Fission-track dating of minerals from the Santa Rosa Mylonite Belt and adjoining rocks of the northeastern Peninsular Ranges batholith indicates that the area was affected by a brief, but intense cooling event approximately 61 m.y. ago. Concordant ages from different mineral species suggest a minimum temperatures drop of ~120¿C during this event. This rapid cooling event probabily reflects major uplift and unroofing of the batholith at the beginning of the Paleocene. These new data coupled with other published isotropic data, firmly establishes that the Santa Rosa Mylonite Belt formed during the Late Cretaceous.