The strain and geometry of meso-scale ductile shear zones and the associated fluid flow

Meso-scale ductile shear zones are structures commonly produced during the deformation of meta-igneous rocks. Their strain can be oblate in geometry. This requires a pure shear component to the deformation that formed the shear zone. The pure shear component can be produced by any combination of volume loss or extrusion of material from within the shear zone. Modeling shear zone deformation of different strains and volume losses demonstrates that, unless the strain is of a specific geometry, even if volume loss occurs, extrusion must also occur. The amount of volume loss also influences the vorticity, thickness, and displacement of shear zones. Two shear zone systems were investigated to compare their character, investigate their geologic significance to the regional tectonics, and to consider their deformation.; Shear zones of the Tarfala Valley in northern Sweden developed at peak metamorphic conditions (∼640°C, ∼10.5 kbar) during infiltration of a Cl-bearing fluid as recorded by the elevated Cl concentrations of hydrous minerals inside the shear zones. The Cl-bearing fluid's infiltration strongly influenced the mineralogy, mineral chemistry, and strain localization in these rocks. These shear zones interlink and possess fabrics consistent with extrusion.; The shear zones of the Diana Syenite in the northwest Adirondacks are, for the most part, straight, parallel-sided, and generally do not interconnect. Deformation of these shear zones is therefore constrained to plane strain produced only by simple shear. These shear zones, plus an associated regional mylonitic fabric, developed at peak metamorphic conditions (700-800°C) as a result of significant oblique-reverse thrusting along the Carthage-Colton Mylonite Zone. The orientation distribution of the shear zones in this area can be explained well by modeling the passive rotation of planes in simple shear, thus providing an explanation for their regional orientation pattern.; Differences in these shear zone systems may be attributed to the fundamental difference in the way the wall rocks behave. At Tarfala, the wall rock is undeformed, whereas in the Diana Syenite, the wall rock has penetrative deformation.