Browsing by Author "Prince, Philip S."

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    Evolution of transient topography on passive margins: A study of landscape disequilibrium in the southern Appalachian Mountains
    Prince, Philip S. (Virginia Tech, 2011-04-22)
    The mechanism through which the Appalachian Mountains have maintained moderate relief some ~300 Myr after the cessation of mountain building has long puzzled geomorphologists. As recent studies have shown that Appalachian exhumation has occurred at slow rates consistent with isostatic rebound of thickened crust, the driving forces behind localized episodes of accelerated incision and the associated rugged topography have been difficult to explain given the absence of tectonic uplift. This study uses previously undocumented relict fluvial gravels and knickpoint location to confirm the role of drainage rearrangement in producing local base level drop and subsequent basin-scale transient incision in the southern Appalachians. This process is fundamentally driven by the high potential energy of streams flowing across the elevated, slowly eroding Blue Ridge Plateau relative to the present Atlantic and landward interior base levels. Gravel deposits confirm that repeated capture of landward-draining Plateau streams by Atlantic basin streams, whose immediate base level is 250-300 m lower, forces episodic rapid incision and overall erosional retreat of the Blue Ridge Escarpment along the Plateau margin. The distribution of knickpoints, bedrock gorges, and relict surfaces in the interior of the Plateau indicate that the New River, which drains to the continental interior, is actively incising the low-relief Plateau surface due to episodic drops in landward base level. The origin of landward base level perturbation is unclear, but it may be the result of glacially-driven shortening and steepening of the lower New River during the Pleistocene. Collectively, these data indicate that rapid base level drop through drainage reorganization can energize streams in otherwise stable landscapes and accelerate fluvial incision and relief production without uplift of the land surface. This process is likely quite significant in post-orogenic settings, where inherited drainage patterns may not reflect the most direct, and thus energetically appropriate, path to present base level. Passive margins may therefore never achieve a topographic steady-state, despite uniformly slow and constant uplift due to isostatic rebound.
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    Headwater stream length dynamics across four physiographic provinces of the Appalachian Highlands
    Jensen, Carrie K.; McGuire, Kevin J.; Prince, Philip S. (Wiley, 2017-09-15)
    Understanding patterns of expansion, contraction, and disconnection of headwater stream length in diverse settings is invaluable for the effective management of water resources as well as for informing research in the hydrology, ecology, and biogeochemistry of temporary streams. More accurate mapping of the stream network and quantitative measures of flow duration in the vast headwater regions facilitate implementation of water quality regulation and other policies to protect waterways. We determined the length and connectivity of the wet stream and geomorphic channel network in three forested catchments (<75 ha) in each of four physiographic provinces of the Appalachian Highlands: the New England, Appalachian Plateau, Valley and Ridge, and Blue Ridge. We mapped wet stream length seven times at each catchment to characterize flow conditions between exceedance probabilities of <5% and >90% of the mean daily discharge. Stream network dynamics reflected geologic controls at both regional and local scales. Wet stream length was most variable at two Valley and Ridge catchments on a shale scarp slope and changed the least in the Blue Ridge. The density and source area of flow origins differed between the crystalline and sedimentary physiographic provinces, as the Appalachian Plateau and Valley and Ridge had fewer origins with much larger contributing areas than New England and the Blue Ridge. However, the length and surface connectivity of the wet stream depended on local lithology, geologic structure, and the distribution of surficial deposits such as boulders, glacially-derived material, and colluival debris or sediment valley fills. Several proxies indicate the magnitude of stream length dynamics, including bankfull channel width, network connectivity, the base flow index, and the ratio of geomorphic channel to wet stream length. Consideration of geologic characteristics at multiple spatial scales is imperative for future investigations of flow intermittency in headwaters.
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    Incipient evolution of the Eastern California shear zone through the transpressional zone of the San Bernardino Mountains and San Gorgonio Pass, California
    Cochran, William J.; Spotila, James A.; Prince, Philip S. (2020-08)
    The nature of the connection between the Eastern California shear zone (ECSZ) and the San Andreas fault (SAF) in southern California (western United States) is not well understood. Northwest of San Gorgonio Pass, strands of the ECSZ may be migrating south and west into the convergent zone of the San Bernardino Mountains (SBM) as it is advected to the southeast via the SAF. Using high-resolution topography and field mapping, this study aims to test whether diffuse faults within the SBM represent a nascent connection between the ECSZ and the SAF. Topographic resolution of <= 1 m was achieved using both lidar and unmanned aerial vehicle surveys along two Quaternary strike-slip faults. The Lone Valley fault enters the SBM from the north and may form an along-strike continuation of the Helendale fault. We find that its geomorphic expression is obscured where it crosses Quaternary alluvium, however, suggesting that it may have a low rate of yet-undetermined activity. The Lake Peak fault is located farther south and cuts through the high topography of the San Gorgonio massif and may merge with strands of the SAF system. We find that this fault clearly cuts Quaternary glacial deposits, although the magnitude of offset is difficult to assess. Based on our interpretation of geomorphic features, we propose that the Lake Peak fault has predominantly dextral or oblique-dextral motion, possibly with a slip rate that is comparable to the low rates observed along other strands of the ECSZ (i.e., <= 1 mm/yr). Comparing the geomorphic expressions of these faults is difficult, however, given that the erosive nature of the mountainous landscape in the SBM may obscure evidence of active faulting. Based on these observations, as well as the occurrence of other diffuse faults in the region, we suggest that dextral strain is overprinting the actively convergent zone of the SBM, thereby creating a throughgoing connection between the ECSZ and the SAF west of San Gorgonio Pass.
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    Investigating the Effects of Lithology on Landscape Evolution Processes across Scales
    Chilton, Kristin Danielle (Virginia Tech, 2021-08-26)
    Geomorphologists have long observed the influence of lithology on landscape form and evolution. However, the specific mechanisms by which this is accomplished are not well characterized. Here, I investigate the role of lithology in landscape evolution processes across spatial and temporal scales and geomorphic domains, to progress our understanding of the basic controls on the processes which shape Earth's surface. These investigations were carried out within the Valley and Ridge province of the Appalachian Mountains, where contrasts in strength of underlying lithologies (juxtaposed by Alleghanian deformation) exert a clear, dominant control on the fabric of the landscape, providing an excellent opportunity to study the influence of lithology on a variety of landscape evolution processes. First, I assess the geomorphic function of boulders found on hillslopes and channels in the Valley and Ridge province of the Appalachians, which are sourced from resistant lithologies capping ridgelines. High-resolution UAV surveys and field mapping of boulder distributions and characteristics reveal that boulders are abundant on hillslopes and highly concentrated in channels, often trap sediment upslope, and appear to be long-lived. These observations suggest that boulders act as armor for hillslopes and channels, shielding weaker underlying units from erosion and inhibiting fluvial incision, and therefore play an important role in preserving topography in the Valley and Ridge landscape, highlighting a specific mechanism by which lithology exerts an influence on topography in this setting. Second, I investigate the relative importance of rock strength and discontinuity spacing in setting fluvial bedrock erodibility by comparing knickpoint and non-knickpoint bedrock, which correspond to end-member erodibility cases, and assess how lithology impacts knickpoint expression. Detailed field surveys of 21 lithologic knickpoints, surrounding non-knickpoint reaches, and corresponding bedrock properties reveal three key outcomes: 1) discontinuity spacing is a stronger predictor of knickpoint occurrence, and therefore more significant in setting bedrock erodibility in this setting, confirming quantitatively the hypothesis that discontinuities exert a dominant control on fluvial erodibility, 2) knickpoint expression is a function of the unique combination of characteristics within a given stratigraphic interval, and therefore highly complex and specific to local conditions, implying that knickpoint morphology should be interpreted with extreme caution, and 3) because all 21 study knickpoints occur within the same unit, inter-unit heterogeneity must be accounted for before lithologic influence on channel profile convexities can be ruled out, rather than comparing to geologic map contacts. These findings represent an important contribution towards a more functional understanding of the influence of lithology on fluvial bedrock incision processes.
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    Supercontinental Inheritance and its Influence on Supercontinental Breakup: The Central Atlantic Magmatic Province and the Breakup of Pangea
    Whalen, Lisa Marie (Virginia Tech, 2016-06-23)
    The Central Atlantic Magmatic Province (CAMP) is the large igneous province (LIP) that coincides with the breakup of the supercontinent Pangea. Major and trace element data, Sr-Nd-Pb radiogenic isotopes, and high-precision olivine chemistry were collected on primitive CAMP dikes from Virginia (VA). These new samples were used in conjunction with a global CAMP data set to elucidate different mechanisms for supercontinent breakup and LIP formation. On the Eastern North American Margin, CAMP flows are found primarily in rift basins that can be divided into northern or southern groups based on differences in tectonic evolution, rifting history, and supercontinental inheritance. Geochemical signatures of CAMP suggest an upper mantle source modified by subduction processes. We propose that the greater number of accretionary events, or metasomatism by sediment melts as opposed to fluids on the northern versus the southern Laurentian margin during the formation of Pangea led to different subduction-related signatures in the mantle source of the northern versus southern CAMP lavas. CAMP samples have elevated Ni and low Ca in olivine phenocrysts indicating a significant pyroxenite component in the source, interpreted here as a result of subduction metasomatism. Different collisional styles during the Alleghanian orogeny in the North and South may have led to the diachroneity of the rifting of Pangea. Furthermore, due to a low angle of subduction, the Rheic Plate may have underplated the lithosphere then delaminated, triggering both the breakup of Pangea and the formation of CAMP.