Paleozoic evolution in the Alborz: Continuous extension from the Gondwanan active margin to Paleotethys post-rift passive margin

The transition from the Ediacaran-Cambrian Gondwana active margin to the rifting and opening of the Rheic and Paleotethys Oceans forms a turning point in the history of the Peri-Gondwanan Terranes. There are few better places to study the Pre-Cambrian and Paleozoic evolution of the northern Gondwana margin better than in the Central Iranian Microplate (CIM). The Alborz Mountains of northern Iran, formed by inversion of the Paleozoic Paleotethys passive margin of the Central Iranian Plate during successive collision phases since the Late Triassic, is one of the places where Paleozoic rocks are well exposed. The aim of this study is to elucidate the Paleozoic structural-tectonic framework and the transition of geodynamic regimes. We present the first paleostress analysis of the Paleozoic evolution of Alborz, were carried out for 49 sites, ranging from Cambrian to Permian outcrops in the Central and Eastern Alborz.

We focus on the orientation of the pre-orogenic Paleotethys margin and the evolution of the Paleozoic stress field by inversion of fault slip data, starting with the Central and Eastern Alborz. Syn-sedimentary faults, i.e. those not reactivated by later tectonic events, were the main target of measurements for the paleostress analysis. Calculated paleostress tensors show a constant north-south extension stress regime throughout the Paleozoic during successive geodynamic regimes, from the Gondwana active margin to the Paleotethys post-rift passive margin. The extension was accommodated by east-west oriented (current reference frame), margin-parallel normal faults. These intra-formational faults are parallel to the main basin-bounding faults, which were repeatedly reactivated during later inversion and extension phases.