Abstract

Relative motions of the southern Chilean forearc are assessed through a paleomagnetic study of teh Eocene-Miocene Ayacara Formation. This marine volcaniclastic unit was studied at three localities, all immediately west of the Liquiñe-Ofqui fault zone (LOFZ), a 1000-km-long, NS-trending intracontinental shear zone with probable Cenozoic right-lateral displacement. A paleomagnetic pole (74.8S; 44.0E; A-95, 8.5°) was calculated by combining 11 site-mean VGP from the Ayacara Fm. with three sites from the nearby Oligocene Cocotue Beach basalts (previously published). When compared with a 20–40 Ma North American cratonic pole rotated into the South American reference framework this pole implies counterclockwise rotation of 14.6±11.8. We speculate that this counterclockwise rotation results from processes that tend to impel a detached sliver of continental crust northward along the continental margin, against a buttress. In our model, northward displacement of the trailing edge of the sliver is accomplished by segmenting the sliver into lens-shaped blocks which rotate counterclockwise as they move northward. The geometry of the LOFZ clearly supports this model, which may have application to other buttressed strike-slip fault zones.