The North Sistan orogen (Eastern Iran): Tectono-metamorphic evolution and significance within the Tethyan realm

Abstract

The Sistan orogen (Eastern Iran) separates the Afghan and Lut continental blocks and stretches along ∼700 km from north to south, at a high angle with respect to other, dominantly E-W trending Alpine-Himalayan orogens. This study reappraises the tectono-metamorphic evolution of the northern part of the orogen, as well as its significance within the Neotethyan realm. Detailed inspection of the Sistan ophiolite indicates that the Sistan Ocean was of a slow-spreading type and that, given its structural patterns, petrological characteristics and age, it opened in a transtensional setting ∼125 Ma ago. Closure of the Sistan Ocean took place through a major NE-dipping subduction zone, formed no later than 90 Ma, as shown by the location and age of bimodal juvenile arc magmatism, the SW vergence of the orogen and the location and age of subducted fragments. The discovery of a metamorphic sole at the base of the ophiolite (∼750 °C–0.65 GPa) argues for the onset of an additional intra-oceanic thrust/subduction zone around 74–72 Ma, which resulted in the south-westward obduction and preservation of the ophiolite onto the continental Lut block. The Sistan Ocean therefore appears to have recorded two major geodynamic events that accompanied the closure of the Neotethys, i.e. the major change in kinematics at ∼105 ± 5 Ma and the northward migration of India from ∼75 to 70 Ma onwards. Subsequent collision, likely started during the Paleocene and mostly completed by the Oligocene, was accompanied by a drastic change of the Eocene sedimentation yet by only moderate shortening (∼30–50 km in total). Since the Late Miocene onwards, post-collisional deformation is dominated by far-field stresses related to the Zagros collision.