New Models of Crustal Fluid Flow Incorporating Magmatism and Porosity Evolution During Orogenesis

We present a 2d numerical model simulating the flow of fluid in an orogenic overthrust setting during magmatic intrusion and metamorphic devolatilization. The model is intended to test the existing hypotheses on the direction of the regional fluid flow in the deep crust. Advective (single-pass, pervasive or focused), convective (multi-pass), and sub-horizontal (up-temperature) fluid flow regimes have been proposed on the basis of various geological observations and models. Our numerical simulation reveals complicated local fluid flow patterns, with fluid moving upward and downward, as well as in the direction of decreasing temperature. Most of the fluid flux is concentrated along the fault zone and around magmatic intrusions, which take the form of repetitive basaltic sills. The sills have variable permeability for fluid depending on their temperature, and may serve as sources/sinks for volatiles and as a barrier for fluid flow, thus introducing anisotropy to our model. In addition, we present a "next- generation" numerical model for fluid flow in a deformable two-phase (fluid, solid) media with visco-elastic rheology. The necessity of the new model for crustal fluid flow is indicated by the difficulty that traditional one- phase formulations have in reproducing the near-lithostatic fluid pressure in the deep crust with realistic rock permeability and in the absence of extra fluid sources. As fluid motion is driven by the gradients in fluid pressure, the use of two-phase models with solid-fluid interaction and evolving porosity may be critical for numerical simulations of deep crustal fluid flow. We test the traditional one-phase formulation by comparing the fluid patterns inferred from the one-phase and the two-phase models. Furthermore, we use the two-phase formulation to reconstruct the profile of the fluid pressure and associated matrix permeability within the model crustal section, and to estimate the characteristic time scales of the fluctuations in fluid pressure due to hydrofracturing and fracture healing.