LATTICE PREFERRED ORIENTATIONS AND MICROSTRUCTURES OF DEFORMED CORDILLERAN MARBLES - CORRELATION OF SHEAR INDICATORS AND DETERMINATION OF STRAIN PATH

Marbles from nine Cordilleran deformation zones (the Raft River and Sn ake Ranges, the Ruby, Rincon and Riverside Mountains, the eastem Penin sular Ranges mylonite zone, and three sites in the Death Valley region ) were sampled for microstructural analysis of thin and ultra-thin sec tions and pole figure measurements by X-ray diffraction to interpret t heir deformation history. Microstructural observations reveal various degrees of recrystallization. Fabrics are predominantly symmetric, but some samples display consistent asymmetries of the long grain axes (s chistosity) and the macroscopic foliation plane defined by composition al layering. These asymmetric microstructures are most pronounced in n on-recrystallized fabrics. Well-developed textures are found at all si tes with c-axes perpendicular to the foliation and a-axes in a girdle in the foliation plane. The sense of shear derived from slightly asymm etric (1120BAR} pole figures is consistent with the shear sense determ ined in the field and by S-C structures in adjacent gneisses. The.magn itude of the obliquity of {1120BAR} girdles suggests that the componen t of simple shear is relatively small even in samples with highly asym metric microstructures. We conclude that the obliquity of the {1120BAR } girdles is a reliable indicator of the strain path during the last s ubstantial increment of strain and indicates that the carbonate rocks studied deformed dominantly in pure shear. This implied strain path fo r carbonate rocks may not necessarily reflect the regional strain path , because strain may be partitioned between rocks of different composi tions, or between zones of predominantly pure and simple shear at all scales.