MID-CRETACEOUS EXHUMATION OF THE NORTHERN APPALACHIAN MOUNTAINS, VERMONT, FROM APATITE (U-TH)/HE THERMOCHRONOLOGY

The Eastern North American Margin (ENAM), like other long-lived Atlantic passive margins, is characterized by significant elevation and relief, yet the mechanisms responsible for retaining or obtaining this topography are unclear. We report pilot apatite (U-Th)/He (AHe; closure temperature = 50-90°C) dates for four samples from the Green Mountains of southern Vermont to investigate the timing, mechanism(s), and magnitude of post-rift topographic evolution of the ENAM. We specifically aim to assess the contributions of Atlantic rifting and associated lithospheric thinning, passage over the Great Meteor Hotspot, and the modern Northern Appalachian mantle anomaly on the exhumation history of the region.

AHe results range from 100 ± 5 Ma to 112 ± 17 Ma, have no correlation with modern elevation, and are consistent with existing low-temperature thermochronology within Northern Appalachian mountain ranges, suggesting a shared region-wide driver of exhumation. The timing and extent of exhumation are most compatible with North America's passage over the Great Meteor hotspot during the Cretaceous and/or a large, shallow mantle anomaly spanning both sides of the Atlantic at that time. We plan to collect mid-temperature zircon (U-Th)/He data (ZHe; closure temperature = 160-210°C) to constrain the upper limit on Mesozoic exhumation.

Low-eU grains (<20 ppm; n = 7) yield relatively old (87-119 Ma) dates and impose a rigid ceiling on the magnitude of late Cretaceous and Cenozoic exhumation, with thermal models indicating less than 1.5 km of cooling since 90 Ma, assuming a surface temperature of 20°C and a geothermal gradient of 25°C/km. The lack of a Cenozoic exhumation signal may point to the limited extent of dynamic uplift or a delay in the erosion and cooling response to uplift induced by the modern mantle anomalies under New England.