Fossil thermal structure of the southern Sanandaj-Sirjan zone (SW Iran): Implications for regional-scale tectonics
Graphical abstract
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.
Section snippets
Geological setting and overview of tectono-metamorphic data in the SSZ
From southwest to northeast, the SSZ can be divided into two sub-zones: the outer belt, near the MZT, where imbricated thrusts and deformed Mesozoic cover series are largely exposed, and the inner belt, which includes both metamorphic rocks deformed during the Paleozoic/Mesozoic and/or Alpine collision (Mohajjel et al., 2003). The inner belt also hosts abundant intrusions, mostly Middle Jurassic (e.g., Baharifar et al., 2004, Berberian and Berberian, 1981, Hassanzadeh and Wernicke, 2016,
Sampling strategy and analytical techniques
One hundred and five metasedimentary samples were collected across the southern SSZ targeting exposures with good lateral continuity and distinct protolith age, based on 1:100,000 scale published geological maps (Golegohar, Kore Sefid and Qatruyeh; for indications on available age constraints see Table 1). Twenty-eight samples (Fig. 2) provided sufficient carbonaceous material for reliable estimates through the RSCM technique. This technique uses the systematic shift of the Raman spectra of
Petrography and mineral evolution
All collected samples are metasedimentary rocks which can be classified as follows.
RSCM results
Table 1 shows the peak temperatures obtained via RSCM. The R2 parameter [D1/(D1 + D2 + G)], defined as the ratio of peak areas of graphite and defect bands (Beyssac et al., 2002), varies from 0.13 to 0.67 (Table 1). The amplitude of the G band in the spectra increases from Late Devonian to Precambrian samples (Fig. 5a). Temperature and metamorphic grade increase too and exceed ~580 °C in sample 20* (Late Precambrian). The G band also becomes more symmetric while the D4 shoulder almost
Comparison between RSCM data and microstructures
The RSCM data of this study indicates that samples experienced maximum temperatures between ~200 and ~600 °C (Table 1). These RSCM estimates match temperature constraints deduced from microstructural and mineral assemblage observations (Table 2), except for two samples (26* and 38). Although sample 26* (Late Permian) reached ~390 °C according to RSCM data, quartz grains are not ductilely deformed (Fig. 6d), which hints to a temperature lower than 250–300 °C. Sample 38 recorded a RSCM
Conclusions
This study of the Southern SSZ shows that (i) maximum metamorphic temperature for Precambrian to Jurassic rocks range from ~200 to ~600 °C, (ii) all Precambrian and Early Paleozoic samples record temperatures >500 °C and (iii) the largest contrast in maximum temperature lies before and after the Late Devonian. Combined with other observations, these data suggest the existence of a 'mid-Variscan' thermal and likely tectonic event at 380–360 Ma on the southern edge of the Paleotethys, whose
CRediT authorship contribution statement
Parisa Gharibnejad: Conceptualization, Data curation, Formal analysis, Investigation, Writing - original draft. Philippe Agard: Conceptualization, Methodology, Validation, Resources, Writing - review & editing, Supervision, Funding acquisition. Claudio L. Rosenberg: Conceptualization, Methodology, Validation, Resources. Jafar Omrani: Conceptualization, Funding acquisition. Ali Kananian: Conceptualization, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This research was conducted as part of Ph.D. thesis (Gharibnejad, 2020; University of Tehran, Iran). We sincerely thank Mr. A. Eshraghi who accompanied us during the fieldwork as a member of the mapping group during the 1990s and who is familiar with the studied area. We thank the Geological Survey of Iran for its logistical support in the field. We are thankful to A.Lahfid at BRGM (Geological Survey of France) and D. Deldicque (ENS Paris, France) for analytical support. This work benefited
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