NEOTECTONICS OF ZANJAN (CENTRAL IRAN): A LEFT LATERAL STRIKE-SLIP SYSTEM AND RELATED STEPOVER STRUCTURES

Escape tectonics developed in the collision zones and strike-slip faults are the main tectonic elements. Arabia- Eurasia continental convergence is continuing in Iran according to seismic and GPS data. This N-S convergence cause right lateral (Tebriz, Zagros) and left lateral (Dashte Beyaz, Elburz) strike-slip systems. Zanjan (Zengan) is located between Tehran and Tebriz and its neotectonic structures are not well determined. In this area, Sultaniye, Geydar and Tarim mountains are located as NW-SE trending highlands parallel to the Zagros and Elburz mountains where the pre-Neogene rocks come to contact with Quaternary deposits occupying the lowlands. Our morpho-tectonic studies indicate that active E-W trending left lateral strike-slip faults control the morphology of the Zanjan area. They are evaluated as western continuation of main left lateral Mosha-Shahrud fault that is the main structure of Elburz mountain. The left and right stepping of the left lateral strike-slip faults creates pull-apart basins and push-ups. Kazvin basin is an example of pull-apart basin and Sultaniye is located on the push-up region where the rivers flow opposite directions towards NW and SE. In the restraining bends single or double vergent thrust faults trending NW-SE is developed and deformed Quaternary sediments. This explains NW-SE trending highlands in the region. Geological record of this uplift is lying in the Upper Miocene-Quaternary alluvial and fluvial sedimentary deposits around Sultaniye. Lower part of coarse clastic sedimentary succession is composed of fragments of Eocene units whereas that of upper part contains metamorphic rock fragments that its original outcrops are located in the core of Sultaniye mountain. The uplift of the NW-SE trending highlands and related strike slip system is operational since Late Miocene. Maximum displacement of the geological units in the area is 25 km. If GPS data of 4mm/year is considered, it is clear that most of the deformation must be absorbed in the sidestep and restraining bends.