HR: 0800h
AN: T11A-0340    [Abstracts]
TI: Sublinear Displacement-Length Scaling of Igneous Dikes in Basalt
AU: * Mege, D
EM: daniel.mege@univ-nantes.fr
AF: Universite de Nantes, Laboratoire de Planetologie et Geodynamique, Nantes, NV 44322, France
AU: Schultz, R
EM: schultz@mines.unr.edu
AF: University of Nevada, Rock Fracture Group, Department of Geological Sciences and Engineering, Reno, NV 89557, United States
AB: Displacement-length ( D-L) scaling relations of faults are well understood, with an exponent of n = 1, whereas dilatant structures such as veins and dikes may scale as n = 0.5. We investigate the scaling relations from a new dataset obtained from 39 igneous dikes from northwestern Ethiopia that intrude 30 Ma Precambrian mylonites and gneisses capped by a 100-m-thick layer of altered basalt. Dike lengths were measured using Spot and ASTER satellite images with measurements of dike thicknesses (maximum opening displacements) obtained in the field. The data demonstrate sublinear displacement-length scaling, with D_max = 0.088 L^0.48 consistent with those of other opening-mode fractures available in the literature, including basaltic dikes intruding shales at Shiprock, New Mexico and two populations of veins. All four datasets are characterized by a power-law slope of about n = 0.5, indicating propagation under conditions of constant rock fracture toughness, rather than constant driving stress, as in the case of faults. The intercept for the population can be solved for fracture toughness to obtain K_Ic = 303-909 MPa m^1/2 for the basalt. Several factors may contribute to the large fracture toughness value. First, the country rock is not intact basalt, but a jointed and altered basaltic rock mass. The small tensile strength of the basaltic rock mass was likely to have been exceeded within a volume surrounding the tips of propagating igneous dikes, thus increasing the intrinsic resistance to propagation. Second, given field evidence for syn- or post-emplacement shearing along several dike margins, the orientations of the dikes were likely misaligned relative to the remote least compressive principal stress direction. Propagation of the dikes was probably controlled by the mixed-mode fracture toughness of the rock mass, which is invariably larger than that associated with pure opening of a dike. Third, fracture toughness of basalt increases nonlinearly with temperature, making dike propagation substantially more difficult for country-rock temperatures adjacent to the dike above °sim700 °C. Similarity in ages of dikes and intruded basalt suggest that the temperature effect may also have contributed to some degree to an increased resistance to dike propagation.
UR: http://mines.unr.edu/geo-eng/schultz
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
DE: 8040 Remote sensing
DE: 8178 Tectonics and magmatism
DE: 8486 Field relationships (1090, 3690)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting