CRYSTALLOGRAPHIC FABRICS OF THE MARTIN BRIDGE LIMESTONE, WALLOWA MOUNTAINS: AN ELECTRON-BACKSCATTERED DIFFRACTION STUDY

During amalgamation of the Blue Mountain terrane, supracrustal rocks of the Wallowa terrane in the eastern Wallowa mountains were deformed by west-directed thrust faults and open to isoclinal folds. Limestone of the Martin Bridge formation was penetratively deformed producing banded, calcite-rich mylonites with a well-defined east-dipping foliation and southeast plunging lineation. Subsequent to this penetrative deformation, the Wallowa terrane was intruded by granitoids of the Wallowa batholith metamorphosing the Martin Bridge limestone. Depending on the distance from the batholith, the metamorphic grade of Martin Bridge marbles varies from talc to wollastonite zone. Locally, lithological contacts and metamorphic isograds are offset along younger (Miocene?) high-angle faults.

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).