THE SIGNIFICANCE OF STRIKE‐SLIP FAULTING IN THE BASEMENT OF THE ZAGROS FOLD AND THRUST BELT

Abstract

Lateral offsets in the pattern of seismicity along the Zagros fold and thrust belt indicate that transverse faults segmenting the Arabian basement are active deep‐seated strike‐slip faults. The dominant NW‐SE trending features of the belt have undergone repeated horizontal displacements along these transverse faults. These reactivated basement faults, which are inherited from the Pan‐African construction phase, controlled both deposition of the Phanerozoic cover before Tertiary‐Recent deformation of the Zagros and probably the entrapment of hydrocarbons on the NE margin of Arabia and in the Zagros area. We have used observations of faulting recognized on Landsat satellite images, in conjunction with the spatial distribution of earthquakes and their focal mechanism solutions, to infer a tectonic model for the Zagros basement.Deformation in the NW Zagros appears to be concentrated on basement thrusts and a few widely‐spaced north‐south trending strike‐slip faults which separate major structural segments. In the SE Zagros, two main structural domains can be distinguished. A domain of NNW‐trending right‐lateral faults in the northern part of the SE Zagros implies that fault‐bounded blocks are likely to have rotated anticlockwise about vertical axes relative to both Arabia and Central Iran. In contrast, the predominance of NNE‐trending left‐lateral faults in the southern part of the SE Zagros implies that fault‐bounded blocks may have rotated clockwise about vertical axes. We propose a tectonic model in which crustal blocks bounded by strike‐slip faults in a zone of simple shear rotate about vertical axes relative to both Arabia and Central Iran. The presence of domains of strike‐slip and thrust faulting in the Zagros basement suggest that some of the convergence between Arabia and Central Iran is accommodated by rotation and possible lateral movement of crust along the belt by strike‐slip faults, as well as by obvious crustal shortening and thickening along thrust faults.