HR: 10:20h
AN: U32A-01    [Abstracts]
TI: Preservation and Erosion of a Cenozoic Volcanic Plateau Assessed by Cosmogenic Nuclides. Constraints for the Morpho-Tectonic Evolution of the Afar Margin
AU: * Pik, R
EM: rpik@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, BP20, Vandoeuvre-Les-Nancy, 54501 Cede France
AU: Blard, P
EM: blard@cerege.fr
AF: CRPG-CNRS, BP20, Vandoeuvre-Les-Nancy, 54501 Cede France
AU: Blard, P
EM: blard@cerege.fr
AF: CEREGE, BP80, Aix en Provence, 13545 France
AU: Vigier, N
EM: nvigier@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, BP20, Vandoeuvre-Les-Nancy, 54501 Cede France
AU: Lave, J
EM: jlave@ujf-grenoble.fr
AF: LGCA, BP 53, Grenoble, 38041 France
AU: Ayalew, D
EM: dereayal@geol.aau.edu.et
AF: Dept. of Geol. and Geoph., Po box 1176, Addis Ababa, 00000 Ethiopia
AU: Yirgu, G
EM: yirgu.g@geol.aau.edu.et
AF: Dept. of Geol. and Geoph., Po box 1176, Addis Ababa, 00000 Ethiopia
AB: The morpho-tectonic evolution of the Afar triple junction has been debated for long, especially concerning the timing of rift development and basement uplift. Based on a low-T (U-Th)/He study, we have proposed recently that the stability of the Blue Nile river (on the main Ethiopian flood-basalts plateau) could last since the emplacement of the lava pile 25-29 Ma ago, implying a very early, large-scale domal uplift and consequently the collapse of the Afar depression from an originally elevated area (Pik et al., 2003). An interesting way to date the timing of the Afar depression collapse is to investigate the development of its margin. However, there exist very few constraints and models range from : (i) a very early development of the scarp with its preservation for tens of Ma or, in contrast, (ii) a very recent evolution of this scarp as a consequence of Quaternary rifting. We focus here on a pre-rift, thick flood-basalt pile that might have recorded tectonic and erosional events having affected the province since the Oligocene. We have therefore investigated the preservation and erosion of this volcanic plateau by measuring helium (3Hec) and beryllium (10Bec) cosmogenic isotopes on detrital olivines and quartz along the plateau and margin rivers, as well as on exposed surfaces on top of the plateau and on flanks of the scraps. The amount of cosmogenic nuclides contained in minerals from the various morphological zones of the plateau is very contrasted and indicates that : (i) the flat top of the plateau was stable with erosion rates as low as 1-5 m/Ma, demonstrating the preservation of such a plateau landscape even in monsoonal climate, and (ii) tributaries from the centrifuge Blue Nile and Tekeze river basins which erode the inner part of the plateau exhibit erosion rates in the range 30-60 m/Ma in very good agreement with erosion rates requested to account for the stability of the Nile since the end of the Oligocene. In contrast, (iii) rivers from the very small and short basins of the margin dominating the Afar depression are characterized by a very low cosmogenic nuclides contents, that are consistent with erosion rates higher than 100 m/Ma. These results will be used to discuss and model the timing of development and preservation of the Afar margin. Pik et al., EPSL, 2003, V215, 73-88
DE: 1130 Geomorphological geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 8109 Continental tectonics: extensional (0905)
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
SC: Union [U]
MN: Fall Meeting 2005