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

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Neotectonic Stress Field and Deformation Pattern Within the Zagros and Its Adjoining Area: an Approach from Finite Element Modeling Journal of Geology and Mining Research Vol. 2(7), pp. 170-182, December 2010 Available online http://www.academicjournals.org/jgmr ISSN 2006 – 9766 ©2010 Academic Journals 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
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