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Role of Fluids in Rock Deformation |
Department of Geology, Utah State University, Logan, UT 84322-4505, USA
Optical and scanning electron microscopy were used to examine the textures and infer deformation mechanisms in rocks deformed at upper crustal conditions in the Geesaman oblique-slip fault, southeastern Arizona, and the Washakie thrust fault, northwestern Wyoming. Feldspar in slightly and moderately deformed rocks in both faults deformed by formation of extension and shear fractures along cleavage planes, and quartz grains deformed by brittle fracture. Fracture density is greater in feldspar than in quartz. Intergranular fractures and faults developed by the linking of intragranular fractured feldspars, which in turn resulted in the development of foliated cataclasites in the Washakie fault and phyllonites in the Geesaman fault. Clays and mica developed by the syntectonic alteration of feldspar, and with increasing mica or clay content quartz grains deformed by fracture in a progressively softer matrix. Fracturing and healing of fractures in quartz resulted in irregularly-shaped, elongate pods of quartz in a mica matrix in the Geesaman fault and in a clay and feldspar fragment matrix in the Washakie fault. The syntectonic alteration of feldspars, the presence of iron oxides in the faults, and late-stage quartz veins attest to the flow of water during deformation. The fault zones developed by early fracture in feldspar which gave way to cataclastic flow in feldspars and subsequent development of clay-rich cataclasites at low temperature and of phyllonites at high temperatures. Most of the slip in mature cataclasites was localized by slip and cataclasis of clays. Slip in the phyllonites was localized by plastic deformation of micas. The brittle to ductile transition in the rocks was the result of fracture and subsequent cataclasitic flow in feldspars, followed by deformation of clay or mica, and for the most part excluded the plastic deformation of quartz.
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