TERTIARY ORIGIN OF THE APPALACHIAN MOUNTAINS

The Appalachians extend ̴ 2500 km from Canada to Alabama with the highest peaks at >2000 m. The last orogeny that affected the Appalachian crust was the 330-260 Ma Alleghanian event. Even with minimal erosion rates, it is highly unlikely that the Appalachians could have sustained such elevations for over 200 m.y. Geomorphic evidence suggests that recent tectonic uplift has produced the topography visible today. The multiple phases of late Mesozoic and Tertiary uplift are related to poorly understood processes, but a large amount of data from within today’s mountain chain and the adjacent Coastal Plains indicate the present high topography is anomalous and clearly not related to Paleozoic crustal processes. Major rivers with headwaters in the eastern Blue Ridge and Piedmont drain westward across the high topography. Westward-migrating drainage divides and anomalous drainage patterns indicate that topography is not controlled by Paleozoic and early Mesozoic tectonics. There is a marked shift in high topography from the Blue Ridge in TN-NC to the Valley and Ridge in VA-W VA. Topographic inversion is recorded in the late Miocene inverted lake at Gray fossil site, NE TN. Occurrence of Cretaceous and Tertiary sediments at >200 m elevations in NE MS may be indicative of the recent uplift. Here, we will use zircon geochronology and age of fluvial terraces as a proxy for time of uplift. LaserChron age dating of zircons from Miocene-Pliocene sediment samples from the Gulf Coastal Plain (in progress) should determine the provenance of the sediments and an indication of time of uplift in the source. Geologic mapping of terraces along Douglas Lake, French Broad River, TN is being carried out to separate the terraces and to collect samples for cosmogenic nuclide dating. Oldest cosmogenic ages obtained on the terraces will be equivalent to or younger than the time of the uplift. Several optically stimulated luminescence ages for the terraces range from 100-200 ka. The late Tertiary uplift of the Appalachians is likely tectonic and may be related to mantle and deep lithospheric processes; visible geomorphology is only consequential.