Misalignment of quartz c-axis fabrics and lineations due to oblique final strain increments in the Ruby Mountains core complex, Nevada

Abstract

To interpret c-axis stereonet patterns of deformed quartz the patterns must be viewed in the XZ strain plane, which is usually assumed to contain the elongation lineation. This study describes several c-axis patterns from a region of complex, amphibolite facies deformation in the northern Ruby Mountains where the late XZ plane is oblique to most of the lineations. Without the lineation as a guide, the proper orientation for interpretation of each pattern is determined by rotating the c-axis data around the normal to the foliation until the c-axes within the foliation plane lie at the center of the stereonet. This new view is inferred to be the XZ plane for the final increments of strain. The rotation produces an internal symmetry in the patterns. In addition, the rotated views across the area show a more consistent geographic orientation than the lineations. This geographic orientation is aligned with strain in a mylonitic shear zone which represents the latest macroscopic evidence for plastic strain along the edge of the study area. The complete reorientation of the quartz textures during the final strain increments implies that they record very little of the finite strain history. Quartz is the only mineral recrystallized during the late strain thereby providing insight into the geometry of strain increments that cannot be determined from other minerals.