Transpression and Exhumation of Granitoid Plutons along the Northern Part of the Nehbandan Fault System in the Sistan Suture Zone, Eastern Iran

摘要

Geometry and kinematics of deformation across the Nehbandan Fault System (NFS) have been studied in the Bibimaryam and Damdameh areas in the Sistan Suture zone (SSZ). Nearly vertical fault planes with sub-horizontal lineations and correlation of these faults analysis with anisotropy of magnetic susceptibility (AMS) indicates a predominantly transpressive regime including right-lateral strike-slip faults with reverse components in the region. An assemblage of en-echelon and doubly plunging folds with slip indicators, such as duplexes in their limbs, confirms progressive transpressive deformation. Accompanying hydrothermal alteration has created ideal conditions for reactivating shear fractures during late Cenozoic times that record the last phase of tectonic activity. Intrusive bodies such as the Bibimaryam and Damdameh granitoid plutons are exposed along the NW-SE faults that branch off the Nehbandan Fault System (NFS). Fault analysis there shows three directions for compression (sigma(1)): N085 degrees, N074 degrees, and N024 degrees from the Late Cretaceous to the Plio-Quaternary respectively, on the borders of the NFS. Our results are consistent with recent paleostress studies, which revealed counterclockwise rotation of sigma(1) during the Late Cenozoic in eastern Iran. Redistribution of sigma(1) directions along the NFS in the context of general N-S striking indicates a decreasing pure shear component, and consequently an increasing simple shear component from the Late Cretaceous to Plio-Quaternary. This reconfiguration of sigma(1) likely triggered the exhumation of the Tertiary Bibimaryam and Damdameh granitoid plutons along the fault zones. Field evidence and lithofacies indicate that exhumation of the intrusive bodies cannot have preceded the late Eocene. Hence, the late Cenozoic uplift and exhumation of the Sistan Belt may largely have resulted from the development of a dextral transpressive regime along the NFS in the SSZ.