PALEOMAGNETISM OF THE MIDTERTIARY AYACARA FORMATION, SOUTHERN CHILE -COUNTERCLOCKWISE ROTATION IN A DEXTRAL SHEAR ZONE

Relative motions of the southern Chilean forearc are assessed through a paleomagnetic study of the Eocene-Miocene Ayacara Formation. This ma rine volcaniclastic unit was studied at three localities, all immediat ely west of the Liquine-Ofqui fault zone (LOFZ), a 1000-km-long, NS-tr ending intracontinental shear zone with probable Cenozoic right-latera l displacement. A paleomagnetic pole (74.8S; 44.0E; A-95, 8.5-degrees) was calculated by combining 11 site-mean VGP from the Ayacara Fm. wit h three sites from the nearby Oligocene Cocotue Beach basalts (previou sly published). When compared with a 20-40 Ma North American cratonic pole rotated into the South American reference framework this pole imp lies counterclockwise rotation of 14.6+/-11.8. We speculate that this counterclockwise rotation results from processes that tend to impel a detached silver of continental crust northward along the continental m argin, against a buttress. In our model, northward displacement of the trailing edge of the silver is accomplished by segmenting the silver into lens-shaped blocks which rotate counterclock wise as they move no rthward. The geometry of the LOFZ clearly supports this model, which m ay have application to other buttressed strike-slip fault zones.