MECHANICAL CONTROLS ON FLUID-FLOW DURING REGIONAL METAMORPHISM - SOMENUMERICAL-MODELS

The control of fluid flow by plastic deformation during metamorphism i s critical to our understanding of metamorphic processes. Various geol ogical observations and field studies demonstrate the consequences of fluid flow control by deformation, so that the concept appears to be a ccepted, 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 mo dels based on fully coupled mechanical-fluid flow concepts; they inclu de deformation of a section of fluid-saturated crust containing a bloc k or a layer of material of different properties from its surrounds. I n particular, rheological and permeability contrasts between rock type s during deformation associated with regional metamorphism are suffici ent to control the form of fluid flow over the range of a few kilometr es. Low contrasts and small strains allow pervasive fluid flow, wherea s greater contrasts and increasing strains cause focusing of the flow. Such focusing is generally associated with localization of the deform ation, especially for a strongly dilatant elastic-plastic material. Ho wever, 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 a nd downward fluid flow may occur, demonstrated here by simple models f or folding and for deformation of regions containing plutons. Therefor e, such modelling may be used as a means of testing the various hypoth eses concerning the volumes of fluid predicted to have passed through some rock volumes. Numerical models of the future will become increasi ngly complex and powerful, allowing greater coupling of thermal, mecha nical, chemical and fluid flow effects, and based more on the physical processes involved. Combined field and laboratory studies will provid e correspondinly greater understanding and will permit the determinati on of the timing of fluid flow and structural controls on fluid flow p atterns.