SLOW LATE PALEOZOIC EXHUMATION IN THE PRESIDENTIAL RANGE OF NEW-HAMPSHIRE AS DETERMINED BY THE 40AR 39AR RELIEF METHOD/

Ar-40/Ar-39 mineral ages from muscovite were used to determine exhumat ion rates in the multiply deformed and metamorphosed Silurian and Devo nian 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 f t [1900 m]) is 304 +/- 3 Ma and the youngest (Route 16, elevation 1350 ft [410 m]) is 274 +/- 3 Ma. The age and elevation data yield an exhu mation rate of 0.04 mm/yr. The exhumation rate indicates that (1) the Middle Pennsylvanian through Early Permian was a period of very slow e xhumation; (2) rapid uplift associated with the Acadian orogeny must h ave 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 deep ly eroded orogenic belts, if the topographic relief is adequate and co oling was slow.