ALBERT

Description: ABSTRACT In 2008-2009, a severe cold snap affected the otherwise temperate mid-latitude coasts of the English Channel of France. In March 2009, we gathered rock spalling observations at Mesnil Val, NW France, to document the effect of frost on platform lowering in macro-tidal environments. Six epochs of frost were recorded in two months, the two longest lasted 16 and 8 days, with minimum air temperature dropping to -9.5°C. Semi-diurnal tides flood the entire platform, imposing up to 25 freeze-thaw cycles below -2.5°C, the freezing temperature of seawater. 19 cycles occurred at neap tidal elevation lasting at most 3.5 hrs. Our integrated observations indicate that these frost cycles were responsible for a platform lowering of about 0.8±0.5 mm during a single winter. No clear spatial trend appears, nor do macroscopic chalk types clearly stand out as being more susceptible to erosion. Assuming that the long-term platform retreat model preserves a constant slope, frost shattering would then account for 10 to 20% of the expected annual platform erosion rate. Under more contrasted climates, frost is thus likely to be a prominent shaping process for rocky coastal platforms. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Millennial catchment-mean erosion rates derived from terrestrial cosmogenic nuclides (TCN) are generally based on the assumption that the lithologies of the parent rock each contain the same proportion of quartz. This is not always true for large catchments, in particular at the edge of mountainous plateaus where quartz-rich basement rocks may adjoin sedimentary or volcano-sedimentary rocks with low quartz content. The western Central Andes is an example of this type of situation. Different quartz contents may be taken into account by weighting the TCN production rates in the catchment. We recall the underlying theory and show that weighting the TCN production rate may also lead to bias in the case of a spatial correlation between erosion rate and lithology. We illustrate the difference between weighted and unweighted erosion rates for seven catchments (16 samples) in southern Perú and northern Chile and show variations up to a factor of 2 between both approaches. In this dataset, calculated erosion rates considering only granitoid outcrops are better correlated with catchment mean slopes than those obtained without taking into account the geological heterogeneity of the drained watershed. This dataset analysis demonstrates that weighting erosion rates by relative proportions of quartz is necessary to evaluate the uncertainties for calculated catchment-mean erosion rates and may reveal the correlation with geomorphic parameters. This article is protected by copyright. All rights reserved.

Description: Attrition rates and mean transport velocities of coarse fluvial sediments averaged over long periods of time ($\gg$100 a) are yet to be fully quantified. A knowledge of long-term transport rates would allow us to predict the response of fluvial systems to changes in external conditions, while determining clast attrition rates would improve our understanding of fluvial abrasion processes. The concentration of terrestrial cosmogenic nuclides (TCNs) in boulders can be modified during river transport by attrition and temporary exposure, which are two competing processes. In order to evaluate the potential of TCNs to quantify these processes, a stochastic model of TCN concentration evolution in clasts of different sizes is developed from exhumation on a hillslope to a sampling point in a river. The model includes shallow landslides or movement in a regolith and episodic transport of clasts in a sediment layer on the river bed. We study the downstream evolution of TCN concentrations for a population of clasts sourced from a localized TCN-rich lithology on a hillslope. When attrition is strong, the model predicts that the variance and maximum of TCN concentrations decrease downstream. Such a trend is possible only if the dominant hillslope erosion corresponds to shallow landslides. Natural variability of hillslope processes can hamper the trend. A way to limit the scatter is to sample the biggest clasts. On the contrary, if clast attrition is small and river transport slow, TCN concentrations increase downstream. In this case, combining the TCN concentrations of clasts gathered at several river stations should provide a method to estimate their mean transport rate. Our results offer guidelines to interpret the downstream evolution of TCN concentration in pebbles and at the surface of boulders.

Description: [1]  At the NW corner of the Pacific region, just south of the Kamchatsky Peninsula, the northern tip of Pacific plate subduction and associated volcanic arc interacts with the western end of the Aleutian-Komandorsky dextral transform plate boundary and associated arc. Study of both Holocene and Pleistocene sequences of uplifted marine terraces and also of fluvial drainage patterns on the Kamchatsky Peninsula allows us to highlight active tectonics produced by complex plate interaction. Our results show that the central - eastern coast of the peninsula is currently divided into four different zones consisting in uplifted blocks associated with various uplift rates in front of a fold and thrust zone to the west. Our main tectonic benchmark--the altitude of the shoreline correlated to last interglacial maximum (MIS 5e)--yields Late Pleistocene uplift rates ranging from 0.2 to 2.74 mm/yr. One of the main active faults bounding the coastal blocks is dextral and is interpreted as a prolongation of an offshore fault of the Aleutian-Komandorsky dextral transform plate boundary. We suggest that structures on the Kamchatsky Peninsula accommodate a part of the transform motion, but that mainly the arc-continent collision of the Aleutian arc against Kamchatka produces a “bulldozer” effect on the Kamchatsky Peninsula.

Description: The study of the Las Tunas River incisions, located in the eastern Andean foreland front (33{ring operator}20' S in Argentina), provides new clues for the interpretation of deep piedmont entrenchments. Both the Las Tunas mountain catchment and its piedmont are strongly entrenched with maximal incision of over 100 m at the mountain front. Three main terrace levels are well exposed and are labelled T1, T2 and T3 from the youngest to the oldest. We combined geological and geomorphological field observations, kinematic GPS data, satellite data and aerial photos with geochronological 40Ar/39Ar and 10Be analysis to provide a detailed description of terrace organization and a discussion of the evolution of the Las Tunas landscape. The surprisingly constant 10Be concentrations in surface layers as deep as 1.5 m show that gently dipping alluvial surfaces can be continuously and deeply mixed. Our data show a first period of deposition (Mesones Fm) before 0.85 Myr (minimum T3 age), followed by deep erosion and a second sedimentation period (Las Tunas Fm) that includes a ca. 0.6 Myr ash deposit. T2 and T1 are inset in the Las Tunas Fm and were abandoned ca. 15-20 kyr ago. The similar ages for T2 and T1 show that post-20 kyr entrenchment occurred very rapidly. Despite Quaternary deformation in the Las Tunas piedmont, terrace entrenchment is best explained by paleo-climatic changes. The terrace organization reveals that the erosion-sedimentation phases affected the entire system from the piedmont toe to 10 km upstream of the mountain front. Finally, contrary to the neighbouring more deeply incised Diamante River system, where late Quaternary piedmont uplift is more likely to have been a factor causing incision, the more stable Las Tunas system provides an incomplete geomorphological record of Pleistocene and Holocene climate variations. We suggest that climate variations are better recorded in uplifting piedmonts than in stable ones, where the magnitude of incision and sedimentation and the fact that they occur repeatedly at the same elevation can erase a large part of the record. © 2013 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists.