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

Abstract

ABSTRACT The Triassic succession of the Nakhlak area in central Iran consists of (1) the LithologicallAlam Formation, which is a sequence of shallowing- and coarsening-upward marine turbidites deposited equon the forearc side of an accretionary prism; (2) the Baqoroq Formation, a resentatsequence of coarse to fine, polymictic, fluvial conglomerates; and (3) the Ashin For- mation, which comprises alternating, distal marine shales and sand- stones that have turbiditic characteristics. These rocks are wnot litholog- ically 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 sedimen- tary framework. Accepting the 135° counterclockwise rotation of the central-east Iranian microcontinent authorwith 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 no rthern side of the paleo-Tethyan oceanic realm, we present here a sequential develop- ment. In this scheme, rocks of the bNakhlak and Aghdarband areas are considered deposits of a forearc, basin-ridge-slope centralenvironment. 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 segmen t of the Turan plate, first detached from Turan and then attached to the Iranian plate, and finally rotated with it in a coun- terclockwise direction to successionits present site; or (2) as a thin thrust slice first obducted over th e Iranian continental shelf and then displaced to cen- tral Iran by its counterclockwise rotation. INTRODUCTION