Abstract By comparing new detrital zircon provenance analysis of Triassic synrift sediments from the Tallahassee graben (FL), the South Georgia rift basin (GA), and Deep River rift basin (NC) with our previous detrital zircon provenance data for the Jurassic Norphlet Formation erg in the Eastern Gulf of Mexico, we have developed a regional model of Triassic-Jurassic erosion and sediment transport. In the Eastern Gulf of Mexico, detrital zircon ages observed in Triassic synrift clastics from the Tallahassee graben and southern South Georgia rift system contain not only Gondwanan-aged and Grenville-aged zircon grains but also an abundance of Paleozoic detrital zircon grains, reflecting sediment influx from rocks associated with the Paleozoic orogens of eastern Laurentia. Although Paleozoic detrital zircon grains are present in the younger Norphlet deposits, they are less abundant than in Triassic rift sediments. In southwest Alabama, the most abundant detrital zircon age population in the Norphlet Formation is Grenville-aged (950-1,250 Ma). In the Conecuh embayment of southeastern AL and western FL panhandle, Norphlet samples show a marked decrease in Grenville detrital zircon and an increase in 525-680 Ma zircon ages, interpreted to represent influx from rocks associated with the Gondwanan Suwannee terrane. In the Apala-chicola Basin, the proportion of Gondwanan zircon ages increases to nearly 40% of the total population and Grenville-aged grains constitute just ~20% of the population. We suggest that the difference between Triassic and Jurassic detrital zircon signatures in the Eastern Gulf of Mexico reflects significant unroofing of Paleozoic rocks during early Mesozoic rifting of the easternmost Eastern Gulf of Mexico, possibly including rocks equivalent with those exposed in the Talladega slate belt units. Subsequent erosion of rift-flanking highlands to expose older Gondwanan and Grenville rocks and/or input from northern sediment sources supplied the older Grenville-aged detrital zircon grains present in the Norphlet erg in the area to the west and within the Conecuh embayment.

We present here the first geochemical data from adakitic rocks from an extensional system—the U.S. East Coast rifted margin. Adakitic magmas are high-K melts that have been petrogenetically interpreted to be partial melts of subducting slab and/or lower crustal lithologies in delamination events. The adakitic rocks presented here are from a small volcanic region in the Valley and Ridge province in Virginia and were probably emplaced around the time of continent rupture and Central Atlantic magmatic province activity. They are bimodal in character (high Si and low Si) and have the typical high- and low-Si adakitic geochemical characteristics such as high K 2 O (up to 9.88 wt%) abundances, steep rare earth element patterns, and significantly high Sr (2473 ppm) and relatively low Rb (35 ppm) contents for high-Si adakitic rocks. The petrogenetic relation of these melts to partial melting of metagabbroic rocks (high-Si adakites) and interaction of these melts with ambient peridotite (low-Si adakites) suggests that the geodynamic process for the formation of the studied Jurassic central Virginia igneous rock succession is delamination of mantle lithosphere and lower crust below the volcanic rifted margin. We present with geodynamic models that negatively buoyant mantle lithosphere instabilities developed below this passive margin during continent rupture. After foundering, warm asthenosphere welled up and heated the lower crust of the East Coast margin. This lithosphere was interspersed in our study area with fragmented hydrated metamorphic mafic to ultramafic lithologies. In situ and/or dripping melting of such meta-igneous rocks reproduces the observed geochemistry of the studied high-Si adakitic rocks. Further recycling processes within the convecting mantle of delaminated floating fertile meta-igneous rock packages could be responsible for Atlantic melting anomalies such as the Azores or Bermuda.

Abstract This volume includes nine field trip guides that explore geological history and visit four regional geologic provinces—Blue Ridge, Valley and Ridge, Cumberland Plateau, and the Nashville dome. Two guides focus on the Cumberland Plateau structure and hydrology. Two explore aspects of the Nashville dome, including Mississippian Waulsortian mounds and meso-scale structural deformation. Various aspects of the Valley and Ridge are visited on three trips, including the 1925 Scopes trial in Dayton, Tennessee, structural aspects of the Sequatchie Valley, and regional Silurian Red Mountain/Rockwood stratigraphy. Two field trips explore features of the Blue Ridge province—one investigates southernmost Appalachian exposures of metamorphosed lower Paleozoic rock, and another focuses on the Appalachian geomorphological response to uplift during the late Cenozoic.

Our models show patterns reflecting local fault control on both shoreline regression and river deflections along the Atlantic Coastal Plain. In these models, maximum displacement is assumed to be at the center of a fault, and both uplifts and downwarps are assumed to be of sufficient magnitude to influence surface processes. Models show regional shoreline regression: (1A) without localized uplifts; (1B) with different rates of regional uplift at either end; (1C) without any localized uplifts but with a large river-dominated delta; (2A) with a fault parallel to the shoreline with seaward side down or (2B) with seaward side up; and (3) with a fault perpendicular to the shoreline. Model 1A has consistently spaced parallel shorelines and an absence of river deflections, such as characterizes most of the late Pleistocene coastal plain across Georgia. Model 1B has divergence of shorelines toward and deflection of rivers away from the end with greater uplift. Model 1C has seaward deflections of shorelines with spacing dependent upon rates of sediment influx and removal by coastal processes. Models 2A and 2B represent interruptions of model 1 patterns. Both produce a seaward deflection and wider spacing of younger shorelines on the uplifted side of the fault with associated river deflections toward the margins of the uplift. Both also produce a landward deflection and closer spacing of younger shorelines coupled with convergence of rivers toward the downdropped basin. Model 3 produces a seaward deflection and wider spacing of older shorelines across the uplift associated with river deflections toward the margins of the uplift on one side of the fault. On the other side, there is a landward deflection and narrower spacing of younger shorelines on the downdropped side of the fault where river deflections merge toward the lowest area. In model 3, shorelines are discontinuous and may be difficult to correlate across the fault, and fault length is constrained by resumption of model 1 shorelines seaward of the fault. Model 3 matches patterns in the vicinity of the 1886 Charleston earthquake, South Carolina, with a NW-trending fault of ~50 km length with the NE side up and uplift continuing since the early Pleistocene. Very similar patterns occur in the vicinity of Beaufort, South Carolina, and Wilmington, North Carolina, which suggest other NW-trending faults of comparable or greater length may be present near these localities. Model 2A matches patterns near the Okefenokee Swamp, which suggests that a 100-km-long, N-trending fault may border the east side of Trail Ridge near the Georgia-Florida state boundary. Model 2B was used by previous workers to explain zones of river anomalies in the Carolinas, but those anomalies do not match this model.