CRETACEOUS UNROOFING RATES FOR THE PRESIDENTIAL RANGE, NEW HAMPSHIRE DETERMINED BY THE RELIEF METHOD USING APATITE FISSION-TRACK AGES

Apatite fission-track ages (AFT) are being determined on eleven samples of the Siluro-Devonian Rangeley and Littleton Formations to calculate an unroofing rate using the relief method. From 408 to 360 Ma, these rocks experienced multiple ductile deformations and several metamorphisms, reaching the sillimanite zone during the Devonian Acadian Orogeny. Following orogenesis, the only tectonic event that occurred to possibly disrupt the slow cooling and erosion of these rocks was extension due to the Mesozoic rifting of Pangea. The samples were taken about every 500 vertical feet from the top (6,288 ft) to the bottom (1,350 ft) of Mt. Washington, NH. The AFT ages we have determined to date are: 152 ± 17 Ma at 6,200', 137 ± 14 Ma at 4,283', 101 ± 10 Ma at 1,674', and 109 ± 10 Ma at 1350'. The lower two AFT ages are within error of existing AFT ages that define a regional northeast-trending zone of young Late Cretaceous (70-118 Ma) AFT ages across central New Hampshire. This zone has been interpreted to be associated with a long-lived thermal anomaly in the Cretaceous (Great Meteor Hotspot) that resulted in regional extension and unroofing (Roden-Tice et al., 2009). The unroofing rate, as determined by the relief method between the older summit age, younger intermediate elevation age, and youngest lower elevation age, is 0.027 mm/yr from the Late Jurassic to Early Cretaceous. This unroofing rate is comparable to that determined for High Peak summits and fault valleys (areas of maximum relief) in the Adirondack Mountains of northern New York (0.033 mm/yr) during the same time period using thermal history modeling of apatite fission-track length distributions (Roden-Tice and Tice, 2005). The AFT relief method rate reported here is consistent with that given by Eusden and Lux (1994) for the same locations using ⁴⁰Ar/³⁹Ar muscovite ages that gave an exhumation rate of 0.04 mm/yr in the Late Carboniferous to Early Permian. These rates collectively constrain the post-orogenic thermal evolution and late Paleozoic to Mesozoic unroofing history of the northern Appalachians. Collectively, the AFT and ⁴⁰Ar/³⁹Ar data for Mt. Washington show a progressive decrease in the unroofing rate from the Permian to the Cretaceous with no apparent acceleration of unroofing during the Triassic to Jurassic rifting.