Scott Southworth

Temporally and spatially uniform rates of erosion in the southern Appalachian Great Smoky Mountains

We measured 1 0 Be in fluvial sediment samples (n = 27) from eight Great Smoky Mountain drainages (1-330 km 2 ). Results suggest spatially homogeneous sediment generation (on the 10 4 -10 5 yr time scale and >100 km 2 spatial scale) at 73 ′ 11 t km - 2 yr - 1 , equivalent to 27 ′ 4 m/m.y. of bedrock erosion. This rate is consistent with rates derived from fission-track, long-term sediment budget, and sediment yield data, all of which indicate that the Great Smoky Mountains and the southern Appalachians eroded during the Mesozoic and Cenozoic at ∼30 m/m.y. In contrast, unroofing rates during the Paleozoic orogenic events that formed the Appalachian Mountains were higher (≥10 2 m/m.y.). Erosion rates decreased after termination of tectonically driven uplift, enabling the survival of this ancient mountain belt with its deep crustal root as an isostatically maintained feature in the contemporary landscape.

Erosion rates in and around Shenandoah National Park, Virginia, determined using analysis of cosmogenic 10Be

We use cosmogenic 10Be analysis of fluvial sediments and bedrock to estimate erosion rates (104-105 year timescale) and to infer the distribution of post-orogenic geomorphic processes in the Blue Ridge Province in and around Shenandoah National Park, Virginia. Our sampling plan was designed to investigate relationships between erosion rate and lithology, mean basin slope, basin area, and sediment grain size. Fifty-nine samples were collected from a variety of basin sizes (<1-3305 km2) and average basin slopes (6-24°) in each of four different lithologies that crop out in the park: granite, metabasalt, quartzite, and siliciclastic rocks. The samples include bedrock (n = 5), fluvial sediment from single-lithology basins (n = 43), and fluvial sediment from multilithology basins (n = 11): two multilithology samples are from rivers with tributary streams draining the eastern and western slopes of the park, respectively (Rappahannock and Shenandoah Rivers), and two samples are temporal replicates. In one sample of each lithology, we measured 10Be in four different grain sizes from fine sand to gravel. Inferred erosion rates for the medium sand fraction of all fluvial samples from all lithologies range from 3.0 to 21 m/My. The area-weighted mean erosion rate for single-lithology basins in the Park is 12.2 m/My. Single-lithology erosion rate ranges for fluvial samples are: granite, 7.0 to 20 m/My; metabasalt, 3.8 to 21 m/My; quartzite, 3.8 to 15 m/My; and siliciclastic rocks, 5.2 to 15 m/My. Multilithology basins erode at rates between 3.0-16 m/My. The Shenandoah River basin (3305 km2) is eroding at 6.6 m/My. Bedrock erosion rates range from 1.8 to 11 m/My across all lithologies, with a mean of 6.5 ± 4.3 m/My. Grain-size specific 10Be analysis of four samples showed no consistent trend of concentration with grain size. Cosmogenic analysis of bedrock and sediment from the Shenandoah National Park area allows us to speculate about why some parts of the Appalachian Mountains erode more slowly and some more rapidly. Overall, it appears that steep drainage basins erode more rapidly than gently sloped basins. Climate and lithology may also influence basin-scale rates of erosion as suggested by the difference in average erosion rates east and west of the divide and the difference between the erosion rates of quartzite- and granite-dominated basins. Data are conflicting in regards to the evolution of relief over time. Analyses made of exposed bedrock along ridgelines suggest that such rock is eroding either more slowly than adjacent drainage basins (Susquehanna River, Shenandoah National Park region) or at similar rates (Great Smoky Mountains) providing a mechanism for growing relief at the scale of individual ridgelines. However, considering relief on a landscape or physiographic province scale, by comparing erosion rates of the highlands versus the lowlands, suggests that relief of the range as a whole is either steady or very slowly decreasing over multi-millennial timescales. The presence of significant erosion rate/slope relationships negates a broad Hackian view of the landscape because there is not uniform erosion across this landscape. The aspect-erosion rate and slope-erosion rate relationships present in the Shenandoah area suggest that the landscape is not fully adjusted to rock strength.

Erosional and depositional history of the Atlantic passive margin as recorded in detrital zircon fission-track ages and lithic detritus in Atlantic Coastal plain sediments

Comparison of fission-track (FT) ages of detrital zircons recovered from Atlantic Coastal Plain sediments to FT ages of zircons from bedrock in source terranes in the Appalachians provides a key to understanding the provenance of the sediments and, in turn, the erosional and depositional history of the Atlantic passive margin. In Appalachian source terranes, the oldest zircon fission-track (ZFT) ages from bedrock in the western Appalachians (defined for this paper as the Appalachian Plateau, Valley and Ridge, and far western Blue Ridge) are notably older than the oldest ages from bedrock in the eastern Appalachians (Piedmont and main part of the Blue Ridge). The age difference is seen both in ZFT sample ages and in individual zircon grain ages and reflects differences in the thermotectonic history of the rocks. In the east, ZFT data indicate that the rocks cooled from temperatures high enough to partially or totally reset ZFT ages during the Paleozoic and (or) Mesozoic. The majority of the rocks are interpreted to have cooled through the ZFT closure temperature (∼235 °C) at various times during the late Paleozoic Alleghanian orogeny. In contrast, most of the rocks sampled in the western Appalachians have never been heated to temperatures high enough to totally reset their ZFT ages. Reflecting their contrasting thermotectonic histories, nearly 80 percent of the sampled western rocks yield one or more zircon grains with very old FT ages, in excess of 800 Ma; zircon grains yielding FT ages this old have not been found in rocks in the Piedmont and main part of the Blue Ridge. The ZFT data suggest that the asymmetry of zircon ages of exposed bedrock in the eastern and western Appalachians was in evidence by no later than the Early Cretaceous and probably by the Late Triassic. Detrital zircon suites from sands collected in the Atlantic Coastal Plain provide a record of detritus eroded from source terranes in the Appalachians during the Mesozoic and Cenozoic. In Virginia and Maryland, sands of Early Cretaceous through late early Oligocene age do not yield any old zircons comparable in age to the old zircons found in bedrock in the western Appalachians. Very old zircons yielding FT ages >800 Ma are only encountered in Coastal Plain sands of middle early Miocene and younger age. Miocene and younger fluvial-deltaic deposits associated with the major mid-Atlantic Coastal Plain rivers that now head in the western Appalachians (the Hudson, Delaware, Susquehanna, Potomac, James, and Roanoke) contain abundant clasts of fossiliferous chert and quartzite and other distinctive rock types derived from Paleozoic rocks of the western Appalachians. These distinctive clasts have not been reported in older Coastal Plain sediments. The ZFT and lithic detritus data indicate that the drainage divide for one or more east-flowing mid-Atlantic rivers migrated west into the western Appalachians, and the river(s) began transporting western Appalachian detritus to the Atlantic Coastal Plain, sometime between the late early Oligocene and middle early Miocene. By no later than late middle Miocene most if not all of the major rivers that now head west of the Blue Ridge were transporting western Appalachian detritus to the Coastal Plain. Prior to the drainage divide migrating into the western Appalachians, the ZFT data are consistent with the dominant source of Atlantic Coastal Plain sediments being detritus from the Piedmont and main part of the Blue Ridge, with possible input from distant volcanic sources. The ZFT data suggest that the rapid increase in the rate of siliciclastic sediment accumulation in middle Atlantic margin offshore basins that peaked in the middle Miocene and produced almost 30 percent of the total volume of post-rift siliciclastic sediments in the offshore basins began in the early Miocene when Atlantic river(s) gained access to the relatively easily eroded Paleozoic sedimentary rocks of the western Appalachians.

Volcanic rift margin model for the rift-to-drift setting of the late Neoproterozoic-early Cambrian eastern margin of Laurentia: Chilhowee Group of the Appalachian Blue Ridge

New data support a model of a volcanic rifted margin for eastern Laurentia and the breakup of the supercontinent Rodinia. Upper Neoproterozoic–lower Cambrian rocks of the Chilhowee Group in the Blue Ridge Province of eastern North America are subdivided into two facies assemblages separated by an unconformity. Historically, the rocks have been correlated as a tripartite division: 1) basal sandstone and conglomerate (Cochran, Unicoi, and Weverton Formations), 2) middle siltstone and shale (Nichols Shale, Hampton Shale, and the Hampton and Harpers Formations), and 3) an upper sandstone and shale (Nebo Quartzite, Murray Shale, Hesse Quartzite, and Helenmode, Erwin, and Antietam Formations). Sedimentary analyses show that boundaries of the newly defined facies assemblages transect the named stratigraphic units. Assemblage A consists of fluvial-lacustrine deposits with interbedded subaerial basalt flows overlain by marine deposits. Fluvial strata formed in anastomosing braided-river channels and are interbedded with mudstones deposited in shallow lacustrine plains. Overlying shallow marine deposits consist of upward-coarsening successions capped by over-thickened sandstones. Shale-rich turbidite deposits characterized by subdued bioturbation and common slump features occur at the top. Assemblage B consists of sandstone and shale deposited on a stable shelf. These upward-coarsening parasequences are stacked to form transgressive-regressive system tracts reflecting long-term sea-level fluctuations. A working hypothesis is that these rocks are comparable to those present in seismic reflection profiles and drill cores of the volcanic rift margins of the Atlantic and Indian Oceans. Assemblage A deposits formed during rapid subsidence associated with the formation of seaward-dipping reflectors. Flood basalts within fluvial sandstones are the landward facies of more voluminous basalt flows in the opening basin. Overlying marine deposits of Assemblage A were coeval with basaltic volcanism and deeper-water deposits to the east. Assemblage B unconformably overlies Assemblage A rocks and laps onto much older continental rocks. It was deposited on the craton into the late Cambrian. Much of the lateral lithologic variability and differences in character and thickness of the Chilhowee Group is attributed to the depositional setting that was influenced by the underlying rift architecture. It was further complicated by juxtaposition of rocks telescoped along numerous Paleozoic thrust faults. This model predicts that coeval rocks in outboard thrust sheets are finer-grained, mafic volcanic-rich marine sequences (Assemblage A), overlain by shelf-margin and basin deposits that lack volcanic rocks (Assemblage B). This model provides more refined sedimentary criteria for examination of other volcanic rift margins.

Implications for late Grenvillian (Rigolet phase) construction of Rodinia using new U-Pb data from the Mars Hill terrane, Tennessee and North Carolina, United States

New data for zircon (external morphology, cathodoluminescence zoning, and sensitive high resolution ion microprobe [SHRIMP] U-Pb ages) from the Carvers Gap granulite gneiss of the Mars Hill terrane (Tennessee and North Carolina, United States) require reevaluation of interpretations of the age and origin of this rock. The new results indicate that the zircon is detrital and that the sedimentary protolith of this gneiss (and related Cloudland gneiss) was deposited no earlier than ca. 1.02 Ga and was metamorphosed at ca. 0.98 Ga. Tectonic models that included the gneiss as a piece of 1.8 Ga Amazonian crust (perhaps as part of the hypothetical Columbia supercontinent) are now untenable. The remarkably fast cycle of exhumation, erosion, deposition, and deep burial also is characteristic of other late Grenvillian (post-Ottawan) Mesoproterozoic paragneisses that occur throughout the Appalachians. These rocks provide new evidence for the duration of the formation of the Rodinia supercontinent lasting until at least 0.98 Ma.

Laurentian and Amazonian sediment sources to Neoproterozoic–lower Paleozoic Maryland Piedmont rocks

Several terranes of variable tectonic affinity and history underlie the central Appalachian Piedmont Province (eastern United States). These terranes mostly consist of widespread metasedimentary and lesser metavolcanic rocks. Intense and pervasive deformation and metamorphism have made the depositional ages and provenance of sediment in these rocks difficult to determine. The lack of tight constraints on such basic information led to a century-long debate about the tectonic significance of these rocks, particularly how they correlate to similar rocks along and across strike in the Appalachian orogen. We address these issues using U/Pb isotopic ages from single spots in 2433 zircon grains from 18 metasedimentary rock samples distributed across the Maryland Piedmont. The resulting age signatures indicate that the Marburg Formation and Prettyboy Schist, heretofore assigned to the Westminster terrane, actually belong to the Potomac terrane, making the Hyattstown thrust the contact between the two terranes. Ediacaran Laurentia could have supplied all Potomac terrane sediment except for the detritus in one sample from the northern part of the terrane that likely came from Amazonia. This is one of the first recognitions of a Gondwana-derived terrane between Carolinia to the south and Ganderia to the north. Maximum depositional ages for Potomac terrane suprasubduction zone sedimentary rocks are latest Neoproterozoic or early Cambrian, and some may have been deposited ca. 510 Ma. Continental rifting ended ca. 560 Ma at the longitude of our study, so the transition from rifting to subduction at this location in eastern Laurentia may have lasted only 50 M.y. Lower Ordovician arc intrusions into these rocks demonstrate that the transition lasted no longer than 90 M.y. The Iapetan margin of central-eastern Laurentia was one of the shortest lived passive margins that formed in Neoproterozoic time.