The eastern iranian orocline
Introduction
The Eastern Iranian ranges, a NS elongated belt with an approximately 900-km length and 200-km width in average, mainly composed of Cenozoic rocks extend between the Gondwana-derived blocks of Afghanistan in the east and Lut in the west (Stöcklin, 1968) (Fig. 1). Owing to the distinct nature and perpendicular orientation of the belt compared with its adjacent Alpine and Himalayan orogenesis (Aghanabati, 1993a), we call it in this research “the Eastern Iranian Orogen”, a part of the greater “Paleogene Orogen” which developed west of the Indian-Eurasian collision zone. The Eastern Iranian Orogen at large map scale is a curved belt hereby defined as “the Eastern Iranian Orocline” in short the EIO. Carey (1955) proposed the name orocline for an orogenic system which has been flexed in plan to a horse-shoe or elbow shape representing an impressed strain. Indeed, they are curves that developed in response to bending or buckling of an existing orogenic belt about a vertical axis of rotation. (See Plate 1, Plate 2.)
One of the most spectacular, largest, but least understood lithospheric-scale structures on Earth are oroclines. Studies of these horseshoe or elbow shape structures including definitions, descriptive terminology, classifying methods, kinematics, relative timing of curvature, and origin, implications for plate tectonics and global geodynamics have been described in the last decades (e.g., Carey, 1955; Ries and Shackleton, 1976; Marshak, 1988, Marshak, 2004; Hindle and Burkhard, 1999; Gutiérrez-Alonso et al., 2004; Van der Voo, 2004; Weil and Sussman, 2004; Rosenbaum, 2012; Johnston et al., 2013; Weil et al., 2013; Musgrave, 2015; Pastor-Galán et al., 2017). Oroclines have been inferred to develop in response to two different applied stresses relative to an existing orogen; stress-perpendicular and stress-parallel models (e.g., Ries and Shackleton, 1976; Johnston and Mazzoli, 2009). Weil and Sussman (2004) classified curved belts based on the angular relationship between structural trend and secondary imposed curvature into three main groups: (1) oroclines, (2) progressive arcs, and (3) primary arcs. Oroclines are those orogens that were originally linear and were curved during a subsequent deformation event. Progressive arcs develop their arcuate nature contemporaneously with growth of the belt. Primary arcs are those orogenic systems that inherit curvature during initial deformation and experience no appreciable tightening during subsequent deformation. Later, Johnston et al. (2013) separated two distinct types of oroclines as progressive and secondary according to the character and tectonic setting of oroclines and the similarity or difference with the tectonic setting that gave rise to the previously formed orogen. Progressive oroclines are restricted to the scale of a thrust sheet to thrust belt, are thin-skinned, and develop during thrust sheet emplacement. Secondary oroclines are larger, occurring at the scale of an orogen, and are plate-scale features that affect crust and lithospheric mantle.
On the other hand, many studies have attempted to understand the mechanism of oroclinal bending and their evolution. Most of these proposed models emphasize distinct processes; for instance, along-strike variations in propagating thin-skinned thrusts (Marshak, 1988, Marshak, 2004), rollback along the subduction segment (Linzer, 1996; Schellart et al., 2004; Cifelli et al., 2008), large-scale drag folding associated with strike-slip faults (Kamp, 1987), wrapping around tectonic obstacles and indentor (Tapponier et al., 1981; Rumelhart et al., 1999), escape tectonics (Burchfiel, 2004; Tatar et al., 2012), and orogenic-scale buckling (Gutiérrez-Alonso et al., 2012; Johnston, 2008; Weil et al., 2013).
The Baluchistan orocline, a wrinkled orogenic system in the Middle East (Carey, 1955), is a V-shaped bend wrapped between the Indian and Arabian plates. Today geologists distinguish obviously that the Carey's Baluchestan orocline is a part of a multiple-bend belt comprising at least two separate collisional events (e.g., Berberian and King, 1981; Searle et al., 1987; Agard et al., 2011) and, accordingly, it should not be considered as a single orocline.
In Iran, the study of such large oroclines is new. For instance, the Alborz Mountains is considered as a secondary orocline associated with the subduction and extrusion of South Caspian block (Mattei et al., 2017). Another example comes from southern Iran, the Bandar Abbas syntaxis, where the curved margin of the Arabian plate indented into the Sanandaj-Sirjan Zone (Molinaro et al., 2005); there, the contrast between the thin-skinned to thick-skinned tectonics were identified. In addition, there are several unpublished dissertations on the topic of tectonics of large structural arcs in the Department of Geology in Sistan and Baluchestan University. Several large structural arcs such as the Shuru mega-antiform at the centre of the Sistan suture zone, the Gohar Kuh mega-antiform in northern Baluchestan, the Naybandan structural arcs at the center of the Tabas block, the Birjand structural arcs in eastern Iran, the Ravar curvature in the south Tabas block, and the horseshoe-shaped Sirjan orocline in the southeastern part of the Sanandaj-Sirjan Zone (Partabian et al., 2020) were presented and their genesis discussed. However, the spatial and temporal relationships of these bends and their origin still remain controversial.
After considerable progression in the understanding of the geology of the Zagros, Alborz and most parts of Central Iran, the geology and geodynamic of the Eastern Iranian ranges have been noticed in recent years (e.g., Tirrul et al., 1983; McCall, 1985; Angiboust et al., 2013; Pang et al., 2013). However, there is still no one unique scheme to compare the regional tectono-stratigraphic zones in the Eastern Iranian Orogen. It is due to our limited knowledge regarding the structural position of tectonic units and the less studied geology of the vast southern part of the Sistan suture zone in the Iranian Baluchestan. Moreover, the previously presented tectonic units in the northern domain of the Sistan suture zone did not correlate to its southern part. In this regard, the continuation of structural zones, principal boundary faults and ophiolitic belts from north to south is generally blurred.
One of the most serious problems in understanding the formation and evolution of the Eastern Iranian Orogen are inconsistencies and disagreements which occur between the existing plate tectonic models. There is no consensus about the evolution of the Sistan Ocean, an oceanic realm which was existing between the Lut and Afghan blocks; for example, the time and mechanism of opening, its extension and connection to the other oceans, subduction polarity of the oceanic lithosphere, existence of island-arc chains and finally the time and cause of its closure (e.g., Saccani et al., 2010; Pang et al., 2013; Mohammadi et al., 2016).
Another issue which was not taken into account in related research is the problem of deformational phases and their interactions. For instance, the refolded structures revealed in a large region in the Sefidabeh basin, in the middle part of the Sistan suture zone, indicates that the origin of the early E–W-trending folds before middle Eocene has been in an unexplainable, NS stress direction (Tirrul et al., 1983). It still remains a serious unresolved problem.
The extensive field studies between 2007 and 2017 supported by satellite image analyses on almost the entire Eastern Iranian Orogen, let us propose a revised model to unravel the formation and evolution of the Eastern Iranian Orogeny and examine its constituents. The structural analysis of the orogen directed us to explore several mega-antiforms, conical folds, interference fold patterns, main thrust systems, strike-slip duplexes and consequently distinguishing deformation phases in key localities of the Eastern Iranian Orogen. These structures have been applied to elucidate how they formed through an oroclinal buckling. On top of the structural data, petrological, geochemical, geochronological, and geophysical documents were gathered from previous studies in Iran and adjacent countries, which led us propose a new tectono-stratigraphic subdivision and to construct an oroclinal model. Then, the orocline was placed within the plate-tectonic framework used for a paleo-tectonic reconstruction.
Section snippets
Iran and surrounding regions
Iran and the surrounding areas present a mosaic of continental blocks separated from each other by complex fold-and-thrust belts within the Alpine–Himalayan orogenic system (e.g., Stöcklin, 1968; Stampfli, 2000). The formation and evolution of such a large orogenic system have been controlled by the role of the Tethyan Oceans (Stampfli, 2000; Stampfli et al., 2013). That region is generally a collage resulting from successive accretions of continental blocks detached from Gondwana, crossed the
A New Tectono-stratigraphic-unit subdivision
The tectonic subdivision arranged by Tirrul and his colleagues (1983) for the geology of eastern Iran covers only the middle part of the Sistan suture zone. Results from remote sensing studies from one hand and widespread petrological, geochemical and geochronological investigations, on the other hand, during the last three decades led to a revision of the previous subdivisions and the development of new ideas on the geology of the eastern Iran. In addition, to achieve better understanding the
The eastern iranian orocline
The large-scale curvatures of about 90 degrees around a vertical axis recognizable on the geological maps are one of the most striking tectonic features that attracted the geologists' attention in recent decades. These structures are known as oroclines (Carey, 1955). Later, a group of these structures were recognized as non-rotational arcs which originated with a curved plane, while another group was defined as rotational arcs, are curved segments of an orogen which have undergone bending in
Phases of deformation
The evidence of fold superimposition has already been reported in the Sefidabeh basin by Tirrul et al. (1983). In addition, interference fold patterns can be observed in most areas of the EIO, however, the Khash complex is one of the most obvious areas exhibiting these types of structures. These structures can be clearly related to at least two deformation phases that can be divided based on their origin and time of formation compared with the time of the oroclinal buckling through three main
Orocline test
At this moment, there are no new paleomagnetic data that could support the oroclinal buckling at the EIO. Moreover, the East Lut fault system and the concomitant strike-slip detachment zone displaced and dissected the western limb of the orocline. Besides, substantial parts of the eastern limb of the orocline underlie the post-oroclinal deposits. These features do not directly permit to compare structurally and paleomagnetically both limbs of the inner arc. However, abundant evidences from the
Lithospheric delamination
Delamination in orogenic belt and subduction zones are the two main mechanism for the recycling of the lithospheric material into mantle (Bird, 1979; Houseman and Molnar, 1997; Ducea, 2011). Late to post-orogenic oroclinal bending of the lithosphere around a vertical axis may cause thickening and eventual detachment of the lithospheric root of orogenic belts (Gutiérrez-Alonso et al., 2004; Johnston et al., 2013). The delamination hypothesis proposed here for the EIO is consistent with the
Syn- to post-oroclinal magmatism
Magmatism involving the entire lithosphere has been used as a remarkable tool related to orocline formation (e.g., Fernández-Suárez et al., 2000, Gutiérrez-Alonso et al., 2012, 2011, 2004; Ducea, 2011). The syn-orocline magmatism spans the entire orogen with late Eocene–early Oligocene ages mainly including mafic to felsic magmatic rocks. Lava, pyroclastic and sub-volcanic rocks and minor gabbroic to considerable granitic intrusives are abundant in the Khorasan outer arc (Karimpour et al., 2011b
Tectonic reconstruction of the EIO
The palinspastic restoration of individual structural units and tectonic domains to their primary position before the onset of deformation, shows that the present-day structural sinuosity in the EIO is a consequence of the secondary rotation of originally linear features and modification and tightening of originally curvilinear features.
Our observations indicate that the Cenozoic tectonic history in the EIO involved at least three temporally discrete deformation phases (Fig. 16). The last and
Age of the eastern iranian orocline
The age of the EIO can be derived from indirect evidences such as the age of the oroclinal buckling during the late Eocene–early Oligocene times, a short period of nearly 10 to 20 Ma. We can add the following points:
- (1)
The entire Lower-Middle Eocene carbonate rocks has participated in the second phase of deformation (D2) or the main phase of buckling.
- (2)
The northwestern end of the Zahedan granitic belt including the mylonitized 44–40 Ma granites, were affected by the D2 phase, and folded with similar
On the origin of the eastern iranian orocline
Most of the structural, magmatic, and geophysical characters of the EIO drive us in the road to the assembly of oroclines which were not involved in thin-skinned thrust belts or the ones associated to subduction zones, related to the strike-slip faults or wrapped about tectonic obstacles (see the cases reviewed in section 1).
The long and wide arcuate belt of the EIO is composed of continental terranes and accretionary complexes curved in response to an applied stress oriented parallel to the
The eastern iranian orocline is a secondary orocline?
The Paleogene orogen of Eastern Iran seems to be buckled into three arcs; two, coupled oroclines, in Iran and one common to the three regions of Iran, Afghanistan and Pakistan. The Convex-to-the-south Western Pakistan as the eastern arc, the convex-to-the-north Eastern Iranian as the middle arc, and the linked, convex-to-the-south Central Iranian to the west. We do not know much about these oroclines. We have just begun studying the middle one. The EIO is approximately “isoclinal” and exhibit
Correlation of tectono-stratigraphic units inside and around the EIO
We correlate the tectono-stratigraphic units first in the hinterland zone setting of the Paleogene orogen and then in the foreland zone setting. There are three reasons which suggest the ribbon continents of Sanandaj-Sirjan, Lut, and Helmand linked together orderly from the west to the east (Stampfli and Borel, 2004; Ghodsi et al., 2016) have been attached and in the same regional tectonic setting before the oroclinal bending.
- (1)
Common geological similarities; The Sanandaj-Sirjan, Lut, and Helmand
Conclusions
A new, regional tectonic model, incorporating a revised tectono-stratigraphic-unit subdivision covering eastern Iran along with structural field studies was used to constrain the formation of the Eastern Iranian Orogeny within the context of oroclinal buckling. The Paleogene Eastern Iranian Orogeny, the westward extension of the NW Himalayan orogeny, was a protracted collisional event that involved the dispersal and amalgamation of several micro-continents and their interspace fold-thrust belts.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
Special thanks to all the graduate students under my guidance, members of the CEIM group, in the last twelve years for all their scientific assistance and the important role of each of them in scoring tectonic studies in eastern Iran. We appreciate Prof. Gerard Stampfli due to his valuable review and all comments and corrections. The final form of this article is indebted to the reviewers Dr. Daniel Pastor-Galán and one anonymous reviewer, much to their compassionate efforts in improving the
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