High fluid pressures during regional metamorphism and deformation: Implications for mass transport and deformation mechanisms

Abstract

Evidence is presented to support the conclusion that pore fluid pressures tresses PF during regional metamorphism are generally greater than or equal to the minimum principal compressive stress S3. The resultant very low effective stresses σ lead to significantly increased porosity and permeability, even at moderate to high metamorphic temperatures. Permeabilities between 10−18 and 10−15 m2 and are considered to be common, resulting in rapid 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‐pressure 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, advective mass transport along fluid pressure gradients can give rise to a solution transfer deformation mechanism that competes with conventional pressure solution. The rate of deformation by advective mass transport could be controlled by a number of processes, including dissolution 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 competitive rates but with stress and temperature dependences of unusual form.

Structural Evolution of Upper Layer 2, Hole 896A

“…data, 1994). There is also a fundamental problem in the argument that the rapid pressure drop during hydrofracturing causes supersaturation of solute species and consequent increment of fiber growth (e.g., Etheridge et al, 1984). Although sudden pressure drops do cause supersaturation of most solute species such as quartz, calcite is more soluble as fluid pressure increases (Bruton and Helgeson, 1982), yet quartz and calcite fibers commonly occur together as fibers in "crack-seal" veins.…”

“…Supersaturation of solute phases caused by the pressure drop during crack propagation, discussed above, has been proposed as the mechanism to cause mineral precipitation resulting in sealing of the crack (Etheridge et al, 1984). This explanation is attractive for minerals such as quartz, but calcite (and aragonite) is more soluble as fluid pressure increases (Bruton and Helgeson, 1982).…”

“…Because extension fractures in rock deformation experiments form normal to the least principal stress (σ 3 ), it is surprising that the fibrous veins are not primarily vertical and parallel to the ridge axis. For example, in regional metamorphic rocks, fibrous extension veins of crack-seal origin are generally uniform in orientation and oriented normal to the regional extension direction (Etheridge et al, 1984). The few small faults having fibrous slickensides that occur below 334 mbsf also show highly variable orientations, and vary from normal, reverse, to oblique (Dilek et al,chapter 18,this volume).…”

“…Fractures initiate at flaws in the rock and grow by propagation (e.g., Pollard and Aydin, 1988), generally occurring by short periods of rapid propagation followed by longer periods during which fluids flow into the fracture. The fractures propagate at a rate higher than the velocity of the fluid entering the crack, resulting in an instantaneous fluid pressure drop inside the crack (Phillips, 1972;Etheridge et al, 1984). For as long as the fluid in the crack is under reduced pressure, any mineral that was in equilibrium with the fluid and that has a positive pressure dependence of its solubility will become supersaturated.…”

“…Crack-seal veins have been interpreted as forming from repeated cycles of hydrofracturing followed by mineral precipitation and sealing of the crack (Ramsay, 1980;Etheridge et al, 1984). Mineral precipitation resulting from individual crack-seal cycles can be often be inferred from bands of fluid inclusions, each band generally being only a few tens of microns thick and each crack-seal vein commonly consisting of hundreds of such bands.…”