Bg. Erskine et al., LATTICE PREFERRED ORIENTATIONS AND MICROSTRUCTURES OF DEFORMED CORDILLERAN MARBLES - CORRELATION OF SHEAR INDICATORS AND DETERMINATION OF STRAIN PATH, Journal of structural geology, 15(9-10), 1993, pp. 1189-1205
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.