HR: 13:30h
AN: GP33A-01 INVITED     [Abstracts]
TI: Paleomagnetism and the Tectonic Evolution of the Central Andes
AU: * Somoza, R
EM: somoza@gl.fcen.uba.ar
AF: Universidad de Buenos Aires, Dpto. GeologĦa, Pab. 2, Cdad. Universitaria, Buenos Aires, C1428EHA Argentina
AB: The curved shape of the Central Andes has long been attributed to oroclinal bending, the so-called Bolivian Orocline. Refined oroclinal models do not consider a former straight orogen but an inherited curvature that was enhanced to accommodate along strike gradients in Late Cenozoic horizontal shortening. They predict regional counterclockwise rotation in the northern branch and regional clockwise rotation in the southern branch of the arc. Although paleomagnetically detected rotations show an overall consistency with this arrangement, a closer inspection indicates that they record a deformation history that is more complex than a simple bending. The orogen evolves since the Mesozoic. The present forearc (northern Chile and coastal areas of Peru) was the locus of the early contractional phases in Late Cretaceous to Paleogene times. Building of the modern Central Andes occurred in the Late Cenozoic when the locus of shortening shifted eastward to the present backarc region (Bolivia and NW Argentina). Paleomagnetic studies in the modern forearc have shown widespread rotations in Mesozoic-Paleogene rocks and no rotation in Neogene units. In contrast, rotation in Neogene rocks is widespread in the modern backarc. This points out that the timing of rotation roughly follows the overall eastward shift of deformation. On the other hand, recent paleomagnetic results seem to favor an Oligocene age for the rotations in the southern Peruvian forearc, whereas rotations in northern Chile seem to be mainly Eocene. This latter suggests that the timing of rotations also changes along-strike in the forearc. Tectonic rotations in the Central Andes are time-transgressive, constituting a characteristic component of the deformation during, at least, the compressive stage of the orogenic development. It is likely that the pre-orogenic shape of the South American margin is one of the factors in controlling the along-strike change in sense of rotations. The fact that Neogene rocks along the curved forearc do not show paleomagnetically resolvable rotation suggests that the prominent curvature delineated by the external Neogene fold-thrust belt, the Central Andean salient, is a primary arc.
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
DE: 8102 Continental contractional orogenic belts
DE: 9360 South America
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly