HR: 11:50h
AN: V42A-07    [Abstracts]
TI: Numerical Models of Dike Propagation Near the Surface
AU: * Gaffney, E S
EM: edgaffney@earthlink.net
AF: Gaffney Associates, 111 N Walnut St, Glenwood, IA 51534, United States
AU: Damjanac, B
EM: branko@itascacg.com
AF: Itasca Consulting Group, 111 Third Ave S Suite 450, Minneapolis, MN 55401, United States
AU: Kreese, O
EM: olga_kresse@yahoo.com
AF: University of Minnesota, 500 Pillsbury Dr SE, Minneapolis, MN 55455, United States
AU: Keating, G N
EM: gkea@lanl.gov
AF: Los Alamos National Lab, PO Box 1663, Los Alamos, NM 87545, United States
AB: We have modelled dike propagation using two different hydrofracture models and a discrete element model. One of the hydrofracture models was driven by a compressible fluid; other magmas were incompressible. With an incompressible hydrofracture code (NPHF2D), the crack tip accelerates markedly as the surface is approached and the open cavity between the crack tip and the magma front may be hundreds of meters long. This is consistent with the several hours lapse between the appearance of a surface crack at Paricutin and continuous eruptions. By concatenating incompressible results, we show the surface instability is enhanced if the magma is expanding. EMSA, a near-surface, compressible fluid hydrofracture code, has magma density in equilibrium with pressure. It shows the near-surface acceleration of the tip is even greater than predicted with NPHF2D. EMSA also models vertical variations in lateral confining stress. This feature shows that an increase in lateral confining stresses due to radioactive heat in the first millennium after waste is placed in the proposed Yucca Mountain nuclear waste repository in Nevada,USA, will not prevent a dike from penetrating the repository; neither will it cause a sill below the repository. This was confirmed by a UDEC model with a mass of randomly oriented polygons filling the upper part of the model. This allowed the magma to seek its own path uninfluenced by preferred orientation of cracks. This produced the same result regarding thermal stresses at Yucca Mountain as EMSA. It also showed the dike had a tendency to bifurcate in the upper 100 m. This agrees with field observations of exhumed Miocene basaltic sites in southern Nevada, USA, where it is reported that dikes less than 5 meters wide at depths of 250 m began to bifurcate and splay into a zone 15 to 25 meters wide at depths about 100 meters and opened into conduits as wide as 100 m in diameter at the surface. We also used UDEC to confirm an approximate analytic solution showing that magma can be diverted more easily into a fault if the fault angle is steep or if the intersection is shallow. Together, these results suggest a complex variety of phenomena may be expected as a dike approaches the surface and confining stresses diminish.
DE: 3642 Intrusive structures and rocks
DE: 3653 Fluid flow
DE: 8434 Magma migration and fragmentation
DE: 8439 Physics and chemistry of magma bodies
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly