Fossil thermal structure of the southern Sanandaj-Sirjan zone (SW Iran): Implications for regional-scale tectonics

Highlights

• Maximum temperatures experienced by Precambrian to Jurassic rocks from the southern Sanandaj-Sirjan zone is between ~200 and ~600 °C.

• There is a large contrast in maximum temperatures between rocks older and younger than the Late Devonian.

• The temperatures obtained by the RSCM method reveal the existence of a Variscan thermal (and likely tectonic) event at 380–360 Ma on the southern edge of the Paleotethys.

Abstract

The Sanandaj-Sirjan Zone (SSZ) is one of the Cimmerian blocks accreted to Eurasia during the Late Triassic and the only tectonic unit of the Zagros orogen affected by Barrovian metamorphism. Both the age and absolute temperature of this metamorphic event are still poorly defined. In the present study, we use the Raman Spectrometry of Carbonaceous Material (RSCM) to estimate maximum temperatures attained in the southern SSZ, along two SW-NE cross-sections. While maximum obtained temperature for Precambrian to Jurassic rocks ranges from ~200 to ~600 °C, all Precambrian and Early Paleozoic samples record temperatures >500 °C. Results thus reveal the existence of a major temperature contrast between Precambrian to Devonian samples (showing T_max > 500 °C) and Late Devonian to Cretaceous ones (T_max < 400 °C). This temperature contrast can be assigned to two distinct end-member scenarios, either to a so far unreported Variscan thermal event or to differential exhumation along later thrusts. The stepwise decrease of temperatures with age, and the tectonic style common to rocks older than the Early Paleozoic, strongly support the existence of a thermal (and likely) deformation event at ~380–360 Ma. The SSZ, which was part of the northern edge of Gondwana at that time, was thus affected by Variscan tectonics, similar to what is known in western Europe or NW Africa.

Introduction

The Sanandaj-Sirjan Zone (SSZ) is an elongate crustal block sandwiched between the Africa-Arabia and Eurasia plates (Hassanzadeh and Wernicke, 2016, McCall, 2002, Stöcklin, 1968). As for Central Iran and the Lut block, the SSZ is one of the Cimmerian blocks that drifted away from Gondwana in the Late Paleozoic and were accreted to Eurasia in the Late Triassic (Ricou, 1974, Stampfli and Borel, 2004). The SSZ is also generally regarded as the subduction margin of the Neotethys beneath Eurasia during the Mesozoic (Agard et al., 2005, Agard et al., 2011, Alavi, 1980, Berberian and King, 1981, Gansser et al., 1981, Mohajjel et al., 2003, Mohajjel and Fergusson, 2014, Stampfli, 2000, Stampfli and Kozur, 2006), after subduction initiation during the Middle Triassic to Early Jurassic (Berberian and King, 1981, Davoudian et al., 2016). Calc-alkaline magmatism, whether arc-related (e.g. Berberian and Berberian, 1981, Hassanzadeh and Wernicke, 2016, Sepahi et al., 2018, Shahbazi et al., 2010) or related to extension and continental rifting (Azizi and Stern, 2019, Hunziker et al., 2015), essentially spans the Middle to Late Jurassic (~165 ± 15 Ma) and is younging towards the north of the SSZ (Agard et al., 2005). These plutons are a landmark of the SSZ and were emplaced in a Neoproterozoic to the Paleozoic crustal basement (Hassanzadeh and Wernicke, 2016).

Exhumation and uplift of the SSZ partly result from the Eocene-Early Oligocene collision between Africa-Arabia and the Iranian continental blocks (e.g., Morley et al., 2009), following the oblique closure of the Neotethys at ~30 ± 5 Ma (Agard et al., 2005, Agard et al., 2011, François et al., 2014, Mulch et al., 2010, Ricou, 1974, Şengör et al., 1988, Vincent et al., 2005). The suture zone is marked by a succession of low angle thrusts forming the Main Zagros Thrust (MZT; Berberian and King, 1981), which separates the Zagros fold and thrust belt in the northeastern Arabian platform from the High Zagros and Crush zone (Fig. 3d). The SSZ is located east of and structurally above the Crush Zone, and is bounded by the Cenozoic Urumieh-Dokhtar Magmatic Arc to the northeast (UDMA, Fig. 1b; Alavi, 1994, Stöcklin, 1968).

The SSZ is the only tectonic unit of the Zagros orogen affected by high temperature metamorphism. Assessing the age of metamorphism and distribution of metamorphic temperatures in the SSZ is therefore, a key to understand the stacking of originally deep-seated units on the rest of the orogenic wedge (i.e. onto non-metamorphic to weakly metamorphosed nappes) as well as the dynamics of the Zagros orogeny.

However, neither the absolute P-T conditions nor the age and spatial distribution of the metamorphosed SSZ units are yet well constrained (see Agard et al., 2005, Davoudian et al., 2016, Hassanzadeh and Wernicke, 2016, Shakerardakani et al., 2015). This is particularly the case for the southern SSZ, for which only a few studies have provided geochronological (Fazlnia et al., 2007, Sheikholeslami et al., 2003), petrological (Fazlnia et al., 2007, Sheikholeslami, 2002, Sheikholeslami et al., 2003, Watters et al., 1970) or structural data (Farahpour and Hessami, 2012, Sarkarinejad et al., 2009, Sarkarinejad et al., 2015, Sheikholeslami et al., 2008, Sheikholeslami, 2015).

To provide additional constraints for geodynamic reconstructions, this study focuses on the spatial and temporal distribution of metamorphic peak temperatures in the southern SSZ (Fig. 1a) using Raman Spectroscopy of Carbonaceous Material (RSCM). The irreversible transformation of carbonaceous material indeed allows estimating maximum temperatures experienced by the rocks and thus to evaluate the past thermal evolution of geological units (Beyssac et al., 2002, Lahfid et al., 2010). Using this fossil thermal imprint, we then discuss the age and importance of metamorphic events in the southern SSZ.