CRETACEOUS UPLIFT AND DENUDATION OF THE GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA: ANOMALOUS APATITE FISSION-TRACK AGES

More than 100 apatite fission-track ages have been determined for rocks from the Blue Ridge Province of North Carolina, Tennessee, Virginia, and Maryland. Along most of the Blue Ridge, from the Great Smoky Mountains of Tennessee and North Carolina northeast to the Shenandoah National Park area of Virginia, apatite ages define very similar trends of decreasing age with decreasing elevation. The age-elevation trends indicate average effective uplift and denudation rates for the Blue Ridge of about 20 m/m.y. during the Mesozoic. Modelling suggests that, on average, uplift and denudation have continued to the present at similar, very slow rates.

In the Grandfather Mountain (GFM) area of North Carolina, Mesoproterozoic to Cambrian rocks are exposed in a window in the Alleghanian (Carboniferous-Early Permian) Blue Ridge thrust sheets. Apatite fission-track ages have been determined for 20 samples collected in the GFM window and adjacent thrust sheets. Sample elevations range from about 1,600 m near the summit of GFM to about 420 m.

Apatite fission-track ages from the Blue Ridge thrust sheets, even close to the window-framing faults, generally define an age-elevation trend similar to the trends observed elsewhere in the Blue Ridge. In contrast, the data indicate an anomalous uplift history for the GFM window. Apatite ages from the higher elevations of GFM are the youngest observed at high elevations anywhere in the Blue Ridge. On the east side of GFM, apatite ages show little or no variation with elevation, averaging about 85 Ma over a vertical relief of almost 1.2 km.

The data suggest one or more period(s) of accelerated uplift and denudation of rocks in the GFM window during the Late Cretaceous. Anomalous uplift and denudation rates in the window appear to have ended about 85 Ma. From ~85 Ma to the present, rocks in the GFM window and surrounding Blue Ridge thrust sheets have most likely been uplifted as essentially a single block.