Investigation of Tectonically Rotation of Gabbroic Blocks along the Sahneh Fault Based on the Analysis of Palaeomagnetic Data, Western Iran

Palaeomagnetic analysis has been applied worldwide on active faults for decades. The palaeomagnetic investigation on the Sahneh Fault, at middle part of the Zagros Main Recent fault, is the main objective of this research. The length of the Sahneh fault, which is about 100 km in study area, cuts the gabbroic blocks exposed on the both sides with NW-SE trend and connects the Morvarid fault in the NW to the Nahavand fault in the SE of the study area. Tectonically, the mechanism of the Sahneh fault is high angle reverse with dextral strike slip component, and is compatible with the earthquakes focal mechanism solution, movement of the Arabian plate towards the Central Iran and the results of palaeomagnetic data. The paleomagnetic analysis results are based on the drilled oriented samples of 17 selected sites along and on both sides of the Sahneh fault. Nine to eighteen oriented samples were collected from each site. The conducted paleomagnetic analysis includes measurement of NRM, magnetic mineralogy (high temperature), and thermal/ AF demagnetizing. The declination, inclination and ChRM directions of each site separated from the overprint directions by means of the thermal demagnetization method. The mean direction of ChRM and VGP for each site is determined using statistics and palaeomagnetic analysis. For 7 sites the ChRM mean direction is calculated. The obtained mean ChRM directions then compared with the reference palaeomagnetic pole position of the ophioliths of Central Iran, and the sense of rotation were determined for all sites. The dextral strike-slip Movement of the Morvarid and Nahavand faults imposed a compressional and shear stress components on the Sahneh fault, resulted in the formation of transpressional stress regime in the study area. Under this tectonic stress regime, the Riedel of shear systematic fractures may be helpful to interpret the palaeomagnetic data. According to this model, the whole synthetic shear fractures (P, R, D) caused clockwise rotation and the antithetic shear fractures (Ŕ) caused counterclockwise rotation in gabbroic blocks.