The Triassic and associated rocks of the Nakhlak and Aghdarband areas in central and northeastern Iran as remnants of the southern Turanian active continental margin

The Triassic succession of the Nakhlak area in central Iran consists of (1) the Alam Formation, which is a sequence of shallowing- and coarsening-upward marine turbidites deposited on the forearc side of an accretionary prism; (2) the Baqoroq Formation, a sequence of coarse to fine, polymictic, fluvial conglomerates; and (3) the Ashin Formation, which comprises alternating, distal marine shales and sandstones that have turbiditic characteristics. These rocks are not lithologically similar to time-equivalent lithostratigraphic units of Late Permian-Triassic age of the Aghdarband area of northeastern Iran (which are interpreted to be forearc deposits), but they may have formed in close association with them in a single tectonic and sedimentary framework. Accepting the 135° counterclockwise rotation of the central-east Iranian microcontinent with respect to the Turan plate since Triassic time, and assuming that the Triassic rocks of the Nakhlak and the Late Permian to Triassic rocks of the Aghdarband formed in a single tectonosedimentary framework on the northern side of the paleo-Tethyan oceanic realm, we present here a sequential development. In this scheme, rocks of the Nakhlak and Aghdarband areas are considered deposits of a forearc, basin-ridge-slope environment. The separation of the Nakhlak succession from the rest of the Turan plate and its transportation to central Iran might have occurred as (1) a lithospheric segment of the Turan plate, first detached from Turan and then attached to the Iranian plate, and finally rotated with it in a counterclockwise direction to its present site; or (2) as a thin thrust slice first obducted over the Iranian continental shelf and then displaced to central Iran by its counterclockwise rotation.