Finite strain analysis in the Seydan anticline using ammonoid spiral shells, Zagros, Iran

• Quantitative strain analysis was made with application of spiral shape fossils.
• Result show variation of shear strain along the Sivand thrust fault.
• Result show relationship between strain magnitude and folding style.
• Creep mechanism probably caused various shear strain along the Sivand thrust.

Fossils with a spiral shape can be used as strain markers in quantitative strain analysis. Several methods have been suggested for strain analyses that exploit the logarithmic spiral laws governing development and growth of ammonoids. In this study, ammonoid spiral shells were measured to estimate strain variations along the northwestern limb of the Seydan anticline in the Zagros Fold-and-Thrust Belt, Iran. Results show an increase in strain from SE to NW along the Seydan anticline. These results, combined with other structural evidence, reveal a logical relationship between the intensity of folding and increasing strain. Strain variations are related to different amounts of slip along the Sivand thrust fault, which played a significant role in the development of the Seydan anticline as a fault-propagation fold.