JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B6, PAGES 4344-4358, JUNE 10, 1984 HIGH FLUID PRESSURES DURING REGIONAL METAMORPHISM AND DEFORMATION: IMPLICATIONS FOR MASS TRANSPORT AND DEFORMATION MECHANISMS 1 M. A. Etheridge, V. J. Wall, and S. F. Cox Department of Earth Sciences, Monash University R. H. Vernon School of Earth Sciences, Macquarie University Abstract. Evidence is p•esented to support the conclusion that pore fluid pressures PF dur- ing regional metamorphism are generally greater than or equal to the minimum principal compressive stress S 3. Theresultant very low effective stresses o lead to significantly increased poro- sity and permeability, even at moderate to high metamorphic temperatures. Permeabilities be- tween 10 -18 and 10 -15 m 2 are consideredto be common, resulting in rapi d fluid migration and the dominance of advective (infiltrative) over diffusive mass transport, even over relatively small distances. In view of the importance of intergranular mass transport to rock deformation during metamorphism, a mobile, high-p•essure fluid will have substantial rheological effects, especially in polyphase rocks. The fluid is capable of influencing the rate of dislocation creep in a number of ways. More importantly, ad- vective mass transport along fluid pressure grad- ients can give rise to a solution transfer de- formation mechanism that competes with convent- ional pressure solution. The rate of deformat- ion by advective mass transport could be control- led by a number of processes, including dissolut- ion kinetics, advective transport rates, and the rate of crack growth. A specific deformation model, based on advective transport rate control, is developed, which can produce strain at com- petitive rates but with stress and temperature dependences of unusual form. Introduction transfer processes involving the fluid [Ramberg, 1952; Elliott, 1973; Fletcher and Hoffman, 1974; Fisher and Elliott, 1974]. In this paper, we will therefore examine some of the rheological consequences of the interact- ion between chemical and deformational processes, especially the ways in which they are influenced by the aqueous fluid phase. We aim to show that the key to this interaction is the presence of a high-pressure, mobile fluid phase and will first discuss the evidence for and mobility of this fluid before considering its implications for mass transport, deformation mechanisms, and crustal rheology. Another paper lEtheridge et al., 1983] discusses the concept of a highly mob- ile fluid more from the point of view of meta- morphic processes, with particular attention to the problems of large-scale fluid circulation, heat transfer, and retrograde metamorphism. Evidence for a High-Pressure Fluid The evidence for fluid pressure PFapproxim- ately equalto lithostatic pressurePLduring low to medium grade regional metamorph•sm has been summarized by Norris and Henley [1976] and Fyfe et al. [1978]. The main lines of evidence are as follows: 1. Extension fractures (veins) filled with minerals that reflect the pressure-temperature conditions of their host rock [Vidale, 1974; Norris and Henley, 1976] and were thus formed during metamorphism are common. Figure 1 shows examples of syn-metamorphic veins, the fibrous Most of our knowledge of the rheology of crus• microstructures and orientation of which, with al rocks comes from simple experiments on single crystals and single-phase aggregates of minerals (see Carter [1976], Nicholas and Poirier [1976], and Tullis [1979] for recent reviews). However, conditions during regional metamorphism are far more complex, with (1) polyphase assemblages undergoing complex chemical reactions during de- formation, (2) different solid phases having different strengths (and even different rate- controlling deformation mechanisms operating), and (3) a ubiquitous fluid present in a variety of intergranular and intragranular sites. It is unrealistic to presume that the chemical and mechanical processes are independent, especially since both are likely to be influenced by mass- 1Now at Bureau of Mineral Resources. Copyright 1984 by the American Geophysical Union. Paper number 3B1803. 0148-0227/84/003B- 1803505.00 respect to foliation and lineation, indicate that they formed as tensile fractures, requiring that P_ exceeded the minimum principal stress St by at least the tensile strength T of the rock during a substantial part of the rock deformation and metamorphism. Throughout this paper, weuseS 1 S 2, S 3 for the total principal stresses imposed ' tRrough the solid phases and o , o_, 03 for the effective principal stresses alroun• fluid-filled openings (thus 0_ = S•-P=, etc.). The best ex- amples of fibrous 3, j •w crack-seal veins are found in low to medium grade rocks, but massive quartzo- feldspathic veins that must have filled tensile fractures are common at higher metamorphic gradera 2. Experimental phase equilibria, determined under conditions such that PF equalled the confin- ing pressure (or mean stress = 1/3(S 1 + SA + S3)), agree with the distributionof natural metamorphic mineral assemblages. At the very lowest and highest metamorphic grades there is limited evid- ence from natural assemblages that PFmay be less than the mean stress [Coombs, 1971; Fyfe et al., 1978], but the body of petrologic evidence over- 4344