New radiometric ages on tuffs from south-central Peru support the postulated flare-up of volcanic activity during early Miocene time. In the region of Huancavelica, Julcani, and Lircay, lower Miocene rocks lie on folded strata of pre-Cenozoic age; the absence of units of Eocene and early Oligocene age indicates that this area remained positive after Incaic deformation in Eocene time. Conglomerate beds reflecting erosion attendant on the first pulse of late Cenozoic (Quechuan) compressive deformation do not appear in the stratigraphic record until after 21.5 m.y. B.P. In one section, beds of coarse conglomerate are underlain by tuff dated at 18.3 ± 0.6 m.y. and overlain by tuff dated at 17.3 ± 0.2 m.y. At another locality, beds of conglomerate conformably overlie tuff dated at 19.6 ± 0.8 m.y. If we incorporate published data from other areas in central and southern Peru, it appears that Quechuan deformation began in early Miocene time between 19.5 and 17 m.y. ago. A firm age for the end of the first pulse of Quechuan deformation in this region is provided by an ash-flow sheet dated at 12 to 12.5 m.y. that unconformably overlies strata of Eocene to early Miocene age.

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