2.a. Regional extent and tectonic subdivisions

The Zagros orogen extends from the Turkish–Iranian border to the NW, to the Makran area in the SE (where oceanic subduction is still active; Ellouz-Zimmermann et al. Reference Ellouz-Zimmermann, Lallemant, Castilla, Mouchot, Leturmy, Battani, Buret, Cherel, Desaubliaux, Deville, Ferrand, Lügke, Mahieux, Mascle, Mühr, Pierson-Wickmann, Robion, Schmitz, Danish, Hasany, Shahzad, Tabreez, Lacombe, Lavé, Vergès and Roure2007; Smit et al. Reference Smit, Burg, Dolati and Sokoutis2010b; Fig. 1c, d). In an orographic sense, the Zagros orogen comprises the following sub-parallel tectonostratigraphic domains, from SW to NE: the Zagros foreland basin, the Simply Folded Belt, the more mountainous part of the Zagros proper (including the High Zagros and the Crush Zone) as well as domains that can be thought of belonging to the Zagros orogen in a broader sense: the SSZ (or Central Iranian active margin; Berberian & King, Reference Berberian and King1981), the UDMA and the western parts of Central Iran (Fig. 1e).

The interested reader is referred to Stöcklin (Reference Stöcklin1968) for historical tectonic subdivisions of the Zagros orogen. The Zagros was initially considered to lie to the SW of the MZT by Stöcklin (Reference Stöcklin1968), whereas he referred to the part to the NE of the MZT as Central Iran. This latter part was later termed the internal Zagros (Reyre & Mohafez, Reference Reyre and Mohafez1970; J. Braud, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987), emphasizing, as already recognized by Stöcklin, the importance of Tertiary deformation over the whole of Iran. In fact, several other adjacent domains located in the far-field of Zagros deformation should be considered whenever studying the Zagros orogeny (e.g. Jackson, Hains & Holt, Reference Jackson, Hains and Holt1995): these are the Alborz, Kopet Dagh, Central Iran and Sistan regions (Fig. 1 c, d). We will indeed show that the history of the Zagros orogeny largely overlaps with the deformation history of most of Iran since the Jurassic. All these domains are further described in what follows (Section 3).

It is convenient, however, to outline from the start a more fundamental division, between a lower and an upper lithospheric plate. Calc-alkaline, arc-related magmatism found in the SSZ and UDMA (Fig. 2) provide compelling evidence for past convergence and northward subduction of the Neo-Tethys Ocean (now vanished everywhere but in front of the accretionary prism of Makran) below Eurasia (Fig. 1b; Berberian & Berberian, Reference Berberian, Berberian, Gupta and Delany1981).

Figure 2.

Compilation of magmatism in Iran through time (upper Eurasian plate) through 250000 scale maps from the Geological Survey of Iran.

This boundary between the upper (Eurasia) and lower (Arabia) plates runs along the MZT (Fig. 2), as supported by the presence of ophiolite fragments (Ricou Reference Ricou1971; Ricou, Braud & Brunn, Reference Ricou, Braud and Brunn1977), high-pressure, low-temperature, mainly blueschist facies rocks (Sabzehei et al. Reference Sabzehei, Berberian, Roshanravan, Azizan, Nazemzadeh, Alavi-Tehrani, Houchmand-Zadeh, Nowgole-Sadat and Madjidi1994; Agard et al. Reference Agard, Monie, Gerber, Omrani, Molinaro, Meyer, Labrousse, Vrielynck, Jolivet and Yamato2006), the style of deformation (Agard et al. Reference Agard, Omrani, Jolivet and Mouthereau2005) and seismic profiles tracing this thrust down to Moho depths (Paul et al. Reference Paul, Kaviani, Hatzfeld, Vergne and Mokhtari2006, Reference Paul, Hatzfeld, Kaviani, Tatar, Pequegnat, Leturmy and Robin2010). It can be useful to recall that Alavi (Reference Alavi1994) proposed an alternative location for the suture zone between the SSZ and the UDMA, because this option was considered in a number of later publications and only recently refuted (Agard et al. Reference Agard, Omrani, Jolivet and Mouthereau2005; Paul et al. Reference Paul, Kaviani, Hatzfeld, Vergne and Mokhtari2006).

The MZT thus defines the boundary between the Arabian, lower plate external zones to the SW and the Eurasian, upper plate internal zones to the NE (cross-section Fig. 1e, Fig. 2). We give below a brief overview of the various domains, for which synthetic stratigraphic columns of the Zagros (Arabia) and Central Iran (Eurasia) are shown in Figure 3 (and for which a wealth of papers are available; for details, see more in-depth publications, e.g. James & Wynd, Reference James and Wynd1965; Motiei, Reference Motiei1993; Homke et al. Reference Homke, Vergés, Serra-Kiel, Bernaola, Sharp, Garcés, Montero-Verdù, Karpuz and Goodarzi2009). Emphasis will be placed here on the internal zones, which provide crucial constraints for the geodynamic evolution of the Zagros orogen. All domains share a common Infracambrian to Middle Triassic ~ 3 km thick sequence of platform deposits, very constant throughout Iran (Stöcklin, Reference Stöcklin1968), overlying a presumably Arabian-type basement structured during late Precambrian ‘Assyntic’ or Pan-African times (Hassanzadeh et al. Reference Hassanzadeh, Stockli, Horton, Axen, Stockli, Grove, Schmitt and Walker2008). Some N–S trends (Oman line and Qatar high; Gansser, Reference Gansser1960), and possibly even early NW–SE trends (e.g. controlling the deposition of the Cambrian Hormuz salt), are thought to be inherited from these deformation episodes. No significant deformation affected Iran during Palaeozoic times, apart from the Permian–Triassic rifting which gave birth to the Neo-Tethys. Major contrasts on either side of what is now the MZT formed from Late Triassic time onwards.

Figure 3.

Simplified stratigraphic columns for the ZFTB (see also Alavi, Reference Alavi2007, fig. 8) and for Central Iran. Modified after Aghanabati & Rezai (Reference Aghanabati and Rezai2009).

Before recalling essential characteristics of the various palaeogeographic domains, we start by giving an outline of the present-day deformation patterns as seen by GPS and tracking of active faults.

2.b. Present-day partitioning of the deformation through the Zagros orogen

Present-day kinematics indicates a northward motion of the Arabian plate relative to Eurasia of 22 ± 2 mm yr⁻¹ (Vernant et al. Reference Vernant, Nilforoushan, Hatzfeld, Abbassi, Vigny, Masson, Nankali, Martinod, Ashtiani, Bayer, Tavakoli and Chery2004). GPS vectors reveal that the overall present-day strain field across the Zagros, in response to the Arabia–Eurasia convergence, is directed N–S to N010 on average (Fig. 1b, d; Masson et al. Reference Masson, Anvari, Djamour, Walpersdorf, Tavakoli, Daignières, Nankali and Van Gorp2007), that is, oblique by ~ 20–30° to the principal long-term SW–NE shortening direction across the orogen (Fig. 1d). Northward convergence is accommodated (Jackson, Hains & Holt, Reference Jackson, Hains and Holt1995; Allen et al. Reference Allen, Blanc, Walker, Jackson, Talebian and Ghassemi2006; Hatzfeld & Molnar, Reference Hatzfeld and Molnar2010) through a combination of:

(1) across-range shortening in the Zagros orogen and in far-field domains (Alborz, Kopet Dagh) at rates of 5 ± 2 mm yr⁻¹ (McClusky et al. Reference McClusky, Balassanian, Barka, Demir, Ergintav, Georgiev, Gurkan, Hamburger, Hurst, Kahle, Kastens, Kekelidze, King, Kotzev, Lenk, Mahmoud, Mishin, Nadariya, Ouzounis, Paradissis, Peter, Prilepin, Reilinger, Sanli, Seeger, Tealeb, Toksoz and Veis2000; Vernant et al. Reference Vernant, Nilforoushan, Hatzfeld, Abbassi, Vigny, Masson, Nankali, Martinod, Ashtiani, Bayer, Tavakoli and Chery2004; Walpersdorf et al. Reference Walpersdorf, Hatzfeld, Nankali, Tavakoli, Nilforoushan, Tatar, Vernant, Chéry and Masson2006; Masson et al. Reference Masson, Anvari, Djamour, Walpersdorf, Tavakoli, Daignières, Nankali and Van Gorp2007). Deformation was concentrated in the Zagros and the Alborz since Pliocene time at least (Oveisi et al. Reference Oveisi, Lavé, Van Der Beek, Carcaillet, Benedetti and Aubourg2009; Allen et al. Reference Allen, Kheirkhah, Emami and Jones2011) at a probably constant rate over the last 5 Ma (Lacombe et al. Reference Lacombe, Mouthereau, Kargar and Meyer2006, Reference Lacombe, Amrouch, Mouthereau and Dissez2007). Out of the ~ 2 cm yr⁻¹ of overall orogen-normal shortening component, two thirds are taken up in the Zagros fold-and-thrust belt (ZFTB) and one third in the Alborz, with domains such as the SSZ behaving more rigidly (Masson et al. Reference Masson, Anvari, Djamour, Walpersdorf, Tavakoli, Daignières, Nankali and Van Gorp2007).

(2) dextral strike-slip movements across the Main Recent Fault (MRF; 3–15 mm yr⁻¹; Talebian & Jackson, Reference Talebian and Jackson2002, Reference Talebian and Jackson2004; Authemayou et al. Reference Authemayou, Bellier, Chardon, Benedetti, Malekzade, Claude, Angeletti, Shabanian and Abbassi2009), the Kazerun fault (Authemayou et al. Reference Authemayou, Chardon, Bellier, Malekzadeh, Shabanian and Abbassi2006, Reference Authemayou, Bellier, Chardon, Benedetti, Malekzade, Claude, Angeletti, Shabanian and Abbassi2009) and subordinate faults east of the Kazerun fault proper (Lacombe et al. Reference Lacombe, Mouthereau, Kargar and Meyer2006). Several N–S-trending faults also cut across the Iranian plateau (e.g. from west to east: the Deshir, Anar, Nayband–Gowk and Ney faults; Walker & Jackson, Reference Walker and Jackson2002; Regard et al. Reference Regard, Bellier, Thomas, Abbassi, Mercier, Shabanian, Feghhi and Soleymani2004; Meyer et al. Reference Meyer, Mouthereau, Lacombe and Agard2006; Meyer & Le Dortz, Reference Meyer and Dortz2007; Le Dortz et al. Reference Le Dortz, Meyer, Sébrier, Nazari, Braucher, Fattahi, Benedetti, Foroutan, Siame, Bourlès, Talebian, Bateman and Ghoraishi2009).

Overall, deformation partitioning across Iran can be viewed as shown in Figure 1d. The predominance of N–S dextral strike-slip faults to the east of the Kazerun–Doruneh faults could correspond to Iran moving north with respect to stable Afghanistan and/or to the existence of a somewhat freer boundary in the still subducting Makran region (at least before Pliocene time; Allen et al. Reference Allen, Kheirkhah, Emami and Jones2011; Regard et al. Reference Regard, Hatzfeld, Molinaro, Aubourg, Bayer, Bellier, Yamini-Fard, Peyret, Abbassi, Leturmy and Robin2010; Smit et al. Reference Smit, Burg, Dolati and Sokoutis2010b). Some of these major faults represent long-lived structures, such as the MRF or the Neh fault, which both reactivate suture zones (Talebian & Jackson, Reference Talebian and Jackson2002; Walker et al. Reference Walker, Gans, Allen, Jackson, Khatib, Marsh and Zarrinkoub2009).

3.a. The Zagros fold-and-thrust belt

The Zagros fold-and-thrust belt (ZFTB) classically comprises the geographic provinces of Lurestan and Fars separated by the Dezful Embayment (Fig. 1d). The ZFTB can be divided into two distinct domains from the SW to NE (Fig. 4a), with major changes in structural style and topography: (a) the Simply Folded Belt (SFB) is found in the vicinity of the Persian Gulf and shows fairly regular wavelength folds running along hundreds of kilometres (Falcon, Reference Falcon and Spencer1974; Sepehr & Cosgrove, Reference Sepehr and Cosgrove2004; Mouthereau, Lacombe & Meyer, Reference Mouthereau, Lacombe and Meyer2006). The SFB is in fact also cut by several hidden major faults and progressively steps up in elevation towards the NE (Berberian, Reference Berberian1995; Leturmy, Molinaro & Frizon de Lamotte, Reference Leturmy, Molinaro, Frizon De Lamotte, Leturmy and Robin2010); (b) the High Zagros shows higher elevation, a sharp increase in elevation and kilometre-scale throws on major thrusts.

Figure 4.

(a) Map of the Zagros orogen. Compilation of shortening estimates (bold numbers, in kilometres) from recently published cross-sections (A: Alavi, Reference Alavi2007; B: Blanc et al. Reference Blanc, Allen, Inger and Hassani2003; M: Mouthereau et al. Reference Mouthereau, Tensi, Bellahsen, Lacombe, Boisgrollier and Kargar2007; Mo: Molinaro et al. Reference Molinaro, Leturmy, Guezou, Frizon De Lamotte and Eshraghi2005; Q: McQuarrie, Reference McQuarrie2004; S: Sherkati, Letouzey & Frizon de Lamotte, Reference Sherkati, Letouzey and Frizon De Lamotte2006). Empty stars with numbers locate available radiometric datings for the SSZ, which are shown in Figure 5c. Other symbols correspond to reference geochemical data given in Figure 5a, b (as in Omrani et al. Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008): black and white symbols refer to Eocene and Upper Miocene to Plio-quaternary UDMA samples, respectively, whereas diamonds correspond to SSZ samples. Places where adakites are found in the UDMA are shown with an orange rectangle. Grey dotted ellipses recall the age range obtained from apatite fission-track and U–Th–He data (after Gavillot et al. Reference Gavillot, Axen, Stockli, Horton and Fakhari2010; Homke et al. Reference Homke, Verges, Van Der Beek, Fernandez, Saura, Barbero, Badics and Labrin2010; J. Omrani, unpub. Ph.D. thesis, Institut des Sciences de la Terre, Paris, Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008). Abbreviations: CC – core-complexes ETMD – Early Tertiary Magmatic Domain; SFB – Simply Folded Belt; SSZ, UDMA, ZFTB – as for Fig. 1c. Ophiolitic bodies in purple. (b) Section across the Crush Zone in the Kermanshah region (A–B shown on (a)) and tectonic evolution since Late Cretaceous time. Adapted from Agard et al. (Reference Agard, Omrani, Jolivet and Mouthereau2005) and Omrani et al. (unpub. data), and Wrobel-Daveau et al. (Reference Wrobel-Daveau, Ringenbach, Tavakoli, Ruiz, Masse and Frizon De Lamotte2010) for the Late Cretaceous stage. Abbreviations: ETMD – Early Tertiary Magmatic Domain; H – Hamadan; K – Kermanshah; MZT, SSZ, ZFTB – as for Fig. 1c, e. Note how little is left from the Neo-Tethyan ophiolite: much (if not all) of the peridotites correspond to the basement of the ETMD and to earlier stretched sub-continental mantle from the Arabian margin. Also note that 70 km is a minimum shortening estimate since internal ductile deformation cannot be constrained precisely. Plutons in the SSZ are already largely exhumed at 20 Ma, as shown by fission-track data (Fig. 4a).

The ZFTB essentially corresponds to the Eo-Cambrian to Quaternary cover of the Arabian plate deformed above the Arabian basement, with a maximum thickness of 12–13 km (James & Wynd, Reference James and Wynd1965; Motiei, Reference Motiei1993; Alavi, Reference Alavi2007) and a deeper Zagros distal basin individualized since Late Triassic time. This sedimentary cover dominantly comprises shelf carbonates, together with several evaporitic low viscosity layers (i.e. Cambrian Hormuz salt and the Early–Middle Miocene Gachsaran Formation; Sherkati et al. Reference Sherkati, Molinaro, Frizon De Lamotte and Letouzey2005; Fig. 3), which acted as major mechanical decollement horizons, thickened fold hinges and favoured spectacular thin-skinned deformation (Blanc et al. Reference Blanc, Allen, Inger and Hassani2003; McQuarrie, Reference McQuarrie2004; Sherkati et al. Reference Sherkati, Molinaro, Frizon De Lamotte and Letouzey2005). Thin-skinned deformation produced regular fold wavelengths (i.e. ~ 15 km), yet only accounts for a subordinate amount of the overall deformation (Agard et al. Reference Agard, Monie, Gerber, Omrani, Molinaro, Meyer, Labrousse, Vrielynck, Jolivet and Yamato2006; Fig. 1e). Numerous salt diapirs from the Cambrian Hormuz salt formation, found to the SE of a N–S line coincident with the Qatar High, also strongly influenced heat advection in the ZFTB (and the extent of oil preservation; Haynes & McQuillan, Reference Haynes and McQuillan1974; Jahani et al. Reference Jahani, Callot, Letouzey and Frizon De Lamotte2009), and halokinesis mechanisms and duration are still debated (Koyi et al. Reference Koyi, Ghasemi, Hessami and Dietl2008; Jahani et al. Reference Jahani, Callot, Letouzey and Frizon De Lamotte2009).

From bottom to top, other important time markers are (Fig. 3):

(i) The Paleocene Amiran flysch (James & Wynd, Reference James and Wynd1965; J. Braud, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987; Homke et al. Reference Homke, Vergés, Serra-Kiel, Bernaola, Sharp, Garcés, Montero-Verdù, Karpuz and Goodarzi2009), which contains ophiolitic fragments including clasts of serpentinite, radiolarite and mafic rocks in the Amiran anticline (James & Wynd, Reference James and Wynd1965), but mainly radiolarites further to the NE (J. Braud, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987). It is thought to represent the frontal foredeep of the Late Cretaceous obduction nappes, whose remnants can be found in Kermanshah, Neyriz and in Oman (Ricou, Reference Ricou1971; Coleman, Reference Coleman1971). This early syntectonic basin is found only in Lurestan and in Iraq, however, and is somewhat younger than its counterpart in Oman (Maastrichtian Muti Formation, Oman; Robertson, Reference Robertson1987).

(ii) A hiatus in sedimentation from the Lower Eocene to the Oligocene (Homke et al. Reference Homke, Vergés, Serra-Kiel, Bernaola, Sharp, Garcés, Montero-Verdù, Karpuz and Goodarzi2009), which could result from forebulge flexural deformation (Mouthereau et al. Reference Mouthereau, Tensi, Bellahsen, Lacombe, Boisgrollier and Kargar2007).

(iii) Shallow water platform carbonates of the Lower Miocene Asmari Formation (Motiei, Reference Motiei1993), showing a large extension (even into Central Iran, where its equivalent is known as the Qom Fm). This Lower Miocene formation is associated with high sea level and worldwide marine transgressions (Haq, Hardenbol & Vail, Reference Haq, Hardenbol and Vail1987).

(iv) Syn-orogenic deposits with growth strata, from the upper Agha Jari onwards (from ~ 10 Ma onwards).

(v) Coarse-grained basin fill known as Bakhtyari conglomerates, classically assumed to be Upper Pliocene to Quaternary along the Zagros (James & Wynd, Reference James and Wynd1965; Homke et al. Reference Homke, Vergès, Garcés, Emami and Karpuz2004).

Revised age constraints, however, point to significant diachronism for the Bakhtyari conglomerates, as well as for the Agha Jari Formation (Fakhari et al. Reference Fakhari, Axen, Horton, Hassanzadeh and Amini2008; Khadivi et al. Reference Khadivi, Mouthereau, Larrasoaña, Vergés, Lacombe, Khademi, Beamud, Melinte-Dobrinescu and Suc2010). These syn-orogenic deposits are thus probably more diachronous along and across strike than acknowledged or demonstrated at present (in line with the fact that, for example, collision started earlier in Turkey than in the NW Zagros or that subduction is still active in Makran).

Renewed fieldwork and exploration since the 1990s have provided a number of shortening estimates for the ZFTB (Fig. 4a). Overall estimates are about 60 ± 20 km, without clear evidence for any gradient along strike (Fig. 4a; Sherkati, Letouzey & Frizon de Lamotte, Reference Sherkati, Letouzey and Frizon De Lamotte2006), despite quite distinct widths on either side of the Dezful Embayment. This observation suggests that these contrasting widths rather relate to variations in the stratigraphic column (such as the presence of deep salt to the SE of the Kazerun fault; Koop et al. Reference Koop, Stoneley, Ridd, Murphy, Osmaston and Kholief1982; Bahroudi & Koyi, Reference Bahroudi and Koyi2003; Sherkati, Letouzey & Frizon de Lamotte, Reference Sherkati, Letouzey and Frizon De Lamotte2006) than to distinct overall shortening estimates. Several authors have also underlined, based on restoration models, that much of the distal part of the Arabian continental margin is missing (Molinaro et al. Reference Molinaro, Leturmy, Guezou, Frizon De Lamotte and Eshraghi2005; Mouthereau et al. Reference Mouthereau, Tensi, Bellahsen, Lacombe, Boisgrollier and Kargar2007). Whether deformation evolved from thin-skinned to thick-skinned after the Mio-Pliocene (Molinaro et al. Reference Molinaro, Leturmy, Guezou, Frizon De Lamotte and Eshraghi2005) or whether both were already more or less coevally superimposed (Mouthereau et al. Reference Mouthereau, Tensi, Bellahsen, Lacombe, Boisgrollier and Kargar2007; Ahmadhadi, Lacombe & Daniel, Reference Ahmadhadi, Lacombe, Daniel, Lacombe, Lavé, Vergès and Roure2007) is still a matter of debate.

Younger SW-directed folds and thrusts are recognized in the ZFTB when moving from NE to SW (e.g. Stocklin, Reference Stöcklin1968; Hessami et al. Reference Hessami, Koyi, Talbot, Tabasi and Shabanian2001; Molinaro et al. Reference Molinaro, Leturmy, Guezou, Frizon De Lamotte and Eshraghi2005). The High Zagros is distinct from the SFB in terms of structural style: it is characterized by earlier deformation, larger offsets on basement faults, steep contacts and more ductile deformation (Ricou, Braud & Brunn, Reference Ricou, Braud and Brunn1977). Shortening is also concentrated mostly in frontal folds on Pleistocene time scales (Navabpour, Angelier & Barrier, Reference Navabpour, Angelier and Barrier2007; Oveisi et al. Reference Oveisi, Lavé, Van Der Beek, Carcaillet, Benedetti and Aubourg2009). This contrast between the ZFTB and the SFB is confirmed by recent apatite fission-track and U–Th–He constraints obtained on the timing of uplift (Homke et al. Reference Homke, Vergés, Serra-Kiel, Bernaola, Sharp, Garcés, Montero-Verdù, Karpuz and Goodarzi2009; Gavillot et al. Reference Gavillot, Axen, Stockli, Horton and Fakhari2010; Axen et al. Reference Axen, Fakhari, Guest, Gavillot, Stockli and Horton2010; Fig. 4a), indicating that the High Zagros was uplifted since c. 10 Ma (Gavillot et al. Reference Gavillot, Axen, Stockli, Horton and Fakhari2010). The SFB, by contrast, may have suffered 2–4 km of uplift on average from balanced cross-sections (Mouthereau, Lacombe & Meyer, Reference Mouthereau, Lacombe and Meyer2006; Leturmy, Molinaro & Frizon de Lamotte, Reference Leturmy, Molinaro, Frizon De Lamotte, Leturmy and Robin2010, fig. 11), but apatite fission-track ages were not reset (Fig. 4a), suggesting that T did not exceed ~ 100°C (hence burial < 2–3 km) since onset of the current deformation stage.

3.b. The Crush Zone: the Zagros suture zone

The Crush Zone (Fig. 4a; Wells, Reference Wells1969; Alavi, Reference Alavi1994; J. Omrani, unpub. Ph.D. thesis, Institut des Sciences de la Terre, Paris, Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008; also known as the Thrust Zone by Stöcklin, Reference Stöcklin1968 or Imbricated Zone by Falcon, Reference Falcon1967) shows extensive thrusts with steep fault contacts and is partly similar to the High Zagros in terms of topography and deformation style. The Crush Zone, however, comprises distinctive geological units which do not belong to the Arabian platform proper and are bounded by the MZT to the NE. The MZT was already considered a major boundary between Arabia and Iran by Stöcklin (Reference Stöcklin1968), prior to the recognition of plate tectonics in the area, and regarded as the suture zone since Berberian & King (Reference Berberian and King1981).

In the Crush Zone near Kermanshah (Fig. 1c), several units are thus stacked on top of the ZFTB and in turn overlain by the SSZ. From bottom to top (Braud, Reference Braud1978, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987; Fig. 4b, present section), one finds: (a) Liassic to Cretaceous radiolarite basins (Gharib & de Wever, Reference Gharib and De Wever2010; as for their Pichakun equivalents in Neyriz; Robin et al. Reference Robin, Gorican, Guillocheau, Razin, Dromart, Mosaffa, Leturmy and Robin2010), (b) thick Mesozoic platform carbonates (e.g. Bisotun limestone, considered an equivalent of the Oman ‘Exotics’; J. Braud, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987; Pillevuit et al. Reference Pillevuit, Marcoux, Stampfli and Baud1997), (c) presumably ophiolitic remnants (both in Kermanshah and Neyriz), and (d) an Early Tertiary Magmatic Domain (ETMD, hereafter).

Structural style, timing of deformation, rough balancing and shortening rates across the Crush Zone were given, for the Lurestan region, by Agard et al. (Reference Agard, Omrani, Jolivet and Mouthereau2005). Together with recent findings on the Crush Zone (Wrobel-Daveau et al. Reference Wrobel-Daveau, Ringenbach, Tavakoli, Ruiz, Masse and Frizon De Lamotte2010), they are recalled in Figure 4b. A crucial observation is that major contacts, including the ophiolite/ETMD, are sealed by the Lower Miocene Qom Formation. This indicates that Arabia–Eurasia collision, marked by the juxtaposition of domains previously situated on both sides of the Neo-Tethys (i.e. plates in contact), must have started prior to the deposition of the Qom Fm (Agard et al. Reference Agard, Omrani, Jolivet and Mouthereau2005), and thus pre-dates the lowermost Miocene. On the other hand, gabbroic intrusions emplaced in the (upper plate) ETMD only (i.e. Tah intrusion in the Kamyaran region dated at 34 ± 1 Ma with whole-rock Rb–Sr; Leterrier, Reference Leterrier1985), bracket collision to be uppermost Eocene to Oligocene. This observation is consistent with recent findings by Fakhari et al. (Reference Fakhari, Axen, Horton, Hassanzadeh and Amini2008) and Homke et al. (Reference Homke, Vergés, Serra-Kiel, Bernaola, Sharp, Garcés, Montero-Verdù, Karpuz and Goodarzi2009; see also discussions by Allen & Armstrong, Reference Allen and Armstrong2008 and Ballato et al. Reference Ballato, Mulch, Landgraf, Strecker, Dalconi, Friedrich and Tabatabaei2010). Pre-Miocene deformation patterns in the Crush Zone also suggest the existence of early dextral strike-slip movements (Agard et al. Reference Agard, Omrani, Jolivet and Mouthereau2005).

Ophiolitic remnants from the Crush Zone are considered equivalent to the Oman ophiolite and to have been emplaced onto Arabia (Ricou, Reference Ricou1971) as a result of obduction processes over the period 95–70 Ma. Only limited radiometric K–Ar or Ar–Ar age constraints are available for the Kermanshah and Neyriz ophiolites, yet cluster at 85 ± 5 Ma (Delaloye & Desmons, Reference Delaloye and Desmons1980; J. Braud, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987) and 90 ± 15 Ma (Lanphere & Pamic, Reference Lanphere and Pamic1983; M. R. Jannessary, unpub. Ph.D. thesis, Univ. Louis Pasteur de Strasbourg, 2003; 92.1 ± 1.7 Ma and 93.2 ± 2.5 Ma, Babaei et al. Reference Babaei, Babaei, Ghazi and Arvin2006). Kermanshah and Neyriz ophiolites significantly differ, however, from the oceanic lithosphere found in Oman (Coleman, Reference Coleman1971, Reference Coleman1981; Glennie et al. Reference Glennie, Boeuf, Clarke, Moody-Stuart, Pilaar and Reinhardt1973): they correspond to fragments from the southern footwall of the Neo-Tethyan margin for Neyriz (M. R. Jannessary, unpub. Ph.D. thesis, Univ. Louis Pasteur de Strasbourg, 2003; for a different interpretation see Sarkarinejad, Reference Sarkarinejad2005) and at least partly for Kermanshah too (J. Omrani, unpub. Ph.D. thesis, Institut des Sciences de la Terre, Paris, Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008; Wrobel-Daveau et al. Reference Wrobel-Daveau, Ringenbach, Tavakoli, Ruiz, Masse and Frizon De Lamotte2010; J. Omrani et al. unpub. data). Part of the Kermanshah ophiolite (near Harsin; Fig. 4b) shows an ocean island-arc signature (Fig. 5a; J. Omrani et al. unpub. data) and in fact rather corresponds to a small oceanic basin located between the radiolarite basin and the Bisotun limestone (the latter representing oceanic islands partly built on a peridotitic substrate; J. Braud, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987). Some authors have recently advocated for the existence of extensional allochthons in the area (Wrobel-Daveau et al. Reference Wrobel-Daveau, Ringenbach, Tavakoli, Ruiz, Masse and Frizon De Lamotte2010; see the Late Cretaceous section in Fig. 4b). In any case, no true mid-ocean ridge (MORB)-type ophiolitic material such as in Oman has been found so far in the Kermanshah ophiolite (Ghazi & Hassanipak, Reference Ghazi and Hassanipak1999; J. Omrani et al. unpub. data), suggesting that much (if not all) of the Neo-Tethyan ophiolite has either been eroded and/or later buried (Fig. 4b).

Figure 5.

(a, b) Reference geochemical data for the Crush Zone (Omrani et al. unpub. data) and the internal zones (Omrani et al. Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008; see location in Fig. 4a). Grey overlay: typical variations of trace element multielement patterns for the SSZ, for comparison with other units. UM-PQ – Upper Miocene to Plio-Quaternary. (c) Age compilation of available radiometric constraints for the SSZ (after Ahmadi Khalaji et al. Reference Ahmadi Khalaji, Esmaeily, Valizadeh and Rahimpour-Bonab2007; Arvin et al. Reference Arvin, Pan, Dargahi, Malekizadeh and Babaei2007; Baharifar et al. Reference Baharifar, Moinevaziri, Bellon and Piqué2004; Fazlnia et al. Reference Fazlnia, Moradian, Rezaei, Moazzen and Alipour2007; Ghalamghash et al. Reference Ghalamghash, Bouchez, Vosoughi-Abedini and Nédélec2009; Hassanzadeh et al. Reference Hassanzadeh, Stockli, Horton, Axen, Stockli, Grove, Schmitt and Walker2008; Masoudi, Yardley & Cliff, Reference Masoudi, Yardley and Cliff2002; Mazhari et al. Reference Mazhari, Amini, Ghalamghash and Bea2009a, Reference Mazhari, Bea, Amini, Ghalamghash, Molina, Montero, Scarrow and Williamsb; Rachidnejad-Omran et al. Reference Rachidnejad-Omran, Emami, Sabzehei, Rastad, Bellon and Piqué2002; Shahbazi et al. Reference Shahbazi, Siebel, Pourmoafee, Ghorbani, Sepahi, Shang and Vousoughi Abedini2010; Sheikholeslami et al. Reference Sheikholeslami, Bellon, Emami, Sabzehei and Pique2003). (d) Discriminant diagram (Defant & Drummond, Reference Defant and Drummond1990) for the Zagros (UDMA) adakites (after Omrani et al. Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008; same symbols as Fig. 4a). (e) Interpretation of the adakitic magmatism in terms of slab break-off: HSA (high-silica adakites) corresponding to slab melts (Martin et al. Reference Martin, Smithies, Rapp, Moyen and Champion2005) are located in the centre only, whereas LSA (low silica adakites) corresponding to mantle wedge melts contaminated by slab components are found on the edges of the adakitic region. See Omrani et al. (Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008, Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2009) for a detailed discussion.

The ETMD is well exposed near Songor and Kangavar, to the N and NW of Kermanshah, and is generally considered a lateral equivalent of the Maden complex in Turkey (Braud & Ricou, Reference Braud and Ricou1975). Intercalated sediments, sheared serpentinites, large gabbroic intrusions and mainly andesitic volcanics indicate that this ETMD formed during Paleocene–Early Eocene times on the Eurasian side of the Neo-Tethys (to the south of the SSZ), on a partly peridotitic substratum. They also show an evolution in time from a back-arc to arc signature (Fig. 5a; J. Omrani et al. unpub. data). Volcanic rocks with a similar arc signature also lie between the ophiolite and the SSZ near Neyriz (Babaie et al. Reference Babaie, Ghazi, Babaei, La Tour and Hassanipak2001), in a probably uppermost Cretaceous melange. It is yet unclear, however, whether they are equivalent to the ETMD, to the crustal section of the Neyriz ophiolite or even to the coloured melange found further to the SE (where high-pressure (HP) rocks are found in places, as described in the following).

In the southeasternmost part of Zagros, restricted blueschist exposures are indeed found sandwiched between the High Zagros and the SSZ, thus in a position equivalent to the Crush Zone (Fig. 1e; Agard et al, Reference Agard, Monie, Gerber, Omrani, Molinaro, Meyer, Labrousse, Vrielynck, Jolivet and Yamato2006; Oberhänsli et al. Reference Oberhänsli, Bousquet, Moinzadeh, Moazzen and Arvin2007). These Zagros blueschists correspond to tectonic slices lying within a more extensive ‘coloured melange’ (Gansser, Reference Gansser1960) made of serpentinites, radiolarites and lower greenschist facies metabasalts. These blueschists reached maximum depths of 30–40 km, on average, for temperatures between 500 and 550°C. Most importantly, they yield ⁴⁰Ar–³⁹Ar ages broadly coincident (90 ± 10 Ma) with obduction processes in the region, as well as with other blueschist exposures known from Western Turkey to the Himalayas (Fig. 1b; Agard et al, Reference Agard, Monie, Gerber, Omrani, Molinaro, Meyer, Labrousse, Vrielynck, Jolivet and Yamato2006; Monié & Agard, Reference Monié and Agard2009). Their exhumation was therefore interpreted as the result of a regional-scale (~ 3000 km along strike) modification of plate–slab coupling in the subduction zone below Eurasia during Late Cretaceous time.

3.c. The Sanandaj–Sirjan Zone (SSZ)

The SSZ (Stöcklin, Reference Stöcklin1968) represented an active Andean-like margin whose calc-alkaline magmatic activity progressively shifted northwards during most of the second half of the Mesozoic (Fig. 1b; Berberian & King, Reference Berberian and King1981; Sengör, Reference Sengör, Robertson, Searle and Ries1990; Agard et al. Reference Agard, Omrani, Jolivet and Mouthereau2005). It already lies within the Iranian plateau in terms of elevation, yet follows the same NW–SE orographic trend as the ZFTB (Figs 1c, 4a). The SSZ forms the continental collage of Iran together with the Lut block and other blocks from Central Iran (see Section 3.e; e.g. Ricou, Reference Ricou1994) and extends from the Bitlis area in Turkey to the western end of Makran (Sengör et al. Reference Sengör, Altiner, Cin, Ustaömer, Hsü, Audley-Charles and Hallam1988; McCall & Kidd, Reference McCall, Kidd and Leggett1982), across the present-day transition zone from collision to subduction (Fig. 1c). The crustal root of the Zagros orogen in fact coincides with the SSZ (Snyder & Barazangi, Reference Snyder and Barazangi1986), with a crustal thickness between 55 and 70 km (Paul et al. Reference Paul, Kaviani, Hatzfeld, Vergne and Mokhtari2006, Reference Paul, Hatzfeld, Kaviani, Tatar, Pequegnat, Leturmy and Robin2010).

As elsewhere in Iran, series of Infracambrian to Triassic rocks are found in the SSZ (Stöcklin, Reference Stöcklin1968), with only few basement outcrops (near Golpaygan and south of Mahabad; Alavi & Mahdavi, Reference Alavi and Mahdavi1994; Hassanzadeh et al. Reference Hassanzadeh, Stockli, Horton, Axen, Stockli, Grove, Schmitt and Walker2008). The SSZ mainly exposes metamorphosed cover series from Permian to Upper Cretaceous (Orbitolina limestone), with thick Permian (Golpaygan and Dorud-Azna regions) and Triassic rocks in places (June complex; Mohajjel, Fergusson & Sahandi, Reference Mohajjel, Fergusson and Sahandi2003), notably where they are thrust along the MZT on top of the Crush Zone (Gidon et al. Reference Gidon, Berthier, Billiault, Halbronn and Maurizot1974). The Songor-Kangavar Triassic to Jurassic volcanic series or lateral equivalents (such as the Upper Triassic–Jurassic turbiditic succession of the Hamadan phyllite; J. Braud, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987) are topped by a Neocomian unconformity, which seems slightly diachronous along strike (Thiele et al. Reference Thiele, Alavi, Assefi, Hushmand-Zadeh, Seyed-Emami and Zahedi1968; J. Braud, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987; Mohajjel & Fergusson, Reference Mohajjel and Fergusson2000). Lowermost Cretaceous marine sediments are absent, contrary to the ZFTB (Fig. 3), and only limited alternating andesitic volcanic rocks, limestone and shales are found. These formations are in turn overlain by the extensive, slightly unconformable Barremian Orbitolina limestone. Eocene sediments are spatially restricted in the SSZ (Mian Kuh basin near Songor; J. Braud, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987) and, unlike underlying formations, not affected by deformation. Continental or shallow marine Neogene deposits are represented by the Oligo-Miocene Lower Red, Aquitano-Burdigalian Qom (equivalent to the ZFTB Asmari Fm) and Upper Red formations, respectively. Quaternary endoreic deposits finally filled in most of the SSZ valleys.

The metamorphosed and deformed SSZ is also characterized by the emplacement of subduction-related, mainly Mesozoic calc-alkaline plutons and lavas (Figs 2a, 4a, 5b). The age compilation of recent radiometric datings (Fig. 5c) confirms that the SSZ is the locus of arc magmatism essentially during the Mesozoic (Berberian & Berberian, Reference Berberian, Berberian, Gupta and Delany1981), but recent data showed local magmatic activity as young as Eocene in the NW SSZ (Mazhari et al. Reference Mazhari, Bea, Amini, Ghalamghash, Molina, Montero, Scarrow and Williams2009b). There is also an apparent time migration from SE to NW (Agard et al. Reference Agard, Omrani, Jolivet and Mouthereau2005; Fig. 2a) that is not accounted for at present. Note also the possible time clusters of magmatic activity (Fig. 5c; Late Jurassic: Ahmadi Khalaji et al. Reference Ahmadi Khalaji, Esmaeily, Valizadeh and Rahimpour-Bonab2007; Fazlnia et al. Reference Fazlnia, Moradian, Rezaei, Moazzen and Alipour2007; Shahbazi et al. Reference Shahbazi, Siebel, Pourmoafee, Ghorbani, Sepahi, Shang and Vousoughi Abedini2010; and Late Cretaceous), but more data is required to ascertain this. The timing of inception of subduction is debated in the literature, whether Late Triassic (which would coincide with the time of closure of the Palaeo-Tethys to the north) to Early Jurassic (Berberian & King, Reference Berberian and King1981; Arvin et al. Reference Arvin, Pan, Dargahi, Malekizadeh and Babaei2007) or Late Jurassic (Mohajjel, Fergusson & Sahandi, Reference Mohajjel, Fergusson and Sahandi2003). Note, however, that Triassic magmatic rocks in Iran are almost completely restricted to the SSZ (Fig. 2b; Berberian & Berberian, Reference Berberian, Berberian, Gupta and Delany1981), some showing a calc-alkaline signature, which could suggest that subduction had already begun by the Late Triassic (J. Omrani, unpub. data).

Metamorphism is marked by mostly low-grade greenschist facies rocks, but reaches amphibolite facies near some plutons (near Hamadan and Azna; Agard et al. Reference Agard, Omrani, Jolivet and Mouthereau2005; J. Omrani, unpub. Ph.D. thesis, Institut des Sciences de la Terre, Paris, Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008). The dominant regional schistosity, S2, is characterized by tight S- to SW-vergent isoclinal folds affecting the Permian to Cretaceous formations to the north (Mohajjel & Fergusson, Reference Mohajjel and Fergusson2000) and to the south of the SSZ (Sheikholeslami et al. Reference Sheikholeslami, Bellon, Emami, Sabzehei and Pique2003). Although seemingly related to intrusions, the age of this Andean-type metamorphism is not well constrained, with possibly a first stage during Late Jurassic time and a second one during Late Cretaceous time (J. Braud, unpub. Ph.D. thesis, Univ. Paris-Sud, 1987). Eclogites have also been locally reported from the SSZ (Davoudian et al. Reference Davoudian, Khalili, Noorbehsht, Dachs, Genser and Shabanian2006, Reference Davoudian, Genser, Dachs and Shabanian2007), but are Triassic in age (Davoudian, pers. comm).

Late Cretaceous strike-slip movements were reported by several authors (Stöcklin, Reference Stöcklin1968; Alavi, Reference Alavi1994; Mohajjel & Fergusson, Reference Mohajjel and Fergusson2000; Mohajjel, Fergusson & Sahandi, Reference Mohajjel, Fergusson and Sahandi2003; Agard et al. Reference Agard, Omrani, Jolivet and Mouthereau2005). The SSZ was nevertheless only thrust onto the Crush Zone from the Mid-Miocene onwards (Agard et al. Reference Agard, Omrani, Jolivet and Mouthereau2005; Fig. 4b). Limited yet consistent apatite fission-track data (J. Omrani, unpub. Ph.D. thesis, Institut des Sciences de la Terre, Paris, Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008; Homke et al. Reference Homke, Verges, Van Der Beek, Fernandez, Saura, Barbero, Badics and Labrin2010) cluster in the range 25–28 Ma (Fig. 4a). These ages suggest that the SSZ, by contrast with the ZFTB, was significantly uplifted and eroded prior to Miocene time.

3.d. The Urumieh–Dokhtar Magmatic Arc (UDMA)

The UDMA (Schröder, Reference Schröder1944), situated between the SSZ and Central Iran, runs parallel to the Zagros and the SSZ (Figs 1c, 4a). It forms a topographic ridge separating the SSZ from Central Iran, and bears huge volcanosedimentary deposits, in places > 10 km thick (Dimitrijevic, Reference Dimitrijevic1973).

The UDMA hosts abundant Tertiary magmatism, dominantly of arc (Berberian & Berberian, Reference Berberian, Berberian, Gupta and Delany1981; Berberian et al. Reference Berberian, Muir, Pankhurst and Berberian1982; Emami, Reference Emami2000) or island-arc type (Shahabpour, Reference Shahabpour2005). Volumetrically, volcanic rocks were mostly produced during Eocene time (Farhoudi, Reference Farhoudi1978; Shahabpour, Reference Shahabpour2005), with the oldest ones dating back to Early Eocene time (Ypresian; c. 55–50 Ma; Emami, Reference Emami2000). Note, however, as already emphasized by Omrani et al. (Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008) that the Eocene magmatic activity was not restricted to the UDMA (Fig. 2a, d). Intrusive rocks, by contrast, are dominantly Oligo-Miocene (Berberian & Berberian, Reference Berberian, Berberian, Gupta and Delany1981), but more recent datings and maps are probably needed to strengthen this. The UDMA was interpreted as subduction-related (with different locations for the suture zone: Berberian & Berberian, Reference Berberian, Berberian, Gupta and Delany1981; Alavi, Reference Alavi1994) or rift-derived (Amidi, Emami & Michel, Reference Amidi, Emami and Michel1984). More recently, Omrani et al. (Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008) showed that the UDMA rocks derive from a supra-subduction mantle source, with comparable multielement patterns to those from the SSZ (Fig. 5b).

Radiometric age constraints for the UDMA are rather poor, but NW to SE gradients can be identified from stratigraphic constraints on available geological maps: from 35 to 16 Ma and from 24 to 12 Ma to the NW and SE of the Doruneh fault, respectively. New age constraints, using zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb and whole-rock Ar–Ar age data (Chiu et al. Reference Chiu, Zarrinkoub, Chung, Lin, Yang, Lo, Mohammadi and Khatib2010; see also Fig. 7) confirm this trend, with a cessation of calc-alkaline magmatism in the UDMA in Late Oligocene time near Tabriz (~ 27 Ma), in Middle Miocene time in Esfahan (~ 16 Ma) and in Late Miocene time near Kerman (~ 7 Ma).

By contrast to normal calc-alkaline magmatism, which is found everywhere along the UDMA, restricted adakitic provinces are found from ~ 10 Ma onwards in the extreme NW of Iran (Jahangiri, Reference Jahangiri2007) and in the central part of the Zagros orogen, around Anar (Omrani et al. Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008; Figs 4a, 5d). Age constraints were based on cross-cutting relationships (across the Upper Red Fm), and recent radiometric constraints indeed range between 10 and 1 Ma (Chiu et al. Reference Chiu, Zarrinkoub, Chung, Lin, Yang, Lo, Mohammadi and Khatib2010). Omrani et al. (Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2008, Reference Omrani, Agard, Whitechurch, Benoit, Prouteau and Jolivet2009), based on the type and location of high-silica adakites (HSA, Fig. 5e; Martin et al. Reference Martin, Smithies, Rapp, Moyen and Champion2005), related this adakitic magmatism to slab break-off below the south-central part of the Zagros. This is also suggested by tomographic images (Fig. 6), which show that the Neo-Tethyan slab is detached below the central Anar region and possibly further to the NW (Hafkenscheid, Wortel & Spakman, Reference Hafkenscheid, Wortel and Spakman2006).

Figure 6.

Tomographic sections across the Zagros orogen (location of transects on map; W. Spakman, unpub. data; for acquisition details see Hafkenscheid, Wortel & Spakman, Reference Hafkenscheid, Wortel and Spakman2006 for example). Tomographic sections A and B constrain our present-day lithospheric-scale 1D reconstruction of the Kermanshah and Anar transects (Fig. 9) and their evolution through time (Figs 10, 11). Note that both A and B sections point to a detached slab below the Zagros, although to a lesser extent for section A. Sections A′–C′, though less precise in terms of velocity anomalies, are used to further suggest that the remnant Neo-Tethyan slab could be somewhat more continuous below the Kermanshah region (NW Zagros) than below the Central Zagros. We interpret this as a more recent slab break-off below the NW Zagros (whether the slab may even be still attached in places, however, cannot be assessed confidently). More extensive slab break-off below the Central Zagros is consistent with the finding of Upper Miocene adakites in the Central Zagros only (Fig. 5d, e) and a northward younging trend for these adakites (from ~ 9 to 1 Ma; Chiu et al. Reference Chiu, Zarrinkoub, Chung, Lin, Yang, Lo, Mohammadi and Khatib2010). Ages on map: age constraints for recent slab break-off in the Alpine convergence zone (between E Turkey and Makran). NAF – North Anatolian Fault.

The most recent magmatic activity is found in the SE part or in the extreme NW of the UDMA (near the Turkish border; Jahangiri, Reference Jahangiri2007; Kheirkhah, Allen & Emami, Reference Kheirkhah, Allen and Emami2009). Alkaline, post-collisional lavas (as in Turkey, e.g. Pearce et al. Reference Pearce, Bender, De Long, Kidd, Low, Guner, Saroglu, Yilmaz, Moorbath and Mitchell1990; Keskin, Pearce & Mitchell, Reference Keskin, Pearce and Mitchell1998; or as reported elsewhere in Eurasia, e.g. Chung et al. 2005) are found in volumetrically large amounts in the latter area only, but small outcrops of (presumably Quaternary) alkaline basalt are found in many places, mainly along fault zones.

3.e. Central Iran and the Iranian plateau

Central Iran comprises metamorphic successions and plutonic suites (e.g. Chapedony and Posht-e-Badam metamorphic complexes: Haghipour, Reference Haghipour1974; Nadimi, Reference Nadimi2007; Precambrian plutonism: Berberian & Berberian, Reference Berberian, Berberian, Gupta and Delany1981) and overlying Jurassic–Cretaceous and subordinate Palaeogene cover formations (see also Fig. 3).

There has been ongoing debate on whether Central Iran represents a central median mass (see Stöcklin, Reference Stöcklin1968 for an historical perspective) or corresponds to a mosaic of microblocks (e.g. Lut, Tabas, Yazd and Anarak blocks). Despite complexities inherited from pre-Pan-African times (e.g. the N–S trend in the Tabas area, or the Lut ‘block’, which was a Precambrian basement horst; Stöcklin, Reference Stöcklin1968; Nadimi, Reference Nadimi2007 and references therein) or Palaeo-Tethys closure (Bagheri & Stampfli, Reference Bagheri and Stampfli2008), Central Iran, the UDMA and SSZ can together be considered to represent the upper plate domain during most of the recent convergence history leading to the Zagros orogeny. The main volcanic activity, as for the UDMA, took place during Eocene time. A stage of extensional tectonic activity, marked by distributed extension and the formation of core-complexes, took place during mid-Eocene time (c. 45 Ma; Verdel et al. Reference Verdel, Wernicke, Ramezani, Hassanzadeh, Renne and Spell2007) and pre-dates later, syn- to post-Oligocene SW–NE shortening (Kargaranbafghi, Neubauer & Genser, Reference Kargaranbafghi, Neubauer and Genser2010).

Most importantly, Central Iran is cross-cut by several ophiolitic domains (Nain–Baft, Sabzevar, Sistan; Fig. 1c) interpreted as minor oceanic seaways showing discontinuous oceanic crust emplacement (e.g. Baroz et al. Reference Baroz, Macaudière, Montigny, Noghreyan, Ohnenstetter and Rocci1984; Arvin & Robinson, Reference Arvin and Robinson1994). These domains correspond to the Upper Cretaceous to Paleocene radiolarite and ophiolite ‘coloured melange’ of Gansser (Reference Gansser1960) and to the inner Mesozoic oceans of McCall (Reference McCall1997).

Both the back-arc Nain–Baft (Arvin & Robinson, Reference Arvin and Robinson1994) and the Sabzevar oceanic domains are thought to have opened during Late Cretaceous time and closed during Paleocene time (c. 95–60 Ma; Davoudzadeh, Reference Davoudzadeh1972; Baroz et al. Reference Baroz, Macaudière, Montigny, Noghreyan, Ohnenstetter and Rocci1984; Sengör et al. Reference Sengör, Altiner, Cin, Ustaömer, Hsü, Audley-Charles and Hallam1988; Arvin & Robinson, Reference Arvin and Robinson1994; Stampfli & Borel, Reference Stampfli and Borel2002; Shojaat et al. Reference Shojaat, Hassanipak, Mobasher and Ghazi2003). Few radiometric age constraints are available for the Nain–Baft (93 Ma, K–Ar dating on hornblende; Shafaii Moghadam et al. Reference Shafaii Moghadam, Whitechurch, Rahgoshay and Monsef2009) and Sabzevar ophiolites (86–87 Ma; Baroz et al. Reference Baroz, Macaudière, Montigny, Noghreyan, Ohnenstetter and Rocci1984).

The Nain–Baft ophiolite is not found north of Nain and the Doruneh fault, which suggests that the SSZ was only partially separated from Central Iran. The suggestion that the Nain–Baft could be equivalent to the Neyriz and Kermanshah ophiolites (i.e. considering them as tectonic allochthons from the Nain–Baft ophiolite across the SSZ) was made by Alavi (Reference Alavi1994) and more recently by Shafaii Moghadam, Stern & Rahgoshay (Reference Shafaii Moghadam, Stern and Rahgoshay2010). The following major objections need to be recalled, however: (1) contrary to the outer ophiolites, which have equivalents in Turkey, there is no continuation to the north of the Nain–Baft domain, (2) blueschists marking the suture are only found to the SW of the SSZ, and (3) the SSZ does not correspond to fore-arc material.

The palaeogeography of the Sabzevar ophiolitic domain is uncertain, with authors connecting it to Nain–Baft (Stampfli & Borel, 2004; Bagheri & Stampfli, Reference Bagheri and Stampfli2008) or to Sistan (Barrier & Vrielynck, Reference Barrier and Vrielynck2008; Saccani et al. Reference Saccani, Delavari, Beccaluva and Amini2010), making it an independent small oceanic domain, or connecting it to both (see Saccani et al. Reference Saccani, Delavari, Beccaluva and Amini2010, fig. 12). High-pressure granulites in one of the Sabzevar sub-units, with ~ 106 Ma zircon overgrowths, could also testify to early subduction events in the region (Rossetti et al. Reference Rossetti, Nasrabady, Vignaroli, Theye, Gerdes, Razavi and Vaziri2009).

The Sistan region, finally, forms a > 700 km long, N–S belt sandwiched between Central Iran and Afghanistan or, more generally, between the Zagros and Himalayan orogens. The Sistan oceanic domain once represented a back-arc domain or a branch of the Neo-Tethys (Tirrul et al. Reference Tirrul, Bell, Griffis and Camp1983; hence breaking through Eurasia like the Alpine Liguro-Piemontese ocean; Agard & Lemoine, Reference Agard and Lemoine2005), and was later closed during Paleocene–Eocene time through an E-dipping subduction. Aptian–Albian radiolarites were reported by Babazadeh & de Wever (2004), suggesting oceanic lithosphere production between 110 and 50 Ma and a somewhat earlier (Early Cretaceous?) rifting than for the Nain–Baft and Sabzevar domains. Subduction is thought to have started between the Turonian–Maastrichtian (Saccani et al. Reference Saccani, Delavari, Beccaluva and Amini2010) and led to the formation of some highly deformed blueschist and eclogite-bearing melanges (Fotoohi Rad et al. Reference Fotoohi Rad, Droop, Amini and Moazzen2005). Note that exhumation of HP rocks took place at the same time as for the Zagros blueschists (c. 86 Ma by Rb–Sr dating on phengite: Bröcker, Fotoohi Rad & Theunisson, Reference Bröcker, Fotoohi Rad and Theunissen2010; these authors dismissed, on the suspicion of excess argon, the earlier Ar–Ar ages of Fotoohi Rad, Droop & Burgess, Reference Fotoohi Rad, Droop and Burgess2009, mainly in the range 124–116 Ma). After closure, the Sistan region was affected by intense Eo-Oligocene to Early Miocene magmatic activity (Camp & Griffis, Reference Camp and Griffis1982; Fig. 2d, e). These authors (as well as Sadeghian et al. Reference Sadeghian, Bouchez, Nedelec, Siqueira and Valizadeh2005) related the Upper Eocene–Lower Oligocene calc-alkaline granitic batholiths to the anatexis of marine sediments, and the mainly Oligocene alkaline volcanics and minor intrusions to major transcurrent faults. As for other parts of Iran, late alkaline activity is found along some major dextral strike-slip faults from Late Miocene time onwards (Fig. 2; i.e. the Nayband and Ney faults; Walker et al. Reference Walker, Gans, Allen, Jackson, Khatib, Marsh and Zarrinkoub2009).

3.f. Far-field deformation areas: Alborz, Kopet Dagh

These regions are only briefly described here (see publications below and references therein for details). The Alborz Mountains (Fig. 1c) represent a good example of an intracontinental belt, with a stretched continental domain inverted during Late Triassic time (Zanchi et al. Reference Zanchi, Berra, Mattei, Ghassemi and Sabouri2006) and the Tertiary (e.g. Guest et al. Reference Guest, Axen, Lam and Hassanzadeh2006; Ritz et al. Reference Ritz, Nazari, Ghassemi, Salamati, Shafei, Solaymani and Vernant2006), as a result of both Palaeo-Tethyan and Neo-Tethyan closures. The Alborz is thus an integral part of the Arabia–Eurasia collision zone, and accommodates present-day convergence (Fig. 1d) through orogen-normal shortening and lateral escape (left-lateral escape of the South Caspian basin in the west, and more complex deformation partitioning, including right-lateral movements, to the east; Hollingsworth et al. Reference Hollingsworth, Jackson, Walker and Nazari2008). The adjacent Kopet Dagh fold-and-thrust belt (Fig. 1d), which developed at the expense of a thick Jurassic to Oligocene trough (~ 6 km of sediments, i.e. more than in the Zagros for that period), also reworks the Palaeo-Tethys suture zone. Across-range shortening estimates for the Tertiary deformation are of the order of 30–35 km for both areas (Allen et al. Reference Allen, Ghassemi, Shahrabi and Qorashi2003; Guest et al. Reference Guest, Axen, Lam and Hassanzadeh2006). Convergence also likely resulted, since 2–5 Ma, in the subduction of the South Caspian basin below the Apsheron–Balkhan sill to the north and the Talesh region to the west (Jackson et al. Reference Jackson, Priestley, Allen and Berberian2002; Masson et al. Reference Masson, Djamour, Van Gorp, Chery, Tatar, Tavakoli, Nankali and Vernant2006; Hollingsworth et al. Reference Hollingsworth, Jackson, Walker and Nazari2008).

Differences between the Alborz region and Central Iran and the SSZ include thicker (up to 5 km!) Middle Jurassic series and, unlike the SSZ, the lack of an Upper Jurassic–Lower Cretaceous unconformity (only a slight period of emersion is reported). There is evidence for a Late Cretaceous–Paleocene compression (Barrier & Vrielynck, Reference Barrier and Vrielynck2008). Minor Cretaceous magmatism is reported in the Alborz, but the main magmatic activity is broadly coincident with that of the UDMA and dated between 45 and 36 Ma (Ballato et al. Reference Ballato, Mulch, Landgraf, Strecker, Dalconi, Friedrich and Tabatabaei2010). It is exemplified by the Eocene Karaj Formation, making up to 4 km thick calc-alkaline tuffs, and generally thought to mark (possibly back-arc) extension. The Damavand, by contrast, stands out as an isolated alkaline Quaternary volcano, possibly associated with deep lithospheric delamination (i.e. partial removal of the lithospheric mantle; Mirnejad et al. Reference Mirnejad, Hassanzadeh, Cousens and Taylor2010).

5.a. Geodynamic and geophysical constraints

5.b. Lithospheric-scale reconstructions across two transects

We briefly describe the lithospheric evolution of the Zagros through seven stages for each transect (Fig. 10): Late Cretaceous (90 Ma), K–T boundary (65 Ma), Late Paleocene (60–55 Ma), Middle Eocene (45–40 Ma), Late Oligocene (25 Ma), Late Miocene (10 ± 5 Ma) and present day (enlarged in Fig. 9). For the sake of discussion, close-up views of the Kermanshah transect are also given in Figure 11.

Figure 10.

Lithospheric-scale cross-sections across two transects (Fig. 9; location in Fig. 1c). The northern section (a–g; termed ‘Kermanshah’) crosses the whole orogen from the foreland, on the Arabian plate, to the Caspian Sea. The second section (a′–g′; ‘Anar’) runs from Arabia to Central Iran, across the Deshir and Anar faults. The present-day sections were established using all available geological, kinematic and geophysical data (see Section 5.a for details). We selected seven stages for each of the transects: Late Cretaceous (90 Ma), K–T boundary (65 Ma), Late Paleocene (60–55 Ma), Middle Eocene (45–40 Ma), Late Oligocene (25 Ma), Late Miocene (10 ± 5 Ma) and present-day cross-sections (enlarged in Fig. 9). For the sake of discussion, close-up views of the Kermanshah transect are also given in Figure 11. Detailed comments are given in Section 5.b of the text. Abbreviations: CC – core-complex; ETMD – Early Tertiary Magmatic Domain; HZ – High Zagros Fault; MZT – Main Zagros Thrust; NB – Nain–Baft; SFB – Simply Folded Belt; SSZ – Sanandaj–Sirjan Zone; UDMA – Urumieh–Dokhtar Magmatic Arc; ZFTB – Zagros fold-and-thrust belt.

Figure 11.

Close-up views of the lithospheric-scale cross-sections presented in Figure 10, emphasizing some of the salient events of Zagros geodynamics (see discussion in Section 6.a). Abbreviations as for Figure 10.