Abstract

40Ar/39Ar mineral ages from muscovite were used to determine exhumation rates in the multiply deformed and metamorphosed Silurian and Devonian metasedimentary rocks exposed on Mount Washington, New Hampshire. The muscovite ages increase progressively from the bottom to the top of the mountain; the oldest (Mount Washington summit, elevation 6200 ft [1900 m]) is 304 ± 3 Ma and the youngest (Route 16, elevation 1350 ft [410 ml) is 274 ± 3 Ma. The age and elevation data yield an exhumation rate of 0.04 mm/yr. The exhumation rate indicates that (1) the Middle Pennsylvanian through Early Permian was a period of very slow exhumation; (2) rapid uplift associated with the Acadian orogeny must have terminated by at least 305 Ma; (3) sometime after 274 Ma, renewed uplift created the present topography of the Mount Washington massif; and (4) the relief method can be used successfully in older, more deeply eroded orogenic belts, if the topographic relief is adequate and cooling was slow.

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