HR: 17:15h
AN: V14B-06    [Abstracts]
TI: Thermomechanical Modeling of Ignimbrite Flare-ups
AU: * de Silva, S L
EM: desilva@space.edu
AF: University of North Dakota, Department of Space Studies, Grand Forks, ND 58201-9008 United States
AU: Gosnold, W
EM: willgosnold@mail.und.nodak.edu
AF: University of North Dakota, Department of Geology and Geological Engineering, Grand Forks, ND 58202-8358 United States
AB: The most spectacular of geologic events on the planet may be ignimbrite flare-ups. Typically triggered by an elevated thermal input from the mantle into the crust, ignimbrite flare-ups can be considered the surface manifestation of the progressive thermal (and mechanical) maturation of the crustal column due to advection of heat by magmatism. The progressive erosion of crustal strength as the thermal impact of intrusion evolves results in failure of the crust and catastrophic eruptions. We support this model with a case study of the Altiplano-Puna Volcanic Complex (APVC) of the Central Andes where delamination of the lower lithosphere is thought to have fueled the eruption of at least 30,000 km³ of ignimbrites in a 10 ma period. Available age and volume data on major ignimbrite eruptions in the APVC shows that they initiated at ~10 ma and have "pulsed" with successively more voluminous episodes of activity at ~8, 6, and 4 ma. Peak eruption rates of ~5000 km³ ma^-1 were reached during the climactic 4 ma episode after which activity diminished rapidly. To evaluate the thermomechanical model we have used a 2-D, finite-difference, conductive heat flow model to determine the thermal disturbances associated with the development of the APVC. The initial condition for the model was established assuming low-angle subduction of the Pacific plate leading to a doubling of crustal thickness (60 km) following the tectonic model of Isacks (1988). Published values for radioactive heat generation within the Andes and typical values for mantle temperatures were used in the initial model. A delamination event was set at 13 ma and was followed at 10 ma, 8 ma, 6 ma, and 4 ma by emplacement of large magma batches of volumes equivalent to eruptive volumes at mid-crustal levels consistent with available geophysical data. Each magmatic emplacement was assumed to be instantaneous on a geological time-scale and its effects were superposed on the preceding thermal field. A single pulse of magma emplacement at 8 to 4 km equivalent to the 5000 km³ erupted during the 4 ma eruptive pulse was used to simulate pre-eruption conditions. The temperature profiles obtained from the model were applied to calculate depth to the brittle-ductile transition zone at various stages during the model evolution using the method of Li (2001). The model approach leads to a brittle-ductile transition that is located to within 3 km of the surface. Strain rates associated with local tumescence and extension above the pre-eruptive magma chamber are sufficient to trigger failure of the roof of the chamber.
DE: 8104 Continental margins: convergent
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8178 Tectonics and magmatism
DE: 8428 Explosive volcanism
DE: 8440 Calderas
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005