An Unexplained Pulse of Late Cretaceous Exhumation in Northern New England

Abstract:

The steep relief of northern New England has long puzzled geomorphologists. Although it is suggestive of recent tectonic rejuvenation, roughly 200 Myr has passed since the opening of the Atlantic. The timing of recent exhumation has proven difficult to constrain because the magnitude of Cenozoic unroofing is often insufficient to reset surface thermochronometers and associated sediments are not well preserved. This study attempts to better resolve the Mesozoic-Cenozoic exhumation history of the White Mountains (New Hampshire) by combining apatite fission track, (U-Th)/He, and 4He/3He thermochronology in a 700 m deep drill core from the White Mountains of New Hampshire. Forward (HeFTY) and inverse (QTQt) modeling of age-elevation profiles point to a period of accelerated exhumation from roughly 85 to 65 Ma. Cooling rates accelerated by a factor of 6-10 around this time, presumably translating to the removal of 2-3 km of overburden in the span of ~20 million years. The timing of this episode is in broad agreement with AFT cooling ages across major faults in the region, which suggest late Cretaceous reactivation. However, an analysis of regional AFT ages suggests that not all of the uplift can be explained by extensional faulting and that dynamic surface uplift might have played a role. Although external forcing mechanisms remain unclear, the timing of this exhumation corresponds well to a reorganization of seafloor spreading in the North Atlantic and to the timing of alkaline volcanism in Iberia.