THE ROLE OF THE JUAN FERNANDEZ RIDGE IN THE TECTONIC EVOLUTION OF THE CENTRAL CHILEAN ANDES (33°S)

^{In the southern edge of the Pampean Flat Slab segment, the Juan Fernandez Ridge (JFR) is presently subducted (33° S). The tectonics effects that are produced by a subducting ridge in the continental margin are still poorly understood. However this process has been correlated in time with denudation and uplift of the Andes. In order to constraint the role of this ridge we have carried out a paleomagnetic study in the Concón Mafic Dyke Swarm (CMDS), located near the city of Viña del Mar in the coast of central Chile (33° S). Recent 40Ar/,sup>39}Ar ages in hornblende between 163 and 157 Ma constrain the emplacement of this unit, that was coeval with sinistral shearing along NW-trending structures. A primary paleomagnetic direction (343.2°/-29.4°) has been found in pyrrhotite-bearing dykes, suggesting that the area was tilted to the NNW in around 23.2°. This tilting is probably associated to extensional tectonics active during the Jurassic – Cretaceous in the area. The paleomagnetic results in magnetite-bearing dykes and country rock Paleozoic granitoids yield a mean paleomagnetic direction of normal polarity of 349.4°/-55.6°. This direction is similar to that expected for the Cretaceous (~100 Ma). This magnetization could have been acquired during a major event of uplift in the Coastal Ranges, as indicated by apatite fission track data. The facts that no significant rotations have been observed in this region (see also Astudillo et al., this congress) allows us to suggest that the subduction of the Juan Fernandez Ridge has no drive oroclinal bending in the region. Extrapolating our results, models that try to explain the Bolivian Orocline using the Nazca ridge should to be re-examined.