We use cosmogenic 10Be analysis of fluvial sediments and bedrock to estimate erosion rates (103 – 106 year timescale) and to infer the distribution of post-orogenic geomorphic processes in the Blue Ridge Province in and around Shenandoah National Park, VA. Our sampling plan was designed to investigate relationships between erosion rate, lithology, slope, and basin area. Fifty-nine samples were collected from a variety of basin sizes (<1 – 3351 km2) and average basin slopes (7 - 26°) 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 of these samples are from rivers draining streams exiting the eastern and western slopes of the Park (Rappahannock and Shenandoah Rivers). Inferred erosion rates for all lithologies for fluvial samples range from 3.8 to 24 m/My. The mean erosion rate for single-lithology basins in the Park is 11.6 ± 4.8 m/My. Singlelithology erosion rates ranges for fluvial samples are: granite (basin size = ~0.4-40 km2 and slope = 11-23°), 7.9–22 m/My; metabasalt (basin size = ~1-25 km2 and slope = 11-19°), 4.8–24 m/My; quartzite (basin size = ~0.1-9 km2 and slope = 12-23°), 4.7–17 m/My; and siliciclastic rocks (basin size = ~0.3-13 km2 and slope = 18-26°), 6.2–17 m/My. The mean erosion rate for multilithology basins (basin size = ~1-3351 km2 and slope = 7-22°) is 10.2 m/My, and individually for the Shenandoah River 7.3 m/My and the Rappahannock River 13.8 m/My. Bedrock erosion rates range from 2.4-13 m/My across all lithologies, with a mean erosion rate of 7.9 ± 5.0 m/My. Grain-size specific 10Be analysis of four samples showed no consistent trend of concentration with grain size. These data support Hack’s dynamic equilibrium model. Slope and erosion rate are not well correlated, and mean erosion rates are similar for different lithologies. Cosmogenicallydetermined erosion rates in Shenandoah Park are similar to or lower than those reported elsewhere in the Appalachians including those of Matmon and others (2003), 25 to 30 m/My for metaclastic rocks in the steep Great Smoky Mountains, Reuter and others (2004), 4 – 54 m/My in Susquehanna River basin for shale, sandstone, and schist, and Sullivan and others (2006), 6-38 m/My in the micaceous schist and gneiss of the Blue Ridge Escarpment. Cosmogenic erosion rates (integration over 104 yrs) in the Blue Ridge province of Shenandoah National Park are consistent with long-term unroofing rates (integration over 107 yrs) estimated from U-Th/He measurements (11-18 m/My) in samples collected near the Blue Ridge Escarpment by Spotila and others (2004), and fission tracks (20 m/My) in the Appalachians by Naeser and others (2005). The consistency of denudation rates integrated over very different periods of time suggests steady erosion most likely in balance with, and driving isostatic uplift of rock.

Hillslopes comprise the majority of unglaciated upland landscapes; they are the primary source for the production of sediment from bedrock, and the routing system by which sediment is delivered to the channel network. Yet the nature of hillslope response to changes in tectonic, climatic or base-level boundary conditions is poorly understood in terms of the spatial and temporal distribution of hillslope morphology. Here I exploit a previously published framework for exploring hillslope morphology in high relief landscapes (Roering et al., 2007), to address several critical questions: Does high resolution topography allow understanding of the processes and rates by which sediment is redistributed on hillslopes? If so, can hillslope morphology be used to map the spatial distribution of erosion rates and facilitate interpretation of the timing and magnitude of tectonic forcing, particularly in transient landscapes which are adjusting their erosion rates? And to what extent does variation in lithology influence hillslope evolution and morphology, and the ability to interpret process rates from hillslope form? In this thesis I sought to explain hillslope adjustment to changing boundary conditions through combining the predictions of analytical and numerical models with detailed analysis of real, high resolution topographic datasets (derived from LiDAR), focusing on two landscapes where the influence of tectonic forcing on base-level history is relatively well constrained, the Middle Fork Feather River in the northern Sierra Nevada, and the Dragon’s Back Pressure Ridge, on the Carrizo Plain, both in California. The Sierra Nevada of California is a west-tilted fault block composed primarily of granitoids formed during Mesozoic arc volcanism. The block underwent acceleration in uplift 5 - 3.5 Ma which is hypothesised to be caused be the drop-off of a dense root from the lower crust and replacement by hot asthenosphere, causing crustal buoyancy. A relict landscape has thus been uplifted and dissected by the major drainage routes crossing the range, which have eroded rapidly to form deep canyons. The fluvial network is characterised by breaks in slope (knickpoints) which migrate into the landscape to transmit the signal of increased erosion, setting baselevel conditions for adjacent hillslopes. Theoretical predictions for the morphology of hillslopes governed by a nonlinear sediment transport law, if the hillslopes have attained steady state (i.e. they are eroding in concert with base-level fall in adjacent valleys) reveal that the curvature of hilltops will be linearly proportional to erosion rates or rate of base-level fall. I present innovative techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates. This work is carried out in granitoid lithologies where the influence of bedrock heterogeneity is assumed no to be a first order control on hillslope morphology. Existing and new cosmogenic radionuclide analyses in the Feather River basin, California, suggest that erosion rates vary by over an order of magnitude from the remnant upland landscape to the incised river canyon. Hilltop curvature increases with erosion rates, allowing calibration of the hillslope sediment transport coefficient, which controls the relationship between hillslope gradient and sediment flux. This in turn allows the estimation of erosion rates throughout the landscape by mapping the spatial distribution of hilltop curvature. Additionally, despite the landscape containing gradient-limited hillslopes, hilltop curvature continues to increase with rising erosion rates, reflecting higher erosion rates than can be predicted by hillslope gradient. The distribution of hillslope morphology conforms well to predictions of a nonlinear sediment transport model, with measured values of hillslope relief varying with the product of hilltop curvature and hillslope length (proxy for erosion rate) in a manner similar to that predicted by Roering et al. (2007). Hilltop curvature can thus be used to estimate erosion rates in landscapes undergoing a transient adjustment to changing boundary conditions provided that the response timescale of hillslopes is short relative to channels. Having focused on a landscape with roughly uniform bedrock geology to isolate drivers of geomorphic change, I sought to evaluate whether these techniques could be extended across lithologic contacts and throughout the landscape. Underlying geology influences the efficacy of soil production and transport on hillslopes, and resistance to erosion by valley-forming processes. Here, quantitative analysis of LiDAR digital terrain models was performed to search for a topographic signature in two distinct lithologies in the Feather River catchment in northern California; granodiorite and deformed volcanics. The two sites, separated by <2 km and spanning similar elevations, are assumed to have similar climatic and denudation histories. Responding to increased erosion rates, transient hillslopes exhibit high gradient but low hilltop curvature in the metavolcanics relative to theoretical predictions for steady state hillslopes. However, hillslopes in the granodiorite have, for the most part, variation in hilltop curvature, hillslope length and hillslope relief similar to model predictions for steady state hillslopes. The curvature of hilltops adjacent to main stem channels implies that the coefficient of sediment transport is two times larger in the granodiorite (c. 8.8 m2 ka-1) than in the metavolcanics (c. 4.8 m2 ka-1). The data suggest that hillslopes get shorter as erosion rates increase due to the increased influence of debris flows in valley incision, suggesting that drainage density increases with erosion rate. The incision wave associated with more rapid erosion in the Feather River has propagated further into a basin developed on the metavolcanics and hence this substrate is less resistant to channel incision. I review an inventory of values for the transport coefficient for hillslope sediment transport but find that no clear patterns emerge with varying lithology. However in unconsolidated substrates, precipitation may play an important role in modulating sediment transport through variation in rain splash impact frequency and the frequency of wetting/drying, freeze/thaw, and expansion/contraction cycles. Finally I apply the same techniques to study hillslope morphology to a landscape where the tectonic history has a documented influence on landscape development. The Dragon’s Back pressure ridge, Carrizo Plain, CA, consists of a series of small catchments adjacent to the San Andreas fault, where previous detailed geologic mapping has allowed the spatial and temporal distribution of uplift to be constrained. This landscape offers a hitherto unique opportunity to study the temporal evolution of hillslope morphology via ergodic substitution. I show that the time evolution of a sensitive indicator of erosion rate, hilltop curvature, can be predicted using a nonlinear sediment flux law. Further to this, the temporal evolution of relief and hilltop curvature experiences hysteresis as the landscape grows and decays. Relative to steady-state predictions, hillslope morphologies exhibit higher than expected values for relief during active uplift or landscape growth, and lower than expected relief during landscape decay. Therefore landscapes growing due to fault activity can be distinguished from those with quiescent faults undergoing topographic decay.

No description available.

Upland rivers control the large-scale topographic form of mountain belts, allow coupling of climate and tectonics at the earth’s surface and are responsible for large scale redistribution of sediment from source areas to sinks. However, the details of how these rivers behave when perturbed by changes to their boundary conditions are not well understood. I have used a combination of fieldwork, remotely sensed data, mathematical analysis and computer modelling to investigate the response of channels to well constrained changes in the forcings upon them, focussing in particular on the effects of glacial remoulding of the catchments draining the south flank of the Ladakh batholith, northwest Indian Himalaya. The last glacial maximum for these catchments is atypically old (~100 ka), and this allows investigation of the response to glaciation on a timescale not usually available. The geomorphology of the catchments is divided into three distinct domains on the basis of the behaviour of the trunk stream – an upper domain where the channel neither aggrades above or incises into the valley form previously carved by glacial abrasion, a middle domain where the channel incises a gorge down into glacial sediments which mantle the valley floor, and a lower domain where the channel aggrades above this postglacial sediment surface. This landscape provides a framework in which to analyze the processes and timescales of fluvial response to glacial modification. The dimensions of the gorge and the known dates of glacial retreat record a time averaged peak river incision rate of approximately 0.5 mm/y; the timescale for the river long profile to recover to a smooth, concave up form must exceed 1 Ma. These values are comparable with those from similarly sized catchments that have been transiently perturbed by changing tectonics, but have never been quoted for a glacially forced basin-scale response. I have also demonstrated that lowering of the upper reaches of the Ladakh channel long profiles by glacial processes can systematically and nonlinearly perturb the slope-area (concavity) scaling of the channel downstream of the resulting profile convexities, or knickzones. The concavity values are elevated significantly above the expected equilibrium values of 0.3-0.6, with the magnitude controlled by the relative position of the knickzone within the catchment, and thus also by the degree of glacial modification of the fluvial system. This work also documents the existence of very similar trends in measured concavities downstream of long profile convexities in other transiently responding river systems in different tectonoclimatic settings, including those responding to changes in relative channel uplift. This previously unrecognised unity of response across a wide variety of different environments argues that such a trend is an intrinsic property of river response to perturbation. Importantly, it is consistent with the scaling expected from variation in incision efficiency driven by evolving sediment flux downstream of knickzones. The pervasive nature of this altered scaling, and its implications for fluvial erosion laws in perturbed settings, have significant consequences for efforts to interpret past changes in forcings acting on river systems from modern topography. I follow this by examining in detail the channel hydraulics of the Ladakh streams as they incise in response to the glacial perturbation. I present a new framework under which the style of erosion of a natural channel can be characterized as either detachment- or transport-limited based upon comparison of the downstream distribution of shear stress with the resulting magnitude of incision. This framework also allows assessment of the importance of sediment flux driven effects in studied channels. This approach is then used to demonstrate that fluvial erosion and deposition in the Ladakh catchments is best modelled as a sediment flux dependent, thresholded, detachment-limited system. The exceptional quality of the incision record in this landscape enables an unprecedented calibration of the sediment flux function within this incision law for three different trunk streams. The resulting curves are not compatible with the theoretically-derived parabolic form of this relation, instead showing nonzero erosion rates at zero sediment flux, a rapid rise and peak at relative sediment fluxes of less than 0.5 and a quasi exponential decrease in erosional efficiency beyond this. The position of the erosional efficiency peak in relative sediment flux space and the magnitude of the curve are shown to be both variable between the catchments explored and also correlated with absolute sediment flux in the streams.

Terrestrial cosmogenic nuclides such as beryllium-10 have recently been used as a way to determine basin-average erosion rates around the world. These erosion rates are useful to geomorphologists investigating landscape evolution. The Hangay Mountains in Mongolia are a prime location to use beryllium-10 because of the granitic rocks that provide the quartz needed for cosmogenic analysis as well as the lack of observed evidence of recent or old mass wasting events that mobilize sediment and bedrock with much lower cosmogenic concentrations that cause underestimations of erosion rates. Basin-average erosion rates observed in seven basins across the eastern Hangay Mountains range from 12 m/My to about 20 m/My. These are of similar magnitude to those found in tectonically inactive regions such as the southern Appalachians. Comparing basin-average erosion rates to basin parameters, whole basin relief had the highest calculated R2 value and elevation had the lowest P-value. No strong relationships were seen between erosion rate and mean slope angle, hypsometric integral, area, or mean local relief. The basin-average erosion rates observed in the Hangay were compared to previous studies by Ahnert (1970), Portenga and Biernman (2011), and Matmon et al. (2009). We found erosion rates from the Hangay to be much lower than expected in our analyses. The differences in erosion rates from the Hangay Mountains compared to other places around the world are likely due to the fact that the streams in the Hangay are eroding into alluvium as opposed to bedrock, and are located in a landscape dominanted by diffusive hillslope sediment transport mechanisms. The erosion rate is limited to the amount of sediment that can be transported by the streams.

abstract: The San Gabriel Mountains (SGM) of southern California provide the opportunity to study the topographic controls on erosion rate in a mountain range where climate and lithology are relatively constant. I use a combination of digital elevation model data, detailed channel survey data, decadal climate records, and catchment-averaged erosion rates quantified from 10Be concentrations in stream sands to investigate the style and rates of hillslope and channel processes across the transition from soil-mantled to rocky landscapes in the SGM. Specifically, I investigate (1) the interrelations among different topographic metrics and their variation with erosion rate, (2) how hillslopes respond to tectonic forcing in "threshold" landscapes, (3) the role of discharge variability and erosion thresholds in controlling the relationship between relief and erosion rate, and (4) the style and pace of transient adjustment in the western SGM to a recent increase in uplift rate. Millennial erosion rates in the SGM range from 0.03-1.1 mm/a, generally increasing from west to east. For low erosion rates (< 0.3 mm/a), hillslopes tend to be soil-mantled, and catchment-averaged erosion rates are positively correlated with catchment-averaged slope, channel steepness, and local relief. For erosion rates greater than 0.3 mm/a, hillslopes become increasingly rocky, catchment-mean hillslope angle becomes much less sensitive to erosion rate, and channels continue to steepen. I find that a non-linear relationship observed between channel steepness and erosion rate can be explained by a simple bedrock incision model that combines a threshold for erosion with a probability distribution of discharge events where large floods follow an inverse power-law. I also find that the timing of a two-staged increase in uplift rate in the western SGM based on stream profile analysis agrees with independent estimates. Field observations in the same region suggest that the relict topography that allows for this calculation has persisted for more than 7 Ma due to the stalling of migrating knickpoints by locally stronger bedrock and a lack of coarse sediment cover. / Dissertation/Thesis / Ph.D. Geological Sciences 2011

With large populations living in tropical regions of the world with volcanic substrates, understanding basalt weathering processes is vital. The Hawaiian Islands are an excellent natural analogue to study chemical weathering rates due to a uniform bedrock (basalt), large variations in rainfall, and varying ages across the islands. Laterite weathering profiles (LWP) develop over time through chemical weathering, where LWP thickness is influenced by many factors, including precipitation and time. Using the rapid, non-invasive horizontal-to-vertical spectral ratio (HVSR) method, LWP thicknesses can be estimated to constrain chemical weathering rates. Studying the laterite weathering profiles developed from basaltic bedrock of varying ages on Oahu (~2 Ma), Molokai (~1 Ma) and Kohala, Hawaii (~0.3 Ma) reveals three profiles in varying developmental stages. Over 200 HVSR soundings were collected on Oahu, Molokai, and Kohala. Shear wave velocity values of LWPs were determined by MASW (multichannel analysis of surface waves), and LWP thicknesses verified from geologic logs and outcrop. Oahu has thick LWPs compared to the other islands and shows a trend of increasing thickness with increasing precipitation across the island. The Molokai LWP follows a trend similar to Oahu, with a noticeable difference of thicknesses (20-40 m) at similar precipitation thresholds. Molokai presented a unique case, where the shear-wave velocity (Vs) boundaries between laterite and basalt were gradational for ~43% of HVSR datapoints, resulting in featureless frequency spectra that could not reliably model laterite-basalt boundary depths. The gradational nature of the LWP of Molokai is attributed to the young age of the island, and primary permeability properties of the thick, post-shield alkalic lavas. Molokai has an aerially average weathering rate of 0.02 to 0.04 m/ka. Kohala HVSR data show a newly developed LWP with varying LWP thickness within the same precipitation isohyet. LWPs on Kohala show a unique trend where LWP is thickest along the coast and is wedge shaped thinning out towards higher elevations. Each island differs in age and has its own unique LWP trends, with older islands tending to have deeper, more developed LWPs at similar precipitation ranges.

<p>Chemical weathering of highly reactive mafic and ultramafic igneous rocks may be a key sink in the global carbon cycle. Understanding how uplift of these rocks during arc-arc and arc-continent collisions through earth history has affected the evolution of global climate, including the onset of icehouse periods, requires improved constraints on the relative sensitivity of their weathering rates to physical erosion vs. climate. If weathering rates depend chiefly on erosion, then tectonic uplift of mafic and ultramafic rocks may have a strongly destabilizing effect on global climate. Conversely, if weathering rates are limited primarily by temperature or runoff, then a negative feedback mechanism between weathering and climate may attenuate the effects of rock uplift. This work characterizes the relationship between chemical weathering rates, physical erosion rates, and climate in tropical, montane watersheds in Puerto Rico that are underlain by volcanic arc rocks and associated ophiolitic serpentinite. Key to this analysis are new constraints on long-term erosion rates on these rocks from cosmogenic Cl-36 produced <em>in situ</em> in magnetite. These cosmogenic erosion rates are paired with classical measurements of stream solute fluxes and sediment geochemistry across runoff gradients to quantify the limits to volcanic arc rock and serpentinite weathering rates. </p> <p><br></p> <p>This work is divided into three chapters. Chapter 2 constrains the altitude scaling behavior of Cl-36 production in magnetite. This allows erosion rates to be determined more accurately in watersheds near sea level in Puerto Rico. Chapter 3 demonstrates that volcanic arc rock weathering rates in the humid tropics are more strongly limited by physical erosion than by climatic factors. However, a positive correlation between erosion and runoff observed in this landscape may enhance the coupling between climate and weathering rates. Chapter 4 finds that, in contrast to volcanic arc rocks, serpentinite weathering is strongly limited by runoff and weakly limited by erosion. These results are presented as empirical power-law relationships that can be readily applied in global carbon cycle modeling.  </p>

Es procedeix a l'estudi dels patrons, taxes i formes d'erosió a les costes rocoses carbonatades de Mallorca a partir de la utilització de diferents tècniques instrumentals i experiències de laboratori i camp per tal de desentrellar la contribució dels agents i processos d'alteració i erosió en la formació del relleu litoral. Es dedica una particular atenció a la contribució dels organismes brostejadors (Patella rustica i Melaraphe neritoides) en la destrucció de les costes rocoses, així com també al paper dels biofilms que habitens sobre la roca o bé que ocupen la porositat de la roca i que estan relacionats bé amb les variacions de microtopografia o bé amb el control de la capacitat agressiva de l'aigua retinguda al conjunt de formes del karren litoral. El principal interès del treball realitzat és posar de manifest que a una escala temporal curta, diària, i considerant aquells processos més continus en la dinàmica erosiva de les costes rocoses, la component biològica juga un paper capital en l'erosió del rocam. / This document deals with the study of the rates; patterns and morphologies of erosion at carbonate rock coasts of Mallorca. The approaches takes into account different instrumental techniques and field an laboratory experiments in order to separate the contribution of the different agents and processes in rock coast landform development. The investigation has been specially oriented towards the analysis of the contribution of grazing organisms and the role of biofilms in the evolution of coastal karren forms. The main contribution of the work is to point up that at short time scales and taking in account the processes that operate in a continuous way on the rock coast erosion dynamics, the biological component is very important in the coast erosion.

Submitted by (lucia.rodrigues@ufrpe.br) on 2016-07-11T14:45:22Z No. of bitstreams: 1 Flavio Adriano Marques.pdf: 3199315 bytes, checksum: a208fb8314e3ffa0d451678350503edb (MD5) / Made available in DSpace on 2016-07-11T14:45:22Z (GMT). No. of bitstreams: 1 Flavio Adriano Marques.pdf: 3199315 bytes, checksum: a208fb8314e3ffa0d451678350503edb (MD5) Previous issue date: 2013-02-21 / The Brazilian region known as Brejo Paraibano, in recent decades has been undergoing a process of replacing its native coverage by crops, mainly for pasture for extensive livestock farm, which has favored the occurrence of soil degradation in the form of water erosion. Given this context, this study aimed to quantify the rates of interrill erosion under simulated rainfall and evaluate the roughness parameters vegetable emerged to laminar flow in a pasture area hilly relief. The experiment was conducted in a completely randomized design with 4 treatments, 15%, 25%, 35% and 45% slope in a Ultisol under pasture, 5 replications, totaling 20 plots. The mean flow velocity and infiltration rates of pasture varied significantly with increasing slope contributing to increased erosion rates up to 35%. In the condition of 45% slope was not the greatest soil losses have occurred because of the removal of the horizon, in which the erosion process is acting on the argilic B horizon. The vegetation drag coefficient to pasture expressed elevation when there was a decrease of turbulent flow between plants. In the condition height flow increase, there was a greater energy flow between the mass of water and the plants structure of Brachiaria decumbens that resulted in decreasing on the force drag tension of plants. / A microrregião do Brejo Paraibano nas últimas décadas vem sofrendo um processo de substituição de sua cobertura nativa por culturas agrícolas, sobretudo por pastagens para exploração de pecuária extensiva, o que vem favorecendo a ocorrência de degradação do solo na forma de erosão hídrica. Diante desse contexto, este estudo teve como proposta quantificar as taxas de erosão em entressulcos sob chuva simulada e avaliar os parâmetros da rugosidade vegetal emersa ao escoamento em área de pastagem em relevo movimentado. O experimento foi conduzido em delineamento inteiramente casualizado nas condições de declive: 15%, 25%, 35% e 45%, em um Argissolo Vermelho-Amarelo sob pastagem, em 5 repetições, totalizando 20 parcelas experimentais. A velocidade média do escoamento e as taxas de infiltração na pastagem variaram significativamente com a elevação do declive contribuindo para elevação das taxas de erosão ate o declive de 35%. Na condição de 45% de declive não ocorreram as maiores perdas de solo em virtude de ter ocorrido a remoção do horizonte A, estando o processo erosivo atuando sobre o horizonte B textural. O coeficiente de arraste vegetal da pastagem expressou elevação quando ocorreu diminuição da turbulência do escoamento entre as plantas. Com o aumento da lâmina de escoamento, houve um maior fluxo de energia entre a massa de água e a estrutura vegetal da gramínea Brachiaria decumbens, que se refletiram na diminuição dos valores de tensão de arraste na planta.