J. metamorphic Geol., 1997, 15, 345–359 Mechanical controls on fluid flow during regional metamorphism: some numerical models A. ORD 1 AND N. H. S. OLIVER 2 1 CSIRO Exploration and Mining and Australian Geodynamics Cooperative Research Centre, PO Box 437, Nedlands WA 6009, Australia 2 School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth WA 6001, Australia ABSTRACT The control of fluid flow by plastic deformation during metamorphism is critical to our understanding of metamorphic processes. Various geological observations and field studies demonstrate the consequences of fluid flow control by deformation, so that the concept appears to be accepted, at least for small-scale systems (for example faults and vein systems). However, the concept appears to be less well recognized at regional scales. Considered here are examples of simple, conceptual models based on fully coupled mechanical–fluid flow concepts; they include deformation of a section of fluid-saturated crust containing a block or a layer of material of different properties from its surrounds. In particular, rheological and permeability contrasts between rock types during deformation associated with regional metamorphism are sufficient to control the form of fluid flow over the range of a few kilometres. Low contrasts and small strains allow pervasive fluid flow, whereas greater contrasts and increasing strains cause focusing of the flow. Such focusing is generally associated with localization of the deformation, especially for a strongly dilatant elastic–plastic material. However, a rate of fluid flow much greater than the rate of deformation may result in pervasive flow, although for most models pervasive flow is difficult to attain over regional distances. Furthermore, lateral and downward fluid flow may occur, demonstrated here by simple models for folding and for deformation of regions containing plutons. Therefore, such modelling may be used as a means of testing the various hypotheses concerning the volumes of fluid predicted to have passed through some rock volumes. Numerical models of the future will become increasingly complex and powerful, allowing greater coupling of thermal, mechanical, chemical and fluid flow effects, and based more on the physical processes involved. Combined field and laboratory studies will provide correspond- ingly greater understanding and will permit the determination of the timing of fluid flow and structural controls on fluid flow patterns. Key words: deformation; deformation–fluid flow coupling; fluid flow; numerical model; permeability; regional metamorphism. fluid pressure. For example, Mandl (1988; fig. II. 10-1) INTRODUCTION demonstrated that fluid can flow up a gradient in fluid pressure, as long as this is associated with a decrease Our field and laboratory observations, and the obser- vations of many other workers worldwide (Table 1 of in the hydraulic head. Fluid focusing commonly occurs in regions of Oliver, 1996), reveal that metamorphic fluid flow is most commonly coupled with plastic deformation, in dilation. Faulting and vein formation may occur at lower stresses than in dry rocks as pore pressure particular with the localization of deformation. Hydrothermal ore deposits formed in metamorphic increases and effective stress decreases (Terzaghi, 1923; Hobbs et al., 1976; Mandl, 1988). Although predictions environments are almost exclusively formed by the passage of large volumes of metal-bearing fluids have been made as to where fluid should flow at any one time, based on inferred stress regimes (e.g. Sibson, through particular structural sites (e.g. zones of localized shear, veining or brecciation), usually in 1981; Etheridge et al., 1983, 1984; Oliver et al., 1990; Ord, 1991b; Ridley, 1993; Power et al., 1994), such combination with a set of chemical controls influencing the precipitation mechanism(s). Thus, the single most predictions may not be correct when a history of deformation and fluid flow is allowed, since the important factor controlling fluid flow is the pattern of deformation. For this reason, we focus our attention physical parameters may change with deformation (Ord, 1991a). in this paper on models of coupled deformation and fluid flow. Where the permeability of the material stays The fully coupled mechanical–fluid flow problem can be incorporated into numerical codes (e.g. Hart, constant, fluid flow in these environments is controlled by gradients in the hydraulic head, and such gradients 1981), along with a range of important rock properties, to couple mechanical and fluid flow behaviour. Here, need not necessarily coincide with gradients in the 345 © Blackwell Science Inc., 0263-4929/97/$14.00 Journal of Metamorphic Geology, Volume 15, Number 3, 1997, 345–359