CRYSTALLOGRAPHIC FABRICS OF THE MARTIN BRIDGE LIMESTONE, WALLOWA MOUNTAINS: AN ELECTRON-BACKSCATTERED DIFFRACTION STUDY
Calcite grain fabrics were largely annealed during metamorphism. Calcite grains are somewhat elongated at a low-angle to both foliation and lineation. Grain boundaries tend to be straight to gently curved with 120 degree triple junctions common. Average grain size in the marbles varies from ~25-400 µm and correlates inversely with the volume fraction of second-phase particles (minerals other than calcite) indicating static grain growth of calcite during metamorphism. Brittle motion along the younger high-angle faults produced very fine-grained (<25 µm) calcite-rich cataclasites. The intensity, thickness and morphology of calcite twins is highly variable and suggesting that the twins formed during this younger low-temperature deformation. Lattice preferred orientations (LPO) were determined by EBSD for 12 marble samples displaying a range of grain size and metamorphic grade. The intensity of the LPO varies from nearly random (c-axis maximum <2 times uniform) to strong (c-axis maximum of >8 times uniform). With increasing intensity of the LPO the c-axis pattern changes from nearly random, to a c-axis girdle ~normal to lineation to a well-defined c-axis maximum ~normal to foliation. Both the c-axis girdle and maxima are slightly inclined to foliation suggesting that the LPO was formed by simple shear. The LPO's are consistent with deformation by a combination of e and r slip at moderate shear strain (γ = 2-5).