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| Titel |
Geomorphic characteristics of the Lannemezan megafan: an insight in the Late-Cenozoic evolution of the northern Pyrenean foreland (France) |
| VerfasserIn |
Margaux Mouchené, Peter van der Beek, Frédéric Mouthereau, Julien Carcaillet |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250092762
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| Publikation (Nr.) |
 EGU/EGU2014-7121.pdf |
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| Zusammenfassung |
| Large alluvial fans, either active or incised, characterize the orogen – foreland-basin
transition in nearly all mountain belts worldwide. Understanding the evolution of fossil
foreland alluvial fans and the incision history of their elevated paleo-surfaces can provide
critical information on their past and present evolution and to distinguish between climatic
and tectonic forcing. The most striking morpho-sedimentary feature of the northern Pyrenean
foreland (SW France) is the Miocene Lannemezan megafan, but little is known about its
evolution: what controlled the development of this fan and how is this related to the orogenic
growth? When and why was the fan abandoned and incised? How large was the feature when
it was active? We combine a quantitative morphometric analysis with field observations,
low-temperature thermochronometry and cosmogenic dating to address these questions. The
Lannemezan fan is exceptionally large (104km2), especially when compared to the other fans
of the northern Pyrenean foreland and the fan area/catchment area ratio is anomalous.
Calculations of the eroded vs. deposited volumes corroborate this unbalanced budget. The
Neste River, which most likely used to feed the megafan, now bends 90° eastwards
near the apex of the fan, indicating it was captured by the larger Garonne River in
Quaternary times. The material forming the fan and the strath terrace system incising
the fan, show a rather unusual sedimentological pattern for an alluvial fan setting,
characterized by a very fine clay and sand matrix supporting sporadic pebbles and boulders
(up to 50 cm in diameter). We show that the terrace slope increases with time and
the current rivers exhibit markedly concave long profiles (reference concavity =
0.7), which could indicate late tilting of the fan. New cosmogenic nuclide analysis
(10Be,26Al) will be used to date the abandonment of the fan surface and the terrace
staircase chronology to provide constraints on incision rates and mechanisms (in
terms of climatic change and/or tectonic deformation). The Neogene source-area
exhumation rate and amount of post-orogenic unroofing will be investigated through the
analysis of new detrital and in-situ apatite fission-track and (U-Th)/He analyses. |
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