Numerical constraints on rates of metamorphism, fluid production, and fluid flux during regional metamorphism

Abstract

One-dimensional numerical models of regional metamorphism were constructed in order to constrain rates of metamorphism, rates of fluid production, and fluid flux as a function of time and position in thickened continental crust. Four end-member numerical experiments result from combinations of sawtooth (single thrust) versus homogeneous crustal thickening geometries and continuous versus discontinuous metamorphic reactions that release up to 5 wt% volatiles from the lower half of the thickened crust. Metamorphic reactions consume substantial amounts of heat and retard the thermal evolution of thickened continental crust by several million years. Peak rates of metamorphism, defined as the rate of advance of the 400 °C isotherm, range from 1.4 to 5.7 mm yr -1 in the saw-tooth models and from 0.5 to 0.6 mm yr -1 in the homogeneous models. Maximum rates of fluid production range from 90 mg (fluid) m -3 yr -1 in the sawtooth, discontinuous-reaction model to 6 mg (fluid) m -3 yr -1 in the homogeneous, continuous-reaction model. Fluid production in the sawtooth models produce a maximum fluid flux of ∼0.2 kg (fluid) m -2 yr -1 out of the 40-km-thick lower plate. The maximum fluid flux in the homogeneous models are lower, -2 yr -1 . Fluid fluxes resulting from all models require significant focusing into channels, spaced greater than 10 km apart, in order to perturb the overlying thermal structure of the continental crust. In these numerical experiments, most fluid flow infiltrates overlying rocks at temperatures substantially below peak metamorphic temperatures, suggesting that strongly focused or multi-pass fluid-circulation systems may be required to explain reported high fluid/rock ratios.