MODERN APPALACHIAN TOPOGRAPHY, PRODUCT OF MIOCENE TO RECENT UPLIFT: NOT A RELIC OF PALEOZOIC OROGENY, AND NOT THE “WORLD’S OLDEST MOUNTAIN CHAIN” (Invited Presentation)

We were taught in junior high the Appalachians (AP) are an “old worn-down” chain, whereas the Rockies are a “young rugged” range: some still believe that. Atlantic-Gulf Coastal Plain (CP) stratigraphy, however, reveals several cycles of uplift and erosion interrupted by carbonate deposition in the Mesozoic (MZ) and Cenozoic (CZ) as the Atlantic and Gulf of Mexico opened. The Late Triassic (TR)-Early Jurassic (J) involved erosion, then Late J-Early Cretaceous (K) development of a carbonate platform. Renewed uplift of SE North America (NA) spilled clastic sediments onto the Atlantic and Gulf margins in the Late Cretaceous (K) and Eocene. Carbonate deposition resumed during the mid-CZ, but AP uplift occurred in the late Miocene-Pliocene (MP) producing the modern southern-central AP topography (TP), supplying voluminous sediment to the CP. Present-day TP and drainage patterns indicate Paleozoic-early MZ tectonics and rock types play a minor role in modern topography: major drainages that drain the W AP head in the E Blue Ridge (BR) and Piedmont (P). The 1.5->2 km maximum AP height shifts from the S AP BR to the VA-WV Valley and Ridge (VR). The Gray Fossil Site in NE TN contains a MP vertebrate assemblage in an ancient lake that rests on a hilltop >100 m above the nearest drainage providing a timeline for uplift.

AP crust today remains >50 km thick beneath parts of the VR, BR, and western P, but thins to <35 km from the central P suture eastward beneath the CP from VA to AL. Late J to Early K reversal of TR-Early J extension to ridge-push-related compression can account for K uplift, but does not account for MP uplift.

Either deep or shallow mantle (M) flow, or erosional processes and sediment deposition disturbing isostatic equilibrium, likely controlled late MZ-early Tertiary and MP AP uplift. Geophysical models involve tomographic (Tm) data in SE NA that suggest flow and reveal a high-velocity, SE-dipping descending M slab W of the AP and a low-velocity M to the E.

Despite great strides in Tm resolution of M structure, the mismatch between modern TP and Tm data reconfirms our imperfect understanding of M structure.