The southern Sierra de Pintas are near the northwest end of the Gulf of California, between a postulated spreading crest of the segmented East Pacific Rise and the uplifted Sierra Juarez of the Peninsular Range. The stratigraphic sequence is: the Pinta Volcanics, a lower basalt to rhyolite sequence and an upper basalt to rhyolite sequence, culminated by a tuff-pumice breccia sequence; and ends with the Grande Volcanics, a series of scattered extrusions of andesitic to rhyolitic rock. Scoria and tuff are dominant to the east and flows, auto breccias, and volcani-clastic rocks are dominate to the west. The volcanic rocks cover a basement complex of pelitic schists, quartzites, and marbles which were metamorphosed before and during an intrusion of late Mesozoic adamellite. Fault and fold patterns correlated with stratigraphic features indicate a settling synclinal basin to the southwest, and an eroded caldera centered to the east. The “3" shaped patterns of most folds and some faults is typical of wrench fault tectonics and their orientation suggests clockwise rotation of the southern Sierra de Pintas block between two right-lateral faults. Mineralization of silver and gold postdates tectonic activity.
