Active tectonics along the Khazar fault (Alborz, Iran)

Abstract

The Alborz Mountain accommodates some of the convergence between Central Iran and Eurasia, is characterized by active range-parallel fold and thrust structures. At present, the kinematics of the range involves a strain partitioning mechanism and clockwise rotation of the South Caspian Basin. Range-parallel, left-lateral strikeslip faulting dominates the central part of the mountain range, while reverse faulting affects its northern and southern borders. Several slip-rate studies have been carried out along active faults in the internal and southern parts of the range. However, the characteristics of the main northern bounding fault (the Khazar [Persian: Caspian] Fault) remain poorly known. Our analysis provides new constraints on the activity of this fault. We first show that the fault generally is a hidden thrust fault, often associated with fault-bend and fault-propagation folds (forebergs). In the central part of the fault, the radiocarbon dating of an uplifted terrace allows estimating minimum vertical and average horizontal slip rates of 2.0 ± 0.5 mm/yr and 3 mm/yr respectively. hence, minimum slip rate along the fault reach to 3.6 mm/yr. About 150 km further east, near the city of Behshahr, within the archeological site of Gohar-Tappe, a paleoseismological trench study on a young detachment fold suggests that at least 5 events occurred in the past 5,300 years, 3 of them with surface-rupturing between 5300- and 3900-years cal BP, our results confirm that the Khazar Fault is a major active structure in northern Iran, and represents a significant seismic hazard for the entire Central Alborz region.