HR: 0830h
AN: H51E-1124    [PDF]
TI: Cosmogenic Nuclide Studies And Geomorphological Implications In The Hyperarid Central Western Andes, Northern Chile
AU: * Kober, F
EM: kober@erdw.ethz.ch
AF: Geological Institute ETH Zurich, Sonneggstr. 5, Zurich, 8092 Switzerland
AU: Schlunegger, F
EM: fritz.schlunegger@geo.unibe.ch
AF: Geological Institute University of Bern, Baltzerstr. 1, Bern, 3012 Switzerland
AU: Ivy-Ochs, S
EM: ivy@phys.ethz.ch
AF: Institute of Particle Physics ETH Zurich, Hoenggerberg, Zurich, 8093 Switzerland
AU: Baur, H
EM: baur@erdw.ethz.ch
AF: Institute of Geochemistry and Mineral Resources, Sonneggstr. 5, Zurich, 8092 Switzerland
AU: Wieler, R
EM: wieler@erdw.ethz.ch
AF: Institute of Geochemistry and Mineral Resources, Sonneggstr. 5, Zurich, 8092 Switzerland
AU: Kubik, P W
EM: kubik@phys.ethz.ch
AF: Institute of Particle Physics ETH Zurich, Hoenggerberg, Zurich, 8093 Switzerland
AU: Schneider, H
EM: schneider@geo.unibe.ch
AF: Geological Institute University of Bern, Baltzerstr. 1, Bern, 3012 Switzerland
AB: We have conducted geomorphological and cosmogenic nuclide studies in the central Andes of Northern Chile (Arica area) in order to quantify landscape evolution at the Western Andean Escarpment. The area is characterized by broad abandoned middle to late Miocene plains that are cut by deeply incised rivers. The major phase of incision has been active since ca. 8 My with different magnitudes and accelerated backward erosion forces through time. Important processes must have been a base level change in the Coastal Cordillera area and climatic changes driven by orogenic growth. Fluvial incision rates are ca. 50m/My across the Escarpment during the middle/late Miocene and early Pliocene and about two to four times higher from the Pliocene until the present, especially in more distal parts relative to the pinned base level control (enhanced backward erosion). Bedrock erosion rates on the broad plains determined with cosmogenic nuclides are several orders of magnitude lower than fluvial incision rates. Erosion rates of ignimbrites determined by paired studies of radionuclides ($^{10}$Be, $^{26}$Al) and stable $^{21}$Ne are 10 to 100cm/My at medium elevations and increase towards the high Andes to $>$250cm/My. This increase is largely controlled by the effect of orographic precipitation that correlates with increasing rates and intensities of rainfall towards the Western Cordillera. Coastal areas, subjected only to coastal fog, show abandoned hanging rivers and smoothed diffusive hillslopes. These landscapes exhibit also extremely low cosmogenically derived erosion rates ($<$100cm/My). Therefore, the large parts of the landscapes at the northern tip of the Atacama Desert have retained their relict landscape aspect since they formed in Middle Micocene. The comparison of river incision and bedrock erosion clearly shows a strong decoupling between both geomorphic landscape units. A change towards a coupled system can only be found in the high, rainy parts of the Andes ($>$3500m), which are dominated by active hillslope processes. Lower elevations ($<$3500m) are controlled by hillslope failure due to river induced slope undercutting and landsliding. We will also report on the influence of climatic variations and uplift scenarios influencing the accumulation of cosmogenic nuclides, especially considering the long temporal scales of millions of years and broad spatial scales such as the Western Escarpment in Northern Chile.
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1815 Erosion and sedimentation
DE: 9360 South America
DE: 9604 Cenozoic
SC: Hydrology [H]
MN: 2003 Fall Meeting