Kinetically limited isotope exchange in a shallow level normal fault, Mineral Mountains, Utah

We have conducted a stable isotope and geochemical study of the Corral Canyon fault zone, a shallow-level, low-angle normal fault on the west flank of the Mineral Mountains in southwest Utah. Our stable isotope and geochemical data document the infiltration of local meteoric water during brittle deformation and hydrothermal alteration of the shear zone during Tertiary Basin and Range extensional faulting. Within the fault zone, O-18 depletion correlates with progressive hydrothermal alteration and cataclasis, demonstrating that the processes of fluid infiltration, hydrothermal alteration, fracturing, comminution of grains, and O-18 exchange took place concurrently. Interpretation of our data using a model of combined fluid flow and kinetically limited isotope exchange leads us to conclude that the exposed fault zone was infiltrated by isotopically unevolved fluids from the wall rock adjacent to the fault zone and that an important component of fluid flow in this fault was between the wall rock and the fault zone rather than strictly within the fault plane. We propose that the rate of isotopic alteration in shallow level fault zones is enhanced by comminution and recrystallization of grains during deformation, so that fluids in the wall rock, because of the relatively coarser grain size, remain relatively unevolved until they infiltrate the fault zone where they rapidly become isotopically evolved due to exchange with cataclasized grains. We present a general model for calculating the duration of isotopic alteration in flow systems characterized by kinetically-limited isotope exchange. Calculated exchange durations for the Corral Canyon fault zone are short, on the order of thousands to a few hundreds of thousands of years, compared to the likely duration of deformation of millions of years. The geologically short duration of isotope exchange leads us to suggest that the fluid flow responsible for the isotopic alteration was episodic, of short duration, and possibly related to discrete seismic events. This result corroborates diverse lines of evidence that support a model of alternating fluid flow and permeability sealing by hydrothermal precipitation within fault zones.