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 that simulate groundwater flow and heat transfer in an idealized cross-section of a metamorphic core complex. The simulations investigate the effects of crustal and fault permeability and porosity, topography, and heat flow on two-way coupled fluid and heat transfer.Our two-dimensional simulations show that our rresults are non-unique, different permeability combinations can produce similar temperature and oxygen stable isotope distribution. However, our results show that fluid migration to mid- to lower-crustal levels is significantly fault-controlled and depends primarily on the permeability contrast between the fault zone and the crustal rock. The results of our simulations reveal that higher 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. Our results also show that the evolution of the rock fabric, in the fault zones, but especially in the shear zone, with the development of planar anisotropy-like foliation, profoundly impacts fluid flow and, hence, has important implication for heat transfer and hydrothermal transport.