HR: 1340h
AN: MR33A-0150    [Abstracts]
TI: Deformation and Recrystallization Mechanisms in Mylonitic Zones in Naturally Deformed Extrusive Salts From Garmsar Hills and Qum Kuh (Central Iran)
AU: * Schleder, Z
EM: z.schleder@ged.rwth-aachen.de
AF: RWTH Aachen University, Lochnerstr. 4-20, Aachen, 52056 Germany
AU: Urai, J L
EM: j.urai@ged.rwth-aachen.de
AF: RWTH Aachen University, Lochnerstr. 4-20, Aachen, 52056 Germany
AB: Microstructural processes in mylonitic zones from extrusions of Eocene-Oligocene rock salts from Garmsar hills and Qum Kuh (central Iran) are described and analyzed. The halite samples were studied with techniques of transmitted light microscopy of gamma-irradiated thin sections, subgrain size palaeopiezometry of polished and chemically etched samples and texture measurements by EBSD. Although in both locations the salt is strongly deformed by sets of recumbent folds, the bedding is subhorizontal. Typically, the mylonitic zones are thin, less than 5 cm thick, and develop parallel to the bedding. Macroscopically the samples are similar in both locations studied: the samples consist predominantly of halite with a few percent of insoluble material. The porphyroclasts are 5-10 mm large, and show slight shape preferred orientation. The mylonitized material is fine-grained (200-500 micron) and shows strong shape preferred orientation. Interestingly, the process, which lead to the grain size reduction and thus to weakening of the material, is different in the two locations studied. One process that leads to grain size reduction is grain dissection by grain boundary migration (Type I) the other is subgrain rotation/new grain nucleation (Type II). The samples from Garmsar hills show microstructural evidence for type I grain size reduction. In this type of salt the porphyroclasts have subgrains with a size of about 100-150 micron. There is no microstructural evidence for new grain nucleation or subgrain rotation, although the elongated subgrains, growth bands (seen in gamma- irradiated sections) points to extensive grain boundary migration. It is inferred that in the mylonitic zone, as the moving grain boundaries intersect grains, the grain size decreases. Samples from Qum Kuh show type II microstructures. The porphyroclasts in the protomylonite have a core and mantle structure, the core is subgrain poor, while the rim is rich in very small subgrains (size about 50 micron). The porphyroclasts are rimmed with small, substructure-free grains (size about 100 micron), which were evolved either by subgrain rotation, or by new grain nucleation. These newly created small grains grow until the porphyroclasts are completely eliminated, thus the grain size decreases. Once the grain size decreased, the main deformation mechanism switches to grain boundary sliding and solution- precipitation creep as indicated by the presence of strain free grains, by fibrous overgrowth structures seen in the gamma-irradiated sections and by the lack of CPO as indicated by the orientation measurements. This deformation mechanism allows the extrusive salt to flow downhill under minor stresses in a geologically extremely fast strain rate of 10^-11 to 10^-9 1/s.
DE: 5104 Fracture and flow
DE: 5112 Microstructure
DE: 5120 Plasticity, diffusion, and creep
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005