NORTHEY HILL "LINE", WESTERN NEW HAMPSHIRE: SINISTRAL SHEAR ZONE DUE TO LATE PALEOZOIC JOSTLING OF DEEP-SEATED LAURENTIA-GANDER SUTURE?

The Northey Hill “line” is a one to two km wide zone (NHZ) that extends from Northey Hill, where it offsets an antiform (Salmon Hole Brook and Garnet Hill synclines of Billings), SSW 80 km to the vicinity of Skitchewaug Mountain. The zone is characterized by steep, post-dome-stage foliation (for example the Sunday Mountain cleavage belt) and by lower metamorphic grade than rocks on either side. It lies east of and sub-parallel to the Mesozoic Ammonoosuc fault (AF).

In the Moosilauke 15’ Quadrangle the NHZ apparently follows the western margin of the Devonian Haverhill granodiorite. Farther south it consists of a zone of highly attenuated and locally intercalated Bronson Hill stratigraphic units, between the eastern limit of the Piermont allochthon (Rangeley stratigraphic sequence) and the western margin of the Indian Pond granodiorite, west of the Bronson Hill arch (BHA). The west side of the zone apparently moved down, dropping the allochthonous rocks to the level of the Bronson Hill rocks. Farther south, the Cornish nappe, which lies under the Piermont rocks, also moved down relative to the BHA. Nappe-stage foliation around the Lebanon dome is drawn into parallelism with and overprinted by the steep NHZ foliation. East of the dome discrete faults within the NHZ sinistrally offset and truncate nappe- and dome-stage folds in the Clough-correlative Hardy Hill Quartzite.

The NHZ continues SSW between the dome-stage Meriden antiform and the BHA to connect with the anastomosing, SSW-striking Westminster West fault system (WWF) south of Skitchewaug Mountain in Vermont. The AF crosses these older faults obliquely, with little apparent offset, because all are nearly vertical. To confirm the correlation of the NHZ and 300 Ma WWF, Ar-Ar ages on micas from the NHZ foliation would be helpful. Together the older faults represent a 140+ km-long, relatively straight, steep fault system, which suggests that a deep-seated discontinuity was responsible. Perhaps the NHZ is the surface manifestation of deep-seated Gander basement jostling against Laurentian basement along the suture during the Northfieldian precursor to the Alleghanian orogeny. It likely was reactivated during the Mesozoic, when the AF also formed along a similar path. Published detrital zircon studies suggest that the suture at the present-day surface lies well to the west.