ACCELERATED LATE CRETACEOUS EXHUMATION IN THE WHITE MOUNTAINS REGION OF NEW HAMPSHIRE BASED ON APATITE FISSION-TRACK, U-TH/HE AND 3HE/4HE ANALYSES

The pace and timing of topographic evolution of New England has proven difficult to constrain. More than 350 Myr have passed since the last period of active mountain building, yet rugged mountainous topography still persists. Slow late Pleistocene erosion rates suggest that much of the modern relief was formed prior to the onset of northern Hemisphere glaciation, but basic questions of when and how fast relief formed, remain unanswered. To explore the Cenozoic erosional history of the White Mountains, we performed Apatite-Fission Track (AFT), U-Th/He (⁴He), and ⁴He/³He analyses on a suite of samples from a ~3000 ft drill core recovered near Redstone, NH. Preliminary results suggest that slow long term slow cooling was interrupted by a period of accelerated cooling between ~80 and 50 Myr ago. A vertical transect of AFT ages spanning the entire 900 m core interval suggests long-term cooling rates of ~1.5°C/Myr between ~82-62 Myr. Differences in cooling ages between AFT and U-Th/He ages from the same samples suggest that cooling rates may have reached 3-5°C/Myr between 68-53 Myr ago. Iterative forward modeling shows that ⁴He/³He profiles in the same samples are also consistent with rapid cooling at this time. If accepted, these cooling rates would be the highest yet reported in New England. This Late Cretaceous cooling event is arguably unrelated to late stage intrusions from the White Mountain Volcanic Series, whose youngest dikes in the region are ~ 94 Myr old. Assuming cooling is driven by exhumation, and adopting a thermal gradient of 30°- 40°C/km yields peak exhumation as high as 0.1-0.15 km/Myr in the latest Cretaceous. The timing of this exhumation is in good agreement with time-temperature models calculated from fission-track length distributions of samples from the summit of nearby Mt. Washington, which show accelerated cooling beginning ~60 Myr ago. Further study is needed to determine the cause of accelerated exhumation in eastern New Hampshire during the Late Cretaceous, although headward river incision into a thick and buoyant plateau is one plausible mechanism.