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 wes t flank of the Mineral Mountains in southwest Utah. Our stable isotope and geochemical data document the infiltration of local meteoric wate r 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 al teration and cataclasis, demonstrating that the processes of fluid inf iltration, 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 infiltra ted by isotopically unevolved fluids from the wall rock adjacent to th e fault zone and that an important component of fluid flow in this fau lt was between the wall rock and the fault zone rather than strictly w ithin the fault plane. We propose that the rate of isotopic alteration in shallow level fault zones is enhanced by comminution and recrystal lization of grains during deformation, so that fluids in the wall rock , because of the relatively coarser grain size, remain relatively unev olved until they infiltrate the fault zone where they rapidly become i sotopically evolved due to exchange with cataclasized grains. We prese nt 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 s hort, on the order of thousands to a few hundreds of thousands of year s, compared to the likely duration of deformation of millions of years . The geologically short duration of isotope exchange leads us to sugg est that the fluid flow responsible for the isotopic alteration was ep isodic, of short duration, and possibly related to discrete seismic ev ents. This result corroborates diverse lines of evidence that support a model of alternating fluid flow and permeability sealing by hydrothe rmal precipitation within fault zones.