HR: 0800h
AN: T21C-0510    [Abstracts]
TI: Testing a Climate-Controll Model for Andean Orogeny
AU: Oncken, O
EM: oncken@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, D-14473 Germany
AU: * Sobolev, S V
EM: stephan@gfz-potsdam.de
AF: GFZ Potsdam, Telegrafenberg, Potsdam, D-14473 Germany
AB: The Altiplano-Puna plateau of the central Andes, Earth's second greatest plateau, has resulted from 300 km of Cenozoic crustal shortening at the western edge of South America. The key question of Andean orogeny is why the high plateau has developed only in the central Andes and only in Cenozoic times, although subduction along the entire western margin of the South America has been ongoing for more than 200 Myr. Lamb and Davis (Nature, 425, 792, 2003) suggest that high shear stresses at the interface between the Nazca and South American plates caused by sediment starvation of the Central Andean trench was crucial for deformation of the upper plate. Based on the correlation between shortening observations in the Central Andes and global sea surface temperature, they inferred that Neogene sediment starvation was caused by progressive, cooling-related aridisation since 30 Ma, making the global climate trend the factor responsible for formation of the high Andes. We use a quantitative reconstruction of shortening evolution and sediment flux into the trench during the Cenozoic along with numerical modelling to test Lamb and Davis hypothesis. We observe that the bulk shortening rate has evolved in three stages: 45-30Ma - gradual increase from 0 to 8mm/yr; 30-10Ma - shortening rate fluctuated between 6 and 10 mm/yr; 10-0Ma - rise of rate to 8-14 mm/yr until the attainment of the modern GPS-value of 9mm/yr. While this trend exhibits an apparent correlation with global cooling, we note that the local response of erosion and sediment flux is not related to sea surface temperature. The trench fill evolution, as based on exhumation data from the forearc, basin fill of forearc basins, and convergence rate, can be shown to have remained below a few 100 m thickness throughout the Cenozoic depending chiefly on the evolution of local topography and convergence rate. Calibrating the relation between interplate friction and trench fill thickness, we next calculate the dependence of shortening magnitude on trench fill for the central as well as the southern Andes, which lack substantial shortening, using the geodynamic model of Sobolev and Babeyko (Geology, 33, 617, 2005). From this calibration and our estimation of the changes of trench fill in the central Andes since 35 Ma, we find that interplate friction in the central Andes has increased by less than 50 percent. Using a numerical model we show that this increase of friction would explain no more than 10 percent of the observed increase of shortening rate. Hence, we conclude that Cenozoic climate change has not been a significant factor for Andean orogeny. However, our modelling supports an idea that high interplate friction at the central Andes latitude in contrast to the southern Andes has contributed to the different magnitudes of tectonic shortening in those regions - indicating that the general position of the belt in the southern hemisphere global arid belt is a key factor as opposed to global cooling.
DE: 0545 Modeling (4255)
DE: 0560 Numerical solutions (4255)
DE: 8102 Continental contractional orogenic belts and inversion tectonics
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005