40Ar/39Ar 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 ∼360-170 Ma and is consistent with a time constant for rebound of ∼80 m.y.

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