Cheskin blind thrust and its probable effect on the missing millennium (Qazvin plain)



The archaeological investigations of three prehistoric tells (Zagheh, Qabrestan, and Sagzabad) located at a distance of ~2 km from each other in the Qazvin plain (NW Iran) indicates that Tepe Zagheh was occupied in 6th and 5th millennia B.C. But due to unknown reasons life at this site ceased and started in Tepe Qabrestan at the beginning of 4th millennium B.C. This tell was occupied for a millennium, and the abundance of pottery workrooms shows that this site was probably an industrial center with an area of ~10 hectares at the time. However, again due to unknown reasons, Qabrestan was deserted, and for ~1000 years (The Missing Millennium, 3000-2100 B.C) there was no sign of habitation around it until the end of the 3rd millennium B.C when this time Iron Age people settled in Tepe Sagzabad.
Among many possibilities from natural reasons such as climate change to human activities like war which could cause the abandonment of Qabrestan, the occurrence of an earthquake is also likely. According to past seismicity and the seismotectonics of the Qazvin region, the occurrence of an earthquake during the Missing Millennium is possible.
One of the active faults in the Qazvin region, whose movement could have led to the destruction of Tepe Qabrestan is the Cheskin Blind Thrust. This fault which was previously unknown has a length of at least 12 km and is capable of generating earthquakes with magnitudes of 6. Because of this and the importance of blind thrusts in general, we decided to determine the activity of this fault. We took a sample from the top of a fold from a ridge which was uplifted due to the activity of the Chekin Blind Thrust beneath. The age of this sample was determined by applying the OSL dating method. Using this age, we estimated an uplift rate of 1.9 ± 0.5 mm/yr. Considering a dip angle of 30°, we also estimated a convergence rate of 2.4-4.2 mm/yr and a slip rate of 2.8-4.8 mm/yr for this blind thrust. Using the Scholz’ relationship (1982) which relates the length of the fault to its average displacement per event, the average displacement on this fault is ~0.6 m. Division of this average displacement per event by the estimated slip rate leads to a return period of 125-214 years for the earthquakes on the Cheskin Blind Thrust.