Finite Strain analysis using Gachsaran formation porphyroclasts, case study in Ramhormoz Anticline

In this research with application of petrofabric studies and finite strain analysis quantitative changes of strain have been evaluated in the Gachsaran rock units in the hanging wall of Ramhormoz thrust fault. Thirteen samples were selected for strain analysis in the normal section to the Fault strike along natural trench in the study area. Porphyroclasts have used as strain markers for future strain studies. The results of petrofabric studies reveal a dextral component of shear in the study area. Results of strain analysis show relative relationship between amounts of strain and distance from the fault. Three dimensional analysis of strain ellipsoid shows three axial oblate ellipsoid shape for strain ellipsoid. Kinematic vorticity analysis in the XZ principal plane of strain ellipsoid shows 0.39<Wk<0.85 and operation of both simple and pure shear components of shear strain and occurrence of strain partitioning in the study area. Percentage amounts of simple and pure shear components of strain reveal general shear with pure shear dominated component of strain.               

 In this research chalk porphyroclasts of Gachsaran Formation have been used as strain markers. Gachsaran Formation in the hanging wall of Ramhormoz fault has conserved the deformation characteristics of shear movements during the deformation history. Shear sense indicators and other strain markers such as porphyroclasts have been evaluated in this formation. These porphyroclasts have made a good fortune for quantitative strain studies in this area. In this research the strain ratios of porphyroclasts were calculated in the XZ principal plane of strain ellipsoid. With application of EllipsFit software the amounts of tectonic strain and long axis orientation of strain ellipsoid were determined using Rf/Ø and polar methods. With using the amounts of tectonic strain and orientation of the long axis of strain ellipsoid and application of (K-γ-θ-Rxz) nomogran (Fossen, 2010) the amounts of shear strain and strain ellipsoid shape factor were determined. Also with application of (Rxz-θ) nomogram (Bailey et al., 1999) the amounts of kinematic vorticity number were calculated for understanding the nature of deformation and percents amounts of pure and simple shear.                  

 The study area has been located in the Rmhormoz Anticline in the 75km NW of Ahvaz city. Structural studies show development two main joint sets that both of them are approximately parallel to the Ramhormoa main thrust fault. According to the petrofabric studies all shear sense indicators such as secondary antitaxial veins, σ type and δ type structures reveal a dextral sense of shear in the area. Using Rf/Ø and polar methods strain analysis have been made in thirteen locations of anticline based on shape factor of chalk porphyroclast. The amounts of strain tectonic in the XZ principal plane of strain ellipsoid were calculated. Also orientations of long axis of strain ellipsoid were determined in each sample. For this aim the horizontal line has used as reference line. Results show relative relationship between amounts of strain and distance from the main thrust fault. With application of (K-γ-θ-Rxz) nomogran (Fossen, 2010) the amounts of shear strain and strain ellipsoid shape factor (K) were determined. Results show 1.3<K<1.7. Therefore according to the amounts of K factor, the general shape of strain ellipsoid is three axial prolate. Kinematic vorticity (Wk) is a dimensionless measure of rotation relative to strain and characterizes the amount of shortening proportional to displacement. Wk was originally defined as an instantaneous rotation relative to the instantaneous stretching at a point (Means et al. 1980). Kinematic vorticity is measured on a scale between 0 (pure shear) and 1 (simple shear). General shear is the term used for flows between pure and simple shear (1>Wk>0) (Passchier and Trouw 2005). Pure and simple shearing components contribute equally to the instantaneous flow at Wk=0.71 (Law et al. 2004). There are several methods for determination of the kinematic vorticity number. In this research kinematic vorticity analysis was made based on Rxz-θ nomogram (Bailey et al., 1999). Rxz and θ are strain ratio in the XZ principal plane of strain ellipsoid and the angle between long axes of the strain ellipsoid related to the reference line, respectively. Results show 0.39<Wk<0.85 and operation of both component of pure shear and simple shear in the area.

 Strucrural and petrofabric studies reveal operation of both ductile and brittle deformation in the study area. According to the study of shear sense indicators, results show a main dextral sense of shear in the area. Results of strain analysis show greater amounts of Rs near the main thrust and lower amount Rs far from the main thrust fault. 3d shape of strain ellipsoid is mainly three axial prolate shape. Results of kinematic vorticity analysis show the occurrence of general shear with operation of both pure and simple shear component of deformation. The amounts of kinematic vorticity number reveals strain partitioning in the area. This strain partitioning mainly is due to oblique convergence between Iranian and Arabian plates.