discontinuity to younger AFT ages of ~70-80 Ma occurs across the Ammonoosuc Fault in the Connecticut River Valley in New Hampshire suggesting possible Cretaceous reactivation of this fault. AFT ages increase to the east away from the fault to 127 Ma for the Concord Granite near Concord, NH. Work is currently in progress across several other orogen parallel faults in New England to determine if these structures were active in the Late Mesozoic. The structures include the Campbell Hill and Flint Hill Faults in southeastern New Hampshire, the Clinton-Newbury and Bloody Bluff faults in Massachusetts and the Lake Char Fault in Connecticut.