POLYPHASE INVERSION TECTONICS IN WESTERN ALBORZ

Recent GPS and geological studies indicate clockwise rotations of northern Iran including north Central Iran,Alborz Mountains as well as South Caspian Block. Here, these data are used to suggest a new tectonic model to explain observed structural features and active deformation in the western Alborz Mountains. Previously proposed modelssuggest that NW component of motion resulted from the clockwise rotation of the South Caspian block can merely cause left lateral motion on NW trending faults in western Alborz. Here, I propose a model in which only if several basement blocks including western Alborz as well as Central Iran and South Caspian Block rotate clockwise with respect to Eurasia the left lateral motion along range-parallel basement faults can occur. This model uses geological data to suggest that the western Alborz Mountains developed as several east-west trending pull-apart basins associated with left lateral motions of the basement faults at least since the Late Triassic time. The inversion of strike slip faults from left lateral to right lateral (an early inversion phase in Neogene), however, could be due to a halt in block rotations. In other words, when the blocks stopped rotating the faults bounding them might be affected by the northward motion of central Iran toward South Caspian Block. This N-S compression would presumably cause dextral motion on NW trending basement faults in western Alborz. The resulting dextral motion on NW strike slip faults, in turn, caused inversion of pre-existing normal faults bounding the pull-apart basins as reverse faults. As rotation of the Central Iran-western Alborz-South Caspian blocks resumed in the Quaternary times, NW trending basement faults regained their left-lateral motions and the associated east-west basement faults inverted as normal faults. However, the reverse faults in the cover sediments being affected by compressional stress between Central Iran and South Caspian Block, remained reverse.

Keywords:

Inversion tectonics , Alborz , Block rotation , Seismicity , Iran