Contrasting river incision in north and south Tian Shan piedmonts due to variable glacial imprint in mountain valleys

Fluvial terraces carved by incising rivers are widely used to investigate external forcing by climate and tectonics. In the Eastern Tian Shan (Central Asia), the north piedmont rivers are much more deeply incised (mean: 124 m) compared to the south piedmont (mean: 17 m) despite very similar tectonic...

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Bibliographic Details
Published in:Geology (Boulder) 2018-07, Vol.46 (7), p.659-662
Main Authors: Malatesta, Luca C, Avouac, Jean-Philippe
Format: Article
Language:English
Subjects:
Accretion
Aggradation
alluvial fans
Aridity
Asia
Catchment area
Catchments
China
Climate
erosion
Far East
fluvial features
Foothills
Forces (mechanics)
Geology
geomorphology
glacial erosion
glaciation
incised valleys
piedmonts
Plate tectonics
Rivers
Sediment
sediment transport
Sediments
Slope
Slopes
Tectonics
Terraces
Tien Shan
topography
transport
Valleys
Water discharge
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Summary:Fluvial terraces carved by incising rivers are widely used to investigate external forcing by climate and tectonics. In the Eastern Tian Shan (Central Asia), the north piedmont rivers are much more deeply incised (mean: 124 m) compared to the south piedmont (mean: 17 m) despite very similar tectonic and climatic settings. We attribute the incision contrast to a difference in glacial imprint between southern and northern catchments. Whereas the upper halves of the valleys in the northern higher subrange are formerly glaciated, wide and gently sloping, U-shaped valleys flowing into V-shaped valleys, the valleys of the lower southern subrange are entirely V-shaped. The glacially widened valleys act as capacitors that accumulate and release glacial and periglacial sediment onto the piedmont. The resulting discrete pulse in sediment flux, Qs, forces aggradation and steepening, followed by incision and gentler slopes of the piedmont rivers. The fluvial valleys do not accumulate sediment, and changes in water discharge primarily control the slope of the piedmont rivers. Today, incision in the north is associated with the drop in Qs that occurred after depletion of the upstream reservoir, while aggradation in the south is due to Central Asian aridity. The same climatic forcing can have strikingly distinct morphological expressions downstream of catchments with different glacial imprints.
ISSN:0091-7613
1943-2682
DOI:10.1130/G40320.1