Geochronologic studies in central New England; II, Post-Acadian hinged and differential uplift

(super 40) Ar/ (super 39) Ar analyses of muscovite, biotite, and K-feldspar from central New England reveal a remarkable pattern of mineral ages: the ages are progressively younger from central to western New Hampshire and rise sharply near the Vermont border to ages more typical of post-Acadian cooling. This distribution is attributed to differential uplift via isostatic rebound of an anomalously thickened crust at the Bronson Hill anticlinorium. This explanation requires that between 6 and 8 km of normal fault motion has occurred on structures in western New Hampshire, not previously recognized to have accommodated this kind or magnitude of displacement. This hinged, differential uplift occurred from approximately 360-170 Ma and is consistent with a time constant for rebound of approximately 80 m.y.