Age control for the latest movements on the Melones fault zone in the Sierra Nevada foothills belt from K-Ar dates for Late Tertiary intrusions

K-Ar dates of 4.12 million years (m.y.) and 4.03 m.y. for two small dacite and rhyodacite intrusive bodies near Mokelumne Hill, Calif., provide important age control for the late Tertiary Mehrten Formation in this area because the Mehrten is both intruded by the dacite and rhyodacite and depositionally overlies the dacite of one intrusion. These dates also establish a maximum age for the latest movement on the Mother Lode fault, a part of the Melones fault zone, because the Mehrten Formation is displaced by this fault and because younger colluvium containing clasts from one of the intrusive bodies is in fault contact with the Mehrten.