Polyphase Inversion Tectonics in Western Alborz Mountains, Northern Iran

Fault kinematics in western Alborz mountains is complicated by rangeparallel left lateral strike slip faults as well as a series of longitudinal zones of thrusts that are considered to be due to the inversion of preexisting right lateral and normal faults, respectively. Previously proposed models suggested that the NW component of the motion resulted from the clockwise rotation of the South Caspian block is responsible for left lateral motion on the NW trending faults in 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 rangeparallel 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 the observed structural features and the active deformation in western Alborz mountains. According to this model the western Alborz mountains initiated as several eastwest trending extensional (pullapart) basins associated with left lateral motions on the largescale 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, they might be affected by the convergence between central Iran and the South Caspian block. This NS compression would presumably cause dextral motion on the NW trending strike slip basement faults in western Alborz. The resulting dextral motion on the NW strike slip faults, in turn, caused preexisting normal faults bounding the pullapart basins to turn into the reverse faults. As rotation of the central Iranwestern AlborzSouth Caspian blocks resumed first in Eocene and then in the Quaternary times, the NW trending basement faults regained their left lateral motions and the associated eastwest basement reverse faults turned into the normal faults. However, geodetically observed shortening across the western Alborz mountains indicates that the eastwest 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 Iranwestern AlborzSouth Caspian blocks.