GEODYNAMIC EVOLUTION OF THE ZAGROS
Prior to continental collision
The Paleo-Tethys (Fig. I-3), which separated Laurasia from Gondwana, has existed since late Palaeozoic times. During the earliest Triassic, the Neo-Tethys Ocean opened between the Afro-Arabian and Iranian blocks ; the Paleo-Tethys began to close at the same time. The Neo-Tethys Ocean started to open along what is now called the “Crush Zone” or “Imbricated Zone” in the northern, internal part of the Zagros. At the beginning of the Mesozoic, due to the development of the Neo-Tethys and the displacement of the Iranian block towards the NE, the Paleo-Tethys Ocean closed and continental collision between the Iranian block and the Eurasian plate took place. The zone of collision stretches out towards NE of the Caucasus and Talesh mountaines in NW Iran and up to the Kopet-Dagh (Sengor, 1979). Throughout the middle Cretaceous the Afro-Arabian and Eurasian plates converged and the Neo-Tethys Ocean began to close. During the Late Cretaceous, the NE margin of the Arabian plate started to be subducted under Central Iran. The closure of the Neo-Tethys was marked by several tectonic events on the Zagros platform. The first was the Early Coniacian – Late Santonian (89.31.0 – 83.50.7) obduction of ophiolites onto the continental crust (Ricou, 1971; Falcon, 1974; Berberian and King, 1981; Berberian, 1995), that changed the architecture of the sedimentary basin. The second event was pronounced reactivation of deep-seated, pre-existing north-south faults following older Panafrican basement trends (Koop and Stoneley, 1982).
Figure I-3. The construction of the Iranian platform and the situation of Paleo- and Neo-Tethys : (A) Lower Triassic, (b) Upper Jurassic, (Trowell, 1995).
Post collisional tectonics
Throughout the upper Oligocene and lower Miocene, the Red Sea opened and subduction of the Arabian plate under central Iran intensified. The separation of the Arabian Plate from Africa along the Red Sea, which is widening at ~1.2 cm per year in a NNE direction, has propelled the Arabian Plate towards Iran. Continent-continent collision between Arabia and Central Iran plates began during middle Miocene time (Alavi, 1994; Berberian, 1995; Hempton, 1987). Zagros folding began during lower Miocene times, with thin-skinned décollement and southwestward propagation of the foreland depocenter (Fig. I-3).
Compression in the Zagros margin continued during the middle Miocene and tectonic units were formed. The Neogene Zagros orogeny partitioned the evolving foreland basin into sub-basins with different sedimentary and tectonic signatures (Bahroudi and Koyi, 2003).
During late Miocene and Pliocene times, regression of the sea and the creation of mountainous relief by folding and thrusting resulted in a continental environment. Large quantities of clastic material and red beds were developed in synclines (Berberian and King, 1981). Hempton (1987) mentioned in some extrusion occurred at the north of the suture zone due to the movement of Arabia toward the North. He suggests that such extrusion at the begining of Pliocene allowed the Arabian plate to move northward faster than Africa. Convergence is partly taken up in the Alborz Mts in the North, as well as by strike slip faults in general located on the border of the different continental blocks (Fig. I-4).
STRUCTURE OF THE ZAGROS FOLD-THRUST BELT
Morphotectonic units of the Zagros Mountains
Various schemes exist for the subdivisions of the Zagros (Stöcklin 1968; Falcon, 1974; Alavi, 1994; Berberian 1995). The Zagros belt can be divided by major faults and/or abrupt changes in geomorphology into five morphotectonic units from northeast to southwest (Fig. I-5), which step down as five prominent topographic levels to the southwest with different characteristics in terms of thrusting, folding, uplift, erosion and sedimentation (Berberian, 1995). Each unit has its own characteristics and deformation style, which will be discussed in the following sections (Fig. I-6):
1) The High Zagros Thrust Belt (HZTB) or Imbricated Zone
2) The Simply Folded Belt (SFB)
3) The Zagros Foredeep (ZF)
4) The Mesopotamian lowland – Zagros Coastal Plain (ZCP), and
5) The Persian Gulf
The boundaries of the units are defined on the basis of (1) surface topography and morphotectonic features, (2) style of deformation, (3) subsurface geologic data, and (4) the regional seismicity.
Salt tectonics (disharmonic folding, flow and diapirism) is an important phenomenon in the Zagros. The styles of structural deformation change in the Zagros from salt-related detachment folding, to salt swells and pillows associated with fault-propogation folding, and fault-bend folding, as the thickness of the paleo-Hormuz salt basin diminishes (McQuarrie, 2004).
Although the amplitude and wavelength of the folds within the Zagros, permit basement shortening, basement deformation is not required by the geometry of the structures or by earthquake focal mechanisms. The geometry of deformation within the Zagros fold thrust belt suggests that many of the folds are cored by faults in the lower Paleozoic strata. The inferred dips and locations of these faults are compatible with the magnitude, depth, and nodal plane orientation of earthquakes throughout the fold–thrust belt (Talebian and Jackson 2004). Quaternary folding becomes younger from northeast to southwest, demonstrating that the deformation front is migrating from the suture towards the foredeep (Berberian 1995, Hessami et al. 2002).
An important change in the fault configuration occurs along strike of the belt. In the NW, overall convergence is oblique to the trend of the belt and the surface anticlines, and is achieved by a spatial separation or partitioning of the orthogonal strike-slip and shortening components on separate parallel fault system (e.g. Authemayou, et. al. 2003). In contrast, in the SE, the overal convergence is orthogonal to the regional strike and achieved purely by thrusting (Talebian and Jackson 2004). In the central Zagros, between these two structural regimes, deformation involves parallel strike-slip faults that rotate about vertical axes, allowing extension along the strike of the belt.
The Zagros Imbricate Zone
The Zagros Imbricate Zone or the High Zagros is a narrow thrust belt up to 80 km wide, with a NW-SE trend between the Main Zagros Thrust (MZT) or the Zagros suture to the northeast and the High Zagros fault (HZF) to the southwest (Fig. I-5 and I-6). The High Zagros is characterised by the highest topography of the belt, with summits over 4000 m above sea level. The belt is strongly dissected by numerous reverse faults and is upthrusted to the southwest along different segments of the High Zagros fault (Berberian, 1995). The High Zagros is characterized by strongly deformed overthrust anticlines, mainly composed of autochonous Jurassic-Cretaceous series with Paleozoic cores along some reverse faults. The belt was affected by Late Cretaceous obduction and Pliocene continent-continent collision (Stocklin, 1968; Falcon, 1969, 1974; Huber, 1977; Berberian, 1976, 1977, 1981, 1983; Berberian and King, 1981; F. Berberian et al., 1982). To the SW, the High Zagros Thrust Belt contains highly imbricated slices of the Arabian margin and fragments of Cretaceous ophiolites (Alavi 1994; Berberian 1995).
The Zagros Simply Folded Belt
The Simply Folded Belt (SFB) of the Zagros (Fig. I-6) is limited to the northeast by the High Zagros fault (HZF) and to the southwest by the Mountain Front fault (MFF). It has an average width of about 250 km to the southeast (Fig. I-6), 120 km to the northwest, and a length of 1375 km in Iran. The Simple Fold Belt contains characteristic elongated hogback or box-shaped anticlines, penetrated by salt plugs from the Hormuz Salt and forming mountainous terrain where calcareous ranges of the Eocene-Oligocene Asmari Limestone and Mesozoic formations dominate the topography. Within this belt, aspect ratios of the folds (length:height) are variable, and have been used to suggest that both forced fold (high aspect ratio > 10) and buckle folds (low aspect ratio < 10) are present (Sattarzadeh et al., 2000). The sedimentary column of the belt (Fig. I-17) is estimated to be up to 12 km thick (James and Wynd, 1965; Falcon, 1974; Huber, 1977; Alavi, 2003). The belt was folded during Miocene-Pliocene continent-continent collision. Incompetent units are present as first- and second-order detachment levels. The Hormuz Salt and the Miocene Gachsaran Evaporites have facilitated décollement in the lower and the upper parts of the Phanerozoic sedimentary cover (Stocklin, 1968; Falcon, 1961, 1969, 1974; Huber, 1977; M. Berberian, 1983). Paleozoic strata are very rarely exposed, but salt from the upper Proterozoic Hormuz Series crops out in diapirs in the east of the Zagros (Gansser 1992; Talbot & Alavi 1996). The Hormuz Salt does not appear in the SFB west of the Kazerun Fault, but crops out in the Zagros Imbricate Zone or the High Zagros to the NW.
The Zagros Foredeep
The Zagros Foredeep is bounded to the northeast by the MFF and to the southwest by the Zagros Foredeep fault (ZFF), which marks the northeastern edge of the Coastal Plain of the Persian Gulf. The formation of the Zagros Foredeep was associated with motion along the MFF and uplift of the Simply Folded Belt (SFB) (Fig. I-6). The Zagros Foredeep, which consists of elongate, symmetrical folds, is characterized by badlands of the Miocene Fars Group sediments (Gachsaran, Mishan and Aghajari formations), sheared off from the subsurface Eocene-Oligocene Asmari Limestone base along décollement thrusts in the Gachsaran Evaporites. There is a thick sequence of the Lower Miocene to Pleistocene synorogenic molasse cover (Aghajari-Bakhtiari formations) in the belt. The growth of the Zagros Foredeep structures was coeval with deposition of the Upper Pliocene-Pleistocene Bakhtiari conglomerate. The anticlines associated with the Zagros Foredeep are actively growing, and the evidence from continuous unconformities in the Pliocene freshwater sediments and folded recent gravels shows they have been active since the beginning of the Pliocene (Lees and Falcon, 1952; Falcon, 1961).
The Zagros Coastal Plain and Mesopotamian lowland
The Zagros Coastal Plain (Figs. I-6) is a narrow feature bounded to the north by the Zagros Foredeep fault (ZFF) and to the south by the Persian Gulf, which is the southern edge of the Zagros orogen. The Coastal Plain slopes very gently to the south at a rate of 0.2%. The morphotectonic unit of Mesopotamian lowland lies south and southwest of the Zagros Coastal Plain and is partly covered by the Persian Gulf.
The Persian Gulf
This morphotectonic unit with an area of about 226,000 km2, 800 km long and from 115 to 185 km wide is a shallow epicontinental sea. The Gulf results from foreland flexure and covers the Arabian shelf platform with shallow water depths (~35 m average depth and ~110 m maximum depth). Some small islands in the Persian Gulf are Hormuz salt plugs, partly fringed by the Neogene clastic and marine deposits and by recent coral reefs. The larger islands near the Iranian coast are gentle anticlines (Seibold and Vollbrecht, 1969; Kassler, 1973; Ross et al., 1986). The Persian Gulf does not seem to deform as much as the Zagros Mountains, which is consistent with the lack of seismicity. Lambeck and Chappell (2001) noted that the sedimentary record of the Gulf indicates that deposition was controlled by the tectonics and sea level changes during Quaternary glaciations. AMS 14C ages indicate that the axial zone of the Persian Gulf experienced rapid flooding leading to the deposition of Holocene marls due to a combination of tectonic subsidence and a major glacial melt water pulse 9500–8500 years ago when sea level rose from ~50 to ~28 m (Lambeck and Chappell ; 2001). It suggests that the region of Gulf-Mesopotamian lowland has undergone regional subsidence disrupted by local uplifts. The rate of this subsidence, in contrast to the northeast side of the Persian Gulf, is much less. It was low enough that sedimentation has been able to keep up (Uchupi et. al., 1999).
Major Fault Structures
The Main Zagros Thrust (MZT) or the Zagros Suture
The original Arabia-Eurasia (Zagros) suture is also known as the Main Zagros Thrust (MZT). The MZT indicates a fundamental change in paleogeography, structure, sedimentary history, morphology and seismicity (Berberian, 1995) (Fig. I-7). It marks the suture between the two colliding plates of the central Iranian active continental margin (to the northeast) and the Afro-Arabian passive continental margin (the Zagros fold-thrust belt to the southwest). It has a NW-SE strike from western Iran to the area north of Bandar Abbas, where it changes to a N-S trend (Minab Fault) and marks the boundary between the Zagros belt (to the west) and the Makran accretionary wedge and the active subduction zone (to the east).
The Main Recent Fault (MRF)
Part of the MZT in the NW Zagros known as the Main Recent Fault (MRF), (Tchalenko and Braud, 1974), a major seismically active right-lateral strike-slip fault with a NW-SE trend that more or less follows the trace of the MZT (Fig. I-7). Geodetic measurement in the Fars arc/Central Zagros indicate that the MZT in the SE is inactive there (Tatar et al. 2002). Based on earthquake focal mechanisms analysis (Talebian and Jackson, 2004), propose that oblique shortening is partitioned into right-lateral strike-slip on the MRF. This proposition is, however, not consistent with mechanical modelling of oblique convergence in the Zagros (Vernant and Chéry, 2006). Strike-slip movement from the MRF appears to be transmitted to the NS-trending right-lateral Kazerun and Karebass faults (Autemayou et al. 2006). Gidon et al. (1974) suggested right-lateral displacement of 10-60 km of a geological marker bed in the northwestern Zagros by the segments of the MRF. If slip began around Pliocene times (5 Ma) this displacement yields an average slip rate of 2-12 mm/yr (Berberian, 1995). The MRF is morphologically and structurally distinct along its entire length, and the component of right-lateral strike-slip motion between Arabia and central Iran takes place preferentially along different segments of the MRF in western Iran (Jackson, 1992). Earthquake mechanisms and seismic potential of the MRF are quite different from those of the earthquakes of the Zagros fold-thrust belt. Notably earthquakes of larger magnitudes than those within the Zagros Fold-Thrust belt have taken place along the MRF. Talebian and Jackson (2002) demonstrated that a right-lateral strike-slip offset of ~50 km on the Main Recent Fault is compatible with a restoration of the drainage, geological markers and the length of the pull-appart basins. They present evidence that show the offset may be as much as ~70 km. The configuration of active faulting and earthquake slip vectors today shows that this offset is geometrically linked to a shortening of ~50 km across the NW Zagros and to a total N-S convergence of ~70 km, which is a substantial fraction of the 85-140 km total Arabia-Eurasia convergence over the last 3-5 Myr (Talebian and Jackson, 2002).
The High Zagros fault (HZF)
The HZF (the southern boundary of the thrust zone of Berberian and Tchalenko, 1976) separates the thrust belt of the High Zagros or Zagros Imbricate Zone (to the northeast) from the Simply Folded belt (to the southwest) (Fig. I-7). The High Zagros is up-thrusted to the southwest along discontinuous segments of the HZF (Berberian and Qorashi, 1986). Geological evidence, based on the present position of the Paleozoic rocks (Huber, 1977) demonstrates vertical displacement along the HZF to be more than 6 km. The Hormuz Salt is intruded along different segments of the HZF, locally reaching the surface. This indicates that the HZF is a deep-seated fault, cutting the Lower Cambrian Hormuz Salt horizon, and its activity extends through the Phanerozoic sedimentary cover (Berberian, 1981). Wedging of the post-Asmari deposits (Miocene Gachsaran evaporites together with the Lower Miocene to Pleistocene Aghajari-Bakhtiari synorogenic molasse) towards the High Zagros (James and Wynd, 1965; Falcon, 1974; Huber, 1977), suggests uplift of the High Zagros along the HZF since the Early Miocene, contemporaneous with subsidence of the Zagros Foredeep and deformation and outward migration of the Zagros basin. In the Khurgu area, north of Bandar Abbas in the southeastern Zagros, the HZF reaches the Mountain Front fault (MFF), and to the northwest of this intersection, it diverts from the MFF and becomes parallel to the Zagros suture (MZRF).
Table of contents :
Goal of thesis
I ) TECTONIC AND GEODYNAMIC SETTING OF THE ZAGROS MOUNTAINS / CONTEXTE TETONIQUE ET GEODYNAMIQUE DU ZAGROS Résumé en français
1. Intruduction and plate tectonic context
2. Geodynamic evolution of the Zagros
2.1. Prior to continental collision
2.2. Post collisional tectonics
3. Structure of the Zagros Fold-Thrust Belt
3.1. Morphotectonic units of the Zagros Mountains
3.1.1 The Zagros Imbricate Zone
3.1.2 The Zagros Simply Folded Belt
3.1.3 The Zagros Foredeep
3.1.4 The Zagros Coastal Plain and Mesopotamian lowland
3.1.5 The Persian Gulf
3.2. Major Fault Structures
3.2.1 The Main Zagros Thrust (MZT) or the Zagros Suture
3.2.2 The Main Recent Fault (MRF)
3.2.3 The High Zagros fault (HZF)
3.2.4 The Mountain Front fault (MFF)
3.2.5 The Zagros Foredeep fault (ZFF)
3.2.6 The Kazerun-Borazjan fault (KF)
3.2.7 The Kareh Bas (KBS) strike-slip fault
3.3. Seismicity in the Zagros
3.4. Geodetic (GPS) measurments
3.5. Crustal and lithospheric structure
3.6. Structural the Zagros belt
3.6.2 Stratigraphy and major decoupling surfaces Hormuz Salt (Lower detachment level) Gachsaran Formation (Upper detachment level)
3.6.3 Structural models of the Zagros belt
II ) METHODOLOGY / MÉTHODOLOGIE
Résumé en français
1. Tectonic geomorphology
2. Geomorphic markers
2.2. Marine terraces
2.3. Fluvial Terraces
3. River long profile
4. Absolute dating methods
4.1. C-14 Dating
4.2. Cosmogenic Radio Nuclieds
4.2.2. Theory of cosmogenic isotope analysis; summary
III) ACTIVE FOLDING EVIDENCED AT THE CENTRAL ZAGROS FRONT EVIDENCES DE PLISSEMENT ACTIVE AU FRONT DU ZAGROS CENTRAL
Résumé en français
Rates and Processes of active folding evidenced by Pleistocene terraces at the central Zagros front (Iran) Abstract
2. Geological and Seismotectonic Setting
2.1. Regional balanced cross section
3. The Mand Detachment Fold
3.2. Fold solution and finite shortening
Case 1: folding above a SW-vergent fault
Case 2: fold detachment with internal deformation
Case 3: Detachment fold accompanied by synclinal flexures
3.3. Fold solution and incremental deformation at the surface
4. Recent deformation of Mand anticline
4.1. Tilted marine and fluvial terraces
Southern site (site A)
Central site (site B)
Northern site (site C)
4.2. Terrace dating and rates of tilting
4.3. Folding model and shortening rate
IV) SYNCLINE CORES EXTRUSION ABOVE A VISCOUS LAYER EMBEDDED IN THE CRITICAL WEDGE OF THE CENTRAL ZAGROS FOLD BELT / EXTRUSON DE COEURS DE SYNCLINUX AU-DESSUS D4UNE COUCHE VISCEUSE DANS LE PRISME CRITIQUE DU ZAGROS Résumé en français
2. Geological setting
3. Surface deformation in western internal Fars province recorded by Bakhtyari Formation
3.1. Stratigraphic description
3.2. Deformation recorded by the Pleistocene Baktiari surfaces
3.2.1 Baladeh monocline uplift
3.2.2. Kuh-e-Pahn North : syncline core extrusion
3.2.3. Kuh-e-Pahn South: flexural flow
3.2.4. Gachsaran formation and syncline core deformation
4. Numerical modelling of syncline reactivation
4.1. Model characteristics
4.2. Results and influence of the syncline geometry on surface deformation
V) THICK- AND THIN-SKINNED DEFORMATION RATES IN THE ZAGROS SIMPLE FOLDED ZONE (IRAN) INDICATED BY DISPLACEMENT OF GEOMORPHIC SURFACES / TAUX DE DÉFORMATION CRUSTALE ET SUPERFICIELLE DANS LA ZONE PLISSÉE DU ZAGROS (IRAN) A PARTIR DU DEPLACEMENT DE SURFACES GEOMORPHOLOGIQUES
Résumé en français
2. Geological setting
2.1. Tectonics and stratigraphy of the Zagros SFZ
2.2. Regional structures and balanced cross sections
3. Deformed Geomorphic Markers fold models and shortening: methodology
3.1. Terrace mapping and dating
3.2. Terrace deformation and fold kinematics
3.3. Fold deformation and absorbed shortening
4. Incision and deformation recorded by Quaternary fluvial and marine terraces in Western Fars
4.1 Deformation of mid-Pleistocene Bakhtyari surfaces
4.1.1. Sedimentology and age of the Bakhtyari Formation
4.1.2. Baladeh monocline
4.1.3. General incision and deformation profile of the Bakhtyari surfaces
4.2. Late Pleistocene fluvial terraces along the Dalaki River
4.2.1. Structural setting and general terrace description
4.2.2. Dalaki River terraces in the hangingwall of the MFF
4.2.3. Incision and fold model
4.3. Late Pleistocene fluvial terraces and river profile along the Mand River
4.3.1. Description of the fill and strath terraces along the Mand River
4.3.2. River and terrace profiles and basement fault activity
4.3.3. Terrace dating and fluvial incision rates in the Kuh e Halikan region
4.4. Deformation recorded by Late Pleistocene marine terraces along the Persian Gulf
4.4.1. Mand anticline
4.4.2. Madar anticline
5. Discussion: spatial distribution of active shortening and seismotectonic implications
5.1. Thin versus thick-skinned tectonics
5.1.1 Surface deformation above inferred basement faults
5.1.2 Evidence for activity of shallow structures
5.2. Fold evolution: from detachment to fault-related folds
5.2.1. Detachment folding above Hormuz Salt
5.2.2. Transition from detachment- to fault-propagation folding above the Gachsaran décollement 5.2.3. Fold evolution with increasing deformation
5.3. Seismotectonic model for the Zagros fold belt in the western Fars
VI) GENERAL CONCLUSION / CONCLUSION GENERALE