Finite strain analysis in the andalusite schists, south eastern Hameden

Strain analysis in the deformed area has been one of the interesting subjects for structural geologist in the last decades. Strain studies based on field data help geologists to understand the parameters such as quantities of amounts of strain in the principal plane of strain ellipsoid, orientation of principal axes of strain ellipsoid, strain ellipsoid shape and the strain type. In this research the strain analysis of andalusite schist has been conducted in the south eastern part of Hamedan.

In this research because of evolution of penetrative planar and linear structures in the andalusite schist rocks of south eastern part of Hamedan, these rocks have been studied as strain markers for strain analysis goals. The orientations of principal axes of strain ellipsoid and geometry of surface of non-finite longitudinal strains have been studied with application of stereography analysis and using structural elements. The geometry of surface of non-finite longitudinal strains was used as criterion for determination of amounts of finite strain in the principal plane of strain ellipsoid. Also with application of some graphical functions and the strain ratio in the principal plane of strain ellipsoid the amounts of kinematic vorticity number and the percentage of each pure shear and simple shear component were determined.

Micro-tectonics and petrofabric studies show the development of three planar and two linear structural elements in the study area. S-C shear bands, delta and sigma types porphyroclasts, strain shadow and hat shadow are the most important structures that developed in the study area. These studies reveal the occurrence of two main deformation phases in the area. Using structural elements and application of stereography analysis of the planar and linear structures in the SpheriStat 2.2 software, the average pole of foliations and the lineation directions were determined as spatial locations of Z and X axis of strain ellipsoid. Therefore the direction of principal axes of strain ellipsoid in the first deformation phase were determined as X= 184/16, Y= 88/3, Z= 350/76 and the direction of principal axes of strain ellipsoid in the second deformation phase were determined as X=195/20, Y=345/59, Z= 95/12 respectively. Using of angular amounts of surface of non-finite longitudinal strain respecting the orientation of principal axes of strain ellipsoid and application of nomogram (Imber et al, 2012) of the amounts of finite strain in the XY and XZ principal planes of strain ellipsoid for both deformation phases were determined. The amounts of kinematic vorticity number for the first and second phase of deformation were calculated as 0.91 and 0.93 respectively. These results show the simple shear dominant deformation in both deformation phases.

Results of strain analysis show the different strain ellipsoid shapes and orientation of principal axes of strain ellipsoid for each of deformation phases. Different amounts of strain parameters reveal the different effect of strain components in D1 and D2 deformation phases. Results of this research show the existence of inclined transpression in the Sanandaj-Sirjan zone as offered by previous geologist researchers. Probably the intrusion of Alvand batholiths causes local changes in the stress and strain properties and the high amounts of kinematic vorticity numbers causes simple shear dominant deformation.