Tectonic Significance of Silicic Dikes Contemporaneous with Latest Miocene Quechua 3 Tectonism in the Rimac Valley, Western Cordillera of Central Peru

In the upper part of the Rimac Valley, central Peru, steeply dipping silicic dikes dated at about 5.8 m.y. trend approximately east-west. The orientation of these vertical dikes indicates that during latest Miocene time the greatest principal stress was oriented about N80°E with a horizontal least principal stress orientation. The dikes appear to have been preferentially emplaced along one of a conjugate set of fractures formed earlier when the greatest principal stress was in a NE to ENE direction. The N80°E stress orientation during the latest Miocene, very similar to that recognized for coeval Quechua 3 deformation in the Ayacucho basin and elsewhere in central Peru, provides additional support for major late Neogene left-lateral movement parallel to the NW-SE structural grain of this portion of the central Andes. The new radiometric ages also provide further evidence for widespread rhyolitic volcanism during latest Miocene and early Pliocene time.