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Geological Time Scales (geological + time_scale)

Distribution by Scientific Domains

Earth and Environmental Science	60%

Selected Abstracts

How does alluvial sedimentation at range fronts modify the erosional dynamics of mountain catchments?

BASIN RESEARCH, Issue 3 2005
S. Carretier
At the geological time scale, the way in which the erosion of drainage catchments responds to tectonic uplift and climate changes depends on boundary conditions. In particular, sediment accumulation and erosion occurring at the edge of mountain ranges should influence the base level of mountain catchments, as well as sediment and water discharges. In this paper, we use a landform evolution model (LEM) to investigate how the presence of alluvial sedimentation at range fronts affects catchment responses to climatic or tectonic changes. This approach is applied to a 25 km × 50 km domain, in which the central part is uplifted progressively to simulate the growth of a small mountain range. The LEM includes different slope and river processes that can compete with each other. This competition leads to ,transport-limited', ,detachment-limited' or ,mixed' transport conditions in mountains at dynamic equilibrium. In addition, two end-member algorithms (the channellized-flow and the sheet-flow regimes) have been included for the alluvial fan-flow regime. The three transport conditions and the two flow algorithms represent six different models for which the responses to increase of rock uplift rate and/or cyclic variation of the precipitation rate are investigated. Our results indicate that addition of an alluvial apron increases the long-term mountain denudation. In response to uplift, mountain rivers adapt their profile in two successive stages; first by propagation of an erosion wave and then by slowly increasing their channel gradients. During the second stage, the erosion rate is almost uniform across the catchment area at any one time, which suggests that dynamic equilibrium has been reached, although the balance between erosion and rock uplift rates has not yet been achieved. This second stage is initiated by the uplift of the mountain river outlets because of sedimentation aggradation at the mountain front. The response time depends on the type of water flow imposed on the alluvial fans domains (× by 1.5 for channelized flow regime and by 10 for the sheet flow one). Cyclic variations of precipitation rate generate cyclic incisions in the alluvial apron. These incision pulses create knick-points in the river profile in the case of ,detachment-limited' and ,mixed' river conditions, which could be mistaken for tectonically induced knick-points. ,Transport-limited' conditions do not create such knick-points, but nevertheless trigger erosion in catchments. The feedbacks linked to sedimentation and erosion at range front can therefore control catchment incision or aggradation. In addition, random river captures in the range front trigger auto-cyclic erosion pulses in the catchment, capable of generating incision,aggradation cycles. [source]

Electrodynamic Disaggregation: Does it Affect Apatite Fission-Track and (U-Th)/He Analyses?

GEOSTANDARDS & GEOANALYTICAL RESEARCH, Issue 1 2010
Jörg Giese
désagrégation électrodynamique; analyse des traces de fission sur apatite; recuit; analyse (U-Th)/He; diffusion de l'He Apatite fission-track and (U-Th)/He analyses require the liberation of intact idiomorphic apatite grains from rock samples. While routinely being carried out by mechanical methods, electrodynamic disaggregation (ED) offers an alternative approach. The high-voltage discharges produced during the ED process create localised temperature peaks (10000 K) along a narrow plasma channel. In apatite, such high temperatures could potentially reduce the length of fission tracks, which start to anneal at temperatures > 60 °C, and could also enhance He diffusion, which becomes significant at 30,40 °C over geological time scales. A comparison of fission-track analyses and (U-Th)/He ages of apatites prepared both by mechanical (jaw crusher, disk mill) and ED processing provides a way of determining whether heating during the latter method has any significant effect. Apatites from three samples of different geological settings (an orthogneiss from Madagascar, the Fish Canyon Tuff, and a muscovite-gneiss from Greece) yielded statistically identical track length distributions compared to samples prepared mechanically. Additionally, (U-Th)/He ages of apatites from a leucogranite from Morocco prepared by both methods were indistinguishable. These first results indicated that during electrodynamic disaggregation apatite crystals were not heated enough to partially anneal the fission tracks or induce significant diffusive loss of He. Les analyses des traces de fission et des rapports isotopiques (U-Th)/He sur apatite nécessitent la séparation de grains intacts et automorphes d'apatite à partir des échantillons de roche. La désagrégation électrodynamique (DE) offre une approche alternative aux méthodes mécaniques utilisées actuellement en routine. Les décharges de haute tension produite pendant le processus de DE entrainent la formation de pics de température (10000 K) localisés le long d'un étroit canal de plasma. Dans l'apatite, de telles températures peuvent potentiellement réduire la longueur des traces de fission, qui commencent à recuire à des températures > 60 °C, et peuvent aussi favoriser la diffusion de l'Hélium, qui devient significative à des températures de 30,40 °C sur des échelles de temps géologiques. Une comparaison des résultats obtenus à partir des analyses des traces de fission et des âges (U-Th)/He sur des apatites séparées par des moyens mécaniques (concasseur à mâchoires, broyeur à disque) et sur d'autres séparées par la méthode de désagrégation électrodynamique offre un moyen de déterminer si le chauffage lié,à la seconde méthode a un effet significatif. Les apatites séparées par la méthode DE à partir de trois échantillons provenant de différents contextes géologiques (un orthogneiss de Madagascar, le tuf de Fish Canyon et un gneiss à muscovite de Grèce) fournissent des distributions de longueurs de trace de fission statistiquement identiques par rapport à des échantillons préparés mécaniquement. En outre, des âges (U-Th)/He obtenus à partir d'apatites provenant d'un leucogranite du Maroc et préparées par les deux méthodes (DE et mécaniques) sont indiscernables. Ces premiers résultats indiquent que, pendant la désagrégation électrodynamique, les cristaux d'apatite n'ont pas été suffisamment chauffés pour soit recuire partiellement les traces de fission soit provoquer une perte significative par diffusion de l'He. [source]

A qualitative and quantitative evaluation of experimental model catchment evolution

HYDROLOGICAL PROCESSES, Issue 12 2003
Dr G. R. Hancock
Abstract Due to the geological time scales required for observation of catchment evolution, surrogates or analogues of field data are necessary to understand long-term processes. To investigate long-term catchment behaviour, two experimental model catchments that developed without rigid boundaries under controlled conditions are examined and a qualitative and quantitative analysis of their evolution is presented. Qualitatively, the experimental catchments have the visual appearance of field scale data. Observation demonstrates that changes in catchment shape and network form are conservative. Quantitative analysis suggests that the catchments reach an equilibrium form while a reduction in the channel network occurs. While the catchments are laboratory scale models, the results provide insights into field scale behaviour. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Relationships between morphological and sedimentological parameters in source-to-sink systems: a basis for predicting semi-quantitative characteristics in subsurface systems

BASIN RESEARCH, Issue 4 2009
Tor O. Sømme
ABSTRACT The study of source-to-sink systems relates long-term variations in sediment flux to morphogenic evolution of erosional,depositional systems. These variations are caused by an intricate combination of autogenic and allogenic forcing mechanisms that operate on multiple time scales , from individual transport events to large-scale filling of basins. In order to achieve a better understanding of how these mechanisms influence morphological characteristics on different scales, 29 submodern source-to-sink systems have been investigated. The study is based on measurements of morphological parameters from catchments, shelves and slopes derived from a ,1 km global digital elevation model dataset, in combination with data on basin floor fans, sediment supply, water discharge and deposition rates derived from published literature. By comparing various morphological and sedimentological parameters within and between individual systems, a number of relationships governing system evolution and behaviour are identified. The results suggest that the amount of low-gradient floodplain area and river channel gradient are good indicators for catchment storage potential. Catchment area and river channel length is also related to shelf area and shelf width, respectively. Similarly to the floodplain area, these parameters are important for long-term storage of sediment on the shelf platform. Additionally, the basin floor fan area is correlative to the long-term deposition rate and the slope length. The slope length thus proves to be a useful parameter linking proximal and distal segments in source-to-sink systems. The relationships observed in this study provide insight into segment scale development of source-to-sink systems, and an understanding of these relationships in modern systems may result in improved knowledge on internal and external development of source-to-sink systems over geological time scales. They also allow for the development of a set of semi-quantitative guidelines that can be used to predict similar relationships in other systems where data from individual system segments are missing or lacking. [source]

Biogeography and diversity among montane populations of mouse shrew (Soricidae: Myosorex) in Tanzania

BIOLOGICAL JOURNAL OF THE LINNEAN SOCIETY, Issue 3 2010
WILLIAM T. STANLEY
We assess variation in morphological and molecular characters among three species of Myosorex (the mouse shrew) ,Myosorex geata, Myosorex kihaulei, and Myosorex zinki, as a means to test previously proposed biogeographic hypotheses for Tanzanian ,sky islands' and systematic hypotheses for Tanzanian mouse shrews. We analyse 17 cranial and dental variables using multivariate statistics and perform phylogenetic and phylogeographic analyses on sequences of mitochondrial and nuclear DNA; samples are drawn from every known Tanzanian population of Myosorex. Morphometric and phylogenetic analyses reveal that M. zinki is distinct, but that currently isolated populations of M. geata and M. kihaulei are relatively similar to one another, and may not have been isolated over geological time scales. Analyses of molecular variance identify statistically significant, but limited, genetic variation within and between isolated populations of M. geata and M. kihaulei. Between two putative regional biogeographic boundaries, greater genetic variation is explained by grouping populations on either side of the Ruaha River than by grouping populations on either side of the Makambako Gap. Our results are in agreement with recent studies illustrating the close relationship between faunas of the Southern Highlands and southern Eastern Arc Mountains, diminishing the apparent importance of the Makambako Gap as a historical biogeographic barrier. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 100, 669,680. [source]