Structural, Morphological, and Seismic Evidence of Abbar Region Active Tectonics in Western Alborz, NW Iran

Abstract

In this research, structures, geometries and directions of dominant stresses in the Abbar region (northwestern Iran) were examined. The fractures and lineaments of the area adequately matches to the brittle left-lateral shear zone model, and the two main fault systems of the area correlate with the P and R2 orders in the shear zones, respectively, where shear occurs along an approximate S60°E direction. Prior to the Quaternary, a compressive tectonic regime has governed the Abbar region along the NW-SE direction. Since the early Quaternary, this regime has changed to left-lateral shear along the WSW-ENE maximum stress direction. Thus, the reverse faults of the old regime have converted to strike-slip faults with reverse components. The geomorphological evidences such as fault scarps, Quaternary displacements and stream shifts indicate a high tectonic activity in the region. The major faults of the north of Abbar area can be divided into two groups with approximate directions of S50°E and N37°E. The Neotectonic activities of the region are in accordance with the left-lateral movement of the Manjil fault (as an important seismic fault in the western Alborz range) which has caused the formation of brittle structures and severe topographical changes.