CONSISTENT SLOW EXHUMATION IN A LATE CENOZOIC GLACIATED LANDSCAPE, THE PRESIDENTIAL AND CARTER RANGES OF THE WHITE MOUNTAINS IN NEW HAMPSHIRE, USA

A comparison of denudation rates averaged over a 10^6-7 yr timescale using apatite (U-Th)/He thermochronology and averaged over a 10^3-4 yr timescale using the in situ terrestrial cosmogenic nuclide (TCN) ¹⁰Be methods allows an examination of the effect of late-Cenozoic glaciation on denuding the Presidential and Carter Ranges of the White Mountains, New Hampshire, USA. Thermal modeling of apatite (U-Th)/He ages from Mt. Washington in the Presidential Range (n=4, 91.7 to 117.3 Ma) indicate that beginning in the Late Cretaceous exhumation rates slowed from ~0.03 to ~0.01 mm yr^-1 in valleys and stagnated over ridges. This resulted in ~1 km of relief production after ~100 Ma, or between 100 - 60 Ma if the data in this study are resolved with previous apatite fission track work (Roden-Tice et al. 2012). Summit bedrock and talus ¹⁰Be concentrations and exposure ages from the Presidential and Carter Ranges (n=8; ~14–190 × 10⁴ atoms g^-1 SiO₂; 14–130 ka) indicate inheritance of ¹⁰Be prior to the last glacial maximum resulting from cold-based ice cover, in agreement with previous work in the region (Bierman et al., 2015). The ¹⁰Be concentrations in till samples across the field area (n=5; ~2–7 × 10⁴ atoms g^-1 SiO₂) are greater than can be ascribed to production since deposition. This implies inheritance across the study area, not just on summits, suggesting that that cold based-ice and limited magnitudes of glacial erosion also affected lower elevation portions of the landscape. Be-10 concentrations from catchment-wide sand and modern flood deposits (n=14; 5–19 × 10⁴ atoms g^-1 SiO₂) are statistically the same as those from till, higher than those from colluvium (n=2; 2–7 × 10⁴ atoms g^-1 SiO₂), and lower than those from bedrock and talus. These ¹⁰Be concentrations suggest that the post-glacial sediment budget is dominantly derived from remobilized glacial sediment. As such, late Cenozoic glaciation did modify existing topography but it did not significantly produce relief or accelerate exhumation in Presidential and Carter Ranges of New Hampshire.