TECTONIC CONSTRAINTS ON LATE TERTIARY UPLIFT IN SOUTHERN APPALACHIANS

Evidence for the southern Appalachian (AP) uplift strongly indicates AP crust under the Blue Ridge (BR), western Piedmont, and Valley and Ridge (VR) has undergone extensive late Tertiary uplift. Anomalous drainage patterns indicate topography is not controlled by Paleozoic and early Mesozoic tectonics. Major rivers with headwaters in the eastern BR and Piedmont drain westward across the high topography requiring that they be superposed streams. A marked shift in the highest topography ( ̴ 2000 m) occurs from the BR in TN-NC to the VR in VA-W VA. Topographic profiles along the westward draining rivers indicate constant and continuous base level changes. The French Broad River (FBR) flows westward across the BR joining the Tennessee River, which flows into the Ohio River draining into the Gulf of Mexico, recording evidence of recent uplift. Constant downcutting and erosion have produced strath terraces along the river directly underlain by Sevier Shale and Knox Dolomite bedrock geology. Geologic mapping (1:24,000 scale) of well exposed river terraces along Douglas Lake, FBR, in E TN has helped understand the occurrence and tectonic development of river terraces in this region. Cosmogenic nuclide dating of higher terrace deposits will help delimit a minimum age of the late Tertiary-Quaternary uplift. Terrace deposits consist of clay, clasts and well-rounded pebbles of quartzite, vein quartz, and granitoid with rare calc-silicate, amphibolite and basement granite indicating common source areas in the eastern BR. LA-ICP-MS detrital zircon (DZ) ages from 18 samples of late Miocene-Pliocene (LMP) sediments from the Gulf Coastal Plain in MS, AL, and FL are currently being processed. We expect dominance of AP provenance of DZ ages, as high topography produced by late Tertiary uplift in the southern AP should be the source for the late Tertiary sediments. Abruptly high sedimentation rates in the LMP indicate active uplift. Current rugged topography, geomorphology, and continental shelf loading are a consequence of the uplift, not the cause. Ridge push and compressional forces along Laurentian margin since the Late Cretaceous drive the active uplift in southern AP. Geophysical data suggest possible lithospheric upwelling due to movement of the underlying Farallon Plate may drive the late Tertiary uplift.