HR: 09:24h
AN: V31D-08    [Abstracts]
TI: Basaltic Dike Propagation at Yucca Mountain, Nevada, USA
AU: * Gaffney, E S
EM: edgaffney@earthlink.net
AF: Gaffney Associates, Inc., 111 No. Walnut St., Glenwood, IA 51534 United States
AU: Damjanac, B
EM: branko@itascacg.com
AF: Itasca Consulting Group, 111 Third Ave South Suite 450, Minneapolis, MN 55401 United States
AU: Warpinski, N R
EM: nrwarpi@sandia.gov
AF: Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185
AB: We describe simulations of the propagation of basaltic dikes using a 2-dimensional, incompressible hydrofracture code including the effects of the free surface with specific application to potential interactions of rising magma with a nuclear waste repository at Yucca Mountain, Nevada. As the leading edge of the dike approaches the free surface, confinement at the crack tip is reduced and the tip accelerates relative to the magma front. In the absence of either excess confining stress or excess gas pressure in the tip cavity, this leads to an increase of crack-tip velocity by more than an order of magnitude. By casting the results in nondimensional form, they can be applied to a wide variety of intrusive situations. When applied to an alkali basalt intrusion at the proposed high-level nuclear waste repository at Yucca Mountain, the results provide for a description of the subsurface phenomena. For magma rising at 1 m/s and dikes wider than about 0.5 m, the tip of the fissure would already have breached the surface by the time magma arrived at the nominal 300-m repository depth. An approximation of the effect of magma expansion on dike propagation is used to show that removing the restriction of an incompressible magma would result in even greater crack-tip acceleration as the dike approached the surface. A second analysis with a distinct element code indicates that a dike could penetrate the repository even during the first 2000 years after closure during which time heating from radioactive decay of waste would raise the minimum horizontal compressive stress above the vertical stress for about 80 m above and below the repository horizon. Rather than sill formation, the analysis indicates that increased pressure and dike width below the repository cause the crack tip to penetrate the horizon, but much more slowly than under in situ stress conditions. The analysis did not address the effects of either anisotropic joints or heat loss on this result.
DE: 8414 Eruption mechanisms
DE: 8434 Magma migration
DE: 8010 Fractures and faults
DE: 8145 Physics of magma and magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting