THE FOSTER HILL-THRASHER PEAKS DETACHMENT, NORTHERN NEW ENGLAND

Contrary to repeated denials of the existence of the Foster Hill detachment fault (FHD), mapping since 1984 along the northern Connecticut Valley documents its presence at least from near Fairlee, Vt. >100 km to North Stratford, N.H. There, the FHD is cut by the Jurassic Gore Mountain plutons. Apparently the same structure reappears north of the plutons as the Thrasher Peaks detachment fault (TPD), which continues another ~100 km through northernmost New Hampshire, thence along the Quebec-Maine border where it wraps around the northwest side of the structurally resistant Chain Lakes massif (CLM). Published geophysical studies across the CLM indicate that the TPD originated as a moderately NW-dipping normal fault that extends to a depth of at least 10 km under the Connecticut Valley trough (CVT). In contrast, structural studies by R.G. Marvinney in an east-trending bend farther NE indicate several km of right-slip displacement. This apparent conflict can be explained by a two-stage sequence of faulting: 1) pre-Acadian normal faulting; 2) Acadian compression. A similar model likely applies to the FHD as well.

These interpretations are based on mapping at 1:48,000 along the FHD, and at 1:250,000 along the TPD. This display presents selected map and section views at critical places along the FHD-TPD. For direct comparison to very different interpretations see Rankin et al (2007, American Journal of Science, v. 307, p. 216-264, esp. figs. 1-3). Both views are ripe for spirited discussion.