CRETACEOUS UNROOFING RATES FROM APATITE FISSION-TRACK AGES, COG RAILROAD, MT. WASHINGTON, NEW HAMPSHIRE

This is a follow up study to Roden-Tice et.al. (2011) using apatite fission-track ages and modeling to determine the Cretaceous thermal history of Mt. Washington. The unroofing rate of Mt. Washington, NH is being calculated using the apatite fission-track (AFT) relief method for thirteen samples along the Cog Railroad on the mountain's western slope. Samples were collected approximately every 150 vertical meters from the summit (1914 m) to the base (540 m) of Mt. Washington. Samples 1 through 7 (from the summit down) are of the Devonian Littleton Schist. Samples 8 through 11 are of the Silurian Rangeley Formation. Samples 12 and 13 are of the Carboniferous Bretton Woods Granite. Each lithology has abundant enough apatite concentrations to perform this study. The AFT ages determined thus far are: 148 ± 15 Ma at 1914 m, 147 ± 15 Ma at 1887 m, 132 ± 14 at 1191 m, 103 ± 11 at 931 m and 89.1 ± 10 at 540 m. These are analogous ages to those determined along Mt. Washington's Auto Road at similar elevations. These values yield a preliminary exhumation rate of 0.023 mm/yr between 150 Ma and 80 Ma, which is comparable to the average exhumation rate of 0.027 mm/yr calculated for the eastern slope of Mt. Washington along the Auto Road (Roden-Tice et. al., 2011). The unroofing history for the Auto Road was divided into three distinct periods of changing exhumation; from 160 Ma to 140 Ma exhumations was 0.03 mm/yr., from 140 Ma to 110 Ma exhumation was 0.01 mm/yr. and from 110 Ma to 100 Ma exhumation was 0.04 mm/yr. Counting is ongoing and periods of faster and slower unroofing, as evidenced from the Auto Road, require completion of Cog Railroad AFT ages. Once done, track length frequency distributions for each sample will be modeled to indicate the time-temperature path through the partial annealing zone (PAZ), 60°C – 100°C, where tracks will shorten proportionally to the amount of time spent at a given temperature within this range. Modeling of the time-temperature path through the PAZ along the Auto Road revealed differential exhumation from 130 Ma to 60 Ma. The base sample initially mirrors the path of the summit sample, but at 110 Ma exhumes much faster than summit samples possibly due to incision of Cretaceous river systems. Based on the Auto Road data, it can be expected that evidence for differential exhumation will be present in the Cog Railroad samples.