LOW TEMPERATURE THERMAL HISTORY OF THE CONWAY GRANITE, NH

It has long been hypothesized that modern relief in the White Mountains, NH is the result of uplift and incision of an ancient low-relief equilibrium landscape. Due to moderate modern relief in New England, surface thermochronology has not provided clear answers to when incision of the modern landscape occurred. In this study, we infer incision history using ⁴He/³He thermochronology on apatite from a ~3000 ft drill core recovered near Redstone, NH. Although high effective uranium concentrations and likely zonation limit meaningful interpretation, the data suggest a period of accelerated cooling during the latest Cretaceous and Paleocene periods. Comparison of bulk U-Th/He ages along a vertical transect suggests exhumation rates up to 0.06 km/Myr occurring around 50-60 Ma (assuming a constant geothermal structure during that interval). Likewise a pair of grains from 403 m depth reveal ⁴He/³He spectra that are consistent with accelerated cooling sometime between ~75-40 Myr, followed by a period of slow cooling from ~45 Myr to present. A sample from the top of the core appears to record a different thermal history, suggestive of rapid cooling sometime between ~130 and 100 Ma. This timing overlaps with late stage volcanism of the White Mountain Volcanic Series, raising the possibility of thermal disturbance at that time. Although preliminary, these results suggest that development of relief in the White Mountains predates regionally recognized erosional events such as Miocene sediment pulses and the onset of northern Hemisphere glaciation ~ 2.8 Myr ago. Further work is required to assess the cause of cooling, and assess its relation to presumed regional uplift during the Cretaceous.