The Santiago Peak Volcanics (SPY) is an elongate belt of volcanic and volcaniclastic rocks exposed along the western margin of the Peninsular Ranges batholith in Southern California. In the San Miguel mountain area exposures of the SPV are dominated by coarse-grained poorly bedded matrix supported lapilli tuff and breccia with lesser tuff and rhyolitic flow rock. These rocks have been statically recrystallized under greenschist facies conditions. The SPY here forms a NW striking NE dipping stratigraphic sequence that is cut by steeply dipping NNE striking cataclastic shear zones. This package is intruded by mid-Cretaceous granitic rocks of Pennisular Range batholith and unconformably overlain by Miocene Rosarito Beach Formation. Tilting of SPY and development of shear zones predates intrusion by granitic rocks. Lithologic characteristics and geometry of map units suggest emplacement on the flanks of an active volcanic center(s). A preliminary U-Pb zircon date from a rhyolite flow on San Miguel Mountain yields an Early Cretaceous age.
