EFFECTS OF PERMEABILITY STRUCTURE, TEMPERATURE FIELD, AND TOPOGRAPHY ON OXYGEN ISOTOPE EXCHANGE BETWEEN FLUIDS AND ROCKS IN METAMORPHIC CORE COMPLEXES

Several field studies of Cordilleran metamorphic core complexes, including the Thor-Odin, Bitterroot, Kettle, Raft River, Ruby Mountains, Snake Range, and Whipple Mountains indicate that meteoric fluids permeated the upper crust down to the detachment shear zone and interacted with recrystallizing rocks. Geochemical data show that fluid flow in such upper crust is primarily controlled by the large-scale fault-zone architecture. Faults cross-cutting lower to upper crust commonly define heterogeneous and anisotropic structures that can serve as conduits or barriers that respectively enhance or impede groundwater flow, heat transport, and ore formation. We conduct continuum-scale (i.e., large-scale, partial-bounceback) lattice-Boltzman fluid, heat, and isotope transport simulations of a simplified cross section of a metamorphic core complex. The simulations investigate the effects of crustal and fault permeability as well as background heating rate, buoyancy, and topography on two-way coupled fluid and heat transfer and resultant exchange of oxygen isotopes between fluid and rock.

The hydrologic system includes 5 different lithologic units: A 4.5 km thick lower crust is linked to the three upper crustal blocks (6, 6.75, 7 km thick) by a 1.5 km shear zone. The upper crustal blocks are divided by two 750 m thick high-angle faults (one for discharge, one for recharge) that take root in the shear zone, adjacent to two 2 km thick basins. 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 allows leakage of the fluids into the crust. Buoyancy affects mainly flow patterns (more upward directed) and, to a lesser degree, temperature distribution (disturbance of the geothermal field of ~35°C). Varying the heat flux does not affect the fluid flow or isotopic distribution. Increasing the topographic gradient (from 5 to 8%) enhances the fluid flow, resulting in lower oxygen isotope compositions along the faults and the shear zone. The oxygen isotope results show profound oxygen depletion (starting value of d¹⁸O = 13‰ down to 4‰) concentrated along the faults and the shear zone.