MID-CRETACEOUS FAULT REACTIVATION IN CENTRAL NEW ENGLAND: EVIDENCE FROM (U-TH)/HE AND APATITE FISSION-TRACK THERMOCHRONOLOGY

Apatite (U-Th)/He and fission-track ages were determined for 14 crystalline rocks from northern Vermont and New Hampshire, southern Maine, Connecticut and eastern Massachusetts. Previous studies have shown apatite fission-track (AFT) age discontinuities across several faults throughout New England. In this investigation, we extend our earlier study of the Norumbega fault zone to the north of Casco Bay. AFT ages from this region were Early Cretaceous and comparable within analytical error ranging from 111-133 Ma on the west side to 109-140 Ma on the east side. (U-Th)/He ages for six of these samples were very close to or indistinguishable from the AFT ages and ranged from 102-126 Ma. The lack of an AFT age discontinuity along this part of the Norumbega fault zone suggests that no significant displacement has occurred in this area during the Early Cretaceous. Motion on the Ammonoosuc fault also dissipates to the north in northeastern Vermont and western New Hampshire where (U-Th)/He and AFT ages from 79-94 Ma show no discontinuity across the boundary.

The Merrimack and Putnam-Nashoba terranes of northeastern Massachusetts yielded comparable Early Cretaceous AFT ages (155-163 Ma) indicating there has been no significant vertical offset on the Clinton-Newbury fault since the Early Cretaceous. In contrast, an ~40 Ma AFT age difference (106 vs.160 Ma) exists between the Merrimack and Central Maine terranes across the Wekepeke fault showing Middle Cretaceous west-side up motion. Displacement on the Wekepeke fault is similar to that of the Norumbega fault zone on strike in southern Maine. Thus, regional kilometer scale mid-Cretaceous reactivation is confirmed for east-central New England, in addition to that previously established for other faults active in the Mesozoic.