Neotectonic Stress Field and Deformation Pattern Within the Zagros and Its Adjoining Area: an Approach from Finite Element Modeling

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Full Length Research Paper

Neotectonic stress field and deformation pattern within the Zagros and its adjoining area: An approach from finite element modeling

Md. Shofiqul Islam1,2* and Ryuichi Shinjo1

1Department of Physics and Earth Sciences, University of the Ryukyus, Okinawa, Japan. 2Department of Petroleum and Georesources Engineering, Shahjalal University of Science and Technology, Sylhet, Bangladesh.

Accepted 23 August, 2010

In this study, finite element modeling (FEM) is performed to analyze the Neotectonic stress field of Zagros fold-and-thrust belt and its adjoining areas. Our modeling results predict that the study area has somewhat complex stress orientation, deformational fashion and faulting pattern under oblique convergence tectonic setting. The modeled results demonstrate that the displacement vectors within the Coastal plain, Persian Gulf, eastern part of Sanandaj-Sirjan-Metamorphic Zone have NE-ward direction, while the Main Recent Fault and some part of Sanandaj-Sirjan-Metamorphic Zone have N-ward and sometimes NE-ward direction. The modeled maximum horizontal compressive stress ( Hmax) orientation within the Lurestan, High Zagros Fault, Main Recent Fault and eastern Zagros Simple Folded Belt are predicted to have NE-SW, NW-SE and N-S, whereas the rest part of the study area displays mainly NE-SW. Fault pattern calculation of the study region predicts that strike slip faults are mainly present at shallow crustal level (up to 10 km), whereas thrust faults with strike-slip component are predominant at deeper crustal level (>10 km). Our modeling results are comparable with available data of focal mechanism solution, seismicity, GPS and world stress map (WSM). Thus we believe that modeling results can be used as a reference dataset, presenting to estimate overall stress condition, plate velocity and faulting pattern of an extensive area, because the other geophysical/geodetic data do not cover wide area of the region.

Key words: Zagros fold and thrust belt, stress, focal mechanism solution, seismicity.

INTRODUCTION

The tectonic stress distribution is directly associated with (Hessami et al., 2001; Vernant et al., 2004). The plate movement and varies place to place over the Earth convergence of Arabia with Eurasia (average motion (Gowd et al., 1992). The convergent boundary condition Aegean relative to Eurasia is 20 - 30 mm/yr) is is responsible for enormous heterogeneous stress field accommodated in large part by lateral transport within the over inter-plate region (Rajendran et al., 1992). The interior part of the collision zone and lithospheric Zagros Fold and Thrust Belt (ZFTB) is a part of the shortening along the Caucasus and Zagros Mountain belt Iranian mountains (Figure 1), which are actively (Reilinger et al., 2006). The deformation is taking place deforming due to shortening between the Arabian and as an oblique convergence and strength of fault Eurasian plates. The Arabia-Eurasia convergence began controlled by faults orientation (Vernant et al., 2006). As a in Southern Iran with the ZFTB at end of Eocene part of Alpine-Himalayan mountain chain, the ZFTB extends for more than 1500 km in NW-SE direction from eastern Turkey to the Minab-Zandan-Palami fault system in the Southern Iran (Stocklin, 1974; Vernant et al., 2004). *Corresponding author. E-mail: [email protected]. Tel: +81- Geologic and geophysical studies on the ZFTB (Alavi, 090-6636-4161. Fax: +81-098-895-8552. 1994, 2004 and 2007; Berberian, 1995; Hatsfeld et al., Islam and Shinjo 171

Figure 1. Regional tectonic map of the Zagros fold-and-thrust belt (modified after Navabpour et al., 2007). UDMA = Urumieh-Dokhtar Magmatic Arc; SSMZ = Sanandaj-Sirjan Metamorphic Zone; MZT = Main Zagros Thrust; MRF, Main Recent Fault; HZF = High Zagros Fault; ZSFB = Zagros Simple Fold Belt; MFF = Mountain Front Fault; IZ=Izeh Zone; BFZ = Borazjan Fault Zone; IFZ = Izeh Fault Zone; KFZ = Kazerun Fault Zone. ZF = Zagros Foredeep; ZFF = Zagros Foredeep Fault; MSZ = Makran Subduction Zone. Black arrow indicates GPS convergence vector from Vernant et al. (2004).

2003; Bachmanov et al., 2004; McQuarrie, 2004; Sepehr, (Navabpour et al., 2007). The High Zagros Belt (HZB) is 2004; Sherkati et al., 2005; Hessami, 2006; Kalviani et an imbricated zone that marks the northeastern part of al., 2007; Navabpour et al., 2007; Stephenson et al., the Arabian passive paleomargin which separates Main 2007) have provided improved understanding the Zagros Thrust (MZT) and Main Recent Thrust (MRF) structure, stratigraphy, tectonics and crustal movement of (Berbarian, 1995; Navabpour et al., 2007). The Sanandaj- the region. Despite of deployment of a lot of geodetic Sirjan Metamorphic Zone (SSMZ) and the Urumieh- instruments, in addition to geophysical works, all these Dokhtar Magmatic Assemblage (UDMA) are two major studies can describe about only several tens of year parallel domains which are interpreted as the product of record. There still is a scope to investigate the study area northeast-dipping subduction processes (late Jurassic- using numerical simulation method with appropriate Cenozoic) of Neo-Tethyan oceanic crust under the proxies. In this paper, we aim to reproduce the study Iranian continental margin (Berbarian and King, 1981). area’s stress distribution, deformation and faulting pattern The Zagros orogenic belt was affected the opening of with help of finite element method (FEM) simulation the Red sea and the closure of Neo-Tethys accompanied program. We believe such data can help us to better by convergence between Arabian plate and Eurasia in interpret regional tectonics of the study area. late Cretaceous to Miocene. First stage of deformation, alkaline pluton had been intruded at SSMZ during the Mesozoic as Andean-type active margin (Agard et al., Geological and tectonic background 2005). The first closing stage of the Neo-Tethys began during the latest Jurassic to early Cretaceous. The ZFTB in Iran (Figure 1) is a consequence of the Northeastward plate subduction beneath the Iranian plate Alpine Orogenic events in the Alpine-Himalayan Mountain caused intense magmatism, forming the UDMA. The range that extends in a NW-SE direction from eastern subsequent major obduction event (ca. 100 - 70 Ma) Turkey to the Strait of Hormuz in the Southern Iran occurred at the SW margin of the Neo-tethys. The island- 172 J. Geol. Min. Res.

arc signature of the Harsin/Sahneh ophiolite suggests is intruded by nearly 120 salt diapers in the southeast. that the obduction could have developed from the volcanic arc setting during Cretaceous as modeled by Boutelier et al. (2003). After emplacement of ophiolite, the The Zagros Foredeep (ZF) and the Dezful embayment Afro-Arabian continent collided with the UDMA in Middle Maastrichtan (approximately 68 Ma) (Alavi, 2007). In the The ZF is enclosed to the northeast by the Main Recent superseding Iranian plates, the collision increased Fault (MRF) and to the southwest by the Zagros differential rotation rate of the Iranian micro-continent, Foredeep Fault (ZFF), which marks the northeastern and the collisional mountain building has continued with edge of alluvial covered coastal plain of the Persian Gulf variable intensity to the present (Alavi, 2004). (Berberian, 1995). Berberian (1995) also reports that the formation of the ZF was associated with motion along the MFF and uplift of the Simple Fold Belt. The ZF consists of Morphotectonic units the Fars Group sediments (Gachsaran, Mishan and Agajari Formations), associated with elongate and Urimiah-Dokhtar magmatic assemblage (UDMA) symmetrical folds. The important phenomenon in this foredeep is sheared off from the subsurface Eocene- The UDMA is a relatively narrow (50 - 80 km wide), linear Oligocene Asmari Limestone base along decollment belt of intrusive and extrusive rocks (Alavi, 2007). Alavi thrust in the Gachsaran Evaporites and Salt tectonics (1994) reports that northeastern side of UDMA is thrusted (Sherkati et al., 2005). The decollement levels separate onto the associated retroarc/retroforland deposits and lithotectonic units that accommodate shortening in transected by number of right-lateral strike-slip faults. The various ways during folding (Sherkati and Letouzey). assemblage in this zone includes a thick (approximately 4 There is a thick sequence of the Lower Miocene to km) pile of calc-alkaline and highly potassic alkaline Pleistocene syn-orogenic molasses cover (Aghajari- andesites, dacites, andesitic basalts, trachyandesites, Bakhtiari formations) within this belt. The Dezful and rhyolites intruded by diorites and various granitoids embayment appears to be a discrete structural unit, with that are associated with extensive pyroclastic layers boundaries defined by Dezful embayment fault to the (Alavi, 2007). north, the Kazerun-Borazjan transverse fault (KFZ) to the east and southeast. It is a sedimentary basin with pronounced subsidence and thickening of the post- The sanandaj-sirjan metamorphic zone (SSMZ) Eocene-Oligocene Aghajari Formation with more than 3 km thickness (Berberian, 1995). Sherkati and Letuozey The SSMZ lies to the southwest of the UDMA and is (2004) referred that the variation of sedimentary considered as a part of Iranian Continental block thickness in the Dezful emabyment is controlled by N-S (Stocklin, 1968). This zone is wide (150 – 250 m) and has and NW-SE faults. structural trends which are parallel to the rest of the Zagros orogenic elements. The northeastern part of the zone contains a series of elongated depressions that are The Zagros coastal plain well developed parallel to the southwestern boundary of the UDMA. Rocks of SSMZ are of Phenerozoic age and The Zagros coastal plain is a narrow feature delimited to considered to be part of Precambrian basement. In some the north by Zagros Foredeep fault (ZFF) and to the places, thin section slices of coarse pyroclastics and south by the Persian Gulf and the Zagros–Arabia tuffaceous layers of Tertiary age (Alavi, 1994) have been boundary. This zone consists of alluvial deposits. found. Volcanic rocks such as basalt and rhyolite are found inter-layered with Carboniferous-Permian-Lower Triassic well-beded, shallow-water, shelf siliciclastic and The Persian gulf (PG) and Mesopotamian lowland carbonate (Alavi, 1994). The PG and Mesopotamian lowland unit lies south and southwest of the Zagros coastal plain. The PG with an The Zagros fold-thrust belt (ZFTB) area of about 226,000 km2, is a shallow epicontinental sea with a tectonic origin. The ZFTB has an average width of approximately 300 km that extends parallel and to the southwest of the Zagros imbricated Zone (Falcon, 1974). From the spectacularly Geodynamics of the study area displayed by satellite images the whole Zagros is visible as en echelon “whale-back” (Alavi, 2007). In the ZFTB, According to NUVEL-1A plate tectonics model, the the sequence is composed of approximately 7 - 12 km of Arabian plate is moving N13°E at a rate of approximately uppermost Neoproterozoic and Phanerozoic strata, which 31 mm/yr relative to Eurasia at longitude of 52°E (DeMets Islam and Shinjo 173

Figure 2. Simplified model geometry from Figure 1 with boundary condition.

et al., 1994). However, the recent GPS data reveal the Navabpour et al. (2007) infer that a N-S compression approximately 20 mm/yr motion of the Arabia with respect (parallel to plate convergence) along the MRF and a NE- to stable Eurasia with the same direction of N13°E SW compression perpendicular to fold axes across the (McQurrie et al., 2000; Sell et al., 2002; Vernant et al., ZSFB. 2004; Hessami, 2006; Reilinger et al., 2006). The GPS study indicates that east of KFZ has 13 - 22 mm/yr velocity toward N7±5°E, whereas the west of KFZ has METHODOLOGY velocity of 14 - 19 mm/yr toward N12±8°W (Hessami, 2006). Walpersdorf et al. (2006) show that the GPS In the study area, we firstly selected a suitable map (taken from Novabpour et al., 2007); the map is simplified (Figure 2) and used velocity in northern Zagros is fairly complex, particularly for simulation considering elastic continuum. The entire study area near Main Recent Fault (MRF) and orientation of velocity is located between latitude 25 - 35°N (1100 km) and longitude 45 - is toward NW direction with magnitude of approximately 2 57°E (1220 km). The area is divided into small triangular elements mm/yr. or domains. A mesh is generated with assemblages of 2262 The historical and instrumental seismicity in Iran elements and 1200 nodes. The simulation is performed by using the FE software package developed by Hayashi (2008). suggests that an intercontinental deformation concentrated in several mountain belt surrounding relatively aseismic blocks like central Iran, Lut and South Model set up Caspian block (Vernant et al., 2004). The strain distribution is different in Central Zagros with respect to Nine rock domains are considered in this study (Figure 2), on the North Zagros. In Central Zagros, the compressional axes basis of morphotectonic units and similarity of lithologic characteristics of the study areas described in earlier. Unit 1 are parallel to each other and perpendicular to fold axes, consists of PG, DE and Strait of Homuz and these are considered to whereas North Zagros exhibits the varied orientation of have similar lithology. The MFF and BFZ are included in unit 2, compressional axes (Walpersdorf et al., 2006). Moreover, while unit 3 also represents a part of MFF. Unit 4 composed of 174 J. Geol. Min. Res.

Table 1. Rock layer property of different domain of the model.

Density Young’s modulus Cohesion Internal angle Layers Vp (km/s) (gm/m3) (GPa) (MPa) friction (Degree) 1 2200 4.7 30 17 34 2 2000 4.0 1 10 12 3 2000 4.0 1 10 12 4 2400 5.6 50 14 35 5 2000 4.0 1 10 12 6 2700 5.8 55 18 35 7 2000 4.0 1 10 12 8 2000 4.0 1 10 12 9 2900 7.5 60 27 38

Poisson’s ratio is 0.25.

Lurestan, IZ, Shiraz and FARS Arc in folded Zagros. Unit 5 side of the model are considered owing to Makran subduction and represents HZF and unit 6 is HZB. Units 7 and 8 indicate MRF and Anatolian subduction motion, respectively. North side is fixed and is MZT, respectively. Unit 9 covers the entire area of Central Iran (CI). to be considered as Eurasian plate.

Rock layer property of the model RESULTS

The deformation within the ZFTB is mainly brittle type (Navabpour et al., 2007). In order to incorporate the brittle deformation, we Considering homogeneous elastic rheology and adopt the rock parameters from Clark (1966), Hatzfeld et al. (2003), assuming 25 mm/yr oblique convergent, we imposed 500 and Kaviani et al. (2007). The prime mechanical properties of m (for 20,000 years), 1000 m (for 40,000 years) and 2000 different layers such as density, Young’s modulus, Poisson’s ratio m (for 80,000 years) displacements in the model, to and cohesion are used in the simulation (Table 1). The density simulate present-day displacement vector, tectonic stress values are taken from Clark (1966) and Paul et al. (2006). We calculate the Young’s modulus by the following equation (1) field, and faulting pattern of the study area. We present (Goodier, 1970; Hayashi, 2008), our simulated results on following topics: (a) displacement vector, (b) Hmax orientation and (c) faulting 2 (1+ υ)(1− 2υ) pattern of the study area. E = ρV (1) p (1−υ)

Displacement vector Where E = Young’s Modulus, Vp is P-wave velocity, is density, andis Poisson’s ratio. We use the value of 0.25 for Poisson’s ratio during calculation. P-wave velocity is taken from Hatzfeld et al. Northward displacement vector is found within the study (2003) and Kaviani et al. (2007). Two other physical parameters area under given boundary condition (Figure 3b). The such as cohesion and internal angle of friction have been taken model results displays similar displacement vector (NNE) from Clark (1966). except SSMZ, CI and MZT. The CI shows uniform displacement vector with smaller magnitudes. Eastern part of SSMZ shows N or NE direction but other parts Boundary condition show NW or WNW. MRF exhibits greater and almost The Zagros mountain belt is the active mountain building stage due similar direction in displacement vector as have observed to collision between the Arabian Plate and Iranian sub-plate. in PG and DE. Folding and corresponding reverse faulting started Miocene in a compressional stress regime that was oriented NNE (Navabpour et al., 2007). Navabpour et al. (2007) also proposed that the HZB orientation underwent two post-fold strike-slip stress regime under two distinct Hmax

NNE and N-S compression in the late Miocene–early Pliocene and post Pliocene, respectively. The movement of Arabian Plate is NE-SW maximum horizontal compressive stress (Hmax) considered to be the main driving force of crustal deformation within orientation is predicted from the modeled result for most the study area. Other aforesaid geodynamic settings are also of the part of model (Figure 4b). The modeled result also reflected in the boundary condition (BC) as shown in Figure 2. We suggest that the Lorestan, HZF, MRF and eastern part of impose oblique convergent to the south and west sides with some the ZSFB display complex and with multiple stress portion of southeast and northwest side, and most of the eastern side keep free to move along both x and y direction. The oblique orientation. The Hmax orientation is predicted in the convergent of the some portion of southeastern and northwestern Lurestan to be having N-S/NNE-SSW, whereas around Islam and Shinjo 175

Figure 3a. GPS velocity of the study area.

MRF (including its western-side and eastern-side), it is DISCUSSION AND CONCLUSION NW-SE. The CI displays NE-SW stress orientation within the model. Recent tectonic activity in the ZFTB is the consequence of continental convergent between Arabia and Asia (Hessami, 2002). To analyze the study area, we imposed Fault pattern realistic boundary condition into our plane stress model with appropriate rheological proxies. We consider that Figures 5a - 5c show the modeled fault pattern of the simulated result with given boundary condition is fitted to study area at depth variation. We modeled fault pattern depict study area more logically and precisely. The GPS for the depth of 1, 10 and 30 km. It is found from velocity (Figure 3a) (Hessami et al., 2006) and our calculation that strike-slip fault is dominant in shallower simulated displacement vector direction (Figure 3b) are depth (up to 10 km), while thrust fault becomes significant comparable. The modeled result exhibits NE-ward with increasing the depth. At 1 km depth (Figure 5a), displacement vector, except for the CI, SSMZ and MZT. MSZ, MZT and MRF are predicted to be thrust fault, and The CI shows uniform displacement vector with the the entire ZFTB shows strike-slip fault. Some strike-slip smallest magnitudes. Eastern part of SSMZ shows N or fault also is predicted in the southern part of CI from NE direction. MRF region shows greater and almost model result. For 10 km depth (Figure 5b), the thrust similar magnitude in displacement vector than PG and faults are associated with strike-slip fault all over the DE regions. So it is obvious that modeling results are ZFTB. However, Lurestan and Fars Arc are predominated consistent with GPS reading. by thrust faults. For greater depth at 30 km (Figure 5c), The High Zagros Belt (HZB) of Fars region is thrust faults are dominant in the ZFTB. The KFZ, BFZ, characterized by a change in state of stress from FZ, HZF, and eastern side of MRF display strike-slip compressional to strike slip regime which is indicated by faults. Moreover, southern CI shows thrust fault along the orientation of maximum horizontal compressive stress with strike-slip component. (Hmax) toward NE-SW or N-S (Navabpour et al., 2007). 176 J. Geol. Min. Res.

Figure 3b. Modeled displacement vector of the study area.

Navabpour et al. (2007) also suggested that orientation of Hmax, whereas Lurestan, HZF, MRF and southwestern Zagros (HZF) shows similar but almost N-S eastern part of ZSFB show different stress such as NE- orientation of Hmax that is parallel to the plate convergent SW, NW-SE/N-S. Eastern side of MRF, northern part of trend along MRF and NE-SW across ZSFB and HZB and east of HZF show NW-SE Hmax orientation. perpendicular to the fold axes. They analyzed 31 Lurestan and eastern part of ZSFB show N-S Hmax earthquakes focal mechanism solution, 13 of which are orientation. From the comparison (Figure 4), it is evident near the MRF having N-S Hmax orientation, and the other that our simulated results seen to be consistent with both 18 events have NE-SW Hmax orientation in the the observed focal mechanism and the WSM data. southwestern part of Zagros (ZSFB, MFF, DE). Based on The ZFTB and Iranian plateau are characterized by fault slip data (Authemayou et al., 2005; Navabpour et al., pure left-lateral strike slip faulting with recent volcanism 2007), N-S stress direction of compression is revealed in and high surface elevation along the Alpine earthquake the HZB of interior Fars and southeastern MRF. belt (Reilinger et al., 2006). Tectonic studies revealed that In the World Stress Map (WSM) (CASMO, 2008), the this area has a very high density of active and recent orientation of present-day Hmax is oriented in NE-SW faults and that many of these fault systems are master direction in the High Zagros, Shiraz and eastern part of CI blind thrust faults, being those have been responsible for (Figure 4a). Fars arc displays NNE-SSW and N-S destructive earthquakes and a serious seismic hazard to orientation of Hmax, whereas HZB shows NW-SE local population (Berberian, 1995). Earthquakes data of direction. East of the Fars arc and area near to Makran Iran indicate that most activity is concentrated along the subduction show multiple Hmax orientation like NE-SW, ZFTB, whereas less activity is observed in central and N-S and NW-SE. Our modeled result (Figure 4b) eastern Iran. The distribution of historical and suggests that most of the area displays NE-SW instrumentally recorded seismic events shows broadly Islam and Shinjo 177

Figure 4a. World stress map.

Figure 4b. Modeled Hmax orientation.

similar pattern, with concentration of epicenters in the recorded at the desert in the central and eastern Iran. Zagros Mountain, although few historical earthquakes are The relatively high level of seismicity within ZFTB and 178 J. Geol. Min. Res.

Figure 5a. Modeled fault pattern for 1 km depth.

Figure 5b. Modeled fault pattern for 10 km depth.

HZB indicate that these are zones of the most active Neotectonic features of the region (Walker and Jackson, faulting and deformation (Figure 6a). Our modeled failure 2004). elements also indicate similar result of the study area Most focal mechanism solution of earthquakes (Figure (Figure 6b). However, the majority of the large 5d) in the ZFTB region indicates the presence of active earthquakes can be associated with Holocene scraps and reverse faults in the uppermost part of the Arabian Islam and Shinjo 179

Figure 5c. Modeled fault pattern for 30 km depth.

Figure 5d. Observed focal mechanism solution.

basement, beneath the Hormuz Salt Formation active Mountain Front Fault is considered to be a major (Hessami, 2006). Berberian (1995) explains that the seismogenic reverse fault in the Zagros basement. Focal 180 J. Geol. Min. Res.

Figure 6a. Distribution of Earthquake of the study area.

Figure 6b. Distribution of failure elements within the model.

mechanism solution also indicates that deformation in the associated with the strike-slip faults results in widespread Zagros basement is shortening and thickening through trust faulting (Walker and Jackson, 2004). numerous faults (Alavi, 2004). Shortening components Our simulated results are consistent with existing focal Islam and Shinjo 181

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