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The erosion caused by jets issued from hydraulic structures progressively develops a confined scour-hole on the riverbed. A realistic scour assessment must consider both the influence of the air entrained when the jet plunges into the pool and the flow patterns induced by bottom geometry. This experimental study systematically analyzes the combined influence of jet aeration and pool confinement on the dynamic pressures affecting the water-rock interface and inside 3D fissures around a block. The results show that confinement reduces mean pressures and pressure fluctuations when the pool is relatively deep, but almost no influence is found when the pool is shallow, while air entrainment has an opposite effect. Three mechanisms are identified, two of them depend on the pool depth. Furthermore, when a block is mobile, pressures are attenuated inside the surrounding joints. The consequent block vibrations and the presence of air reduce pressure waves celerity inside the fissures. Key words: air entrainment, plunging jets, flow pattern, rock scour, plunge pool, high-velocity jets, lateral jet confinement. L'erosion causee par des jets issus de structures hydrauliques developpe progressivement une fosse confinee sur le lit. Une evaluation realiste de l'affouillement considere l'influence de l'air entraine quand le jet plonge, ainsi que des flux induits par la geometrie du fond. Cette etude experimentale analyse systematiquement l'influence combinee de l'aeration et du confinement de la fosse sur les pressions dynamiques affectant l'interface eau-roche et les fissures 3D autour d'un bloc. Les resultats montrent que le confinement reduit les pressions moyennes et ses fluctuations si le bassin est relativement profond, mais l'influence est negligeable quand le bassin est peu profond, tandis que l'aeration a un effet inverse. Trois mecanismes sont identifies, deux d'entre eux dependent de la profondeur du bassin. Quand un bloc est mobile, les pressions sont attenuees a l'interieur des fissures. Les vibrations du bloc, comme la presence d'air, reduisent la celerite des ondes de pression dans les fissures. Mots-cles: entrainement d'air, jets plongeants, caracteristiques d'ecoulement, affouillement, fosse d'erosion, jets a haute vitesse, confinement laterale du jet., Introduction High-velocity jets issued from high-head hydraulic structures, such as dams, have the potential to generate erosion when impacting rocky riverbeds, where a confined scour hole progressively develops (Schleiss 2002). [...]

Soil loss due to water erosion, in particular hillslope erosion, can be estimated using predictive models such as the Revised Universal Soil Loss Equation (RUSLE). One of the important and dynamic elements in the RUSLE model is the cover and management factor (C-factor), which represents effects of vegetation canopy and ground cover in reducing soil loss. This study explores the potential for using fractional vegetation cover, rather than traditional green vegetation indices (e.g. NDVI), to estimate C-factor and consequently hillslope erosion hazard across New South Wales (NSW), Australia. Values of the C-factor were estimated from the emerging time-series fractional cover products derived from Moderate Resolution Imaging Spectroradiometer (MODIS). Time-series C-factor and hillslope erosion maps were produced for NSW on monthly and annual bases for a 13-year period from 2000 to 2012 using automated scripts in a geographic information system. The estimated C-factor time-series values were compared with previous study and field measurements in NSW revealing good consistency in both spatial and temporal contexts. Using these time-series maps, the relationship was analysed between ground cover and hillslope erosion and their temporal variation across NSW. Outcomes from this time-series study are being used to assess hillslope erosion hazard, sediment and water quality (particularly after severe bushfires) across NSW at local, catchment and regional scales. Additional keywords: cover and management factor, fractional vegetation cover, GIS, hillslope erosion, MODIS, RUSLE., Introduction Soil erosion by water is a natural process whereby soil particles are detached from the soil surface and transported by the movement of water. The three dominant processes of [...]

The ~51 Ma Montagnais impact crater on the outer Scotian Shelf is well known, but the potential effects from the impact event on the slope and rise seaward of the crater have, until now, remained poorly understood. Through detailed seismic stratigraphic correlation and ties to available wells, we define a threefold seismic stratigraphic subdivision for Upper Cretaceous to Eocene strata on the shelf and slope, calibrated to the most recent biostratigraphic results. Using this framework, we identify a number of depositional and erosional products that are temporally consistent with a late Ypresian impact event (within the limits of seismic and biostratigraphic resolution). We link a series of prominent failure scarps on the outer shelf and upper slope to a single widespread mass-transport deposit (MTD) on the lower continental slope, rise, and abyssal plain. Failed material amassed in a large debris field referred to here as the 'Montagnais MTD'. It covers an area of ~93 000 [km.sup.2] and travelled up to 580 km from the impact site where its distal termination onlaps the New England Seamounts, making it one of the largest known debris avalanches on Earth. We interpret these deposits, and the associated pattern of erosion landward of them, as products of widespread margin collapse caused by a combination of ground shaking and ensuing tsunamis triggered by the Montagnais impact event. This study provides insight into the potential effects of outer-shelf marine impact events immediately downslope from impact sites, and their diminished effects with increasing distance along the margin. Le cratere d'impact Montagnais, ~51 Ma, sur la plateforme externe Scotian, est bien connu mais les effets potentiels de l'impact sur la pente et l'elevation du cratere du cote de la mer sont, jusqu'a present, demeures mal compris. Grace a une correlation sismique detaillee et l'etablissement de liens avec des puits disponibles, nous definissons une triple subdivision stratigraphique sismique pour les strates du Cretace superieur a l'Eocene sur la plateforme et la pente, etalonnee en se basant sur les plus recents resultats biostratigraphiques. En utilisant ce cadre, nous identifions un certain nombre de produits de deposition et d'erosion qui concordent dans le temps avec un evenement d'impact a l'Ypresien tardif (a l'interieur des limites de la resolution sismique et biostratigraphique). Nous relions une serie de ruptures d'escarpements proeminents sur la plateforme externe et la partie superieure de la pente a un vaste depot unique de transport de masse (MTD) sur la partie inferieure de la plateforme continentale, la partie en elevation et la plaine abyssale. Le materiel de rupture s'est amasse dans un grand champ de debris que nous nommons ici le << Montagnais MTD >>. Il couvre une superficie ~93 000 [km.sup.2] et il atteint une distance de 580 km du site d'impact ou son extremite distale chevauche la chaine de monts sous-marins de la Nouvelle-Angleterre, ce qui en fait l'une des plus grosses avalanches de debris sur Terre. Nous interpretons ces depots, et le patron associe d'erosion du cote continental, comme etant les produits d'un vaste effondrement de la bordure cause par une combinaison de tremblements de terre et des tsunamis qui en ont decoule, declenches par l'evenement d'impact Montagnais. La presente etude donne un apercu des effets potentiels des evenements d'impact marins sur la plateforme externe, tout juste a l'aval des sites d'impact, et de leurs effets, lesquels s'attenuent avec l'accroissement de la distance le long de la bordure. [Traduit par la Redaction], Introduction Ground shaking associated with earthquakes is known to be a primary mechanism for triggering submarine mass failures that erode slopes and transfer large quantities of sediment into deeper water [...]

A thermo-erosion gully has been monitored in the valley of glacier C-79 on Bylot Island since 1999. The main channel of the gully reached 390 m in length a few months after its initiation and grew between 38 and 50 m/year over the following decade, for an overall approximated average of 75 m/year. In 2009, the total gully length and area, including the main and relict channels, were 2500 m and 25 000 [m.sup.2], respectively. Gullies affect snow accumulation, and therefore ground temperature, local water flow, and drainage. Sinkholes, gully heads, pools, baydzherakhi, tunnels, and collapses were grouped as a function of time since gully formation in that area. Sinkholes and tunnels were formed every year after gully inception, and baydzherakhi were found in 3-10 year old sections of the gully. Stabilization of the gully floor and sides took about a decade. Le chenal principal du ravin a atteint une longueur de 390 m quelques mois apres son amorcage et il s'allongeait de 38 a 50 m/a durant la decennie suivante, pour une moyenne approximative de 75 m/a. En 2009, la longueur totale du ravin atteignait 2500 m et sa superficie 25 000 [m.sup.2], en incluant le chenal principal et les chenaux reliques. Les ravins affectent la couverture de neige et donc la temperature du sol, l' ecoulement local de l' eau et le drainage. Des trous de suffosion, le recul de tete de ravins, des bassins, des baydjarakhs, des tunnels et des effondrements etaient regroupes en fonction du temps ecoule depuis la formation du ravin dans la region. Des trous de suffosion et des tunnels se formaient chaque annee apres la formation du ravin et les baydjarakhs se trouvaient dans les sections du ravin formees de 3 a 10 ans auparavant. La stabilisation du fond du ravin et de ses pentes prenait environ une decennie. [Traduit par la Redaction], Introduction Ice-wedge polygons are common in continuous permafrost landscapes (Markov 1961; Popov 1961; Pewe 1966). During winter in areas with mean annual air temperatures of -2°C or less (Hamilton et [...]

Gully erosion is a significant process for delivering sediment to streams, and can be the dominant erosion process in some regions. As with other forms of erosion, we need methods to predict the extent and patterns of gully erosion across large areas. Such methods also improve our understanding of the environmental controls on gully erosion. Here, patterns of gully density are predicted across the 1 x [10.sup.6] [km.sup.2] Murray--Darling Basin in Australia, using aerial photograph mapping of gullies across part of the Basin and a multivariate statistical model of a range of environmental factors. Across the Basin, at a 10-km grid resolution, gully density is predicted to vary from 0 to 1.2 km [km.sup.-2], with 22% of the Basin having a gully density >0.1 km [km.sup.-2] and 3% a density >0.5 km [km.sup.-2]. The model is reasonably successful at predicting the variations in mapped gully density compared with similar attempts to predict erosion processes at this scale. Hillslope gradient and mean annual rainfall are the most important single factors across the region. The predicted mean gully density across the Basin is 0.08 km [km.sup.-2] and gullies contribute up to 27 x [10.sup.6] t [year.sup.-1] of sediment to the fiver network. This is more than the amount that has been estimated from the combined contribution of hillslope (14 x [10.sup.6] t [year.sup.-1]) and riverbank (8.6 x [10.sup.6] t [year.sup.-1]) erosion by other studies within the Basin., Introduction Gully erosion is a significant land degradation process and a source of increased sediment load to rivers (Wasson et al. 1998). Sand and fine gravel eroded from gullies can [...]

Most rust fungi are highly host specific, but Puccina psidii has an extremely broad host range within Myrtaceae and gained notoriety with a host jump in its native Brazil from common guava (Psidium guajava) to commercial Eucalyptus plantations. When detected in Hawai'i in April 2005, the first invasion outside the neotropics/subtropics, there was immediate concern for 'ohi'a (Metrosiderospolymorpha). 'Ohi'a composes 80% of native forest statewide, providing stable watersheds and habitat for most Hawaiian forest birds and plants. Within months, rust spores spread statewide on wind currents, but 'ohi'a was found to be only a minor host, showing very light damage. The primary host was nonnative rose apple (Syzygiumjambos), severely affected at a landscape scale, but the epiphytotic subsided as rose apple was largely defoliated or killed within several years. The limited and stable host range in Hawai'i (versus elsewhere) led the local conservation community to explore possibilities for excluding new genetic strains of P. psidii. Although national/international phytosanitary standards require strong scientific justification for regulations involving an infraspecific taxonomic level, hopes were buoyed when genetic studies showed no apparent genetic variation/evolution in Hawai'i's rust strain. A sophisticated genetic study of P. psidii in its home range is near completion; genetic variation is substantial, and host species strongly influences rust population structure. To prevent introduction of new strains, the Hawai'i Department of Agriculture is moving ahead with establishing stringent measures that restrict entry of Myrtaceae into Hawai'i. Meanwhile, P. psidii poses a major threat to Myrtaceae biodiversity worldwide., ACCELERATED MOVEMENT of plants and plant products between biogeographical zones by human activities has become rampant worldwide (Brasier 2008, Wingfield et al. 2011). Fungi and insects that may have little [...]

The occurrence of abandoned shoreline bedrock erosional features at the edge of the Niagara Escarpment at Cabot Head indicates the existence of a group of islands in glacial Lake Algonquin during early postglacial time, referred to herein as the Cabot Head Archipelago. The abandoned shoreline features are situated as much as 80 m above the level of contemporary Georgian Bay. The range of abandoned shoreline bedrock erosional features, including shoreline cliffs, adjacent wave-cut platforms, wave-cut notches, shore stacks, shore caves, and other features, are described. The occurrence of these features is thought to be the result of the interaction between wave action in glacial Lake Algonquin and two distinct lithological facies representing the Wiarton--Colpoy Bay and Lions Head members of the Amabel Formation. The exceptional development of the abandoned shoreline bedrock erosional features in massive reefal dolostone between elevations of ~250 and 255 m above sea level (asl) is interpreted as representing the relatively long-lived and stable Main stage of glacial Lake Algonquin 000-10 200 years BP). Shoreline erosional forms at elevations between ~240 and 250 m asl may be indicative of declining lake levels partially controlled by bedrock structural factors. The final abandonment of the glacial Lake Algonquin shoreline in this area occurred when the eastern outlets of the lake became ice-free and its level dropped rapidly some 10 200 years BP. The Cabot Head Archipelago and the associated suite of raised and abandoned shoreline bedrock erosional features represent a rare assemblage of landforms within the Great Lakes basin, and possibly within Canada. La presence de lignes de plages abandonnees et les caracteristiques d'erosion du socle a la bordure de l'Escarpement du Niagara a Cabot Head indiquent l'existence, au debut de l'ere postglaciaire, d'un groupe d'lles dans le lac glaciaire Algonquin; ce groupe sera appele ici l'archipel de Cabot Head. Les caracteristiques des lignes de plages abandonnees sont situees a des elevations pouvant atteindre 80 m au-dessus du niveau contemporain de la baie Georgienne. L'etendue de lignes de plages abandonnees, les caracteristiques d'erosion du socle, incluant les falaises littorales, les plateformes adjacentes erodees par les vagues, les encoches, les piliers sur les plages, les grottes et d' autres caracteristiques sont decrites. La presence de ces caracteristiques serait le resultat de l'interaction entre l'activite des vagues dans le lac glaciaire Algonquin et deux facies lithologiques distincts representant les membres Wiarton--Colpoy Bay et Lions Head de la Formation d'Amabel. Le developpement exceptionnel des caracteristiques d' erosion des plages abandonnees dans la dolomie recifale entre les elevations d'environ 250 et 255 m au-dessus du niveau de la mer suggere qu'elles aient ete formees durant l'etage Main, stable et de longue duree, du lac glaciaire Algonquin (environ 11 000 a 10 200 ans avant le present). Les formes d'erosion littorale a des elevations entre environ 240 et 250 m au-dessus du niveau de la mer pourraient indiquer la presence de niveaux a la baisse partiellement controles par les facteurs structurels du socle. L'abandon final des plages du lac glaciaire Algonquin dans ce secteur s'est produit lorsque les exutoires du cote est du lac ont ete degages des glaces et que son niveau a baisse rapidement il y a environ 10 200 ans avant le present. L'archipel de Cabot Head et la suite associee de lignes de plages soulevees et abandonnees ainsi que les caracteristiques d'erosion du socle representent un assemblage rare, sinon unique, de formes de relief dans le bassin des Grands Lacs et possiblement au Canada. [Traduit par la Redaction], Introduction The Bruce Peninsula is a large promontory between Lake Huron and Georgian Bay in southern Ontario (Fig. 1). The eastern and northern edges of the peninsula mark the location [...]

Pediments are gently sloping, low-relief bedrock erosional surfaces at the bases of mountain ranges. Pediments tend to form more readily in arid climates and in weathering-resistant lithoiogies, but the processes responsible for pediment formation are still not widely understood after more than a century of debate. In this paper, I investigate the behavior of a coupled numerical model for the evolution of mountain ranges and their adjacent piedmonts that includes bedrock erosion in channels, soil production and erosion on hillslopes, and the flexural-isostatic response of the lithosphere to erosional unloading. For relatively small values of the flexural parameter, erosion of the mountain range leads to sufficient flexural-isostatic tilting of the adjacent piedmont that a suballuvial bedrock bench is exhumed to form an erosional surface on the piedmont. In addition, slope retreat at the mountain front and subsequent tilting of the abandoned surface can contribute to pediment formation by lengthening the pediment in the upslope direction. The rate of erosion on the piedmont must also be greater than or equal to the rate of soil production, thereby creating an erosional surface that has, at most, a thin veneer of soil or regolith. The rate of soil production depends primarily on climate and lithology, with lower soil production rates associated with more arid climates and more resistant lithologies. The model predictions are compared to morphometric analyses of pediments in the southwestern United States and to the detailed morphology of two classic pediments in southern Arizona. doi: 10.1130/B30128.1

Vast amounts of methane hydrate exist beneath continental margins, but whether this methane releases from sediment on a large scale and affects the oceans and atmosphere remains unclear. Analysis of newly acquired three-dimensional seismic images and drilling data from a large gas hydrate province reveal a recently eroded v-shaped depression. The depression sharply cuts through a relic bottom simulating reflection (BSR) and hydrate- laden sediments. The shape of the relic BSR indicates that the seafloor depression was once a large anticline that has recently been eroded and released an estimated 1.51 x [10.sup.11] [m.sup.3] of methane. We hypothesize that erosion of the seafloor via bottom-water currents unroofed buoyant hydrate- laden sediments and subhydrate overpressured free gas zones beneath the anticline. Once triggered, gas-driven erosion created a positive feedback mechanism, releasing gas and eroding hydrate-bearing sediment. We suggest that erosive currents in deep-water methane hydrate provinces act as hair triggers, destabilizing kilometer-scale swaths of the seafloor where large concentrations of underlying overpressured methane exist. Our analysis suggests that kilometer- scale degassing events are widespread, and that deep-water hydrate reservoirs can rapidly release methane in massive quantities. doi: 10.1130/G31491.1

Hydrograph variability and channel morphology influence rates of fluvial bedrock incision, but little data exist on these controls in natural channels. Through field monitoring we demonstrate that (1) short-term bedrock channel incision can be rapid, (2) sustained floods with smaller peak discharges can be more erosive than flash floods with higher peak discharges, due to changes in bed alluviation, and (3) bedrock channel morphology varies with local bed slope and controls the spatial distribution of erosion. We present a three-year record of flow depths and bedrock erosion for a human-perturbed channel reach that drains the Henry Mountains of Utah, USA. Starting from a small and steep (~30% slope), engineered knickpoint in Navajo sandstone, erosion has cut a narrow, deep, and tortuous inner channel in ~35-40 years. Along the inner channel, we measured up to 1/2 m of vertical incision into Navajo sandstone over ~23 days, caused by the 2005 season of exceptional snowmelt flow. In contrast, flash floods caused little bedrock incision even when peak discharges were much higher than the peak snowmelt flow. Flash floods were net depositors of coarse sediment while snowmelt flow cleared alluvial cover. We document the formation of a pothole and interpret that it was abraded by bedload rather than fine suspended sediment. Finally, several slot canyons (Peek-a-boo, Spooky, and Coyote Gulch narrows) in the nearby Escalante River drainage basin have erosional morphologies similar to the monitored channel reach. Feedbacks between flow, sediment transport, and transient erosion provide a plausible explanation for the evolution of channel slope, width, and bed roughness of these natural bedrock channels. doi: 10.1130/B30126.1

The spatiotemporal behaviour of earthquakes within continental plate interiors is different from that at plate boundaries. At plate margins, tectonic motions quickly reload earthquake ruptures, making the location of recent earthquakes and the average time between them consistent with the faults' geological, palaeoseismic and seismic histories. In contrast, what determines the activation of a particular mid-continental fault and controls the duration of its seismic activity remains poorly understood (1). Here we argue that the concentration of magnitude-7 or larger earthquakes in the New Madrid seismic zone of the central United States (2,3) since the end of the last ice age results from the recent, climate-controlled, erosional history of the northern Mississippi embayment. We show that the upward flexure of the lithosphere caused by unloading from river incision between 16,000 and 10,000 years ago caused a reduction of normal stresses in the upper crust sufficient to unclamp preexisting faults close to failure equilibrium. Models indicate that fault segments that have already ruptured are unlikely to fail again soon, but stress changes from sediment unloading and previous earthquakes may eventually be sufficient to bring to failure other nearby segments that have not yet ruptured., Large earthquakes within continents, far from plate boundaries, are episodic, clustered and seem to migrate through time. Although this behaviour is recognized in many continental interiors (1), the best-studied case [...]

Over geologic timescales, C[O.sub.2] is emitted from the Earth's interior and is removed from the atmosphere by silicate rock weathering and organic carbon burial. This balance is thought to have stabilized greenhouse conditions within a range that ensured habitable conditions (1). Changes in this balance have been attributed to changes in topographic relief, where varying rates of continental rock weathering and erosion (1,2) are superimposed on fluctuations in organic carbon burial (3). Geological strata provide an indirect yet imperfectly preserved record of this change through changing rates of sedimentation (1,2,4). Widespread observations of a recent (0-5-Myr) fourfold increase in global sedimentation rates require a global mechanism to explain them (4-6). Accelerated uplift and global cooling have been given as possible causes (2,4,6,7), but because of the links between rates of erosion and the correlated rate of weathering (8,9), an increase in the drawdown of C[O.sub.2] that is predicted to follow may be the cause of global climate change instead (2). However, globally, rates of uplift cannot increase every where in the way that apparent sedimentation rates do (4,10). Moreover, proxy records of past atmospheric C[O.sub.2] provide no evidence for this large reduction in recent C[O.sub.2] concentrations (11,12). Here we question whether this increase in global weathering and erosion actually occurred and whether the apparent increase in the sedimentation rate is due to observational biases in the sedimentary record (13). As evidence, we recast the ocean dissolved [sup.10]Be/[sup.9]Be isotope system as a weathering proxy spanning the past ~12 Myr (ref. 14). This proxy indicates stable weathering fluxes during the late-Cenozoic era. The sum of these observations shows neither clear evidence for increased erosion nor clear evidence for a pulse in weathered material to the ocean. We conclude that processes different from an increase in denudation caused Cenozoic global cooling, and that global cooling had no profound effect on spatially and temporally averaged weathering rates., Studies of both the suspended and the dissolved loads of the world's largest rivers and of hill-slope denudation have shown a strong link between physical erosion and chemical weathering (8,9). [...]

Precise estimation of sediment transport capacity ([T.sub.c]) is critical to the development of physically based erosion models. Few data are available for estimating [T.sub.c] on steep slopes. The objectives of this study were to evaluate the effects of unit flow discharge (q), slope gradient (S), and mean flow velocity on [T.sub.c] in shallow flows and to investigate the relationship between [T.sub.c] and shear stress, stream power, and unit stream power on steep slopes using a 5-m-long and 0.4-m-wide nonerodible flume bed. Unit flow discharge ranged from 0.625 x [10.sup.-3] to 5 x [10.sup.-3] [m.sup.2] [s.sup.-1] and slope gradient from 8.8 to 46.6%. The diameter of the test riverbed sediment varied from 20 to 2000 [micro]m, with a median diameter of 280 [micro]m. The results showed that [T.sub.c] increased as a power function with discharge and slope gradient with a coefficient of Nash-Sutcliffe model efficiency (NSE) of 0.95. The influences ors on [T.sub.c] increased as S increased, with [T.sub.c] being slightly more sensitive to q than to S. The T was well predicted by shear stress (NSE = 0.97) and stream power (NSE = 0.98) but less satisfactorily by unit stream power (NSE = 0.92) for the slope range of 8.8 to 46.6%. Mean flow velocity was also a good predictor of [T.sub.c] (NSE = 0.95). Mean flow velocity increased as q and S increased in this study. Overall, stream power seems to be the preferred predictor for estimating T for steep slopes; however, the predictive relationships derived in this study need to be evaluated further in eroding beds using a range of soil materials under various slopes. Abbreviations: ANSWERS, Areal Nonpoint Source Watershed Environment Response Simulation; NSE, coefficient of Nash-Sutcliffe model efficiency; RE, relative error; WEPP, Water Erosion Prediction Project.

We present new age data for the evolution of the suture zone between lower plate South Penninic and upper plate Austroalpine units in the Central European Alps. Rb/Sr deformation ages for mylonitized rocks of the South Penninic palaeosubduction melange and for deformed Austroalpine basement (Eastern Switzerland) shed light on the pre-Alpine and Alpine deformation history along the suture, as well as on synsubduction interplate mass transfer. Rb/Sr age data define two age groups. The first group reflects pre-Alpine events within the upper plate basement, with varying degree of resetting by subsequent Alpine overprints. The second group marks the waning of subduction-related deformation along the South Penninic Austroalpine suture zone, at around 50 Ma, and termination at c. 47 Ma. Identical Rb/Sr ages for pervasively deformed Austroalpine and South Penninic lithologies point to tectonic erosion of the upper plate during subduction. We propose that underplating of the Middle Penninic micro-continent at c. 50 Ma led to the cessation of deformation within the South Penninic melange, and shifted the zone of active deformation into the footwall. This also caused a contemporaneous upper plate surface uplift and shutoff of sedimentation in Alpine Gosau-type forearc basins.

New apatite fission-track data, paleoelevation estimates from paleobotany, and recently acquired geological data from the Eastern Cordillera of Colombia document the onset of increased exhumation rates in the northeastern Andes at ca. 3 Ma. The Eastern Cordillera forms an efficient orographic barrier that intercepts moisture-laden winds sourced in the Amazon lowlands, leading to high rainfall and erosion gradients across the eastern flank of the range. In contrast, the drier leeward western flank is characterized by lower rates of deformation and exhumation. In light of the geological evolution of the Eastern Cordillera, the combination of these data sets suggests that the orographic barrier reached a critical elevation between ca. 6 and ca. 3 Ma, which ultimately led to protracted, yet more focused erosion along the eastern flank. Sequentially restored structural cross sections across the eastern flank of the Eastern Cordillera indicate that shortening rates also have increased during the past 3 Ma. From fission-track and structural cross-section balancing, we infer that accelerated exhumation led to increasing tectonic rates on the eastern flank, creating a pronounced topographic and structural asymmetry in the Eastern Cordillera. The tectonic and climatic evolution of this orogen thus makes it a prime example of the importance of climatic forcing on tectonic processes. Keywords: Apatite fission tracks, deformation, balanced cross sections, erosion, orographic barriers, paleoelevation, paleoclimate.

The Philippine Trench marks a nascent plate margin where subduction initiation is propagating from north to south. Magma compositions in the East Philippine Arc record thinning of arc lithosphere as it is eroded from below. Lithosphere is thicker beneath the younger, southern part of the arc, causing basaltic magma to stall and fractionate garnet at high pressure. In the mature, northern section, basaltic magma differentiates at shallower levels, at pressures where garnet is not stable. Local variations in lithosphere thickness suggest that thinning is rapid and may be piecemeal. Fluctuations in arc lithosphere thickness throughout the history of this margin appear to control spatial and temporal variations in magma fluxes into the arc crust. Varying fractionation depths of hydrous basalt may help to explain the andesitic composition of bulk continental crust. Keywords: subduction initiation, arc lithosphere erosion, crust growth, adakitic magmatism, high-Mg# andesite.

Hot lava is a viscous fluid that, driven by gravity, moves along the Earth's surface. Intuitively, one attributes constructional properties to lava--it accumulates in volcanic landforms, compound lava fields and, in the end, entire mountains. On the other hand, there are also examples of the erosive power of lava: on Earth and especially on other planets in the Solar System, there exist channels incised by flowing lava. The origins of these erosive features have long been debated among volcanologists and planetologists. The dominant paradigm is thermal erosion, although it leaves many questions open. After the 2001 eruption on Mount Etna we found a lava channel whose features cannot be explained in the frame of thermal erosion. On the basis of our observations, we have developed a model for mechanical erosion that explains the main field observations, and opens alternative ways to describe erosion by flowing lava. Keywords: erosion by flowing lava; lava channel formation; mechanical erosion; abrasion and wear

Conditions for unique geolocation of multiple radio-frequency emitters using a general observation model are derived.

We present [sup.238]U-[sup.230]Th -[sup.226]Ra disequilibria and cosmogenic [sup.3]He and [sup.36]Cl data for the Bluewater flow of the Zuni-Bandera volcanic field in western New Mexico. The [sup.238]U-[sup.230]Th disequilibria measured on separated groundmass phases yield an internal isochron age of 68 ka (+24/-20 ka; 2[sigma]). This value cannot be directly compared with surface exposure ages unless erosion rates are known. The apparent (zero erosion) ages determined from both the [sup.3]He concentration (47.5 [+ or -] 5 ka; 2[sigma]) and the [sup.36]Cl concentration (41.2 [+ or -] 8.8 ka; 2[sigma]) are significantly younger than the U-Th isochron age. When minimum estimates of surface erosion based on flow morphology are considered, the [sup.3]He concentrations indicate a minimum exposure age of 60 ka, in good agreement with the U-Th isochron age, with a minimum erosion rate of 1.7 mm/k.y. and an erosion rate as high as 5 mm/k.y. in other locations. Correcting for erosion has little effect on the model [sup.36]Cl age and, as a result, the [sup.36]Cl age is significantly younger than the U-Th isochron age and erosion-corrected [sup.3]He ages; this discordance is attributed to a lack of closed-system behavior in the [sup.36]CI system. These new ages have local significance for the geochronology of the Zuni-Bandera volcanic field; however, their larger significance is in their applicability to dating Quaternary basalts and quantifying erosion rates. Keywords: dating Quaternary basalts, U-Th-Ra disequilibria, [sup.3]He and [sup.36]Cl cosmogenic ages, erosion rates.

Sedimentation in the main pool of a deep (maximum depth: 50 m), 227 [km.sup.2] hydropower reservoir was modeled using a three-dimensional numerical model of hydrodynamics and sedimentation for different wind, inflow, and outflow conditions. Short-term velocity measurements made in the reservoir were used to validate some aspects of the hydrodynamic model. The effects of thermal stratification on sedimentation patterns were investigated, since the reservoir is periodically strongly stratified. Stratification alters velocity profiles and thus affects sedimentation in the reservoir. Sedimentation of reservoirs is often modeled considering only the deposition of sediments delivered by tributaries. However, the sediments eroding from the shorelines can contribute significantly to sedimentation if the shorelines of the reservoir erode at sufficiently high rates or if sediment delivery via tributary inflow is small. Thus, shoreline erosion rates for a reservoir were quantified based on measured fetch, parameterized beach profile shape, and measured wind vectors, and the eroded sediments treated as a source within the sedimentation modeling scheme. The methodology for the prediction of shoreline erosion was calibrated and validated using digital aerial photos of the reservoir taken in different years and indicated approximately 1 m/year of shoreline retreat for several locations. This study revealed likely zones of sediment deposition in a thermally stratified reservoir and presented a methodology for integration of shoreline erosion into sedimentation studies that can be used in any reservoir. DOI: 10.1061/(ASCE)0733-9429(2007)133:3(255) CE Database subject headings: Sedimentation; Deposition; EFDC; Shoreline erosion; Reservoir; Stratification.

Rock uplift and erosional denudation of orogenic belts have long been the most important geologic processes that serve to shape continental surfaces, but the rate of geomorphic change resulting from these natural phenomena has now been outstripped by human activities associated with agriculture, construction, and mining. Although humans are now the most important geomorphic agent on the planet's surface, natural and anthropogenic processes serve to modify quite different parts of Earth's landscape. In order to better understand the impact of humans on continental erosion, we have examined both long-term and short-term data on rates of sediment transfer in response to glacio-fluvial and anthropogenic processes. Phanerozoic rates of subaerial denudation inferred from preserved volumes of sedimentary rock require a mean continental erosion rate on the order of 16 m per million years (m/m.y.), resulting in the accumulation of-5 gigatons of sediment per year (Gt/yr). Erosion irregularly increased over the ~542 m.y. span of Phanerozoic time to a Pliocene value of 53 m/m.y. (16 Gt/yr). Current estimates of large river sediment loads are similar to this late Neogene value, and require net denudation of ice-free land surfaces at a rate of ~62 m/m.y. (~21 Gt/yr). Consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confined to drainage headwaters; ~83% of the global river sediment flux is derived from the highest 10% of Earth's surface. Subaerial erosion as a result of human activity, primarily through agricultural practices, has resulted in a sharp increase in net rates of continental denudation; although less well constrained than estimates based on surviving rock volumes or current river loads, available data suggest that present farmland denudation is proceeding at a rate of ~600 m/ m.y. (~75 Gt/yr), and is largely confined to the lower elevations of Earth's land surface, primarily along passive continental margins; ~83% of cropland erosion occurs over the lower 65% of Earth's surface. The conspicuous disparity between natural sediment fluxes suggested by data on rock volumes and river loads (~21 Gt/yr) and anthropogenic fluxes inferred from measured and modeled cropland soil losses (75 Gt/yr) is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from eroding croplands over the history of human agriculture. Accumulation of postsettlement alluvium on higher-order tributary channels and floodplains (mean rate ~12,600 m/m.y.) is the most important geomorphic process in terms of the erosion and deposition of sediment that is currently shaping the landscape of Earth. It far exceeds even the impact of Pleistocene continental glaciers or the current impact of alpine erosion by glacial and/ or fluvial processes. Conversely, available data suggest that since 1961, global cropland area has increased by ~11%, while the global population has approximately doubled. The net effect of both changes is that per capita cropland area has decreased by ~44% over this same time interval; ~1% per year. This is ~25 times the rate of soil area loss anticipated from human denudation of cropland surfaces. In a context of per capita food production, soil loss through cropland erosion is largely insignificant when compared to the impact of population growth. Keywords: erosion, denudation, humans, soils, rivers, alluvium.

The geometry of foreland basins is controlled by a dynamic balance between thrust-belt migration, flexural subsidence, and fluvial deposition. To improve our ability to quantify the relationships between tectonics, climate, and foreland basin geometry, I developed analytic solutions for basin topography and stratigraphy using a classic two-dimensional mathematical model of foreland basin evolution. Model predictions for basin topography are successfully tested against observed profiles along a humid-to-arid climatic gradient in the central Andes. Using published estimates for the thrust-belt migration rate, flexural parameter, and thrust-front basin depth in this region, I show that basin topographic profiles can be used to estimate the upstream erosion rate at any point along a foreland basin. Basin-averaged erosion rates estimated in this way vary from 0.025 to 0.045 mm/yr in the central Andes, increasing from semiarid to humid climates. Keywords: foreland basin, flexure, diffusion, modeling.

Sediment erosion laws form the basis for most landscape evolution models and guide geomorphologists in the pursuit of understanding how landscapes evolve. This focus on the alluvial surface, however, ignores the role of intrinsic feedbacks between sediment transport and bedrock weathering in shaping Earth's landforms. Here, we present a new, parsimonious explanation for the origin and maintenance of pediments, piedmont junctions, and tors, which emerge spontaneously in a numerical model coupling bedrock weathering and sediment transport. The spatial uniformity of the thin regolith mantle that often characterizes pediments is a manifestation of a negative feedback between bedrock weathering and regolith thickness: if regolith thins (thickens) by sediment transport, the regolith production rate will increase (decrease), maintaining an equilibrium regolith thickness on the piedmont. We propose that high infiltration capacities and the instability of ephemeral channel banks in arid and semiarid environments suppress fluvial incision and promote the smoothness of pediments. A positive feedback between bedrock weathering and regolith thickness causes tor growth: if regolith thins locally below a critical value, regolith production slows while surrounding areas continue to weather and erode more rapidly. We suggest that many pedimented and tor-studded landscapes may therefore be a consequence of intrinsic sediment transport-weathering feedbacks mediated by climatic and tectonic conditions, not by lithologic templates. Keywords: pediments, tors, regolith, weathering, deserts.

Episodic debris flows scour the rock beds of many steepland valleys. Along recent debris-flow runont paths in the western United States, we have observed evidence for bedrock lowering, primarily by the impact of large particles entrained in debris flows. This evidence may persist to the point at which debris-flow deposition occurs, commonly at slopes of less than ~0.03-0.10. We find that debris-flow-scoured valleys have a topographic signature that is fundamentally different from that predicted by bedrock river-incision models. Much of this difference results from the fact that local valley slope shows a tendency to decrease abruptly downstream of tributaries that contribute throughgoing debris flows. The degree of weathering of valley floor bedrock may also decrease abruptly downstream of such junctions. On the basis of these observations, we hypothesize that valley slope is adjusted to the long-term frequency of debris flows, and that valleys scoured by debris flows should not be modeled using conventional bedrock river-incision laws. We use field observations to justify one possible debris-flow incision model, whose lowering rate is proportional to the integral of solid inertial normal stresses from particle impacts along the flow and the number of upvalley debris-flow sources. The model predicts that increases in incision rate caused by increases in flow event frequency and length (as flows gain material) downvalley are balanced by rate reductions from reduced inertial normal stress at lower slopes, and stronger, less weathered bedrock. These adjustments lead to a spatially uniform lowering rate. Although the proposed expression leads to equilibrium long-profiles with the correct topographic signature, the crudeness with which the debris-flow dynamics are parameterized reveals that we are far from a validated debris-flow incision law. However, the vast extent of steepland valley networks above slopes of ~0.03-0.10 illustrates the need to understand debris-flow incision if we hope to understand the evolution of steep topography around the world. Keywords: geomorphology, erosion, debris flows, river incision, landscape evolution.

We present a new method for tracing sediment using detrital apatite (U-Th)/He (AHe) thermochronometry, and use this to quantify the spatial distribution of catchment erosion in the eastern Sierra Nevada, California. Well-developed age-elevation relationships permit detrital AHe ages to track the elevations where sediment grains were shed from bedrock. We analyzed sediment exiting nonglaciated Inyo Creek and adjacent (formerly) glaciated Lone Pine Creek. Statistical comparison of measured AHe age probability density functions (PDFs) with predicted PDFs based on catchment hypsometries suggests that Inyo Creek is eroding uniformly, consistent with field observations of weathered hillslopes tightly coupled to the fluvial system. In contrast, significant mismatch between measured and predicted PDFs from Lone Pine Creek reveals that sediment derives primarily from the lower half of the catchment. The dearth of older ages is likely due to sediment storage in cirques and moraines and/or focused erosion at intermediate elevations, both potential consequences of glacial modification. Measured PDFs can also improve cosmogenic nuclide-based erosion rates by more accurately scaling nuclide production rates. Our results demonstrate the utility of detrital AHe thermochronometry for quantifying erosion in fluvially and glacially sculpted catchments. Keywords: detrital thermochronometry, apatite (U-Th)/He, cosmogenic nuclides, erosion, Sierra Nevada.

At the end of the Miocene, the European Alps ceased outward expansion, and tectonic uplift and exhumation shifted into the orogen interior. This shift is consistent with a change from orogenic construction to orogenic destruction, reflecting an increase in the ratio of erosional flux to accretionary flux. The coincidence of this change with an increase in sediment yield from the Alps suggests a climate-driven increase in erosional flux. The timing of deformation and sediment release from the southern Alps indicates that the tectonic change occurred synchronous with the last phase of the Messinian salinity crisis. We attribute the increase in erosional flux to a climatic shift to wetter conditions throughout Europe, likely augmented by the base-level fall that occurred during the Mediterranean dessication. This climate change is represented in the stratigraphic record by the Lago Mare deposits of the Mediterranean salinity crisis. Keywords: Mediterranean salinity crisis, European Alps, Miocene, climate, erosional flux, accretionary flux.

This paper describes the experimental methods developed to understand the self-healing or progressive erosion of core cracks in earth dams. Concentrated leaks through two types of core cracks are simulated experimentally in flow cells of different configurations. Results of experimental investigations indicate that the current empirical filter criterion for base soils with fines content more than 85%, which stipulates that ([D.sub.15F]/[d.sub.85B]) should be less than 9, is conservative. However, analysis of experimental results suggests that the ([D.sub.15F]/[d.sub.85B]) ratios can not indicate the rate of self-healing or progressive erosion. It also leads to a conclusion that mechanistic understanding, rather than another improved empirical criterion, may be needed for quantitative prediction of self-healing or progressive erosion. In this study, we identified two groups of quantitative parameters influencing the mechanism of self-healing: (1) characteristics of base soils and filters, and (2) hydraulic, geometric, and physicochemical conditions. Experimental methods are presented to evaluate these parameters for which no standardized methods are reported in the current literature. CE Database subject headings: Dams, earth; Dam safety; Soil structure; Pipes; Erosion; Cracking.

This paper describes a mechanistic model developed to understand the self-healing mechanism of two types of cracks in impervious cores of earth dams; Type A core cracks which extend from the interior of the core to the downstream filter, and Type B core cracks which extend from the upstream face of the core to the downstream filter. The base soil-crack-filter system is idealized using a four-element one-dimensional continuum to consider various processes in the core and the filter. The model is numerically implemented to predict self-healing in the idealized domain. The model predictions are validated using results from experimental investigations. A parametric study conducted with the model indicates two conditions essential to foster self-healing: a nominal erosion of the base soil, and a seepage velocity in the filter that is less than its critical seepage velocity. This study suggests that the mechanism leading to different rates of self-healing is the interplay of several parameters, viz, characteristics of base soils and filters, geometrical features of cracks, hydraulic conditions, etc. Application of the one-dimensional mechanistic model to a three-dimensional field-scale scenario is demonstrated. CE Database subject headings: Dams, earth; Dam safety; Pipes; Erosion; Cracking.

Local clear-water and live-bed scour tests were performed for a range of water depths and flow velocities with two different uniform cohesionless sediment diameters (0.27 and 0.84 mm) and a circular pile with a diameter of 0.15 m. The tests were performed in a tilting flume (1.5 m wide, 1.2 m deep, and 45 m long) located in the Hydraulics Laboratory at the University of Auckland in Auckland, New Zealand. These tests extend local scour data obtained in controlled experiments to velocity ratio (V/[V.sub.c]) values as high as 6. This is near the velocity where the peak live-bed scour occurs for the sediment and flow conditions. Scour depth predictions are made with four different local scour equations for the conditions of the tests and the results compared with the measured values. DOI: 10.1061/(ASCE)0733-9429(2006) 132:7(635) CE Database subject headings: Scour; Experimentation; Piers; Piles; Bridge foundation; Erosion; Bedforms.

Methods for interpreting data from in situ flume measurements of cohesive sediment dynamics are evaluated and a technique for estimating various erosion parameters using in situ measurements is proposed. There is currently a lack of uniformity in analysis techniques for cohesive erosion data collected in flumes and with in situ instruments and the proposed technique resolves some of these inconsistencies. The data set used in this study was derived from field experiments conducted with a straight benthic in situ flume in different aquatic environments in New Zealand. The experiments with stepwise increases in flow velocity revealed that peaks in the erosion rate at the beginning of each velocity step are most likely associated with heterogeneous bed structure, as transient hydrodynamic effects due to the experimental procedure were found to be insignificant. The field data showed an exponential decay of the erosion rate with time that is indicative of depth-limited erosion. These data are used to illustrate methods for the parameterization of the proposed semiempirical erosion equation, taking into account the time dependency of the erosion process. DOI: 10.1061/(ASCE)0733-9429(2006)132:6(581) CE Database subject headings: Cohesive sediment; Shear stress; In situ tests; Erosion; Flumes; Data analysis; Measurement.

Despite a long research history over the past 150 years, the geometry, kinematics, and dynamic evolution of the Himalayan orogen remain poorly understood. This is mainly due to continued emphasis on the two-dimensionality of the Himalayan orogenic architecture and extrapolation of geologic relationships from a few well-studied but small areas to the rest of the orogen. Confusion and misconception are also widespread in the Himalayan literature in terms of the geographic, stratigraphic, and structural divisions. To clarify these issues and to provide a new platform for those who are interested in studying the geologic development of this spectacular mountain belt, I systematically review the essential observations relevant to the along-strike variation of the Himalayan geologic framework and its role in Cenozoic Himalayan exhumation, metamorphism and foreland sedimentation. A main focus of my synthesis is to elucidate the emplacement history of the high-grade Greater Himalayan Crystalline Complex (GHC) that occupies the core of the orogen. Because the north-dipping Main Central Thrust (MCT) above and South Tibet Detachment (STI)) below bound the GHC in most parts of the Himalaya, it is critical to determine the relationship between them in map and cross-section views. The exposed map pattern in the central Himalaya (i.e., Nepal) indicates that the MCT has a flat-ramp geometry. The thrust flat in the south carries a 2-15-km-thick slab of the GHC over the Lesser Himalayan Sequence (LHS) and creates a large hanging-wall fault-bend fold continuing > 100 km south of the MCT ramp zone. In the western Himalayan orogen at the longitude ~77[degrees]E, the MCT exhibits a major lateral ramp (the Mandi ramp). West of this ramp, the MCT places the low-grade Tethyan Himalayan Sequence (THS) over the low-grade LHS, whereas east of the ramp, the MCT places the high-grade GHC over the low-grade LHS. This along-strike change in stratigraphic juxtaposition and metamorphic grade across the MCT indicates a westward decrease in its slip magnitude, possibly a result of a westward decrease in total crustal shortening along the Himalayan orogen. Everywhere exposed, the STD follows roughly the same stratigraphic horizon at the base of the THS, exhibiting a long (>100 km) hanging-wall flat. This relationship suggests that the STI) may have initiated along a preexisting lithologic contact or the subhorizontal brittle-ductile transition zone in the middle crust. Although the STD has the THS in its hanging wall everywhere in the Himalayan orogen, no THS footwall cutoffs have been identified. This has made slip estimates of the STD exceedingly difficult. The southernmost trace of the STD either merges with the MCT (e.g., in Zanskar) or lies within 1-2 km of the MCT frontal trace (e.g., in Bhutan), suggesting that the MCT may join the STD in their up-dip directions to the south. This geometry, largely neglected by the existing models, has important implications for the deformation, exhumation, and sedimentation history of the entire Himalayan orogen. Keywords: Himalayan orogen; Main Central Thrust; South Tibet Detachment; passive-roof fault; active-roof fault; erosional exhumation

The research department Energetic Materials within TNO Defence, Security and Safety is involved in the development and (safety and insensitive munitions) testing of conventional (nitro cellulose based) and thermoplastic elastomer (TPE) based gun propellants. Recently our testing capabilities have been extended with vented vessel tests in order to investigate whether or not newly developed (low vulnerability (LOVA)) gun propellants or propellant formulations perform better in erosion tests. At the moment we have two vented vessel tests available, making it possible to determine the extent of erosion from a relatively low loading density to one comparable to a large caliber gun (maximum allowable pressures from 20 to 400 MPa). Test pieces of various materials have been used and a number of LOVA and conventional gun propellants have been tested. From the results of these experiments the wear and wear rate have been determined. The erosion dependency on propellant properties like impetus, flame temperature, and combustion gas composition has been determined. Furthermore, an internal ballistics code has been adapted to model the pressure and temperature development in a vented vessel. The results are compared with the experimental results. This model will be described in short and the results presented. [DOI: 10.1115/1.2172956]

[1] The Costa Rica portion of the Middle America Trench (MAT) is characterized by active tectonic erosion, a process that causes the removal of material from the base of the upper plate as the plate boundary migrates upward. Offshore studies demonstrate accelerated subduction erosion starting at the Miocene-Pliocene boundary, as the result of subduction of thickened Galapagos related crust, as presently represented by the Cocos Ridge. The subduction of the Cocos Ridge also caused uplift and exposure of the outer forearc on the Osa Peninsula, which offers a window to explore the tectonic evolution of the area. The rocks outcropping on Osa Peninsula are a middle Eocene-middle Miocene melange dominated by basalt, chert, and limestone resulting from accretion of seamounts. The accretion-dominated period of the MAT evolution ended at the Miocene-Pliocene boundary, when thickened crust, a paleo-Cocos Ridge, produced at the Galapagos hot spot arrived at the trench. The thick crust caused uplift and severe tectonic erosion of the accretionary edifice allowing exhumation of the Osa Melange. The change from accretion to erosion caused the outer forearc to be offset along subvertical faults that define small (kilometer to tens of kilometers size) blocks that are going through differential vertical movements in response to the morphology of the subducting ridge. Subduction accretion and erosion are two processes that can alternate in time and space or coexist along the same margin, so that mass removal can develop on a previously growing margin and completely remove an accretionary prism. Citation: Vannucchi, P., D. M. Fisher, S. Bier, and T. W. Gardner (2006), From seamount accretion to tectonic erosion: Formation of Osa Melange and the effects of Cocos Ridge subduction in southern Costa Rica, Tectonics, 25, TC2004, doi: 10.1029/2005TC001855.

The Jurassic Talkeetna volcanic arc of south-central Alaska is an oceanic island arc that formed far from the North American margin. Geochronological, geochemical, and structural data indicate that the arc formed above a north-dipping subduction zone after ca. 201 Ma. Magmatism migrated northward into the region of the Talkeetna Mountains ca. 180 Ma. We interpret this magmatism as the product of removal of the original forearc while the arc was active, mainly by tectonic erosion. Rapid exhumation of the arc after ca. 160 Ma coincided with the sedimentation of the coarse clastic Naknek Formation. This exhumation event is interpreted to reflect collision of the Talkeetna arc with either the active margin of North America or the Wrangellia composite terrane to the north along a second north-dipping subduction zone. The juxtaposition of accreted trench sedimentary rocks (Chugach terrane) against the base of the Talkeetna arc sequence requires a change from a state of tectonic erosion to accretion, probably during the Late Jurassic (before 150 Ma), and definitely before the Early Cretaceous (ca. 125 Ma). The change from erosion to accretion probably reflects increasing sediment flux to the trench due to collision ca. 160 Ma. Keywords: Alaska, terrane accretion, subduction, tectonic erosion, collision.

Recently acquired 3-m-resolution 244 kHz multibeam seafloor bathymetry (0.5 m depth precision) reveals geomorphology at sufficient detail to interpret small-scale features and short-term processes in the upper 4 km of Monterey Canyon, California. The study area includes the continental shelf and canyon features from 10 m to 250 m depth. The canyon floor contains an axial channel laterally bounded by elevated complex terrace surfaces. Sand waves with 2 m height and 35 m average wavelength dominate the active part of the canyon floor. The sand waves are strongly asymmetrical, indicating net down-canyon sediment transport in this reach. Terraces, including a broad 25-m-tall terrace complex near the head of the canyon, bear evidence of recent degradation of the canyon floor. Slump scars and gullies having a variety of sizes and relative ages shape the canyon walls. Serial georeferenced digital elevation models were analyzed to detect net changes in bathymetry or morphology occurring during both a six month period (September 2002 to March 2003) and a 24-h period (24 March to 25 March). Significant changes over the six month period include: (1) complete reorganization of the sand waves on the channel floor, (2) local channel degradation creating new 2-m-tall erosional terraces on the channel margins, (3) local channel widening that laterally eroded older channel margin terraces, and (4) 60 m extension of one minor gully head on a steep canyon wall. There were no discernable changes in morphology during the 24-h study period. Raster subtraction of serial bathymetric grids provides estimates of sediment erosion and deposition that occurred between the canyon head and a point 2 km down canyon during the six month study. Erosion of 320,000 [m.sup.3] ([+ or -] 80,000 [m.sup.3]) of sediment occurred mainly in the tributaries, along the margins of the axial channel, and in the lowest 700 m of the analyzed reach. This eroded volume was approximately balanced by 260,000 [m.sup.3] ([+ or -] 70,000 [m.sup.3]) of sediment deposition that was concentrated in the nearshore region along the rim of the canyon head. There was no measurable sediment gain or loss during the 24-h study period. Keywords: Monterey, submarine, canyon, sediment, morphology, bathymetry.

Whether variations in the spatial distribution of erosion influence the location, style, and magnitude of deformation within the Himalayan orogen is a matter of debate. We report new [sup.40]Ar/[sup.39]Ar white mica and apatite fission-track (AFT) ages that measure the vertical component of exhumation rates along an ~120-km-wide NE-SW transect spanning the greater Sutlej region of northwest India. The [sup.40]Ar/[sup.39]Ar data indicate that first the High Himalayan Crystalline units cooled below their closing temperature during the early to middle Miocene. Subsequently, Lesser Himalayan Crystalline nappes cooled rapidly, indicating southward propagation of the orogen during late Miocene to Pliocene time. The AFT data, in contrast, imply synchronous exhumation of a NE-SW-oriented ~80 x 40 km region spanning both crystalline nappes during the Pliocene-Quaternary. The locus of pronounced exhumation defined by the AFT data correlates with a region of high precipitation, discharge, and sediment flux rates during the Holocene. This correlation suggests that although tectonic processes exerted the dominant control on the denudation pattern before and until the middle Miocene; erosion may have been the most important factor since the Pliocene. Keywords: Himalaya, exhumation, erosion, uplift, geochronology.

Refolding structures in the Aliaga area (Iberian Chain, Spain) show how erosion can exert a control on the geometry of buckle fold interference. Early Miocene ENE-trending buckle folds are superposed on a large NNW-SSE anticline (Eocene-Oligocene). North of Aliaga, the earlier hinge zone in the competent limestone unit controlling buckling (Urgon facies, Lower Cretaceous) constitutes a mechanical obstacle to refolding, which induces development of essentially conical type 2 interference. To the south, erosional removal of the hinge zone at that competent unit allowed the near-vertical eastern limb to behave as independent layers, being refolded into cylindrical, near-vertical-axis folds showing a snake-like map pattern. Lower Miocene conglomerates containing pebbles of Jurassic origin, whose source area was the core of the earlier NNW-SSE-trending anticline, indicate that this Jurassic core had been effectively exhumed before refolding occurred. In this way, snake-like folds constitute a new case of erosion-controlled tectonic structure. Keywords: superposed folds, erosion control, Iberian Cordillera, Spain.

Soil erosion is a major global challenge. An increased understanding of the mechanisms driving soil erosion, especially the storms that produce it, is vital to reducing the impact on agriculture and the environment. The objective of this work was to study the spatial distribution and time trends of the soil erosion characteristics of storms, including the maximum 30-min precipitation intensity ([I.sub.30]), storm kinetic energy of the falling precipitation (KE), and the storm erosivity index (EI) using a long-term 15-min precipitation database. This is the first time that such an extensive climatology of soil erosion characteristics of storms has been produced. The highest mean [I.sub.30], KE, and EI values occurred in all seasons in the southeastern United States, while the lowest occurred predominantly in the interior west. The lowest mean [I.sub.30], KE, and EI values typically occurred in winter, and the highest occurred in summer. The exception to this was along the West Coast where winter storms exhibited the largest mean KE and EI values. Linear regression was used to identify trends in mean storm erosion characteristics for nine U.S. zones over the 31-yr study period. The south-central United States showed increases for all three storm characteristics for all four seasons. On the other hand, higher elevations along the West Coast showed strong decreases in all three storm characteristics across all seasons. The primary agricultural region in the central United States showed significant increases in fall and winter mean EI when there is less vegetative cover. These results underscore the need to update the storm climatology that is related to soil erosion on a regular basis to reflect changes over time.

This paper presents the results of an experimental study of the scour created by a siphon flow through a vertical tube of diameter d of 9.7 to 20.4 mm in a cohesionless sand bed of approximately uniform size D of 0.58 mm. The position of the inlet of the siphon tube ranged from -101.6 to 6.4 mm relative to the surface of the sand bed and the velocity of flow through the tube [U.sub.0] varied from 0.34 to 6.41 m/s. Data from this and previous studies are used to show the radial extent of the scour hole and the maximum depth of scour depend primarily on the densimetric Froude number [F.sub.0], where [F.suv.0] = [U.sub.0] / [square root of (g([DELTA[rho]/[rho])D, g is the gravitational acceleration. [DELTA][rho]is the difference in density between the sediment and fluid, and [rho] is the density of the fluid. A theoretical analysis of the problem is used to further assess the data and it is found that viscous effects are also important to consider. The profile of the scour hole at equilibrium, for all tube positions, is found to be similar when the scour depth is measured relative to the tube inlet. DOI: 10.1061/(ASCE)0733-950X(2005)131:3(115) CE Database subject headings: Siphons; Abatement and removal; Suction; Dredging; Cohesionless sediment; Sand; Scour; Erosion.

Peneplanation of mountain ranges is generally considered the result of long-term erosional processes that smooth relief and lower elevation near sea level. Therefore peneplain remnants at high elevation in mountain ranges are used to infer posttectonic surface uplift. Such an interpretation has been proposed for the Pyrenees where high-elevation, low-relief erosional surfaces rose up to more than 2000 m. Because the Pyrenean foreland basins are filled with very thick continental deposits, which have buried the early jagged landscape, we challenge this hypothesis by pointing out that relief applanation does not necessarily require elevation lowering. We propose an alternative interpretation in which piedmont aggradation of detrital sediment that comes from erosion of the high chain induces the rise of the base level of the range, therefore reducing strongly the erosive efficiency of the drainage system and resulting in the progressive smoothing of the relief. Such a process allows a high-elevation, low-relief erosional surface to develop at the scale of the range. In the Pyrenees, occurrence of high-elevation, low-relief erosional surface remnants does not imply a posttectonic uplift, but is instead due to the dissection of the initial Miocene high-elevation, low-relief surface by the recent drainage system, the erosive activity of which has been enhanced by global climate change from the late Pliocene onward. doi:10.1029/2004TC001697.

The western UK basins of the Irish Sea have provided one of the best natural laboratories for investigating the causes and consequences of intracratonic uplift and erosion (exhumation). To date, the emphasis has been on igneous underplating as the chief process driving their exhumation. In Ibis paper, we demonstrate that tectonic inversion (the shortening of formerly extensional basins and reactivation of their constituent faults) dominated the exhumation of the St. George's Channel basin, offshore Wales. Based on mapping of an extensive 2D seismic grid, evidence is presented for at least two major inversion episodes in the Late Cretaceous and the Neogene, plus minor shortening during the Eocene. Inversion was distinctly noncoaxial, especially during the Neogene when coeval transpression and transtension was focused at discrete bends and stepovers on the basin-bounding St. George's. Bala and Northwest Flank faults. That the principal mechanism driving these uplift episodes was reversion (as opposed to igneous underplating) is corroborated by analysis of thermal history data (apatite fission track and vitrinite reflectance). They reveal late Cretaceous and Neogene geothermal gradients that were comparable with that at the present clay, i.e. no significant increase in basal heat flow. Sonic velocity profiles logged in hydrocarbon borcholes constrain the minimum thickness of the eroded section, which varies between c. 1000 m in the centre and c. 2240 m at the margins of the basin. Given the strength of evidence for tectonic reversion in the St. George's Channel basin, our favoured model invokes superimposition of the effects of inversion and igneous underplating to account for the complex exhumation history of the St. George's Channel basin in particular, and the western UK basins in general. Keywords: St. George's Channel basin, exhumation, inversion, geophysical seismic methods, thermal history.

Laboratory experiments have been performed to investigate the recession of soft cliffs due to oblique wave actions. The experiments were conducted in a wave tank with irregular waves. A test cliff was constructed of wet sand and (in one test) a sand/clay mixture, which was found to be appropriate for simulating the principal failure mode of soft cliffs fronted by erodeable beaches. The recession process is characterized by the following sequence: toe erosion--cliff failure--erosion of debris--toe erosion. The recession of the cliff top was measured from still photographs taken by an overhead camera. The longshore and cross-shore extents of individual failures and the time interval between two successive failures are obtained. CE Database subject headings: Cliffs; Erosion; Failure modes.

The fundamentally geographic issue of the amounts and spatial patterns of erosion necessary to produce classic glacial landforms such as U-shaped valleys has been debated by scientists for over a century. Terrestrial cosmogenic nuclide (TCN) measurements in glacially abraded bedrock were used to determine patterns of glacial erosion and to quantify the amount of rock removed during the last glaciation along valley-side transects in Sinks Canyon, Wind River Range, Wyoming, and the South Yuba River, Sierra Nevada, California. Surface exposure ages from bedrock and erratic samples obtained during this study indicate last deglaciation between 13-18 ka in the South Yuba River and 15-17 ka in Sinks Canyon. These ages are in agreement with previously published glacial chronologies. In both areas, samples from valley cross sections revealed a pattern of erosion during the last glaciation that decreased toward the lateral limit of ice extent, as predicted by numerical models, while transects further upstream recorded > 1.4 meters of bedrock removal throughout. The effects of varying interglacial erosion and surface exposure histories on modeled glacial erosion depths were tested, validating the methodology used. The results demonstrate that the TCN technique, applied at the valley scale, provides useful insight into the spatial pattern of glacial erosion. Extensive sampling in areas with limited erosional loss may provide detailed records of erosion patterns with which to test predictions generated by models of ice dynamics and erosion processes. Key Words: cosmogenic nuclides, glacial erosion, Sierra Nevada, Wind River Range.