HR: 1340h
AN: V53A-0604    [Abstracts]
TI: Space-time distribution of ignimbrite volcanism in the southern SMO: From Eocene to Pliocene
AU: * Nieto-Obregon, J
EM: nieto@servidor.unam.mx
AF: Centro de Geociencias, UNAM, Campus UNAM Juriquilla, Queretaro, Qro 76230 Mexico
AU: Aguirre-Diaz, G J
EM: ger@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, Campus UNAM Juriquilla, Queretaro, Qro 76230 Mexico
AB: A distinct variation in the age of the ignimbrites of the Sierra Madre Occidental (SMO) is observed in the southern portion, which includes the area between Tepic, Nayarit (-105ø W) and Aguascalientes, Ags (-102ø W). Older, high-grade ignimbrites are Eocene and occur as scattered outcrops. These are in turn covered by a widespread and voluminous sequence of high-grade ignimbrites and silicic to intermediate lavas that ranges in age from Middle Oligocene to Middle Miocene. The peak of this ignimbrite volcanism was at about 21 Ma to 22 Ma, but there is evidence showing that it initiated since about 30 Ma and ended at about 17.5 Ma. This ignimbrite and lava sequence is in turn covered by another series of lavas, predominantly mafic to intermediate, in the southern part of the area. This latest volcanism represents the initiation of the Mexican Volcanic Belt. Ignimbrite volcanism apparently initiated at the NE part of the study area, and migrated to the SW with time, that is from the area Presa Calles to the valley of Bola¤os. Isotopic ages reported on these rocks, cluster in various groups reflecting the time evolution of volcanism. Rocks older than 30 Ma tend to occur on the raised blocks of Sierra de El Laurel and Northern Sierra de Morones, in the eastern part of the area. The interval from 30 to 20 Ma comprises a discontinuous set of ages that are concentrated in the blocks of Southern Sierra de Morones, Tlaltenango, Bola¤os and the area around Cinco Minas-San Pedro Analco-Hostotipaquillo. An apparent gap of ages occurs between 12 to 18 Ma, followed by a predominantly mafic volcanism scattered mainly to the south of the area, that represents the transition of SMO to MVB. Finally mafic volcanism of the MVB of 3 to 4 Ma is present in the south, in the area excavated on the vicinity of Rio Grande de Santiago. A similar migration pattern has been reported in general for the whole SMO by Aguirre-Diaz and Labarthe-Hernandez (2003), from NE Chihuahua to SW Nayarit between ca. 50 Ma to 18 Ma. Thus, in this study we confirm this pattern in a more local scale. These authors also mention that such large ignimbrite units may have been produced by the extrusion of pyroclastic material through linear conduits. In the Aguascalientes area, we have found linear fissure vents for the local ignimbrites, confirming this fact in this area.
DE: 8414 Eruption mechanisms
DE: 9350 North America
DE: 9604 Cenozoic
DE: 8109 Continental tectonics--extensional (0905)
DE: 8110 Continental tectonics--general (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting