RELATIONS BETWEEN PROGRESSIVE DEFORMATION AND FLUID-ROCK INTERACTION DURING GROWTH OF FAULT AND SHEAR ZONES IN A BASEMENT-CORED THRUST SHEET, SEVIER OROGENIC BELT, UTAH

Variations in microtextures, chemistry, mineralogy, fluid inclusions, and fracture network properties record complex interactions between deformation and fluid-rock processes within a basement-cored thrust sheet. Fault and shear zones formed at depths of 12-15 km, temperatures of 300-350 °C, and elevated fluid pressures. The main Ogden thrust fault zone contains a thick core of phyllonite and cataclasite, bounded by a damage zone of highly fractured chloritic gneiss. Shear zones contain thinner cores of phyllonite and damage zones of chloritic gneiss, which grade outward into relatively undeformed granitic gneiss. Granitic gneiss consists mostly of coarser-grained feldspar and quartz; phyllonite consists of fine-grained quartz-mica-rich matrix produced by pervasive plastic and mass transfer processes; and cataclasite contains finely comminuted matrix cut by multiple sets of variably deformed veins. Alteration of granitic gneiss into phyllonite involved significant depletion of Ca, Na, and K, and significant enrichment of Mg and H2O. Similar average contents of relatively immobile elements between granitic gneiss and fault/shear zone cores indicate isovolumetric alteration at a macroscopic scale, but variations between individual samples record up to +/-30% local volume change. Changes in mineral compositions and fluid inclusion characteristics record depletion in Mg from fluids during alteration. Changes in whole-rock chemistry and fluid composition yield geochemical fluid-rock ratios of 10+2 to 10+4, requiring large influxes of reactive fluids. Fluid pathways included regional flow along shear/fault zones, with local outward flow into damage zones along fracture, microcrack, and grain boundary networks. Average fluid fluxes, estimated from fluid-rock ratios and pathway geometries, require average permeabilities of 10-12 to 10-15 m2 in fault/shear zone cores and 10-15 to 10-18 m2 in damage zones for moderate fluid pressure gradients. These values are consistent with mean permeabilities calculated from fracture network properties for a range of histories, in which feedback mechanisms, including sealing and equilibration of fluid pressure gradients, modulate apertures. Fluid influx resulted in alteration of strong, feldspar-rich rock to weak, fine-grained, quartz-mica-rich matrix, with strain softening leading to concentrated deformation within fault/shear zones.