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| Titel |
Late-Cenozoic evolution of the northern Pyrenean foreland inferred from the incision of the Lannemezan megafan |
| VerfasserIn |
Margaux Mouchené, Peter van der Beek, Frédéric Mouthereau, Julien Carcaillet |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250108975
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| Publikation (Nr.) |
 EGU/EGU2015-8820.pdf |
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| Zusammenfassung |
| Deciphering the incision history of elevated piedmont paleosurfaces can provide critical
information to distinguish between climatic and tectonic forcing on the current landscape. A
measure of the temporal and spatial incision patterns at Quaternary timescales can
be obtained through the analysis of geomorphic features and their dating using
cosmogenic nuclides. In the Pyrenees, Quaternary glaciers did not migrate far into the
foreland, thus leaving elevated surfaces relatively well preserved. In particular,
the Lannemezan megafan is the most striking morpho-sedimentary feature of the
northern Pyrenean foreland. It was built from Early Miocene to Pliocene times, while
active deformation of the central Pyrenees, in particular its northern retro-wedge,
had already ceased. 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. A river network that spreads out
radially from the apex has been incising the fan itself since its abandonment. A
well-developed alluvial terrace staircase was cut in the riverbanks by successive episodes of
incision. We combine a quantitative morphometric analysis of the river network with
field observations and cosmogenic dating to assess the Quaternary evolution of the
Lannemezan fan and surrounding northern central Pyrenean foreland. Our geomorphic
analysis of the incising rivers (long profiles, steepness indices, knickpoints) shows no
evidence of an active tectonic feature situated directly beneath the fan that could
have imprinted the incision of the fan. We use a proxy for steady-state channel
elevation (Ï) to analyze drainage network reorganization through divide migration and
river capture. We show that the terrace slope increases with time and the modern
rivers exhibit markedly concave long profiles, which could indicate late tilting of
the fan, possibly under extensional tectonic constraints. New cosmogenic nuclide
analyses (10Be, 26Al) are used to date the abandonment of the fan surface and the
terrace staircase chronology to provide constraints on incision rates and mechanisms. |
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