NUMERICAL MODELING OF FLUID FLOW AND HEAT TRANSFERS IN POROUS MEDIA: IMPLICATIONS FOR THE HYDROLOGY OF MID-CRUSTAL SHEAR ZONES

We investigate crustal -scale fluid flow in areas of crustal extension subjected to normal and/or detachment faulting using numerical modeling. In areas subjected to continental extension, brittle normal faulting of the upper crust leads to steep topographic gradients that provide the driving force (head gradient), and pathways (fractures) to groundwater flow. Ductile extension in the lower crust is characterized by high heat fluxes, granitic intrusion, and migmatitic gneiss domes. When downward fluid flow reaches the detachment shear zone (DSZ) that separates the upper and lower crust, high heat flux combined with magmatic / metamorphic fluids cause density inversions leading to buoyancy-driven upward flow. Therefore DSZs represent crustal-scale hydrothermal systems characterized by buoyancy-driven fluids convection.

We present the results of hydrologic-thermal finite-element numerical models built using ABAQUS/Standard. The simulations investigate the effects of crustal and fault permeability and porosity, topography, and heat flow on two-way coupled fluid and heat transfer. The geometry of the model represents such a simplified metamorphic core complex during the onset of extension, prior to exhumation of the detachment zone. The modeled upper crust is divided into three blocks, separated by two 50m thick vertical fault zones that root into a 100m thick shear zone. The elevation difference between the lowest and the highest block generate a topographic that can be easily changed. This slope induces some topography-driven flow and ensures that recharge will occur in the higher elevation fault; flow toward the lower fault will maintain a fairly stable fluid circulation during modeling. The base of the model includes a 500m thick crust below the shear zone.

Results show that fluid migration to mid- to lower-crustal levels has to be fault controlled and depends primarily on the permeability contrast between the fault zone and the crustal rock. High fault/crust permeability contrast leads to channelized flow in the fault zone and shear zone while lower contrast allow leakage of the fluids in the crust.