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Titel |
Spatial variations in fluvial incision across the eastern margin of Tibet reveal locus of thickening in the deep crust |
VerfasserIn |
Eric Kirby, Huiping Zhang, Jie Chen |
Konferenz |
EGU General Assembly 2016
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Medientyp |
Artikel
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Sprache |
en
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 18 (2016) |
Datensatznummer |
250125601
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Publikation (Nr.) |
EGU/EGU2016-5194.pdf |
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Zusammenfassung |
The manifestation of coupling among climate, erosion and tectonics along steep topographic
margins of orogenic plateaus is strongly dependent on the processes driving crustal
thickening. Along the eastern margin of the Tibetan Plateau, a long-standing an vigorous
debate persists over whether mountain building occurred largely along upper-crustal faults or
was the consequence of distributed thickening in the lower crust. Here we revisit this debate
and show how surface deformation recorded by geomorphology over millennial timescales
(104-105 yr) can yield insight into the role the deep crust along plateau margins.
In contrast to the intensively studied Longmen Shan, the topographic margin of the
Tibetan Plateau north of the Sichuan Basin follows the north-south Min Shan and cuts
orthogonally across the structural grain of the Mesozoic West Qinling orogen. The lack of a
direct association of topography with upper crustal faults affords an opportunity to evaluate
the patterns of differential rock uplift from geomorphology. First, we employ an
empirical calibration of river profile steepness (channel gradient normalized for
drainage basin area) and erosion rate from cosmogenic 10Be concentrations in modern
sediment. Application to the channels draining the plateau margin reveals a locus
of high (300-500 m/Myr) erosion rate coincident with the Min Shan. Second, we
present new results of surveying and dating of fluvial terraces developed along
the Bailong Jiang, one of the major rivers draining across the plateau margin. A
preliminary chronology of terrace tread deposits based on radiocarbon and OSL
samples reveals systematic spatial gradients in fluvial incision, with highest incision
rates (1000-2000 m/Myr) localized along the axis of the Min Shan and decreasing
toward both the foreland and the plateau. This locus of incision has apparently been
sustained through multiple generations of terrace formation and abandonment since
ca. 80ka and thus is interpreted to reflect sustained differential rock uplift along
this axis. The wavelength of the region of highest incision rates is ∼80 km and
requires either 1) a deeply buried tip of a blind fault, or 2) thickening in the deep
crust. We argue that terrace deformation and associated rock uplift likely reflects
flow and thickening of deep Tibetan crust against the foreland of the West Qinling. |
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