POST-PALEOZOIC UPLIFT OF THE SOUTHERN AND CENTRAL APPALACHIANS: THE GREATEST REMAINING UNSOLVED PROBLEM IN APPALACHIAN TECTONICS

The post-Paleozoic (PZ) history of the Appalachian (AP) region is marked by long periods of erosion during the early Mesozoic (MZ) following the Alleghanian orogeny and development of a carbonate platform during opening of the Atlantic Ocean. Uplift of SE North America (NA) occurred again in the Late Cretaceous (K) spilling large quantities of sediment onto the Atlantic and Gulf continental margins from the Late Cretaceous well into the Eocene. Carbonate deposition resumed during the mid-Cenozoic (CZ), but uplift of the AP region occurred again beginning in the late Miocene-early Pliocene (MP) producing the modern topography (TP) of the exposed southern AP and supplying sediment to the Atlantic-Gulf Coastal Plain (CP). Present-day TP and major drainages in the non-glaciated AP reveal anomalous patterns that alone indicate PZ-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 1,500->2000 m maximum AP height shifts from the S AP BR to the central AP foreland. Appalachian crust today remains >50 km thick in some regions beneath the Valley and Ridge, BR, and western P, but thins abruptly to <35 km thick from the central P suture eastward beneath the CP from VA into GA. Late Jurassic (J) to Early K reversal of Triassic-Early J extension to ridge-push-related compression could account for the K uplift, but does not account for the MP uplift event.

Mantle (M) processes undoubtedly controlled the Late MZ-early Tertiary and Miocene-Pliocene uplift events. Tomographic (Tm) data in SE NA reveal a high-velocity (HV), SE-dipping M slab (high density?) W of the AP and a low-velocity (LV) M (low-density?) to the E. This has been interpreted as a dynamic system of downward flow of the HV slab and upward flow beneath the high southern AP, but smaller HV zones underlie the high TP, and LV zones underlie the P. Despite great strides in Tm resolution of M structure, the mismatch between modern TP and TM data reconfirms our imperfect understanding of M structure.