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 degrees 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.(c) 2022 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

The N-S trendingSistan orogen (E Iran) stretches along-700 km at a high angle compared to other Alpine -Himalayan ranges marking the Neotethyan suture (including the nearby Zagros, Makran or Alborz ranges). Both the geometry and timing of closure of the Sistan ocean are currently debated. We provide geochemical data on Late Cretaceous (-78 +/- 8 Ma) magmatic samples collected on the eastern side of the Sistan suture zone. Petrography and major element compositions reveal two coexisting groups: a low-K calc-alkaline series with basaltic to rhyolitic composition, and a set of calc-alkaline intermediate to felsic samples. The low-K calc-alkaline series reflects classical arc magmatism and is characterized by negative anomalies in high field strength elements and positive anomalies in large ion lithophile elements. The calc-alkaline intermediate to felsic samples correspond to high-silica adakites characterized by strong positive anomalies in Sr and higher La/Yb ratios. Sr and Nd isotopic compositions of the low-K calc-alkaline series support partial melting of a DMM-like source contaminated by sediment-derived fluids, consistent with slab-dehydration in a juvenile subduction setting. An additional fraction of slab-derived melt is necessary to model trace element patterns of our adakites. Altogether, results indicate formation of a Late Cretaceous magmatic arc associated with NE-dipping subduction of the Sistan ocean below the stretched continental Afghan margin. The emplacement of adakites postdate the formation of the suture zone eclogites by a few Ma at most. Upwelling of hot asthenosphere following slab break-off would best explain the necessary warming-up of the subduction thermal regime.

Suture zones preserve metamorphosed relicts of subducted ocean floor later exhumed along the plate interface that can provide critical insights on subduction zone processes. Melange-like units are exceptionally well exposed in the Sistan suture (Eastern Iran), which results from the closure of a branch of the Neotethys between the Lut and Afghan continental blocks. High pressure rocks found in the inner part of the suture zone (i.e., Ratuk complex) around Gazik are herein compared to previously studied outcrops along the belt. Detailed field investigations and mapping allow the distinction of two kinds of subduction-related block-in-matrix units: a siliciclastic-matrix complex and a serpentinite-matrix complex. The siliciclastic-matrix complex includes barely metamorphosed blocks of serpentinized peridotite, radiolarite and basalt of maximum greenschist-facies grade (i.e., maximum temperature of 340 degrees C). The serpentinite-matrix complex includes blocks of various grades and lithologies: mafic eclogites, amphibolitized blueschists, blue-amphibole-bearing metacherts and aegirine-augite-albite rocks. Eclogites reached peak pressure conditions around 530 degrees C and 23 GPa and isothermal retrogression down to 530 degrees C and 0.9 GPa. Estimation of peak PT conditions for the other rocks are less-well constrained but suggest equilibration at P <1 GPa. Strikingly similar Ar-Ar ages of 86 +/- 3 Ma, along similar to 70 km, are obtained for phengite and amphibole from fourteen eclogite and amphibolitized blueschist blocks. Ages in Gazik are usually younger than further south (e.g., Sulabest), but there is little age difference between the various kinds of rocks. These results (radiometric ages, observed structures and rock types) support a tectonic origin of the serpentinite-matrix melange and shed light on subduction zone dynamics, particularly on coeval detachment and exhumation mechanisms of slab-derived rocks. (C) 2018 Elsevier B.V. All rights reserved.

We conducted a stress field analysis of the northern part of the similar to 700 km long north-south trending, seismically active Sistan orogenic belt of Eastern Iran formed as a result of the closure of a branch of the Neo-Tethys during the early Cenozoic. Fault kinematic data reveal drastic changes in the stress regime of Eastern Iran during the late Cenozoic, with three successive directions of compression (sigma(1)), from 90 degrees N during the middle-late Miocene to 60 degrees N during the late Pliocene and 25 degrees N during the Plio-Quaternary, thereby evidencing a counterclockwise rotation of about 65 degrees of sigma(1) in less than 10 Myr. As shown by compilation of paleostress data, Plio-Quaternary direction of compression in Sistan coincides with the one recorded across the whole of Iran and with present-day Arabia-Eurasia convergence direction. This result suggests effective stress transfer from the Zagros collision and that Sistan is at present mechanically coupled and shortened along with the rest of the Iranian crust/lithosphere. By contrast, Miocene compression is markedly different in the Iranian hinterland (e.g., Sistan, Central Iran, and Kopet Dagh) and in the Zagros orogen. This could tentatively be related to the end of Sistan collision and/or to the imprint of active deformation occurring further to the east. The intermediate late Pliocene compression (i.e., 60 degrees N) could correspond to the progressive reorientation of the stress regime, as Sistan gets mechanically coupled to the Zagros collision.