Paleomagnetic Constraints on the Extent of the Miocene Tuff of San Felipe/Tuff of Hermosillo, Sonora, Mexico

The Tuff of San Felipe is a peralkaline rhyolite pyroclastic density current deposit 12.4 Ma in age cropping out in NW Mexico. It is a key horizon in the reconstruction of the Gulf of California Extensional Province because it erupted at the onset of extension in part of the Gulf. Rifting of the Gulf of California divided the ignimbrite leaving part of it on the North America Plate in Sonora, and part on the Pacific Plate, northern Baja California. An exceptional feature of this ignimbrite is its unique, low-inclination, reversed magnetization. Along with its mineralogy and age, this magnetization is vital for the correlation of the tuff across the region because the deposits are highly disrupted by subsequent normal faulting and outcrops are sparse and discontinuous away from the vent. We carried out paleomagnetic and other studies of the Tuff of San Felipe to determine its geographic extent in Sonora and Baja California. We examined new possible exposures of the Tuff of San Felipe in four widely separated Sonoran localities: San Miguel de Horcasitas, El Gavilán, Rancho El Pilar (Sierra Libre), and Cerro La Ceja. Oriented samples were analyzed for Natural Remnant Magnetization (NRM), Low Temperature Demagnetization (LTD), Alternating Field demagnetization (AF, 25mT-75mT), and High AF demagnetization (100mT- 800mT). The results from the High AF demagnetization were extremely linear and heading for the origin for all the cores studied. Tilt-corrected magnetization direction was nearly horizontal, slightly reversed with a southwest direction. This is consistent with previous paleomagnetic results for the Tuff of San Felipe from Baja California, Tiburon Island, coastal Sonora and Hermosillo, inland Sonora. We conclude that these outcrops, spread out over 125 km from north to south, comprise yet another major set of exposures of the Tuff of San Felipe.