The Late Cretaceous–early Paleogene tectonic evolution of the Iranian Plateau is not well understood in comparison to its well-studied late Paleogene–Neogene evolution. Exhumation, metamorphism, and changing sedimentary environments are documented away from the plateau margin for this time interval, however the nature and mechanism of deformation in the interior of the Iranian Plateau remains controversial to the point that both compressional and extensional mechanisms are proposed. We use K-feldspar and mica 40Ar/39Ar thermochronology to determine the thermal evolution of metamorphic rocks exposed in northeastern Great Kavir Basin on the Iranian Plateau. Multi-domain diffusion modeling of K-feldspar and field relationships reveal a stage of rapid cooling via tectonic exhumation starting in the Late Cretaceous and lasting until the early Eocene. We attribute this to continental extension as being accommodated on detachment faults that exhumed the Shotor Kuh–Biarjmand metamorphic core complex. The metamorphic rocks of this core complex underlie a southeastward-younging structural-stratigraphic sequence that includes at its top Late Cretaceous (Campanian–Maastrichtian) carbonates. We interpret this sequence as a crustal section that was exhumed by a NW-dipping master detachment fault. Based on the locations of the syn-extensional detrital rocks we propose that this system accommodated ∼100 km of NW-SE–oriented extension. Our results indicate that extensional deformation started in the Iranian Plateau much earlier (Late Cretaceous) than previously thought (Eocene–Oligocene).
During the last decade, many studies have focused on the late Paleogene–early Neogene tectonic evolution of the Iranian Plateau, when pervasive deformation, surface uplift, exhumation, and changes in sedimentary environment occurred as a result of the Arabia-Eurasia collision along the Zagros suture zone (Fig. 1) (e.g., McQuarrie and van Hinsbergen, 2013; Guest et al., 2007; Fakhari et al., 2008, Agard et al., 2011; Ballato et al., 2011; Allen and Armstrong, 2008). However, limited information about Late Cretaceous and early Paleogene tectonics of this region exists. Understanding the tectonic evolution of the Iranian Plateau during this time is important for three reasons. First, it will contribute to filling a gap in our understanding of the Late Cretaceous tectonic setting of the Iranian sector of the Alpine-Himalayan orogenic belt. In both the Alps and Himalaya, compressional deformation started during Late Cretaceous–mid-Paleogene time (Dumont et al., 2011; Bousquet et al., 2002; Stampfli et al., 1998; O’Brien, 2001; Bellahsen et al., 2014; DeCelles et al., 2014; Wu et al., 2014; Hu et al., 2015), but this event has not been previously documented in the Iranian sector. Second, understanding the tectonic evolution of the Iranian Plateau in late Cretaceous–early Paleogene time, when the area was in the upper plate of a subduction system, will shed light on the style of deformation in the upper plate of Neotethyan subduction system just before continental collision. Third, in a larger scale, it will help to better understand the style of continental deformation in convergent systems.
The Late Cretaceous was a time of considerable change in tectonic activity in the Iranian Plateau. During this time interval, plate convergence between Arabia and Eurasia accelerated (Monie and Agard, 2009) and blueschist exhumation occurred along the southern margin of the plateau (Agard et al., 2006). Several important tectonic features formed all around the Iranian Plateau. For instance, an extensive angular unconformity separating terrestrial detrital deposits from older epicontinental and continental shelf sediments is recognized all around the plateau (Davoudzadeh and Schmidt, 1985; Mahboubi et al., 2001; Stöcklin, 1968; Stöcklin and Setudehnia, 1991; Rahimzadeh, 1983). Greenschist facies metamorphism affected the Sanandaj-Sirjan zone during the Late Cretaceous–Paleocene (e.g., Malayer region [Alavi, 1994]; northwest of Arak [Radfar and Kohansal, 2004]; Soursat complex in Takab region [Jamshidi Badr et al., 2010]; Khondab [Kohansal, 2005]). Guest et al. (2006) documented a stage of rapid cooling starting at ca. 80 Ma in the central and western Alborz range (Fig. 1). In addition, the Late Cretaceous is the time of the Doruneh-Kashmar ophiolites obduction in northeastern Iran (Fig. 2), as recorded by the existence of ultramafic clasts in early Paleocene conglomerates (e.g., Shahrabi et al., 2006). These observations imply that not only the southern margin of the Iranian Plateau but also its interior was affected by Late Cretaceous tectonic activity. Nevertheless, the nature of deformation during the Late Cretaceous–early Paleogene remains uncertain. The deformation may have been limited to only compression, or extensional deformation may have occurred, as documented in the Anatolian plate (Advokaat et al., 2014; Lefebvre et al., 2011; Gautier et al., 2008; Genç and Yürür, 2010).
In order to address these issues, we have studied an area in the northeastern Great Kavir Basin between the eastern Alborz and the Central Iran microcontinent (Figs. 1 and 2). The study area extends from the town of Biarjmand toward the southeast to the Doruneh-Kashmar ophiolites where a series of igneous, metamorphic, and sedimentary rocks with ages ranging from Neoproterozoic to Tertiary are exposed. In the northwestern segment of this region, a unique complex of intrusives and amphibolite- to greenschist-facies metamorphic rocks are exposed. We call this segment the Shotor Kuh–Biarjmand complex (SKBC).
This region has been studied for two reasons. First, unlike many deep crustal rocks exposed along the Sanandaj-Sirjan zone that were likely affected by the late Eocene–Miocene tectonics, our study area is located ∼500 km away from the plate margin, and the older deformation and thermal events are not significantly overprinted by younger mid- to late Tertiary events. Second, our preliminary investigation (Hassanzadeh et al., 2005) indicated that metamorphic rocks in the study area cooled rapidly during Late Cretaceous–Paleocene time. Hence, this area provides an exceptional opportunity to decipher the Mesozoic to Paleogene tectonics of the interior of the Iranian Plateau at shallow to mid-crustal levels.
Recently, the age and tectonic setting of metamorphism of the SKBC have been studied, but much disagreement about timing and structure persists. Proposed ages of amphibolite-grade metamorphism range widely: Precambrian (Hushmandzadeh et al., 1978a, 1978b), pre-Permian (Rahmati-Ilkhchi, 2003), pre-Jurassic (Khalatbari-Jafari, 2000), Triassic (Navab-Motlagh, 2005), and Early to Late Jurassic (Rahmati-Ilkhchi et al., 2010). Similarly, both compressional and extensional events have been proposed for the exhumation of this complex. Rahmati-Ilkhchi et al. (2010; 2011) proposed that this complex was metamorphosed by compressional and extensional events in the Early–Middle Jurassic and subsequently affected by a compressional event in the Late Cretaceous and Neogene. In contrast, Hassanzadeh et al. (2005) and Bagheri et al. (2009) have proposed an extensional mechanism for exhumation of this complex during the Late Cretaceous and the Early Cretaceous, respectively.
We developed a model for the tectonic evolution of the study area using field observations, 40Ar/39Ar thermochronology of K-feldspar (multi-domain diffusion modeling) and mica, and zircon U-Pb geochronology. In our model, exhumation and rapid cooling of mid-crustal rocks in this region started in the Late Cretaceous (ca. 76 Ma) by northwest-directed slip along a low-angle normal fault. Finally, the magnitude of the extension and the regional tectonic implications of our findings will be discussed.
GEOLOGY OF NORTHEASTERN GREAT KAVIR BASIN
The Great Kavir Basin, which is located in the interior of the Iranian Plateau (Fig. 1), is a vast area bounded by the Alborz range to the north and northwest, the Sabzevar ophiolite and Kopeh Dagh range to the northeast, the Doruneh fault and Doruneh-Kashmar ophiolites to the south and southeast, and the Urmieh-Dokhtar magmatic belt to the west and southwest (Figs. 1 and 2).
Most of the Great Kavir Basin is covered by late Cenozoic sedimentary rocks, but older crystalline and sedimentary rocks are exposed along the northeastern side in our study area (Fig. 2). In this region, extending for ∼140 km from the town of Biarjmand in the northwest to the Doruneh fault in the southeast (Fig. 2), a series of intrusive, metamorphic, and sedimentary rocks as old as the Neoproterozoic (Hassanzadeh et al., 2008) is exposed. In general, the age and initial depth (stratigraphic position) of rocks in this area are greatest in the northwest near Biarjmand and both decrease toward the Doruneh fault to the southeast (Figs. 4 and 5). The geological maps on which this information is based are compiled from several maps of the Geological Survey of Iran (Khan Nazer, 1992; Hushmandzadeh et al., 1978; Khalatbari-Jafari, 2000; Rahmati-Ilkhchi, 2003; Ghasemi, 2005; Navai et al., 1987; Salamati, 1999; Kohansal, 2007; Navab-Motlagh, 2005) and our field observations. Our field observations were focused on the igneous and metamorphic rocks that are exposed in the Shotor Kuh, Biarjmand, and Band-e-Hezar Chah (BHC) ranges (Fig. 3).
GEOLOGY OF THE SHOTOR KUH–BIARJMAND METAMORPHIC CORE COMPLEX
The mid-crustal and high-temperature tectonic evolution of the Great Kavir Basin is potentially preserved in the SKBC. Rocks of the SKBC are exposed in three ranges: the BHC in the northwest, the Shotor Kuh–Majerad range in the south, and the Biarjmand range in northeast (Fig. 3), which are separated by young structurally controlled basins covered by Quaternary alluvium.
Structurally, the core complex is composed of three plates (Figs. 3 and 4). (1) A lower plate is made primarily of amphibolite-grade metamorphic rocks, which are overlain by a greenschist-grade ductile shear zone (the Majerad detachment). (2) The middle plate is composed of greenschist facies metasedimentary and metavolcanic rocks (in the Biarjmand, Majerad, and Shotor Kuh ranges. The middle plate is bounded above by the Yazdu brittle detachment fault. (3) The upper plate is composed largely of Lower Cretaceous carbonate strata in klippen in the Biarjmand and Shotor Kuh ranges; an assortment of other rocks in the BHC area might also be part of the upper plate (discussed in more detail below).
Lower-plate exposures constitute: (1) Late Proterozoic to Early Cambrian granite and granodiorite (unit gr, Fig. 3) (ages based on U-Pb zircon ages; Hassanzadeh et al., 2008; Rahmati-Ilkhchi et al., 2011; Balaghi Einalou et al., 2014). (2) Amphibolite-grade metamorphic rocks, including mica schist, amphibolite, marble, and orthogneiss (unit gn, Fig. 3), with protolith ages ranging from Proterozoic (Hassanzadeh et al., 2008) to Permo-Triassic (Rahmati-Ilkhchi, 2003). Hassanzadeh et al. (2005) reported a 40Ar/39Ar hornblende age of 210 Ma from the amphibolite facies rocks. Considering the peak metamorphic temperature of these rocks (650 °C; Rahmati-Ilkhchi et al., 2010) and closure temperature of hornblende (∼500°C; Harrison, 1982), this age records cooling of these rocks during the Late Triassic, after peak metamorphism, thus the metamorphism could not have been during the Middle Jurassic as suggested by Rahmati-Ilkhchi et al. (2011) based on a muscovite 40Ar/39Ar age.
Middle plate constitutes greenschist facies metamorphic rocks (Sch1 and Sch2, Figs. 3 and 4). These rocks have sedimentary and volcanic protoliths as young as Late Jurassic–Early Cretaceous (Hushmandzadeh et al., 1978; Rahmati-Ilkhchi et al., 2010; Ghasemi, 2005). Our mica ages (discussed below) from these rocks indicate a Late Cretaceous (70–75 Ma) age for metamorphism.
Upper plate is composed of unmetamorphosed sedimentary and volcanic rocks, which can be divided in two series: (1) Lower Cretaceous carbonates of the upper plate (unit K1l, Fig. 3) that are in tectonic contact with the middle plate, and (2) Paleocene or younger, mainly terrestrial deposits and volcanic rocks that are depositional on older rocks of all three plates. Figure 6 shows the position of these structural units to the southeast of Kuh-e-Yazdu mountain.
The BHC range displays some differences and similarities to the Biarjmand, Shotor Kuh, and Majerad ranges. Unlike in these areas, where the lower plate contains various amphibolite-facies metamorphic rocks (e.g., metapelites, marbles, and orthogneisses), the only amphibolite-facies rocks in the BHC are orthogneisses (Fig. 3). The unmetamorphosed part of these BHC crystalline rocks is leucogranite and granodiorite. Both the orthogneiss and leucogranite yielded late Proterozoic–early Cambrian zircon U-Pb ages, similar to those from the Biarjmand and Shotor Kuh–Majerad ranges (Hassanzadeh et al., 2008), suggesting similar origins. In addition, a series of unmetamorphosed sedimentary rocks, mainly shale and sandstone, are exposed in the west and central parts of the BHC. Based on litho-stratigraphical similarities to the Jurassic rocks, the age of the western part is assigned to the Jurassic (Amini-Chehragh, 1999), in agreement with the Middle Jurassic age suggested by Hassanzadeh et al. (2008).
Similarities between the BHC area and the ranges to the south and east include the following.
(1) A series of greenschist facies metamorphic rocks, including phyllite, slate, metasandstone, and quartz-chlorite schists, is exposed in the eastern and southwestern BHC. The age of metamorphism and the nature of upper and lower contacts of this unit are not well known. However, in the Meyamey region (∼40 km north of the BHC; Fig. 2) where similar rock units are exposed, Amini-Chehragh (1999) found Campanian–Maastrichtian fossils in the slates and phyllites indicating that the metamorphism was syn- or post-Maastrichtian.
(2) All of these ranges share a similar thermal history between 160 and 80 Ma (see argon thermochronology section).
(3) Similar to relations in the Shotor Kuh range (Rahmati-Ilkhchi, 2003) (shown by a red star in Kuh-e-Rezveh in Fig. 3) and unlike those in the Biarjmand range, the Early Cretaceous Orbitolina limestones overlie greenschist facies metamorphic rocks in the BHC (Kuh-e-Kiki, Fig. 3) with an apparently depositional contact (see discussion section for significance of this contact).
By the late Paleocene (Rahmati-Ilkhchi, 2003) to early middle Eocene, all structural levels of the SKBC were exposed and shedding detritus into a widespread basin mainly containing conglomerates. The thickness of the conglomerate ranges from a few tens of meters in the northeastern Biarjmand block (Khalatbari-Jafari, 2000; out of the mapped area in Fig. 3) to 500 m on western Kuh-e-Molhedu mountain (Ghasemi, 2005) and to 1000 m in the BHC (Amini-Chehragh, 1999) and southwest of Shotor Kuh range (Rahmati-Ilkhchi, 2003). The detrital sequence was covered by series of volcanic and volcaniclastic rocks during the middle to late Eocene.
During Neogene time, marl and gypsiferous sediments were deposited in the SKBC, indicating closed basin deposition. Development of basins could be related to the compressional deformation that affected the region. This deformation produced folding in the west of the Biarjmand range. The reverse faults in the south of Kuh-e-Yazdu and Kuh-e-Molhedu in the Biarjmand range might also be related to this compressional phase (Fig. 3).
Three main groups of dikes intrude the SKBC rock units and provide some age constraints. The oldest group has a mafic composition, is intruded into the lower-plate rocks, and is metamorphosed to amphibolite schists (Rahmati-Ilkchi, 2003). Therefore, its intrusion age is older than that of amphibolite-grade metamorphism. The second group consists of unmetamorphosed diabasic dikes intruded into the lower plate. The youngest group cuts Eocene rocks. Below, we will focus on the second group because it gives insight about the pre-extensional thermal evolution of the lower plate.
All dike swarms of the second group in the SKBC probably have the same Late Jurassic–Early Cretaceous age and are related to the same magmatic system, although detailed geochemical analyses are required to confirm this interpretation. In the Biarjmand block, 40Ar/39Ar amphibole dating from a dike of the second group (Fig. 7) yielded an age of 144 ± 6 Ma (see argon thermochronology section), consistent with an older but less precise U-Pb apatite age of 152 ± 35 Ma (Balaghi Einalou et al., 2014). The Late Jurassic–Early Cretaceous age is consistent with the fact that the dikes are not found in Aptian–Albian limestones of the upper plate and also in accordance with a 210 Ma hornblende 40Ar/39Ar age that we have from the lower-plate amphibolite facies rocks of the Biarjmand range (Hassanzadeh et al., 2005). Ages for dikes in two other ranges (BHC and Shotor Kuh) are not known. However, they cut rocks as young as middle Jurassic (Ghasemi, 2005), so are post–Middle Jurassic. On the basis of the above data, we interpret that the dike swarms in the BHC and Shotor Kuh–Majerad blocks are Late Jurassic–Early Cretaceous.
Deformational and Metamorphic Events
Four main deformation events affected the Shotor Kuh and Biarjmand blocks. The oldest, D1, affected the lower-plate rocks during amphibolite facies metamorphism with peak conditions at ∼650 °C and 7.5 kbar (Rahmati-Ilkhchi et al., 2010). Pervasive mylonitic foliation and stretching lineation developed in this event (Fig. 8A), which affected the Shotor Kuh–Majerad and Biarjmand blocks, where kyanite, staurolite, and sillimanite schists, amphibolites, marbles, and orthogneiss were observed. Microstructural features (Fig. 9) such as feldspar core and mantle structures (Fig. 9A), myrmekites in low-strain segments of feldspar porphyroclasts (Figs. 9B and 9C), grain boundary migration between quartz and feldspar (Fig. 9D), and thick quartz ribbons containing fully crystallized grains (Fig. 9B) indicate deformation in high temperatures (>600 °C) (Passchier and Trouw 2005; Tullis, 2002), which is compatible with the calculated temperature for peak metamorphism. It is not known if the orthogneisses in the BHC formed in this event; we found only chloritized orthogneiss in the northern side of the block during reconnaissance.
Event D2 occurred during greenschist facies metamorphism of the middle plate along the Majerad ductile detachment. D2 deformation and metamorphism overprint amphibolite-grade fabrics in the uppermost lower plate. The intensity of deformation diminishes in the upper parts of the middle plate, so we infer that greenschist-grade deformation is related to the Majerad ductile detachment. The original bedding is still preserved in the middle-plate rocks (Fig. 8B). Rahmati-Ilkhchi et al. (2010) believed that D2 in the Shotor Kuh range occurred during the Early to Middle Jurassic. However, as will be discussed, the age of metamorphism in the middle plate is Late Cretaceous. We did not observe stretching lineation in the middle-plate rocks neither in the Biarjmand nor in the Shotor Kuh blocks. However, Rahmati-Ilkhchi et al. (2010) reported a NW-SE–oriented lineation in the Shotor Kuh range (their “cover series”).
Mineral assemblage and deformation features show that both greenschist facies metamorphism and D2 deformation occurred in the same temperature range. In the sample that we used for 40Ar/39Ar thermochronology (sample 83Ea132), quartz, muscovite, biotite, and graphite are the main mineral phases suggesting a metamorphic temperature between 300 and 400 °C (Spear, 1995). The sample displays only one ductile shearing event, which produced mylonitic foliation along C shear bands (Fig. 9E). Biotites and muscovites are mainly localized in the shear bands in high-strain domains (Fig. 9F) implying their nucleation during deformation possibly by solution of detrital grains (e.g., phyllosilicates, quartz, and carbonates) in low-grade metamorphism (Vernon, 2004). Lack of stretching lineation prevented preparation of a thin section parallel to shear direction; however, the presence of sigmoidal porphyroclasts (Fig. 9E) indicates that shear was non-coaxial. Quartz porphyroclasts, as large as 1.2 mm, display internal crystal plasticity by sweeping and patchy undulose extinction (Fig. 9G) and crystallization of new grains by bulging (Hirth and Tullis, 1992) in the boundary of these smaller grains (Fig. 9H), which are indicative of deformation at temperatures ranging between 300 and 400 °C (Tullis, 2002). Lack of brittle fracturing in quartz porphyroclasts implies that deformation occurred above 300 °C (Passchier and Trouw, 2005). These microstructural observations suggest that deformation occurred at low to middle greenschist facies conditions (300–400 °C) (Tullis, 2002) similar to the metamorphic conditions.
Upright folds of D3 affected D2 fabrics of the lower and middle plates in the Biarjmand block at both map and outcrop scales (Figs. 3 and 8C). Fold axes have NE-SW trends. In the middle plate south of Kuh-e-Yazdu (Fig. 3), plunging anticlines and chevron folds are present (Fig. 8C). Kink folds also developed locally in the middle-plate rocks in the Biarjmand block (Fig. 8D). The upright folds are truncated by low-angle normal faults (D4). Therefore, D3 folding occurred between D2 and D4.
Event D4 is characterized by displacement along the Yazdu detachment fault (Fig. 8E) in both the Biarjmand and Shotor Kuh blocks. This fault was interpreted as a thrust fault in both the Biarjmand and Shotor Kuh blocks by several authors (Rahmati et al., 2010, 2011; Hushmandzadeh et al., 1978; Navab-Motlagh, 2005; Ghasemi, 2005; Navai et al., 1987). In contrast, Hassanzadeh et al. (2005) interpreted this as an extensional detachment fault in the Biarjmand block. This interpretation is supported by the observation that in both the Biarjmand and Shotor Kuh areas, this fault places younger sedimentary units above the older metamorphic rocks that originated at deeper levels. Furthermore, the upper plate contains normal faults that end downward at the detachment. Considering that this fault cuts D3 folds that post-date Late Cretaceous D2 metamorphic fabrics and that it does not affect the early Eocene rocks, the age of the low-angle detachment fault can be bracketed between Late Cretaceous and early Eocene.
In the Biarjmand block, the detachment fault has characteristics similar to those in the North American Cordillera (Davis, 1980). The fault generally dips gently (∼10°) toward the NW. The fault zone in this area includes several centimeters to meters of fault gouge and breccia (Fig. 8F). Breccia is developed in both the hanging-wall and footwall rocks; however, in the hanging wall it is much thinner (several centimeters). Phacoidal structures (clasts of unbrecciated rocks) with different sizes exist in the gouge zone (Fig. 8G). South of Kuh-e-Yazdu, fault striations and mineral slip fibers on the detachment fault that are preserved under the brecciated hanging wall display top-to-the-NNE sense of displacement. The gouge gradually changes downward to chlorite breccia and disappears finally in the middle plate <5 m from the fault zone. Both the middle-plate and upper-plate rocks along the detachment fault are intensely cut by quartz and calcite veins, implying high fluid circulation during detachment faulting.
No kinematic information or detailed field observations are available from the fault in the Shotor Kuh block. However, based on younger-over-older juxtaposition we interpret it as equivalent to the detachment system in the Biarjmand block. The presence of a low-angle detachment fault with unmetamorphosed and younger sedimentary rocks in the hanging wall and older metamorphic rocks in the footwall implies that both the middle and lower plates in the Shotor Kuh and Biarjmand blocks comprise a metamorphic core complex.
In the Shotor Kuh range, the deformation history of the footwall rocks of the brittle detachment fault is uncertain. Hushmandzadeh et al. (1978) suggested that all rock units in the footwall of the brittle detachment fault (unit dl in Fig. 3) belong to the Jurassic (protolith age), where they overlie the amphibolite-grade metamorphic rocks by a ductile shear zone. Based on this structural description, all thrust faults in their map inside the footwall of the brittle detachment can be interpreted as normal faults. In contrast, Rahmati-Ilkhchi (2003) divided the footwall of the brittle detachment (unit dl in Fig. 3) into two segments: a structurally higher segment composed of Permian-age rocks that is in thrust contact with underlying Eocene volcanic rocks (this division is not shown in Fig. 3). If true, this implies a period of post-Eocene compression in the SKBC, which would be consistent with the regional folding that affects all of the Neogene units. The Eocene volcanic rocks shown by Rahmati-Ilkhchi (2003) at the bottom of unit dl were mapped by Hushmandzadeh et al. (1978) as “metamorphosed volcanics”, which implies a record of ductile deformation on these rocks. Because we did not observe any metamorphism younger than D2 in this area, we attribute this ductile deformation to D2 and consequently the entire unit dl to the middle plate; thus we question the Eocene age assignment. Rahmati-Ilkhchi et al. (2010) interpreted the complex structure of the western Shotor Kuh range as an extensional duplex.
We did not observe the brittle detachment fault in the BHC, and existing maps (Fig. 3) do not suggest the existence of a low-angle fault in the region. In the BHC, greenschist facies metamorphic rocks are overlain depositionally by Lower Cretaceous carbonates (Amini-Chehragh, 1999; Kahn Nazer, 1992) (Fig. 3). In contrast, in Kuh-e-Peyghambar and Kuh-e-Garmab (Figs. 4 and 5), low-angle faults similar to the Kuh-e-Yazdu fault exist. In Kuh-e-Garmab, the Late Cretaceous hanging-wall sedimentary rocks overlie greenschist facies metamorphic rock along a low-angle fault that we correlate with the Yazdu detachment.
Three other fault sets affect the SKBC: (1) NE-SW–striking high-angle faults; (2) NW-SE–striking high-angle faults; and (3) NE-SW–striking reverse faults. We do not have enough information about their relative ages with respect to each other to determine the structural evolution of these fault sets. The NE-SW–striking faults in both the Biarjmand and Shotor Kuh–Majerad blocks are range bounding and control the current distribution of exposures of the lower plate. The faults that bound the southern and western edges of the Shotor Kuh are probably continuations of the Torud fault (Fig. 2) (Hushmandzadeh et al., 1978; Jafarian, 1995). Hushmandzadeh et al. (1978) described the Torud fault as a high-angle, south-dipping fault exhibiting both dip-slip normal and left-lateral slip. Normal displacement along a NE-striking fault is also reported from west of Kuh-e-Molhedu, between the early Eocene conglomerate and the lower-plate rocks (Fig. 3; Ghasemi, 2005). A NE-SW–oriented horst in the southwest part of the Great Kavir Basin reported by Jackson et al. (1990) indicates that this fault set and its related extension have probably affected the entire basin. The initiation age of these faults is not known; however, they were active either as normal or strike-slip faults in the Eocene or later, given that they cut the Eocene rocks (Fig. 3).
Northwest-striking faults are present in several places (Fig. 3): cutting the lower plate in the Shotor Kuh, in the BHC, and in the southeast-tilted crustal section. In places, these faults cut Neogene sediments, so they may all post-date the extension of interest here. The kinematics and onset age of these faults are not known; however, offset of the Neogene strata indicates that these structures were active during this time.
In the Biarjmand range, south of Kuh-e-Yazdu, NE-SW–striking reverse faults (Fig. 3) juxtapose the lower plate onto the middle plate. The age of these faults are not known. However, considering that their orientation is parallel to the axis of the Neogene plunging syncline (Amini-Chehragh, 1999) south of Meyamey (see Fig. 2 for location), we interpret these faults as minor compressional structures that affected the entire region during the Neogene.
GEOCHRONOLOGY AND THERMOCHRONOLOGY
In order to determine the timing of deposition of the middle-plate protolith rocks in the Biarjmand range, U-Pb zircon analyses were performed on a metatuff (sample 84Ea6; Fig. 3; Supplemental File 11). We used secondary ion mass spectrometry (CAMECA IMS 1270) for the analysis at the University of California, Los Angeles (UCLA), using the method described by Faramarzi et al. (2015). This sample was taken from the stratigraphically lowest part of the middle plate, which crops out in southern part of Kuh-e-Yazdu (Fig. 3). Six analyzed zircon grains (Supplemental File 1) yielded a concordia age of 140.7 ± 4.8 Ma (Fig. 10). We interpret this date as the age of tuff volcanism in the Early Cretaceous. This date, which will be used for constraining the pre-extensional thermal history of the region, implies that the age of the middle-plate protolith in the Biarjmand range is not older than the Early Cretaceous. Our analysis improves on the previously suggested wide range of Late Jurassic–Early Cretaceous for these rocks based on fossil ages (Ghasemi, 2005; Navab-Motlagh, 2005).
Sampling Strategy and Mineral Separation
To constrain the thermal evolution of the SKBC, we applied 40Ar/39Ar furnace step heating on multigrain aliquots of K-feldspar, biotite, and amphibole from nine lower-plate rock samples and one middle-plate rock sample. All four analyzed samples in the Shotor Kuh block were taken from the amphibolite facies orthogneisses of the lower-plate rocks. In the Biarjmand block, two samples were taken from the amphibolite facies orthogneisses of the lower plate, and one sample was taken from diabasic dike swarms injected into the lower-plate rocks. All six samples from the amphibolite facies rocks of the Biarjmand and Shotor Kuh ranges were strongly foliated, coarse-grained, and strongly lineated orthogneisses. All three samples from the BHC were taken from Neoproterozoic–Early Cambrian (Hassanzadeh et al., 2008) leucogranites.
In addition, in order to determine the age of the middle-plate metamorphism and deformation, we applied laser step-heating on biotite and muscovite of a metasandstone from the middle plate. Sample 83Ea132 (Fig. 3) is a meta–quartz wacke from the Majerad range close to (within 10 m) the contact between the lower and middle plates.
Sample EaBi6 (amphibole; Table 1) was analyzed in 2004 at the UCLA argon geochronology laboratory (Supplemental File 22) following procedures described by Quidelleur et al. (1997). All other samples (Table 1) were analyzed at the New Mexico Geochronology Research Laboratory (NMGRL) (Supplemental File 33) following the procedures described below.
Most hand samples were crushed and sieved to 250–425 μm, and minerals were concentrated using standard Frantz magnetic and heavy liquid techniques. A finer grain size of <100 μm was used for sample 83Ea132. Final purification was conducted by choosing the most pristine grains via hand picking under a binocular microscope. Prior to irradiation, all separates were ultrasonically cleaned with acetone followed by a 3–5 min exposure to distilled water.
Approximately 5–10 mg of the mineral separates of each sample were placed in a 20-hole aluminum disk along with neutron flux monitor FC-2 sanidine (28.201 Ma) placed in every fourth hole. Each disk contained several samples. The package was irradiated at the TRIGA reactor at the U.S. Geological Survey Denver (Colorado) facility in the central thimble. For samples step-heated in the double-vacuum Nb resistance furnace, the gas was gettered during heating using a SAESGP 50 getter operated at 450 °C. Following heating, gas was expanded into a second stage and reacted with two SAESGP 50 getters, one heated to 450 °C and the other at room temperature. Gas was also exposed to a filament operated at 2000 °C while in the second stage. K-feldspar samples were analyzed over 36 heating steps, with times ranging from 13 to 118 min. In order to better resolve 39Ar diffusion parameters and to identify excess argon, isothermal duplicates were measured from 500 to 1150 °C. Argon isotopes were analyzed with a MAP 215 50 mass spectrometer fitted with a Balzers 217 multiplier.
Shotor Kuh. All samples from the Shotor Kuh block (Fig. 3) are amphibolite-grade orthogneiss that yield similar age spectra with two plateau-like segments in low- and high-temperature steps (Fig. 11). K-feldspars are characterized by initially old apparent ages that define a sawtooth pattern for isothermal duplicates (Fig. 11). Based on Harrison et al. (1993; 1994), the sawtooth pattern is likely differential release of excess 40Ar from fluid inclusions. In most cases, the initial steps level out into an overall flat segment ranging from a few to tens of percent of the total 39Ar released. Following this, the ages rise in an overall monotonic pattern and commonly level off into another plateau-like segment. The low-temperature flat segment yields consistent ages of ca. 65–70 Ma. The high-temperature flat segments are more variable with three samples terminating ca. 100 Ma whereas sample 05TO2A climbs to ca. 140 Ma.
Biotite ages for this block follow those of the K-feldspar high-temperature flat segments (Fig. 11). The ages of three samples (05TO40, 05TO5, and 05TO10) are ca. 100 Ma, whereas like its coexisting K-feldspar, 05TO2A is older, ca. 150 Ma. Biotite spectra are quite complex and typically record an overall young apparent age for initial heating steps that rises to an intermediate maximum, then falls before rising again for the final heating steps. This pattern mimics that of a crank shaft and has been reported many times in the literature, most notably by Lo and Onstott (1989). They interpreted that pattern to result from 39Ar recoil within an interlayed biotite-chlorite mixture. They showed that the integrated age was an accurate cooling age, similar to the conclusion of Heizler et al. (1988). The biotites dated here have low K contents (5%–6%) based on 39Ar yield and sample weight and thus are likely chloritized. We therefore adopt the past explanations and accept the integrated age as most accurate. We note that choosing the higher-temperature steps where the spectra are flattest would lead to “plateau” ages that are older by only a few percent compared to the integrated ages, and thus either choice for the closure age has no impact on the final estimated thermal history. We also note that many coexisting K-feldspars have high-temperature flat segments that approximately equal the biotite ages. Considering that most large domains of K-feldspars have argon closure temperatures of ∼300 °C, similar to the nominal closure temperature of biotite (Harrison et al., 1985; Grove and Harrison, 1996), it is not surprising that the K-feldspars and biotites share this common age.
Biarjmand Range. K-feldspar samples EaBi20 and EaBi14 have initial ages between 60 and 70 Ma and rise to maximum ages between 140 and 160 Ma (Fig. 12). These samples contrast somewhat with those of the Shotor Kuh, as the initial and the high-temperature flat segments are not as pronounced.
A single hornblende was dated from a mafic dike in the Biarjmand block (sample EaBi6) and yields a complex spectrum (Fig. 12; Supplemental File 2 [footnote 2]). Young initial ages between ca. 100 and 150 Ma rise steeply and complexly to values between 600 and 800 Ma. We show three steps with a weighted mean age of 144 ± 6 Ma, and as discussed below, suggest that this may be accurate based on a U-Pb zircon age that is similar from another volcanic unit (metatuff in the Biarjmand range; Figs. 3 and 10). The old ages likely result from a mineral phase incorporated as an inclusion in the analyzed amphibole. The younger ages also might be related to another mineral phase with a different composition as recorded in K/Ca values.
The age of hornblende from the mafic dike may provide insight about the history of magmatism in the Iranian Plateau. It is worth noting that this amphibole 40Ar/39Ar age is analytically equal to the zircon U-Pb age (140.7 ± 4.8 Ma) obtained from a metatuff in the middle plate of the Biarjmand range, implying that magmatism in the form of dike intrusion and volcanism was active in northeastern Iran during the Late Jurassic and Early Cretaceous. We note that this magmatism is coincident with the late Cimmerian tectonic event (Rawson and Riley, 1982) in Iran (Berberian and King, 1981) and hypothesize that they might be related to each other.
BHC. Three leucogranitic samples were analyzed from this block. Samples 05TO45 and 05TO46 yielded very similar age spectra (Fig. 12) with low-temperature steps beginning ca. 100 Ma, climbing to their intermediate age hump ca. 145–150 Ma, decreasing to 135–140 Ma before their final rise to ca. 170 Ma. Sample 05TO53 yielded a similar spectrum, but the ages are generally ∼20 m.y. younger and a less-pronounced hump occurs in lower-temperature steps. In addition, this sample only climbs to a maximum of ca. 150 Ma. The “intermediate age hump” (e.g., samples O5T045, O5T046; Fig. 12) has been suggested to be caused by alteration of K-feldspar to adularia, low-temperature deformation, and deformation in shallow and high-strain environments adjacent to brittle faults (Lovera et al., 2002).
Biotite and muscovite of the middle plate. Both biotite and muscovite from a meta–quartz wacke (sample 83Ea132) from the Majerad range display similar age spectra, and the biotite records slightly older ages (Fig. 13). The muscovite and biotite yielded integrated ages of 70.367 ± 0.020 and 72.456 ± 0.019 Ma, respectively. Both samples yield gently climbing spectra with the muscovite climbing from 70 to 71 Ma and the biotite from ca. 72 to 75 Ma.
We modeled K-feldspar step-heating analyses following the multiple diffusion domain (MDD) method (Lovera and Richter, 1989, 1991) to assess the thermal history of the SKBC. Our procedures closely follow those outlined by Sanders et al. (2006), who provided a detailed data methodology for extracting thermal histories from K-feldspars analyzed at the NMGRL. In summary, argon diffusion parameters are determined from the fractional release of 39Ar and the laboratory heating time and temperature to construct an Arrhenius plot. Model age spectra are determined by imposing thermal histories on the kinetic parameters and are accepted when model age spectra match the measured age spectrum (Supplemental File 44). Generally, forward thermal models are run until at least 20 thermal histories are generated that yield acceptable fits between measured and model age spectra. Results of the MDD modeling are provided in Figure 14.
Lower-plate samples. For the Shotor Kuh and Biarjmand samples, modeling yields excellent fits to the Arrhenius data, log r/r0 (used for determining the volume fraction and size of diffusion domains; Lovera et al., 1991) and age spectra plots, and the 90% confidence interval of MDD solutions (Supplemental File 4 [footnote 4]). All samples show similar histories of early cooling followed by isothermal periods, in turn followed by subsequent rapid cooling that was contemporaneous for all samples (Figs. 14A–14F).
All samples from both blocks consistently record a stage of rapid cooling starting from ∼250–300 °C in the Late Cretaceous (ca. 68–76 Ma) and cooling below 150 °C ca. 60 Ma with cooling and lasting until the late Paleocene–early Eocene (ca. 50Ma). All six samples from the Biarjmand and Shotor Kuh ranges indicate temperatures between ∼250 and 300 °C at 100 Ma with an apparent isothermal segment between 100 and ca. 70 Ma.
BHC samples. In contrast to the remarkable Late Cretaceous–Early Tertiary rapid cooling of the Biarjmand and Shotor Kuh ranges, the BHC samples cooled much earlier through 200 °C (Figs. 14G–14I). However all three locations share an overall similar pre–100 Ma thermal history, where on average rocks were cooling from ∼350 °C to 300 °C between 150 and 100 Ma. We do note that the robustness of the BHC samples is somewhat compromised by the intermediate age hump and thus mainly state that the BHC samples resided at somewhat cooler temperatures at 100 Ma in comparison to the samples from the Biarjmand and Shotor Kuh ranges.
Interpretation of Mica Age Spectra
Lower-plate mica. Whether we accept the total gas age of the altered biotites as suggested by Lo and Onstott (1989) and Heizler et al. (1988) or the slightly older flat segments of the biotites as suggested by Ruffet et al. (1991), the basic interpretation of the biotite results (Fig. 11) is straightforward. Biotite ages are ca. 100 Ma for the Shotor Kuh block and indicate temperatures of ∼300 °C at this time, which is consistent with the K-feldspar MDD thermal histories.
Middle-plate mica. Considering the youngest age recorded by low-temperature step-heating of muscovite (70 Ma) and the oldest age recorded by high-temperature step-heating of biotite (75 Ma) (Fig. 13), we propose that the metamorphism and its related ductile deformation occurred in a short period of time between 70 and 75 Ma.
Both the muscovite and biotite separates display staircase-shaped age spectra (gradually increasing ages) with younger ages in the low temperatures and older in the highest temperatures of degassing. This type of age spectra is frequently reported from mylonitic rocks and is attributed to a mixture of mica populations with different grain sizes, a mixture of detrital and thermally overprinted mica, and a mixture of detrital and neocrystallized mica (Kirschner et al., 1996, and references therein). The presence of detrital mica with Late Cretaceous ages in the analyzed sample is unlikely considering its Jurassic deposition age (Rahmati-Ilkhchi et al., 2011).
The ages recorded by ∼94% of 39Ar released in both biotite and muscovite can either be the cooling age of older minerals or the age of mica neocrystallization. The first seems unlikely because none of the samples from the lower plate, which is structurally lower than sample 83Ea132 (middle plate), display similar biotite and muscovite ages (they are much older). Therefore, we interpret the recorded ages as the age of neocrystallized mica nucleated during greenschist facies metamorphism and deformation. We attribute the age difference and stair step–shaped age spectra (young to old ages) to degassing of smaller (newer) and less retentive mica at low temperatures and larger (older) and more retentive mica at high temperatures. Considering that only one metamorphic event is recorded in the analyzed sample, this interpretation implies that muscovites and biotites crystallized in a short time span between 71 and 70 Ma and between 75 and 72 Ma, respectively. Microstructural observations (see the section on deformational and metamorphic events) also support the abovementioned interpretation that muscovite and biotite record crystallization and not cooling ages.
The most striking feature in the age spectra of sample 83Ea132 is that the biotite yielded significantly older ages than its coexisting muscovite (Fig. 13), which is not in agreement with recognized closure temperatures for these minerals (McDougall and Harrison, 1999). The older age recorded by biotites might be related to the presence of excess argon and therefore be geologically meaningless. However, for two reasons they might be geologically meaningful:
1. The shape of biotite age spectra imitates the one for the coexisting muscovite, so that the presence of randomly distributed excess argon is unlikely (Fig. 13).
2. K-feldspar MDD modeling from the Shotor Kuh range requires a stage of ductile deformation in mid-crustal depths between 70 and 76 Ma during which the greenschist facies metamorphism and ductile deformation in the middle plate is inferred (Malekpour-Alamdari, 2017). All ages recorded by biotite step-heating (72–75 Ma; Fig. 13) are well inside the expected time range predicted by MDD modeling. It seems unlikely that excess argon only increased the ages up to 75 Ma and not older than 76 Ma (the oldest possible time for ductile deformation as predicted by MDD modeling).
The older ages of biotite with respect to its coexisting muscovite might be related to the effect of chemical composition. It has been suggested that fluorine content of mica plays a significant role in argon retentively of mica (Dahl, 1996; Grove and Harrison, 1996). Dahl (1996) proposed that increasing fluorine in tri-octahedral mica (e.g., biotite) enhances its interlayer K-O bond, decreases the diffusion rate of argon in its lattice, and therefore causes an older biotite40Ar/39Ar age than that of its coexisting muscovite.
To summarize, stair step–shaped age spectra from the middle plate of the Majerad range record crystallization ages of the biotite and muscovite and are geologically meaningful. The reason that mica in the middle plate is younger than biotites in the lower plate lies on their different structural position and thermal evolution. Biotites in the lower plate were formed in high-temperature deformation considering that they are in equilibrium with high-temperature deformation of K-feldspars (see Figs. 9B and 9D). These biotites passed through their closure temperature (∼350 °C) at ca. 100 Ma (samples 05TO5, 05TO10, and 05TO40) and ca. 140 Ma (05TO2A) (Figs. 11) but remained above the temperature level for new mica growth (>280 °C). Later, in Late Cretaceous time, neocrystallization of mica in the middle plate occurred in a localized shear zone on top of the lower plate.
The Shotor Kuh and Biarjmand Ranges as Metamorphic Core Complexes
In both the Shotor Kuh and Biarjmand ranges, a younger-and-low-temperature over older-and-high-temperature relationship exists between the upper, middle, and lower plates. In both ranges, Late Cretaceous greenschist facies metamorphic rocks (D2, middle plate) overprinted on the Late Triassic amphibolite facies rocks. Similarly, a low-angle fault (D4) separates non-metamorphic rocks in the hanging wall from the middle-plate rocks in the footwall. We attribute the exhumation of these ranges to normal faulting and call them metamorphic core complexes considering that this relationship implies a normal displacement for both shear zones. The existence of high-angle normal faults in the hanging wall of the low-angle fault is in agreement with this interpretation and makes the possibility of their (high-angle normal faults) geneses by reverse faulting unlikely.
We do not have enough data to interpret the BHC as a separate metamorphic core complex. The maps of the BHC range (Khan Nazer, 1992; Amini-Chehragh, 1999; Fig. 3) do not show the presence of a low-angle fault between the Early Cretaceous non-metamorphic rocks and their underling greenschist facies rocks. The greenschist facies rocks in this range might be equivalent to those in the Shotor Kuh and Biarjmand ranges (see below for further discussion). However, based on the available data, this range has a somewhat different geologic history from the Shotor Kuh and Biarjmand ranges.
Timing of Extension and Thermal History
Both the Shotor Kuh and Biarjmand blocks exhibit rapid cooling starting from the Late Cretaceous following a long period of thermal stability (Figs. 15A and 15B). We attribute this rapid cooling to exhumation of the lower-plate rocks by slip on the low-angle detachment fault (D4 deformation phase), similar to what has been well documented for other metamorphic core complexes. In the Shotor Kuh range, cooling of the lower plate started at ca. 76 Ma (recorded by sample 05TO2A) and continued with almost the same rate until the late Paleocene when the lower plate was depositionally overlain by detrital sedimentary rocks of this age (Fig. 15A). In the Biarjmand range, rapid cooling of the lower plate rocks started later at ca. 70 Ma. Cooling continued until the early Eocene when detrital sediments covered the lower-plate rocks in this region (Fig. 15B).
In contrast, the BHC range does not record the Late Cretaceous rapid cooling (Fig. 15C). Three samples taken from this block consistently show rapid cooling starting from ca. 140–150 Ma transitioning to a phase of thermal stability and then a second phase of rapid cooling in the Cretaceous, although each sample gives a slightly different timing for Cretaceous cooling. Due to a lack of enough field observations and isotope ages from this block, interpreting these temperature-time curves and tectonic evolution of the BHC is not straightforward. However, using the available data two possible scenarios will be discussed: (1) the BHC has a different thermal history than the two other ranges and therefore is the hanging wall of a large-scale detachment fault; and (2) the BHC and the two other ranges similarly belong to the footwall of the detachment and have a similar thermal evolution.
Scenario 1: The BHC is the Hanging Wall of a Large-Scale Detachment Fault
The Neoproterozoic–early Cambrian plutonic rocks and orthogneisses from which the BHC samples were taken (Fig. 3) are overlain by a series of greenschist facies metamorphic rocks at Kuh-e-Kiki (Fig. 3). This greenschist facies metamorphic unit in turn is overlain by Aptian detrital sedimentary rocks (Amini-Chehragh, 1999) (Fig. 3). As discussed below, the type of contacts between the greenschist metamorphic rocks and overlying strata and underlying orthogneiss are controversial. The age of the greenschist metamorphism is not well known, but Precambrian and Jurassic ages have been suggested (Khan Nazer, 1992; Amini-Chehragh, 1999). If the contact is depositional, this stratigraphic position requires that the Neoproterozoic–early Cambrian plutonic rocks and orthogneisses were exhumed to the surface by the Aptian (path A in Fig. 15C; Fig. 16A). This interpretation is in conflict with MDD thermal modeling, which suggests that these rocks were in temperature ranges between ∼150 and 270 °C from 130 Ma to 80 Ma. This implies that the younger segment (post-Aptian) of the thermal history recorded by MDD modeling is either related to a burial reheating or geologically meaningless. The burial reheating scenario is unlikely because the entire thickness of the Early Cretaceous strata is not more than 5 km in this region. If post-Aptian thermal history is geologically meaningless, juxtaposition of the BHC block beside two subsequently exhumed blocks implies a faulted contact between them and requires that the BHC block be considered as the hanging wall of a larger detachment system that cut this block at depth.
Scenario 2: The BHC Belongs to the Footwall Similarly to the Other Ranges
The alternative interpretation is that the greenschist facies metamorphic rocks might represent a mid-crustal shear zone that became active between the structurally shallower Aptian and younger sedimentary rocks and deeper Neoproterozoic–early Cambrian plutonic and metamorphic rocks (Fig. 16B). This kind of shear zone is known in other extensional terrains. For instance, Wells (2001) showed that a syn-extensional shear zone with at least 24 km length initially developed parallel to an unconformity between Archean basement rocks and overlying Proterozoic quartzite and schist in the Raft River detachment system of the Basin and Range (western United States). This scenario implies that the BHC block may have never been exhumed to the Earth surface during the Aptian and that the MDD modeling yielded an accurate thermal history. Assuming this scenario to be true, the BHC block may also have been affected by the extensional exhumation in the footwall of a large-scale detachment fault similarly to the Biarjmand and Shotor Kuh blocks but at a shallower crustal depth that was not recordable by MDD modeling (path B in Fig. 15C).
Three observations support the second scenario.
(1) In the Meyamey region, a continuation of the BHC block ∼40 km northward (see Fig. 2 for location) where rock units similar to the BHC are exposed, Amini-Chehragh (1999) reported crystallized limestones, slates, and phyllites containing Campanian–Maastrichtian fossils, implying a syn- or post-Maastrichtian metamorphic age for that region. Similarly, considering that the BHC and Meyamey are southern and northern limbs of a Neogene plunging syncline (Amini-Chehragh, 1999), the age of metamorphism in the BHC may also be syn- or post-Maastrichtian.
(2) All rapid cooling stages recorded by the BHC samples are also recorded in the two other blocks (domains shown by vertical dashed lines in Fig. 15) implying that all three blocks may have had the same thermal evolution.
(3) A similar relationship of Aptian detrital sediments overlying the greenschist facies metamorphic rocks is also exposed in the Shotor Kuh range (Kuh-e-Rezveh; shown with red star in Fig. 3) where Rahmati-Ilkhchi et al. (2010, p. 103) described the overlying Aptian detrital rocks as “very-low-grade metamorphosed”. In this region, the age of the protolith of the greenschist facies rocks is Jurassic. A Late Cretaceous metamorphic age for greenschist facies rocks in the Shotor Kuh with “very-low-grade metamorphosed” Aptian detrital rocks on top of them implies that metamorphism occurred while both the protolith of the greenschist facies rocks and Aptian detrital rocks were at mid-crustal depths (Fig. 16B). A similar case might be true for the greenschist facies metamorphic rocks and overlying sediments in the BHC.
Although assuming the greenschist facies metamorphism as a horizontal mid-crustal shear zone of Late Cretaceous age explains the similar thermal history of the BHC (160–80 Ma) compared to the Biarjmand and Shotor Kuh ranges, this model is not consistent with the presence of non-metamorphic Jurassic sedimentary rocks (Fig. 3). More field and isotope dating data are required to fully clarify the BHC’s tectonic evolution. In the next section, we will argue that development of a mid-crustal syn-extensional horizontal or subhorizontal shear zone was important in the development of the Shotor Kuh block.
Pre-Extensional Thermal History
The available geological and thermochronological data indicate that the lower-plate rocks in the Shotor Kuh and Biarjmand ranges have been previously exhumed and subsequently buried during their geological history. In both ranges, the lower-plate rocks passed through hornblende closure (∼500 °C) during the Late Triassic (Hassanzadeh et al., 2005) (Fig. 15A). The lower plate was exhumed to the surface by the Early Jurassic and subsequently buried during the Jurassic by the Shemshak Formation. Both the lower plate and its overlaying sediments were buried to mid-crustal depths by the Late Jurassic. The burial could have resulted from sedimentary load and/or a tectonic event (e.g., mid-Cimmerian event). Between the Late Jurassic and the Late Cretaceous, the lower plate and overlying sediments (which later became the middle plate) remained at mid-crustal levels although several cooling phases affected the lower-plate rocks (Fig. 15). The last pre-extensional cooling phase occurred between 80 and 90 Ma, which we attribute to ophiolite emplacement and continent-continent collision in this region. In the Late Cretaceous (ca. 76 Ma), extension started and the Jurassic and younger sediments overlying the lower-plate rocks were metamorphosed to the greenschist facies in a subhorizontal ductile shear zone (see above for further discussion).
Unlike in the Shotor Kuh range, the middle plate in the Biarjmand range does not record the Late Triassic–Late Jurassic burial and exhumation as the oldest middle-plate rocks have Late Jurassic–Early Cretaceous ages (140 ± 5 Ma; see U-Pb geochronology section). Two possible explanations for this difference are as follows. (1) Unlike in the Shotor Kuh range, the lower plate in the Biarjmand was never exhumed to the surface during the Late Triassic–Early Jurassic (path A in Fig. 15B). In this case, after deposition the protolith of the middle-plate rocks was juxtaposed beside the lower plate by a tectonic event (e.g., ophiolite obduction in the Late Cretaceous) before being metamorphosed during the Late Cretaceous extension. (2) The lower plate in the Biarjmand range had a similar thermal history to that in the Shotor Kuh range (path B in Fig. 15B). In this case, the lack of the Early–Middle Jurassic sediments in the middle plate of the Biarjmand range might be related to an original lack of sedimentation or subsequent removal of the sediments.
Geometry of the Extended Terrain
The SKBC is the oldest part of a southeastward-younging sequence that is exposed in northeastern Great Kavir Basin. To the southeast of the Majerad (Fig. 4), the sequence continues with the Early Cretaceous limestones and gradually changes to younger tuff, radiolarian chert, Orbitolina limestone, and calcareous shale of upper Cretaceous (Campanian–Maastrichtian) age and the Doruneh-Kashmar ophiolites (Fig. 2). This southeastward-younging sequence is internally deformed by many folds (Salamati, 1999) and faults. We interpret this southeastward-younging sequence as a crustal sequence tilted by extension. Furthermore, we interpret the non-metamorphic rocks in the hanging wall of the low-angle faults at Kuh-e-Garmab and Kuh-e-Peyghambar (Figs. 4, 5, and 17) as extensional nappes, similar to those in the Biarjmand and Shotor Kuh ranges, that were emplaced on the tilted crustal sequence during its progressive exhumation. This interpretation implies that the tilted crustal sequence is the lower plate of a large-scale detachment fault, similarly to the crystalline rocks in the Shotor Kuh and Biarjmand ranges.
The southeastward-younging sequence is also unconformably overlain by the early Paleocene–early Eocene detrital sediments (Figs. 4, 5, and 17). We interpret these detrital rocks as syn-extensional sediments that were deposited on the exhumed footwall of a large-scale detachment fault. The age of these detrital sedimentary rocks is older (early Paleocene) in the southeast (southeast of Kuh-e-Peyghambar) than in the northwest (early Eocene; the Biarjmand range) (Naderi-Mighan, 1999; Kohansal, 2007; Salamati, 1999; Rahmati-Ilkhchi, 2003; Ghasemi, 2005). The age variation from southeast to northwest is consistent with the gradual exposure of the footwall rocks by slip along the detachment fault.
Two complexities regarding this southeastward-younging tilted crustal sequence require further explanation: First, in Kuh-e-Garmab, the contact between the lower-plate rocks in the Majerad and the middle-plate rocks is not visible. Second, in Kuh-e-Peyghambar, an older-over-younger relationship exists between footwall and hanging-wall rocks.
The first of these complexities is the structural position of the greenschist facies metamorphic rocks in Kuh-e-Garmab, which are exposed in a tectonic window under the Late Cretaceous shale and limestones (Figs. 3, 4, and 17). The contact between these rocks and the lower-plate rocks is nowhere exposed. However, we hypothesize two scenarios to explain the potential nature of this contact zone (Fig. 18).
(1) The contact is depositional between the lower plate at Kuh-e-Majerad and stratigraphically and structurally higher strata to the southeast. In this case, the entire southeastward-younging crustal sequence is tilted by a detachment fault and the greenschist facies rocks at Kuh-e-Garmab are the southeastward continuation of the middle plate in the Biarjmand and Shotor Kuh ranges (Fig. 18A).
(2) The greenschist facies rocks in Kuh-e-Garmab belong to a SE-dipping shear zone that developed during activation of an older detachment fault (Fig. 18B). This type of shear zone is recognized as a mylonite front in other metamorphic core complexes (e.g., Lister and Davis, 1989). This interpretation requires the existence of another low-angle normal fault under the tilted crustal sequence in this region. Detailed field observation, geophysical data, and isotope dating are required to elucidate whether the middle plate in Kuh-e-Garmab is correlated with the middle-plate rocks in the Biarjmand and Shotor Kuh ranges.
However, two observations support the second model.
(1) The existence of another low-angle normal fault under the tilted crustal sequence requires the existence of a sedimentary basin in the southeast of the tilted crustal sequence (Fig. 18B). It is worth noting that a sedimentary basin with Paleogene age exists in the southeast of the tilted crustal sequence (Fig. 2). The thickness of Paleocene and early Eocene (recorded time of extension in the SKBC) sedimentary rocks, which are mainly detrital, exceeds 3500 m in this basin (Vahdati Daneshmand and Nadim, 1999), which is considerably thicker than any sedimentary sequence with the same age on the tilted crustal sequence. In agreement with the second model, we hypothesize that this thick sedimentary sequence was syn-extensional and developed adjacent to the breakaway zone (Fig. 18B).
(2) We note that rock units similar to the Doruneh-Kashmar ophiolites (ultrabasic and Late Cretaceous volcanic and sedimentary rocks known as the Southern Sabzevar ophiolites; Shafaii-Moghadam et al., 2014) are repeated to the northwest of the Doruneh-Kashmar ophiolites (Jafarian and Jalali, 2000; Vahdati Daneshmand and Nadim, 1999; Eftekhar-Nezhad et al., 1976; see Fig. 2 for location). We interpret the northwestern segment as a segment detached from the Doruneh-Kashmar ophiolites by a detachment fault that penetrates under the tilted crustal sequence (Fig. 18B).
The second complexity is that, unlike in the Kuh-e-Garmab, Biarjmand, and Shotor Kuh ranges, an older-over-younger relationship exists at the Kuh-e-Peyghambar low-angle fault, suggesting reverse kinematics for this fault. Three possibilities regarding this low-angle fault at Kuh-e-Peyghambar exist: (1) the fault is a normal fault but the age of the footwall and hanging-wall rocks is poorly identified; (2) the fault is a thrust fault with an unknown age; or (3) pre-extensional structural complexity existed in the region. We could not evaluate the first possibility because we did not have access to the field area at the time of writing this paper. Reverse faulting, as the second possibility, could be related to Neogene or Late Cretaceous compression in this region. The third possibility considers the low-angle fault in Kuh-e-Peyghambar as a normal fault assuming the existence of a structural disorder in the southeastward-younging crustal sequence; that is, the older rocks existed in the southeasternmost part of the tilted crustal sequence due to pre-extensional tectonic activities (e.g., ophiolite emplacement; Figs. 2 and 18). We favor this hypothesis because we notice that a rock unit similar to the hanging-wall rocks in Kuh-e-Peyghambar (unit K1l; Fig. 4) exists with reverse fault contacts adjacent to the Doruneh-Kashmar ophiolites (Eftekhar-Nezhad et al., 1976; Ghaemi and Moussavi Harami, 2007; Shahrabi et al., 2006) (see Fig. 2 for location and Fig. 18). Therefore, it is possible that the hanging wall of the low-angle fault at Kuh-e-Peyghambar detached from its original place beside Doruneh-Kashmar ophiolites in the early stages of extension. With this interpretation, the breakaway of the brittle detachment fault must be somewhere close to the Doruneh-Kashmar ophiolites (Figs. 2 and 18).
Kinematics of Extension
The extension direction in the Shotor Kuh–Biarjmand metamorphic core complex is not well known from field relationships. Therefore, we use a combination of other features to propose a NW-SE–oriented extension for the SKBC. These features include:
(1) Presence of a SE-tilted crustal section in the southeastern part of the SKBC. Tilting and exhumation of footwall rocks through slip along a master detachment fault is shown in many extensional terrains to result in exposure of a tilted partial crustal section (Wernicke and Axen, 1988; Bartley et al., 1990; Cosca et al., 1995; Foster et al., 1991). We interpret the southeastward-younging stratigraphic sequence with amphibolite facies metamorphic rocks of the SKBC on the bottom and the Late Cretaceous volcano-sedimentary rocks on top (Figs. 2, 4, 5, and 17) as the footwall of a northwest-dipping master detachment fault. Northwestward younging of the syn-extensional sedimentary rocks deposited on the tilted crustal section (Fig. 17) and the absence of the crystalline lower-plate clasts (e.g., amphibolite facies rocks of the SKBC) in southeastern outcrops of the syn-extensional sedimentary rocks support this interpretation. The non-metamorphic Cretaceous rocks in the Biarjmand and Shotor Kuh blocks (Fig. 3), those at Kuh-e-Garmab and Kuh-e-Peyghambar (Fig. 4 and 5), and possibly the entire BHC block (see Timing of Extension and Thermal History section for the uncertainty about the BHC) define the hanging wall of this detachment fault.
(2) Northeast-southwest elongation of the metamorphic core complex. In some cases (e.g., the Catalina, Death Valley, Ruby Mountains, Anaconda, and Bitterroot metamorphic core complexes in Basin and Range, USA, and Hohhot metamorphic core complex in China (Fayon et al., 2000; Foster et al., 2007, 2010; Axen et al., 1990; Davis et al., 2002), footwall rocks are elongated perpendicular to the extension direction due to progressive removal of the hanging wall which imposes isostatic rotation of the footwall by buoyancy forces (Spencer, 1984; Wernicke and Axen, 1988). Similarly, NE-SW–elongated exposure of the lower-plate rocks in the SKBC may imply a NW-SE–directed extension in this region. This mechanism is expected to produce folds with axes perpendicular to extension (Axen, 2004; Wawrzyniec et al., 2001). Analogously, we interpret NE-SW–oriented upright folds (D3 deformation phase) (Figs. 8C and 8D) and map-scale plunging anticlines (Fig. 3) that we have documented in the Biarjmand range (south of Kuh-e-Yazdu) as a manifestation of this mechanism. Considering that D3 folds are cut by the Yazdu detachment fault, they cannot be attributed to the Neogene compression. However, the ranges in our study area are largely controlled by young (post-Eocene) faults that are likely to be unrelated to the Late Cretaceous extension direction.
(3) Younging direction of rapid cooling in the lower plate. In the Shotor Kuh range, the age of rapid cooling recorded by K-feldspar MDD modeling progressively gets younger from the southern to northern samples, suggestive of a component of northward dip for the detachment fault.
(4) Northeast-southwest–oriented high-angle, range-bounding normal faulting in the footwall rocks. High-angle normal faults and strike-slip faults striking perpendicular to the extension direction and affecting footwall rocks are reported from highly extended terrains (e.g., Axen et al., 1995; Yin and Dunn, 1992; John and Foster, 1993; Martínez-Martínez et al., 2002). Generation of these faults is attributed to footwall failure when the elastic strength of the crust is exceeded in the upper crustal levels (Wernicke and Axen, 1988). In the SKBC, these faults affected the Biarjmand and Shotor Kuh–Majerad ranges. Normal displacement for ∼80° SE-dipping faults is reported by Hushmandzadeh et al. (1978). Horst-graben structures inferred in the SKBC are bounded by these steep faults (Jackson et al., 1990). These faults are also present in the area southeast of Kuh-e-Majerad (Fig. 4). The initiation age of the high-angle normal faults is not known; however, considering that their strike is in accordance with the extension direction represented by other indicators, we attribute the orientation of these faults to the extensional regime and metamorphic core complex development in this region. We do not disregard the possibility that the NE-SW faults might be entirely younger than Late Cretaceous, or reactivated as strike-slip faults during a younger tectonic event.
Our suggested NW-SE–directed extension is not in agreement with a NNE-oriented fault striation that we observed in a small and isolated outcrop of the brittle detachment fault in the Biarjmand range (southeast of Kuh-e-Yazdu). Three possible mechanisms might be responsible for this discrepancy.
(1) The NNE-oriented fault striation might be related to a younger deformation reactivating the same surface. We do not have any data to test this possibility.
(2) Both NW-SE and NNE-SSW orientations might have been generated contemporaneously. More than one extension direction, which is reported from several other metamorphic core complexes, might result from strain partitioning of extension in more than one direction because of lateral inward flow of material toward the region where upper-crustal thinning occurs (e.g., MacCready et al., 1997; Gautier et al., 2008; Andronicos et al., 2003). For two reasons, this second mechanism seems not to be in accordance with what we know from the study area. First, inward flow of material requires ductile deformation; second, such a mechanism is suggested for cases where two contemporaneous mylonitic lineations are found. We are not aware of this being the case in our study area.
(3) There might have been a difference in extension direction and dip direction of the detachment fault. The extension direction inferred from a detachment fault striation would be correct if the dip direction of a detachment fault is parallel to direction of extension. If not, the resulted striation would be oblique to extension direction.
Magnitude of Extension
We use two independent thermochronometers and field observations to constrain the magnitude of extension in the northeastern Great Kavir Basin.
Based on K-feldspar MDD modeling data, the deepest part of the lower plate in the Shotor Kuh range (sample 05TO40, ∼320 °C) was located at depths of ∼7–21 km, assuming a geothermal gradient of 15–45 °C/km. Assuming 10°–30° dip for the brittle detachment fault, a slip magnitude as small as 14 km and as large as 125 km is required to exhume the lower-plate rocks to the surface. For this calculation we assume that the extension was accommodated by a single detachment fault.
Several methods based on field observation are usually used to evaluate extension magnitudes in metamorphic core complexes. For example, John and Foster (1993) determined the minimum amount of extension in the southern Basin and Range using the width of the exposed footwall rocks. This method gives a minimum magnitude of ∼25 km in the SKBC (short double-headed arrow in Fig. 2). Extension magnitude can also be identified by the structural offset between unique markers in the footwall and hanging wall of a detachment fault (Fosdick and Colgan, 2008; Gans, 1997; Glazner et al., 1989). However, this method is difficult to apply to the SKBC because of a lack of matching rocks in the hanging wall and footwall of the detachment fault.
The most reliable tool for our case is using the length of the tilted crustal sequence that is exposed in the southeast part of the SKBC (Fig. 5). As previously discussed, we interpret this tilted crustal sequence to be the footwall of a large-scale detachment fault (Figs. 17 and 18). Considering the location of the oldest syn-extensional sedimentary rocks in the southeast of Kuh-e-Peyghambar, and detaching of the hanging-wall rocks along the low-angle fault of Kuh-e-Peyghambar from a place adjacent to the Doruneh-Kashmar ophiolites (see above for discussion about Kuh-e-Peyghambar), a maximum extension estimate is yielded if the detachment breakaway was near the Doruneh-Kashmar ophiolites (Figs. 2 and 18). The distance between the possible breakaway and the Biarjmand block (Kuh-e-Molhedu) is ∼135 km and therefore provides the maximum magnitude of extension along the detachment fault (shown by the long double-headed arrow in Fig. 2). This magnitude is comparable to the 125 km slip magnitude derived from the extreme case determined from thermochronology data, which in turn implies a low-angle detachment dip (∼10°) and low geothermal gradient (∼15 °C/km).
Our suggested slip magnitude is considerably larger than slip determined for most well-documented single detachment faults (e.g., Whipple detachment in Southern California, USA: Yin and Dunn, 1992; Northern Snake Range detachment in Nevada, USA: Bartley and Wernicke, 1984; Catalina detachment in southern Arizona, USA: Dickinson, 1991). However, it is comparable with 120 km down-dip slip along the Banda detachment in eastern Indonesia (Pownall et al., 2016). In this region, the extension occurred adjacent to the trench due to the Banda slab rollback. The significance of our finding is that it indicates that a high-magnitude extension can occur in the interior of a continent away from its margin.
We emphasize that for several reasons the estimated magnitude should be regarded with caution. First, syn- or post-extensional faults, which probably affected the tilted crustal sequence, would have modified the length of the exhumed crustal sequence. Second, our estimation is based on the assumption that the BHC is located in the hanging wall of the detachment fault. If we consider the BHC range to be in the footwall, the estimated extension magnitude would be considerably increased (see the section on timing of extension and thermal history for the uncertainty about the structural position of the BHC). Third, another assumption in our estimation is that the extension is accommodated by a single detachment fault. Accommodation of the extension by more than one detachment fault (e.g., including one that is unknown to the northeast) would reduce the required slip on the detachment system described here.
Regional Tectonic Implications
Our study in the Shotor Kuh–Biarjmand metamorphic core complex provides significant contributions to understanding the tectonic evolution of the Iranian Plateau. First, it indicates that extensional exhumation of deep crustal rocks was not limited to the plate margins but also occurred ∼500 km away from the margin in northeastern Iran. Two out of three previously known metamorphic core complexes (Takab-Zanjan: Stockli et al., 2004; Golpaygan: Moritz et al., 2006) are located along the margin of the plateau. The third metamorphic core complex (Saghand; Verdel et al., 2007) instead is located away from the plate margin, although extensional exhumation in this region has been attributed to be related to a plate margin process (Verdel at al., 2007; Kargaranbafghi et al., 2012a, 2012b). The fourth metamorphic core complex (the Shotor Kuh–Biarjmand) presented in this study is located in the interior of the plateau away from its margin. This observation implies that the governing mechanism for extension affected the entire plateau and was not limited to the margins.
Second, the results of our K-feldspar MDD modeling show that extension in the Iranian Plateau started much earlier than previously thought. Unlike for other metamorphic core complexes in Iran, the extension in the Shotor Kuh started in the Late Cretaceous, ca. 76 Ma, which pre-dates Eocene extension in the Saghand and Golpaygan regions (Moritz et al., 2006; Verdel et al., 2007) and Oligo-Miocene extension in the Takab region (Stockli et al., 2004). However, it is not known whether this old extension age is limited to only the Shotor Kuh region because the timing of extension initiation is not well constrained for some of the other metamorphic core complexes. For example, in the Takab region, timing of extension is constrained by apatite (U-Th)/He low-temperature thermochronology (closure temperature of 55–80 °C; Farley, 2000). This method would be incapable of recording the timing of extension initiation if extension started at an earlier time when the footwall of the detachment fault was at higher temperatures. Recently, Shafaii-Moghadam et al. (2016) showed that partial melting and migmatite generation, which they attributed to the Takab core complex exhumation, occurred in the Oligocene (28–25 Ma). However, even this time may not precisely indicate the initial timing of extension in this region. Rey et al. (2009), using numerical modeling, showed that exhumation of crystallized migmatite cores in extensional terrains can postdate extension initiation by several tens of millions of years. Therefore, further structural and isotope dating investigation is required to precisely constrain the initial timing of extension in some of the Iranian metamorphic core complexes.
This work was partially supported by Geological Society of America Graduate Student Research Grant awarded to the first author. Funding for analyses was provided by the New Mexico Geochronology Research Laboratory. We thank Amir Esna-Ashari for facilitating access to our samples in Iran. Jolante van Wijk, Shari Kelly, and William McIntosh are thanked for their helpful reviews of early drafts of this manuscript. The authors also thank Bernard Guest and two anonymous reviewers for thorough and constructive comments.