Relations between progressive deformation and fluid-rock interaction during shear-zone growth in a basement-cored thrust sheet, Sevier orogenic belt, Utah

Abstract

Variations in microtextures, strain, whole-rock chemistry, mineralogy, fluid inclusion characteristics, and fracture network properties record complex interactions between deformation and fluid-rock processes during progressive growth of a shear zone within crystalline basement rocks. This shear zone formed at a depth of about 15 kilometers, T ≈ 350°C, and at elevated fluid pressures. The shear zone (SZ) has a 10 meter thick core of highly deformed phyllonite, and is bounded by a 3 to 8 meter thick transition zone (TZ) of variably fractured chloritic gneiss, which grades outward into relatively undeformed granitic gneiss. Granitic gneiss consists mostly of coarser-grained feldspar and quartz; chloritic gneiss consists of varying amounts of quartz, feldspar, and fine-grained micaceous matrix produced by a mixture of cataclastic, plastic, and alteration processes; and phyllonite consists mostly of very fine-grained quartz-mica-rich matrix produced by pervasive plastic deformation and alteration processes. Estimated strain ratios are 10:1 within the SZ. Whole-rock chemical compositions record significant depletion of Ca and Na, and enrichment of Mg and H2O during progressive alteration of granitic gneiss into phyllonite. Similar average contents of Al, Ti, and Fe, as well as Si, indicate that alteration was about isovolumetric at outcrop scale, although variations between individual samples record up to ±20 percent local volume change. Alteration produced significant changes in mineral abundances and compositions, with conversion of feldspar to muscovite, mafic minerals to Mg-rich chlorite, and dissolution/precipitation of quartz. Fluid inclusion characteristics and mineral compositions indicate that SZ fluids were moderately saline, and became depleted in Mg during alteration. Changes in mineralogy and fluid composition record large influxes of reactive fluids, with geochemical fluid-rock ratios on the order of 102 to 103. Variably deformed, cross-cutting veins, fractures, and microcracks record repeated episodes of cataclasis, fluid influx, and sealing along complex networks. Fluid flow appears to have been concentrated within the SZ along grain-scale and vein networks, with a component of outward flow into the TZ along fracture and microcrack networks. Estimated average fluid fluxes are on the order of 10−7 to 10−10 m/s in the SZ, and 10−9 to 10−12 m/s in the TZ, consistent with average permeabilities of 10−14 to 10−17 m2 in the SZ and 10−16 to 10−19 m2 in the TZ for moderate fluid pressure gradients. These average permeabilities are consistent with observed grain-scale and fracture network properties for a range of sealing and fluid pressure histories. Permeabilities may have been transiently greater during episodes of very high fluid pressure, but decreased as fluid pressure gradients equilibrated and fractures sealed. Strain softening in the SZ was likely produced by reaction softening, grain size reduction, and hydrolytic weakening of quartz. A model of shear zone growth involves: (1) initial fracturing of relatively strong, coarser-grained, quartz-feldspar-rich rock; (2) episodic influx of reactive fluids during periods of fracturing and high fluid pressure, with intervening periods of sealing and reduced fluid pressure; (3) progressive alteration of feldspar and mafic minerals to micas and recrystallization of quartz to form relatively weak, fine-grained matrix; and (4) concentrated deformation and focused fluid flow in a growing SZ core.