Collapse structures in Dowgonbadan region, Zagros fold- thrust belt

Allochthonous masses are common structures in Zagros fold-thrust belt. They are generally considered as collapse structures formed by the influence of gravity and in rock units with competency contrast. However, large allochthonous masses mapped in Dowgonbadan area in Dezful Embayment near the Mountain Front Fault (MFF) of Zagros show characteristics different from the belt common collapse structures. In this paper, the influences of both gravity and thrusting on development of these masses are presented. Evidences such as the volume of the masses, the greater spacing between the masses and their origin, and the occurrence of crushed zone on the base of the masses are considered as criteria to separate these masses from common collapse structures and are classified as thrust related collapse structures. Thus, thrust faults as well as gravity are proposed as the main features controling the development of these types of collapse structures in the Zagros.