Abstract The present Zagros mountain belt of SW Iran is known to be the former NE Arabian passive continental margin of the southern Neo-Tethyan basin, which originated by Permian–Triassic rifting, and has a late Cenozoic collisional imbricate structure. We carried out brittle tectonic analyses of syndepositional normal fault slip data in the High Zagros Belt of the Fars Province to reconstruct the extensional deformation of the passive margin during the Mesozoic era in terms of stress tensor inversion. This reconstruction revealed two main directions of extension, developing from a north–south margin-oblique trend to a NE–SW margin-perpendicular one. Considering the basement structures and the existence of the basal Infracambrian salt detachment, we infer that a transtensional extension could have initiated two major periods of crustal stretching: a Permian–Triassic thick-skinned phase with the basement faults developing in an oblique rifting, and a Mesozoic thin-skinned phase with the sedimentary cover being affected by successive extensional structures and block tilting. This extensional tectonic history probably continued during the early Tertiary period, prior to the continental collision. Fault slip geometries and structural patterns of both the Mesozoic extension and the late Cenozoic compression indicate inversion of the inherited structures in the Zagros collision during the subsequent thin- and thick-skinned stages of crustal shortening.

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Study of more than 1,000 strike-slip and dip-slip faults in Miocene rocks along a 10-km segment of Rainbow Canyon reveals a well-constrained paleoextension direction of 235°, representing a main stage of syndepositional extensional deformation. This direction is consistent with northeast-southwest extension computed or estimated over a large part of the Basin and Range for main-phase extension. It can be computed from separated or combined subsets of strike-slip and dip-slip faults and is independent of fault size or geographic subarea. The deformation is characterized by synfaulting deposition of volcanic and sedimentary sequences that thicken toward predominantly northwest-striking block-boundary growth faults resulting in fan-shaped patterns in northeast-southwest cross section. Concave-upward faults are common, and fault-to-bedding angles in vertical sections containing the extension direction average about 90°. The coefficient of extension at stratigraphically median levels is 1.9 (90 percent), 10 to 20 percent of which is associated with block-interior displacements on sub-map-scale faults. Though the deformation is primarily synvolcanic, it is interpreted to be more closely associated with regional extension and low-angle normal faulting than with volcano-tectonic processes. The study provides evidence for a young stage of west-northwest extensional deformation that primarily utilized existing faults in combined strike-slip, oblique-slip, and dip-slip modes and accounted for less than 5 percent of the total observed deformation. Extension directions are less well constrained than for the early deformation owing to smaller sample sizes. As with the early deformation, those computed from subsets of strike-slip and dip-slip faults are similar to one another (average 290°). The young deformation occurred following a 55° clockwise rotation of σ 3 sometime in the last 10 m.y. The mixture of dip-slip and strike-slip faulting during each of the deformations is interpreted as resulting from vertical and horizontal constriction normal to the extension direction, rather than from alternations of paleostress conditions. An estimated 10 to 30 percent of the total brittle strain is associated with the strike-slip faulting. This study provides strong verification that our understanding of deformation intensity is highly dependent on the scale of investigation, depth of exposure, and knowledge of slip-sense characteristics.