HR: 14:30h
AN: V23C-03    [Abstracts]
TI: Fluid Flow in Subduction Zones and Mountain Belts: The Importance of Permeability Heterogeneity and Anisotropy
AU: * Ague, J J
EM: jay.ague@yale.edu
AF: Dept. Geology and Geophysics, Yale University, PO Box 208109, New Haven, CT 06520-8109 United States
AB: Fluids are generally expected to be driven upward in the deep parts of orogens, but permeability heterogeneity and anisotropy must also be considered to properly interpret fluid infiltration and kinetic reaction histories preserved in the rock record. This paper focuses on new 2-D models of Darcian fluid flow incorporating permeability contrasts between rock units, the permeability tensor, and reactive fluid sources (e.g., dehydration). Factor of ten contrasts between the minimum and maximum permeability values in anisotropic rocks can strongly divert flow, but contrasts of as little as a factor of two still influence flow behavior. The first example considers fluid flow in subduction zone m\'{e}lange, Syros, Greece. Geochemical evidence suggests that the interiors of meta-mafic blocks of oceanic crust in the m\'{e}lange underwent limited fluid-rock reaction, despite extensive dehydration and decarbonation of the subduction complex. Modeling shows that if the blocks have lower permeability than the surrounding serpentine-rich matrix, then flow is diverted around the blocks resulting in little infiltration except at block margins, consistent with field relations. In this way, the subducted oceanic crust could preserve little evidence of fluid infiltration, even though considerable flow occurred through the m\'{e}lange. The largest fluid fluxes are concentrated in matrix where blocks are in close proximity, and this effect increases as the anisotropy of the matrix increases. The lack of fluid infiltration into blocks could account for the observed limited metamorphism and strong kinetic overstepping of reactions that in some cases allowed preservation of ocean-floor mineral assemblages even at blueschist-eclogite facies conditions. The second example examines fluid flow through a folded sequence in which the direction of maximum permeability is parallel to the folded layering, and is based on field relations of Barrovian metamorphic sequences in CT, USA, and Scotland. As the degree of anisotropy increases, flow is directed along fold limbs and away from fold hinges, becoming more and more concentrated at the tips of antiforms and synforms where cross-layer flow components dominate. Surprisingly, the largest model fluxes are limited to relatively small regions at the tips of fold hinges. The model results predict that permeability heterogeneity and anisotropy have the potential to strongly divert metamorphic fluid flow directions and, thus, influence the spatial distribution of fluid fluxes and the sites of fluid-rock reaction. Consequently, models of deep orogenic flow that consider only homogeneous rock should be re-examined, and new measurements of permeability contrasts and anisotropy relevant for deep crust/upper mantle conditions are needed to better constrain models and, ultimately, field observations.
DE: 8045 Role of fluids
DE: 5114 Permeability and porosity
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting