Lattice preferred orientations and microstructures of deformed Cordilleran marbles: correlation of shear indicators and determination of strain path

Abstract

Marbles from nine Cordilleran deformation zones (the Raft River and Snake Ranges, the Ruby, Rincon and Riverside Mountains, the eastern Peninsular Ranges mylonite zone, and three sites in the Death Valley region) were sampled for microstructural analysis of thin and ultra-thin sections and pole figure measurements by X-ray diffraction to interpret their deformation history. Microstructural observations reveal various degrees of recrystallization. Fabrics are predominantly symmetric, but some samples display consistent asymmetries of the long grain axes (schistosity) and the macroscopic foliation plane defined by compositional layering. These asymmetric microstructures are most pronounced in non-recrystallized fabrics. Well-developed textures are found at all sites with c-axes perpendicular to the foliation and a-axes in a girdle in the foliation plane. The sense of shear derived from slightly asymmetric {11 2 0} pole figures is consistent with the shear sense determined in the field and by S— C structures in adjacent gneisses. The magnitude of the obliquity of {11 20 } girdles suggests that the component of simple shear is relatively small even in samples with highly asymmetric microstructures. We conclude that the obliquity of the {11 2 0} girdles is a reliable indicator of the strain path during the last substantial increment of strain and indicates that the carbonate rocks studied deformed dominantly in pure shear. This implied strain path for carbonate rocks may not necessarily reflect the regional strain path, because strain may be partitioned between rocks of different compositions, or between zones of predominantly pure and simple shear at all scales.