Lattice-preferred orientation development in experimental and natural fine-grained dolomite shear zones

Abstract

We compare microstructures and lattice preferred orientations (LPO) in shear zones formed in two experimentally deformed and two naturally deformed dolomites. Experimental samples were deformed at 900 °C and laboratory strain rates, and naturally deformed dolomites were collected from the Pioneer Landing thrust and Town Knobs thrust of the Southern Appalachians, deformed at ~300 °C and ~240 °C, respectively, at tectonic strain rates. Coarse-grained host dolomite, in all samples, exhibits undulatory extinction, subgrains, recrystallized grains, and, in the naturally deformed dolomites, fractures. Two types of fine-grained shear zones are characterized in experimentally deformed dolomites: 1) a fine-grained zone of recrystallized dolomite generated during axial shortening of coarse-grained dolomite that initially deformed by dislocation creep, with strain localization initiated by grain-size sensitive deformation processes and 2) a synthetic, fine-grained dolomite layer placed between coarse-grained dolomite shear pistons. Microstructures and lattice preferred orientations of the experimentally sheared dolomite reflect the concomitant operation of dislocation creep, diffusion creep, and grain boundary sliding. Microstructures, including subgrains, recrystallized grains, and four-grain junctions, and lattice preferred orientations that consist of weak c-axis maxima in the natural shear zones are similar to the experimental shear zones, however, a slightly stronger LPO, more common subgrains and the lack of four-grain junctions indicate that dislocation creep dominates within dolomites deformed at lower temperatures (240 °C).