Polyphase Inversion Tectonics in Western Alborz Mountains, Northern Iran

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Fault kinematics in the western Alborz mountains is complicated by range-parallel left lateral strike slip faults as well as a series of longitudinal zones of thrusts that are considered to be due to inversion of pre-existing right lateral and normal faults, respectively. Previously proposed models suggested that NW component of motion resulted from the clockwise rotation of the South Caspian block is responsible for left lateral motion on NW trending faults in the western Alborz. However, a more recently proposed model suggests that only if several basement blocks rotate clockwise about vertical axes the left lateral motion along range-parallel basement faults can occur. Recent GPS studies indicate that northern Iran including north central Iran, Alborz mountains as well as South Caspian block are rotating clockwise with respect to Eurasia. Here, I invoke the previously published evidence to suggest a new tectonic model to explain a sequence of invert deformation episodes as well as observed structural features and active deformation in the western Alborz mountains. According to this model the western Alborz mountains initiated as several east-west trending extensional (pull-apart) basins associated with left lateral motions on large-scale strike slip basement faults and intervening normal faulting at least since the Late Triassic time. The inversion of left lateral strike slip faults in Late Cretaceous and Neogene times, however, could be due to a halt in block rotations. In other words, when the blocks stopped rotating the NW trending faults bounding them might be affected by the convergence between central Iran and the South Caspian block. This N-S compression would presumably cause dextral motion on NW trending strike slip basement faults in western Alborz. The resulting dextral motion on NW strike slip faults, in turn, caused pre-existing normal faults bounding the pull-apart basins to turn into reverse faults. As rotation of the central Iran-western Alborz-South Caspian blocks resumed first in Eocene and then in the Quaternary times, NW trending basement faults regained their left lateral motions and the associated east-west basement reverse faults turned into normal faults. However, geodetically observed shortening across the western Alborz mountains indicates that the east-west reverse faults within the cover unit have remained reverse. This contradiction may be explained by simultaneous shortening of the cover unit resulted from convergence between central Iran and South Caspian block, and extension of the basement due to clockwise rotation of the central Iran-western Alborz-South Caspian blocks.Keywords: Inversion tectonics, Alborz, Block rotation, Seismicity, Iran