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| Titel |
Incision in alluvial piedmonts indirectly records climate forcing after modulation by glaciation in the E. Tian Shan high range |
| VerfasserIn |
Luca C. Malatesta, Jean-Philippe Avouac, Nathan D. Brown, Sebastian Breitenbach, Marie-Luce Chevalier, Jiawei Pan, Edward Rhodes, Dimitri Saint-Carlier, Wenjing Zhang |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250146778
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| Publikation (Nr.) |
 EGU/EGU2017-10819.pdf |
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| Zusammenfassung |
| Rivers flowing on alluvial piedmonts can abandon high resolution terrace records by incising
fast in unlithified conglomerate. Fluvial entrenchment can have an autogenic origin but more
often it is driven by climatic forcing and terraces record this forcing. The climatic cause
for incision is generally due to a modification in the ratio of sediment flux over
water discharge Qs∕Qw and it is often assumed to be directly driven by a change in
precipitation.
We show here, with an example from the Eastern Tian Shan (Northwest China) that the
state of fluvial incision is not necessarily directly reflecting a climatic forcing. The north and
south piedmonts of the range undergo the same climatic and tectonic forcing. Yet their
alluvial piedmonts stand in sharp contrast: the north is incised by 100 to 300 m at the alluvial
fan apex while the south is barely incised at all.
In the north, where glaciers carved U-valleys in the upper half of the catchments, incision is
primarily driven by a fast drop in the sediment flux out of the high range after glacial
sediments are flushed out of the high range. The drop in Qs lags behind the initial forcing that
is the onset of deglaciation and the terraces mainly record the postglacial internal
routing of sediments en route to the basin. All rivers of the north piedmont are
similarly entrenched, but the timing of the incision onset differs from one river
to the next (between 13 and 5 ka), precluding a unique external forcing. Vertical
incision rates can be further modified by autogenic processes and changes in water
discharge.
In the south, glaciation is restricted to cirque glaciers or long glaciers that reached the
piedmont. As a result there is no important accumulation of loose sediment up in the range to
be flushed out and starved. The fully aggraded southern piedmonts reflect the current Central
Asian aridity in the absence of a postglacial alluvial readjustment.
A sediment routing system with accumulation of sediment high in the mountain can
amplify environmental signals instead of dampening them. The same climatic forcing can
result in widely different morphological expressions depending on the extent of the glacial
overprint. Strategies to study past climate can take advantage of exacerbated signals to
measure climatic trends provided that the local sediment routing is understood. |
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