CRETACEOUS EXHUMATION OF KILOMETER-SCALE RELIEF AND DEVELOPMENT OF STEADY-STATE TERTIARY TOPOGRAPHY AT MT. WASHINGTON, NH BASED ON APATITE FISSION-TRACK ANALYSIS

Earlier this year (Roden-Tice et al. 2012) we proposed that the topography around Mt Washington was established in Late Cretaceous to Paleogene times, and has been preserved ever since. We came to this conclusion by combining the muscovite ⁴⁰Ar/³⁹Ar cooling ages (304 to 274 Ma; Eusden and Lux, 1994) with AFT (cooling) ages for sample sets collected on the east side of Mt. Washington on the Auto Road. From this integration we have determined that Mt. Washington cooled throughout the Permian and Triassic with a geothermal gradient of ~40°C/km. Time-temperature models based on the AFT ages, track length measurements and wt. % Cl content of the apatites yielded a comparable geothermal gradient of ~43°C/km for the same samples at ~120 Ma. Differential cooling and exhumation between the base and summit samples from ~120 and 60 Ma is indicated by these model time-temperature histories. Surprisingly, from the latest Cretaceous to the present day, cooling histories for the base and summit of Mt Washington are indistinguishable. Our explanation for this was that the topographic profile was established in the Late Cretaceous, and preserved to the present day. However, with no data on the west side of Mt Washington, we could not rule out tectonic tilting as an alternative explanation.

In this contribution we test this hypothesis by analyzing 12 new apatite samples collected from the west side of Mt Washington from along the Cog Railroad. These AFT ages for samples from the Littleton and Rangeley formations and the Bretton Woods Granite range from ~150 Ma at ~1900 m to 89 Ma at the base at ~540 m. Apatite fission-track length measurements for the summit and base samples for each transect are comparable with the summit samples yielding a shorter mean track length of ~12.3 ± 1.8 µm and the base samples giving a longer mean track length of ~13.1 ± 1.7 µm. Qualitatively, longer mean track lengths suggest more rapid cooling through the apatite partial annealing zone of 60° to 90°C. The results of this test confirm the interpretations made from the data exclusively on the east side of Mt Washington. That is, the model thermal histories calculated from the AFT lengths converge at about 60 Ma. thus there is little doubt that the >1 km relief exposed on Mt Washington was established by the end of the Cretaceous, and the amount of relief has survived > 1 km of subsequent erosion.