Hydrolytic weakening and penetrative deformation within a natural shear zone
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Processes of fluid infiltration, hydrolytic weakening, and penetrative deformation within a small ductile shear zone within granitic rocks of the central Sierra Nevada have been investigated using integrated field observations, strain analysis, infrared spectroscopy, and transmission electron microscopy. Several lines of evidence suggest that tensile fracturing accompanied by fluid infiltration preceded the ductile shearing event and that shear strains have localized on a pre-existing sealed fracture. Finite shear strains within an aplite dike and granodiorite host increase sharply from nominally O outside the shear zone to values of 102 near its center. Water contents of quartz grains exhibit similar spatial trends to that of strain, rising from 60 and 2000 ppm within the undeformed aplite and granodiorite, respectively, to 4000 and 11,000 ppm within their highly sheared equivalents. Infrared signatures of absorptions measured at room temperature and at 77 K show that most of the intragranular water within quartz and feldspar resides in fluid inclusions. Two distinct populations of fluid inclusions have been observed by optical and electron microscopy; one decorating healed microcracks and the second decorating dislocations. We interpret these relations to record interactions between fluids and processes of brittle failure and ductile flow. Fluid inclusions, forming planar arrays along the traces of healed microcracks, are relatively large (0.43 m in diameter) and irregular in shape. A second set of fluid inclusions consists of extremely fine (20140 nm in diameter), more nearly spherical inclusions which consistently lie along free dislocations and dislocation nodes, and exhibit relationships with dislocations similar to those observed in hydrolytically-weakened synthetic quartz. These observations suggest that water-related defects gained access to grain interiors and dislocation cores by fluid infiltration along open microcracks followed by pipe diffusion along mobile dislocations. And that's just half the story! Hugh Heard
