Latest Paleozoic through Mesozoic faults in north-central Massachusetts and their correlations with New Hampshire

Several faults in south-central New Hampshire can be extended into Massachusetts (MA) as a result of detailed mapping in both states since publication of the MA state bedrock geologic map in 1983. Many of these faults delineate and/or cut Devonian metamorphic isograds in the Silurian Merrimack Belt in northern MA, and juxtapose chlorite-grade rocks in the Nashua sub-belt (NSB) between lithologically similar middle- to upper amphibolite-facies rocks on either side.Recent mapping in the NSB, combined with previous studies, suggest it may represent a graben initially formed during latest Paleozoic transtension contemporaneous with formation of the Narragansett Basin in southeastern MA and RI. Mylonites along the Silver Hill-Wekepeke Fault (Robinson, 1981), bounding the western edge of the NSB, show east-side-down normal motion and west-side down normal motion along the Clinton-Newbury Fault Zone (CNFZ; Goldstein, 1994) which bounds the NSB’s southeastern margin. A possible extension of the Flint Hill fault system (NH) forms the eastern edge of the NSB offsetting the CNFZ with normal west-side down motion near Ayer, MA. Late brittle normal faults in the NSB are abundant. Late, low-T˚, west-side-down shear zones in the Nashoba Terrane and similar rocks to the south may also be related to down-dropping of the NSB.AFT ages were collected across north-central MA to constrain its late uplift history. A ~127 Ma AFT age in the NSB is discontinuous with AFT ages in the belts adjoining it, with ~182-144 Ma ages west across the Wekepeke fault and ~160-167 Ma east across the CNFZ. To the west, the brittle southern extension of the Pinnacle Fault in NH (Stodge Meadow Pond fault of Peterson, 1984) follows the western edge of the Fitchburg plutons in MA while a well-exposed west-side down brittle normal fault system, possibly the southward extension of the Campbell Hill Fault (NH), is developed along their eastern edge. AFT ages of ~144-136 Ma immediately west of the Pinnacle Fault in MA are discontinuous with ~117-115 Ma ages immediately to the east within the Fitchburg plutons. A single ~106 Ma age in the plutons west of the Campbell Hill Fault in MA is discontinuous with ~128-123 Ma ages to the east of it. The discontinuities amongst AFT ages across these faults suggest that they may have been active through the Cretaceous.