HR: 15:30h
AN: V23C-07 INVITED     [Abstracts]
TI: Miocene to Recent Volcanism in NE Baja California and its Correlation to Adjacent Regions
AU: * Stock, J M
EM: jstock@gps.caltech.edu
AF: Seismo Lab, Caltech 252-21, Pasadena, CA 91125, United States
AB: This paper reviews the status of knowledge of volcanic rocks from part of NW Mexico. Rocks of the Puertecitos Volcanic Province, NE Baja California, span the time period from subduction-related volcanism (the "early to middle Miocene arc" of Gastil et al.) through to modern rift-related volcanism. Ages and major element geochemistry of the principal rock packages were summarized by Martin-Barajas and colleagues in various publications. Additional geochronology and paleomagnetic studies were done by Lewis (1994) and Nagy (1997) to characterize the ages, magnetic polarities, and distributions of the major pyroclastic flow deposits and lavas. The arc-related rocks consist of voluminous epiclastic material of 20-17 Ma and local vents of mafic to intermediate lavas as young as 15.5 Ma. These were eroded prior to deposition of the ca. 12.5 Ma Tuff of San Felipe, a peralkaline high-Si rhyolite pyroclastic flow deposit that is inferred to indicate post-subduction volcanism. Intermediate lavas were erupted in this region ca 9 Ma, followed by a 6.4-6.1 Ma sequence of rhyolite domes and ignimbrites. In the southern Puertecitos Volcanic Province, another pulse of volcanism from 3.3 to 2.7 Ma produced a series of at least 20 thin, high-temperature pyroclastic flow deposits. Volcanism continued with Pliocene and Quaternary andesites and more evolved lavas. This volcanic history is compared to that of surrounding regions. The 20-15 Ma arc rocks are partly younger than similar rocks farther north in the Baja California peninsula and are coeval with volcanic arc rocks farther south. Northward, the centers decrease in frequency in map view; volumes decrease and the relationship of these lavas to coeval lavas near the border (e.g., Alverson Fm in S. California) is not clear. This may be an effect of the northern limit of the corresponding subduction zone. The ca. 6 Ma volcanism is related to rifting of the northern Gulf of California, particularly due to its structurally controlled location (an accommodation zone in the rift system). The ca 3 Ma pulse of volcanism has been related to a "ridge jump" type event (relocation of the plate boundary from the Lower Tiburon basin to the Lower Delfin Basin, within a single spreading segment of the Pacific-North America rift). Both the 6 Ma pulse and the 3 Ma pulse thus seem to be controlled by local processes rather than by regional events. The ca. 12.5 Ma Tuff of San Felipe erupted before the Gulf opened, when Baja California and Sonora were adjacent; the likely vent location is on the modern Sonoran coast north of Bahia de Kino. Work by Oskin (2002), and ongoing studies, allow outcrops of this unit to be correlated over a modern distance of at least 430 km from NE Baja California to east of Hermosillo, Sonora. It has been included by Vidal-Solano and others (2005) as part of a significant episode of post-subduction peralkaline volcanism in Sonora, attributed to regional extension and lithospheric thinning.
DE: 8178 Tectonics and magmatism
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly