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| Titel |
Putting weathering into a landscape context: Variations in exhumation rates across the Colorado Front Range |
| VerfasserIn |
Suzanne P. Anderson, Melissa A. Foster, Scott W. Anderson, Miriam Dühnforth, Robert S. Anderson |
| 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 |
250108243
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| Publikation (Nr.) |
 EGU/EGU2015-7992.pdf |
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| Zusammenfassung |
| Erosion rates are expected vary with lithology, climate, and topographic slope, yet assembling
these variations for an entire landscape is rarely done. The Front Range of the southern
Rocky Mountains in Colorado, USA, exhibits contrasts in all three parameters. The range
comprises ~2300 m in relief from the Plains to the crags of the Continental Divide. Its abrupt
mountain front coincides closely with the boundary between marine sedimentary rocks to the
east and Proterozoic crystalline rocks (primarily granodiorite and gneiss) to the
west. Mean annual temperature declines and mean annual precipitation increases
with elevation, from ~11Ë C/490 mm at the western edge of the Plains to -3.7Ë
C/930 mm on Niwot Ridge near the range crest. The range contains regions of
low relief with rolling topography, in which slopes rarely exceed 20Ë , as well
as deeply incised glacial valleys and fluvial canyons lined by steep slopes (>25Ë
).
Cosmogenic 10Be based erosion rates vary by a factor of ~5 within crystalline rock
across the range. The lowest rates (5-10 mm/ka) are found on low relief summit tors in the
alpine, where temperatures are low and precipitation is high. Slightly higher erosion rates
(20-30 mm/ka) are found in low relief crystalline rock areas with montane forest cover.
Taken together, these rates suggest that on low slopes, rock-weathering rates (which
place a fundamental limit on erosion rates) are lower in cold alpine settings. Over
the 40-150 ka averaging time of 10Be erosion rates, lower rates are found where
periglacial/tundra conditions have prevailed, while moderate rates occur where conditions
have varied from periglacial/tundra in the past to frigid regime/montane forest in the
Holocene.
Higher basin-averaged erosion rates of 40-60 mm/ka are reported for “canyon edge”
basins (Dethier et al., 2014, Geology), which are small, steep basins responding to fluvial
bedrock incision that formed the canyons in the late Cenozoic. Are higher erosion rates in
canyon-edge basins evidence that topographic slope affects weathering rates? We argue that it
is more likely that these high erosion rates reflect faster weathering in areas with thinner soil
cover. A recent major storm unleashed landslides and debris flows from ~10% of these
canyon-edge basins. On average, the volume of material evacuated in these basins was
equivalent to ~300 years of soil production by weathering at these rates, approximately the
recurrence interval of the storm. The conceptual model that emerges is that agents that cut
into rock (bedrock rivers, glaciers) set the pace for exhumation. Adjoining hillslopes erode at
a pace set by weathering in the prevailing climate/vegetation regime, conditioned by
the ability of sediment transport processes to limit soil thickness on the slopes. |
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