Uplift-induced residual strain release and late-thrusting extension in the Anaran mountain front anticline, Zagros (Iran)

• The Anaran anticline is located at the mountain front of the Zagros thrust belt.
• Major syn-folding extensional faults affect the anticline.
• Syn-folding extension was induced by the release of residual compressive strain.

In this work we present and discuss the pattern of extensional faults of the Anaran Anticline (Lurestan province of the Zagros, Iran), one of the most aesthetically appealing mountain front anticlines of the Zaros. This 100 km long structure is affected by extensional faults occurring in different along-strike sectors of the anticline, running parallel and oblique to the local trend of the fold hinge line, and having displacements of hundreds of metres. Relationships between fold growth and extensional deformation are investigated by quantitatively analysing the attributes (i.e. curvature, elevation and throw across faults) of a 3D surface representing the top of the Ilam formation. Results indicate that the main stage of extensional deformation occurred during folding. In particular, we deduce that extensional faulting was associated with the erosional uplift of the crestal sector of the anticline, where exhumation of previously shortened strata implied release of residual compressive strain, with consequent annihilation of the horizontal stress component paralleling the tectonic transport direction. This determined the onset of a large-scale extensional regime at the shallower structural levels of the growing anticline.