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Comparisons of geometric relations, microtextures, and the nature of fluid-rock interaction for deformed basement rocks in parts of the Rocky Mountain foreland, Wyoming, and the Sevier orogenic belt, northern Utah, reveal regional variations in structural style. Basement within the Wind River Range, Wyoming, is deformed by large-scale reverse faults and by intervening major brittle deformation zones (BDZs). Slip along faults and BDZs produced 30% regional shortening by bulk pure shear. Basement away from BDZs is cut by widely spaced fractures, but fracture intensity increases toward BDZs, and complex fracture networks occur along BDZ margins. BDZs contain anastomosing zones of breccia and cataclasite that developed by accumulation of fine-grained matrix due mostly to intergranular cracking and production of wear fragments along shear fractures. Small amounts of chlorite, clay, and calcite record local alteration along major BDZs. Cataclasis appears to be the dominant deformation mechanism, although very fine-grained foliated cataclasite along major faults may record a switch to pressure solution, particulate flow, or grain boundary sliding.

Basement within the northern Wasatch Range, Utah, is deformed by large-scale imbricate thrust faults, by intervening networks of major ductile deformation zones (DDZs), and locally by networks of minor DDZs. Slip along thrusts and DDZs produced 60% shortening, with regional-scale simple shear and significant components of pure shear within individual thrust sheets. Basement rock away from DDZs is cut by widely spaced fractures and displays limited alteration. A transition zone along DDZ margins displays variable alteration concentrated along complex fracture and microcrack networks. Variably deformed grains and mineral-filled veins record temporally overlapping cataclasis, fluid-rock interaction, and plastic deformation. DDZs contain phyllonite that formed by accumulation of fine-grained matrix due to pervasive alteration, plastic deformation, and periodic microcracking. Quartz in DDZs is highly strained and recrystallized, and feldspar is almost completely altered to foliated aggregates of mica. Cataclasite is mixed with phyllonite along major thrust fault zones.

Concentration of deformation within BDZs and DDZs reflects overall strain softening along these zones relative to the host rock. Relations between displacements and thicknesses of zones record growth of BDZs and DDZs with time, and may reflect periods of strain hardening during progressive deformation. The behavior of any particular zone is determined by a number of competing strain softening and hardening processes that are related to grain size reduction, fluid-rock interaction, and geometric evolution of internal structures.

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