STRAIN LOCALIZATION PROCESSES IN EXPERIMENTALLY AND NATURALLY DEFORMED DOLOMITE

Strain localization is common in dolomites over a wide range of conditions. The development of fine-grained shear zones with a lattice preferred orientation (LPO) suggests these grains deform by dislocation creep. However, experimentally deformed fine-grained dolomites have stress exponents that are consistent with diffusion creep. Microstructural evidence from experimentally and naturally deformed dolomite suggests that fine-grained shear zones develop through a combination of dislocation and diffusion processes.

We analyzed two experimentally deformed dolomite samples (T = 900 °C, Pc = ~1150 MPa, bulk strain rate of 10-4 s-1). The first is a coarse-grained dolomite (240 μm) deformed to an axial strain of 41%. Serrated grain boundaries in the coarse-grained dolomite suggest recrystallization-accommodated dislocation creep as the primary mechanism for grain size reduction, which resulted in a localized zone of fine-grained dolomite (~1.6 μm). The second sample contains a fine-grained dolomite (~2.5 μm) within a precut shear zone placed between two coarse-grained (240 μm) dolomite shear pistons and deformed to a shear strain of 10. Both experimentally deformed shear zones contain fine-grains (~2 μm), aligned grain boundaries, and four-grain junctions, consistent with diffusion creep. However, a weak c-axis LPO in these samples suggests a combination of diffusion and dislocation creep.

We also examined two naturally deformed shear zones: Town Knobs thrust (100-240 °C) and Pioneer Landing thrust (250-350 °C), both southern Appalachians, USA. Coarse-grained (35-45 µm) host rock contains twins, lobate grain boundaries, subgrains, and fractures, suggesting brittle and dislocation creep processes contributed to grain size reduction (~3 µm) and strain localization. Fine-grained dolomite along the Town Knobs thrust form a stronger c-axis maxima, compared to the experimentally formed LPO, whereas fine-grained dolomite along the Pioneer Landing thrust form only a weak c-axis LPO. The development of a LPO in these low-moderate temperature dolomites is the result of dislocation creep; however, the decrease in strength of LPO with increasing temperature in the natural samples suggests diffusion processes may also accommodate strain as temperatures increase.