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

TLDR

The study investigates how mobile, high‑pressure fluids influence rock deformation during regional metamorphism, particularly in polyphase rocks. The authors model advective fluid transport along pressure gradients as a deformation mechanism that can drive strain rates comparable to dislocation creep and pressure solution, with rates controlled by dissolution kinetics, transport rates, and crack growth, and they develop a model predicting strain with unusual stress–temperature dependence. High pore fluid pressures during metamorphism produce very low effective stresses, greatly increasing porosity and permeability (10⁻¹⁸–10⁻¹⁵ m²), enabling rapid advective fluid migration that dominates over diffusion even over short distances, and the proposed model predicts strain rates comparable to conventional mechanisms but with atypical stress–temperature behavior.

Abstract

Evidence is presented to support the conclusion that pore fluid pressures tresses P F during regional metamorphism are generally greater than or equal to the minimum principal compressive stress S 3 . 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 m 2 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.