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Insights from scaled analogue modelling into the seismotectonics of the Iranian region
Tectonophysics 376 (2003) 137 – 149 www.elsevier.com/locate/tecto
Insights from scaled analogue modelling into the seismotectonics of the Iranian region Marco Bonini a,*, Giacomo Corti b, Dimitrios Sokoutis c, Gianfranco Vannucci d, Paolo Gasperini e, Sierd Cloetingh c a
C.N.R., Istituto di Geoscienze e Georisorse, Sezione di Firenze, via G. La Pira, 4, 50121 Florence, Italy b Dipartimento di Scienze della Terra, Universita` degli Studi di Firenze, via G. La Pira, 4, 50121 Florence, Italy c Netherlands Centre for Integrated Solid Earth Science, Faculty of Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands d Istituto Nazionale di Geofisica e Vulcanologia, Viale Berti Pichat, 8, 40127 Bologna, Italy e Dipartimento di Fisica, Settore di Geofisica, Universita` degli Studi di Bologna, Viale Berti Pichat, 8, 40127 Bologna, Italy Received 13 February 2003; accepted 29 July 2003
Abstract We show how analogue modelling provides insights into complex crustal block kinematics and the seismotectonics of active areas undergoing contraction, such as the Iranian region. The seismotectonic model deriving from this analysis is consistent with partitioning of the N-directed Arabia indentation into a composite system of collision-oblique and collision-parallel seismogenetic belts. These features include (1) two main conjugate transpressive belts (the dextral NW – SE-trending Zagros belt and the NE – SW-trending sinistral Elburz-Aran-Torud (EAT) belt), and (2) a modest lateral escape of Central Iran towards the Lut block along the Nayband belt. This cinematic model fits the seismic activity and the current fault pattern of the region. Also, the velocity field scaled from the model to nature shows a good similarity with previous numerical modelling as well as with deformation rates along the first-order seismogenetic belts determined by plate tectonic models and GPS data. D 2003 Elsevier B.V. All rights reserved. Keywords: Continental collision; Iran; Seismotectonics; Analogue modelling; Moment tensor sum
1. Introduction and geological setting Seismotectonics in regions undergoing collision results in complex deformation patterns characterized by the interaction between strike-slip and reverse * Corresponding author. Tel.: +39-55-2757528; fax: +39-55290312. E-mail address: [email protected] (M. Bonini). 0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2003.07.002
faulting. This is particularly evident in Iran where intraplate deformation generated by the N-directed Arabia indentation is partitioned into a composite system of collision-oblique and collision-parallel seismogenetic belts (McKenzie, 1972; Berberian, 1981; Jackson and McKenzie, 1984; DeMets et al., 1990; Jackson et al., 1990, 1995; Jestin et al., 1994). These belts (Fig. 1) delimit two adjacent continental blocks, the Central Iran block to the West and the Lut block to
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Fig. 1. Schematic tectonic map of the Iranian region (modified after Geological Survey of Iran, 1973) superposed onto a digital elevation model. Topography elevation was taken from ETOPO-2 global data set, using GMT mapping tool (Wessel and Smith, 1991).
the East (Jackson et al., 1995 and references therein). These crustal blocks behave rigidly within a deforming zone that is well constrained by the relatively undeformed cratons of Arabia to the SW, Eurasia to the North and by its promontory (Afghan block) to the East (Jackson and McKenzie, 1984, 1988). Main bulk shortening occurs across the most active seismic zone in Iran, the NW – SE-trending Zagros mountain chain
bounding the Central Iran block along its South – Western margin. This chain is markedly oblique to the convergence direction and it is characterized by an important orogen-parallel dextral strike-slip component of displacement (e.g., Tchalenko and Braud, 1974; Berberian, 1981, 1995; Jackson and McKenzie, 1984, 1988). Further North, the collision-related deformation extends well beyond the Zagros, concen-
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trating into the N –S-trending fault belts bounding the Lut block (Nayband and Sistan belts), as well as into the Elburz-Kopet Dagh mountain chain system, at the northern boundary of the deformed zone (Fig. 1). In plan view, the Elburz-Kopet Dagh chain attains a sigmoidal, E –W-oriented, shape and it is characterized by complex kinematics dominated by sinistral transpression along its NE-trending segment (Priestly et al., 1994; Jackson et al., 2002). The dextral (e.g., Zagros fold-and-thrust belt) and sinistral (e.g., Elburz mountains, Great Kavir Fault) conjugate belts have been interpreted to accomplish the lateral escape of Central Iran towards the Lut block (Boccaletti and Dainelli, 1982). Reconciling the complex kinematics of each distinct belt within a deforming region into a unique seismotectonic model is obviously rather difficult and requires the understanding of the overall deformative evolution of the system. For this purpose, we get insights into this complex active region by further developing the analysis of the analogue model reported in Sokoutis et al. (2000) that considered a model continent shortened parallel to a lateral thickness variation. In Sokoutis et al. (2000), the model structural pattern was qualitatively compared to that of the Iranian region, while here we attempt to quantify the model kinematics and to scale it to nature. Particularly, based on the mechanical consistency of the model with respect to nature, we show how the results of modelling can be extrapolated to the natural case in order to (1) identify the main crustal blocks and the deformation belts resulting from their interactions, (2) correlate the available seismic data to the natural deformation pattern (e.g., Koyi et al., 2000), and (3) predict both the kinematics and the velocities of main crustal blocks, as well as (4) the deformation rates along the first-order seismogenetic belts.
2. Analogue modelling The model was designed to take into account the initial main boundary conditions acting in the region, in particular (1) the N – S convergence, (2) the rigid boundaries of the deforming zone (Fig. 1) and (3) the lateral variation in crustal thickness between Central Iran block (thickness z 50 km; Hearn and
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Ni, 1994; Pasyanos and Walter, 2002; block A in Fig. 2) and the adjacent Lut block (thickness < 40 km; Dehghani and Makris, 1983; Sobouti and Arkani-Hamed, 1996; block B in Fig. 2). The experimental set-up reproduced a two-layer continental crust floating above a gypsum –glycerol mixture simulating the upper mantle and asthenosphere (Fig. 2a). The lower crust was reproduced by using a 0.8cm-thick layer of a near-Newtonian mixture of finely ground baryte into polydimethylsiloxane (PDMS; Weijermars, 1986), whereas the upper crust was simulated by using sand. The thickness of this latter varied from 0.4 cm (block B) to 0.7 cm (block A), creating a lateral crustal thickness variation trending parallel to the push direction (Fig. 2a). The model was shortened at a collision rate of 1.8 cm/h in order to scale the 3.5 cm/year convergent rate of Arabia with respect to Eurasia during the late Miocene –Present (DeMets et al., 1990). The dynamic and geometric scaling of the model are discussed in detail in Sokoutis et al. (2000) and reported in Table 1. 2.1. Limitations and simplifications of analogue modelling The analogue model by Sokoutis et al. (2000) involved important limitations, mainly related to the lithospheric rheology and to thermal effects during deformation. Particularly, by assuming an initial setup characterized by two main blocks (A and B) with different thickness, the model necessarily simplified the complex blocks puzzle composing the deforming zone between Arabia and Eurasia. Most notably, the model set-up emphasised the lateral thickness variation between Central Iran and the Lut block, whereas the rheological variation related to the Caspian block was not included. The model did not also include pre-existing discrete discontinuities between (or internal to) the crustal blocks that may have been reactivated during continental collision. During the experiment, the lithospheric rheology was simplified assuming a weak upper mantle. Although the most accepted view of continental rheology suggests the presence of a strong upper lithospheric mantle (e.g., Ranalli, 1995 and reference therein), in the case of Iran geophysical
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Fig. 2. (a) Experimental set-up (from Sokoutis et al., 2000) of a three-layer system, where quartz sand representing the upper crust and PDMS + barite mixture representing the lower crust floated above a gypsum – glycerol mixture simulating the upper mantle. (b) The initial model strength profiles of blocks A and B are compared with a lithospheric profile involving wet lower crust and upper mantle (from Jackson, 2002), as inferred for the Iranian region (Hearn and Ni, 1994; Maggi et al., 2000, 2002). (c) Top-view photograph of the model at the end of deformation (38% bulk shortening); light is from the left. The fixed distal wall represented Eurasia in nature, while the moving wall simulated the Arabia indentation. (d) Line drawing of the main structures; gray dotted lines indicate the deformed initial square passive grid.
investigations support the presence of a weak upper mantle, as both a strong seismic shear wave attenuation (between 50 and 150 km depth) beneath the plateau and a long wavelength gravity high are consistent with an increase in mantle temperature beneath Iran (Hearn and Ni, 1994; Maggi et al., 2000, 2002). As a consequence, the model strength
profile has been simplified as characterized by a strength peak in the upper crust and a ductile lower crust stronger than the mantle (Maggi et al., 2000; Jackson et al., 2002; Fig. 2b). This implies that the lithospheric strength resides within the crust, so that the thinner structural domain in the model configuring the Lut block (see following Section 3.2) is
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Table 1 Model and natural parameters used in the experiments (BC, brittle crust; DC, ductile crust) Parameter
Model
Nature
Model/nature ratio
Density BC (kg/cm3) Density DC (kg/cm3) Gravity, g (m/s2) Horizontal scale, l (m) Crustal thickness, h (m) Gravitational stress BC, r (Pa) Gravitational stress DC, r (Pa) Cohesion BC (Pa) Coefficient of friction BC, l Viscosity DC, g (Pa s) Velocity of displacement (m/s) Time
1300 1370 9.81 0.25 0.012 – 0.015 51 – 89 107 negligible 0.6 5 105 5 10 6 f8 h
2700 f 2900 9.81 9 105 4.2 – 5.3 104 3.7 108 – 6.6 108 8 108 6 107 0.6 – 0.85 6 1022 5.7 10 10 f 15 Ma
0.48 0.47 1 2.8 10 7 2.8 10 7 1.3 10 7 1.3 10 7 – f1 8 10 18 4.5 103 6.2 10 11
assumed to be weaker than the thicker domain corresponding to the Central Iran block in nature. Additionally, the experimental set-up did not consider thermal variations during the progressive deformation of model, reducing the complex thermomechanical process of continental collision to a purely mechanical process. Also, syn-shortening erosion and sedimentation were not considered during the experiment, although the thick sedimentary wedge in the Caspian Basin suggests that these processes may locally affect significantly the evolution of deformation (e.g., Brunet et al., 2003). Despite the abovementioned experimental simplifications, our study represents a first attempt to relate seismic activity to first-order structures in nature through analogue modelling, an approach that can be greatly refined in the future.
This undeformed domain slightly moved towards the thinner block B that showed comparatively higher internal deformation. Another main transpressional belt composed of en echelon thrusts formed across the distal corner of block B (Fig. 2c and d). All these structures were already formed by about 26% bulk shortening, such that during the latest stages of deformation the model merely deformed along pre-existing structures. In the following sections, we focus on the most deformed stages that are directly comparable to the kinematics of active deformation in Iran. We compare the model fault pattern with the first-order structures and seismic activity of the Iranian region attempting to integrate the available information in a comprehensive seismotectonic model. 3.2. Comparison of model results with the deformation pattern of the Iranian region
3. Experimental results and comparison with the Iranian region 3.1. Summary of modelling results Indentation of an experimental continent with a lateral variation in strength parallel to the convergence direction (Fig. 2c and d), resulted in the development of deformative belts oblique and parallel to the shortening direction (Sokoutis et al., 2000). Two main conjugate oblique belts of transpressional shear developed within block A, resulting in a main triangular-shaped domain characterized by no significant internal deformation (Fig. 2c and d).
The model deformation exhibits a marked similarity with that of the Iranian region, suggesting a close similarity of dynamic processes. The essential points of similarity between model results and geological and geophysical information on Iran are listed below: (1) The main dextral transpressional belt in the model is comparable to the NW –SE Zagros fold-and-thrust belt in nature, which bounds the Central Iran block to the SW. As in the model, this deformation belt in nature accommodates most of the shortening related to the Arabia
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indentation. Additionally, dextral displacement associated with hinterlandward thrusting along the model belt is matched by similar displacements in the Zagros belt (e.g., Tchalenko and Braud, 1974). An obvious difference between model and nature is related to the width of the deformation belts, which in the Zagros is wider than in the experiment. This difference may be related to the presence of a thick salt layer at the base of the sedimentary cover that strongly weakens the eastern Zagros and enhances the lateral differences in strength (Talbot and Alavi, 1996). Despite these differences, the model dextral transpressional belt is directly comparable to the overall kinematics exhibited by the Zagros fold-and-thrust belt. (2) The conjugate main sinistral transpressional belt in the experiment is correlative, in our interpretation, with a main ENE –WSW-oriented deformative belt bordering the Central Iran block to the NW (Fig. 1). This system of structures is well expressed in the NE – SWtrending segment of the Elburz mountains, separating the Caspian block from the Central Iran (Fig. 1). However, an evident difference between the analogue model and nature is that only minor expression of the NE –SW-trending deformative belt observed in the model is clearly recognisable in the region between the Elburz mountains and the Zagros belt. This difference is probably associated with the strong influence exerted by the southern boundary of the Caspian block on the structural pattern, by controlling reactivation of structures and strain localization along this main inherited rheological anisotropy. In any case, the presence of several active NE-oriented left-lateral transpressional splays along the Aran-Torud belt branching south-westwards from the NE-trending Elburz segment, as well as the occurrence of earthquakes with oblique (transpressional)
(3)
(4)
(5)
(6)
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motion (see Section 4), may be compatible with the presence of a main deep sinistral transpressional zone transferring deformation from the northwestern Zagros up to Kopet Dagh (Fig. 1). We refer to this inferred main deformation belt as Elburz-Aran-Torud belt (EAT). Both conjugate belts configure a triangularshaped block that is correlative with the triangular Central Iran block. On the basis of model block kinematics, we suggest that these main conjugate belts (Zagros and EAT) accomplish the overall N – NE-directed movement of both Central Iran block and Lut block (Jackson et al., 1995; see Section 5 below). The modest overthrusting of triangular block A over block B (Fig. 1) accords with the presence of N –S trending and E-verging thrust folds and provide evidence for shortening along the Nayband belt (Geological Survey of Iran, 1973), although other geological data suggest a dominant strike-slip kinematics for this belt (Walker and Jackson, 2002). The lack of significant transcurrence along the correspondent Nayband belt in the model was prevented by the experimental set-up that did not include a pre-existing discrete crustal discontinuity separating blocks A and B. The structures accommodating deformation at the right margin of the experimental block B strikingly fit those resulting from the dextral shearing of the Lut block against the rigid Afghan block, which acts as the fixed lateral walls of the Plexiglas box in the model (compare the Sistan belt and the structures propagating into the Northern Lut block in Fig. 1 with the deformed model in Fig. 2c and d). The lateral motion of main crustal blocks in the model is inhibited by the confinement of the deforming zone between rigid boundaries, determining that the uplifting and the thickening are the main deformation modes, similarly to what
Fig. 3. (a) Instrumental earthquakes (red open circles) from the on-line ISC Catalogue (2003). Focal solutions (in black) are taken from the literature (Vannucci and Gasperini, 2003 and reference therein) and Harvard CMT (2003) and ETH (2003) on-line Catalogues. The interpreted main deformational belts, indicated by the blue oblique pattern, are: ‘‘EAT’’, Elburz-Aran-Torud belt; ‘‘Mk’’, Makran; ‘‘Na’’, Nayband belt; ‘‘St’’, Sistan belt; ‘‘Zg’’, Zagros belt. The black dashed line indicates the trend of the Elburz-Kopet Dagh mountain chain system. (b) Sums of moment tensor components (Kostrov, 1974) (in blue) on a regular grid with mesh of 1j. Superimposed red lines indicate the best double couple. Topography (from ETOPO-2 global data set), plot of epicentres and focal mechanisms are obtained using GMT mapping tool (Wessel and Smith, 1991).
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has been suggested for the Iranian region (Sobouti and Arkani-Hamed, 1996).
4. Comparison with seismic data The deformation pattern is consistent with the seismic activity (Fig. 3a). This analysis has been carried out considering (1) the spatial distribution of instrumental earthquakes taken from the on-line International Seismological Centre (ISC, 2003) Catalogue, (2) the available focal mechanism solutions (Dziewonskii et al., 1981 and subsequent papers appeared quarterly on Phys. Earth and Planet Inter.; Braunmiller et al., 2002; Vannucci and Gasperini, 2003 and references therein) and (3) the seismic moment sum (Kostrov, 1974) over a regular grid. Earthquakes in the Iranian region show a marked nonuniform distribution, being mostly concentrated within the belts surrounding the more stable, relatively aseismic, Central Iran and Lut blocks (e.g., ShojaTaheri and Niazi, 1981; Jackson and McKenzie, 1984; Jackson and McKenzie, 1988; Jackson et al., 1995; Berberian and Yeats, 1999; Fig. 3a). Such a nonuniform distribution of epicentres is well explained by the pattern of deformation in the models showing localization of faulting into discrete transpressive belts surrounding more undeformed regions, although preexisting topography has been shown to be capable of inducing gravitational stresses with magnitudes similar to those of tectonic origin (Bada et al., 2001). Consistent with the transpressive kinematics inferred from the analogue model, focal mechanism solutions indicates that most events are compressive and strike-slip. In Fig. 3a, we show focal solutions reported by Harvard Centroid Moment Tensor-CMT (2003) and Eidgeno¨ssische Technische Hochschule (ETH, 2003) on-line moment tensors Catalogues, as well as by papery literature (Vannucci and Gasperini, 2003 and references therein). For these latter mechanisms, the correctness and the consistency of the original parameters has been checked by Vannucci and Gasperini (2003). This procedure also included the choice of a ‘‘best mechanism’’ when more than one solution is given in the literature and/or in the online catalogues. The comparison between nature and experiment is implemented by the analysis of complessive mecha-
nisms obtained as the sum of moment tensors of individual earthquakes (Kostrov, 1974) on a regular grid with mesh of 1j (Fig. 3b). Notwithstanding the different resolution between the moment tensor in nature and the analysis of the strain in the model, the overall kinematics exhibited by the main deformation belts show a nearly good correspondence (compare Fig. 3a and b with Fig. 2d). For example, focal mechanisms along the Zagros belt (‘‘Zg’’ in Fig. 3a) indicate mainly compressive deformation, with direction varying from N – S to NE –SW, associated with transcurrent faulting resulting in dextral transpression (Berberian, 1995). Focal mechanism solutions along the EAT zone (Fig. 3a) also exhibit an interaction between strike-slip and reverse faulting that result in an overall sinistral transpressional kinematics along the same deformative belts, as also reported by Priestly et al. (1994) for the northeastern EAT. Additionally, mostly compressional events in the Nayband belt (‘‘Na’’ in Fig. 3a) and the dextral component of movement highlighted by focal mechanism solutions along the Sistan belt (‘‘St’’ in Fig. 3a) fit well the model kinematics. More problematic is instead the comparison between model deformation and the focal mechanisms in the Makran zone (‘‘Mk’’ in Fig. 3a) where compressive, strike-slip and extensional solutions are related to subduction processes, which were not considered in the modelling.
5. Model relevance to prediction of deformation rates in Iran The striking correspondence between the actual deformation pattern outlined by seismic activity and the model structural pattern suggests that analogue modelling can be used to trace the first-order seismogenetic structures in nature. However, it is important to mention that experimental results cannot explain all the structural details that develop on a scale irrelevant to this approach. To achieve a comprehensive comparison of modelling results with nature, we reconstructed the velocity field on the analogue model surface by monitoring the displacement of selected nodes of the passive grid during deformation. The overall blocks kinematics is efficiently described by the velocity vectors in both absolute and relative reference frames (Fig. 4a and b).
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The absolute reference frame allows to compare the velocity field with (1) numerical modelling results, (2) cinematic analyses based on elaboration of seismicity and seismic moment and (3) GPS data. On the other hand, the relative reference frame describes the relative movement between points within the model, thus allows displaying the relative displacement between blocks and the local kinematics along individual deformation belts. This procedure consents the comparison of modelling results with field data, because geologists usually consider the deformations they study in relation to a locality or a region that has probably moved with respect to the absolute reference frame.
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In the absolute reference frame, each moving node (AA) of the passive grid is identified by Cartesian coordinates (xA, yA) with respect to the origin (OA) fixed at the left corner of the distal stable Plexiglas box wall (Fig. 4a). At the end of deformation, the displacement vector of the selected nodes is defined by the velocity (v) and direction of movement (a). In particular, considering the new coordinates (xVA, yVA), the velocity (v) can be calculated dividing the Euclidean distance between initial and final position of each grid node (the amount of displacement, d) by the total duration of experiment (8 h; Fig. 4b and c). Doing so we have assumed a constant deformation rate, calculated as the average value during the experiment. The
Fig. 4. (a) Initial position of the nodes used to calculate the velocity vectors in the absolute (OA; black squares) and relative (OR; white symbols) reference frames. (b) Final position of the selected nodes together with the absolute (black arrows) and relative (white arrows) model velocity vectors, reported in mm/hour. Note that the main structures in the model are labelled as the deformation belts in nature (Fig. 3a), to which they are hypothesized to correspond. (c) Trigonometric relations between initial and final positions of selected nodes used to estimate the velocity vector in terms of velocity modulus and direction. See text for details.
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direction of movement of the selected nodes is given by the clockwise angle a between the velocity vector and the axis Y (representing the North in nature; Fig. 4a and b). In the relative reference frame, the origin of the Cartesian reference frame (OR) has been conveniently fixed within one main crustal block and the corresponding moving point (AR) chosen within one adjacent block (Fig. 4a). During deformation both points are absolutely moving with respect to the stable Plexiglas box wall, but the relative displacement of point A can be still calculated by considering the initial (xR, yR) and the final (xVR, yVR) coordinates (Fig. 4b and c). In this case, to better define the displacement rates along the individual deformation belts, we have calculated the velocity of the relative nodes by averaging the relative movement over the time interval of activity of the single belt. Thus, the term t in equations reported in Fig. 4b is the time interval between the development of the considered belt and the cessation of the experiment. Both absolute and relative velocity vectors highlight the oblique motion of the main blocks in the central part of the model, with a modest relative convergence-orthogonal overthrusting of the triangular block A domain over block B (Fig. 4b). An intriguing hypothesis resulting from the mechanical consistency between experiment and nature is that the model velocity field can be extrapolated to natural conditions by considering a velocity scaling factor v* = vm/vn=(1.8 cm/hour)/(3.5 cm/year) = 4.5 103, where subscripts m and n refer to model and nature, respectively (see Table 1). Best fitting of the model deformation pattern to the Iranian region allows the positioning of the scaled experimental velocity vectors on the correspondent crustal blocks and deformation belts in nature (Fig. 5a). The scaled velocity vectors confirm that the overall kinematics can be described in terms of N25jE-directed movement of both Central Iran and Lut blocks with respect to the fixed Eurasia. This analogue modelling-derived velocity field (Fig. 5a) shows a good agreement with previous numerical models (Fig. 5b; Jackson et al., 1995; Sobouti and Arkani-Hamed, 1996). However, whereas the direction of vectors in the numerical models is rather constant across the various crustal blocks, in the analogue modelling abrupt changes at the block boundaries are observed, particularly across the Zagros
Fig. 5. Comparison between (a) the scaled model velocity field (in mm/year) rotated 5j clockwise to fit the geometry of the deforming Iranian region, and (b) the velocity of crustal blocks estimated from the spatial variation in the strain rates indicated by earthquakes (from Jackson et al., 1995). Symbols as in Fig. 4.
belt (cf. Fig. 5a and b). This difference is likely related to the development of discrete shear zones, allowing the differential movement of crustal blocks. Strain partitioning among deformational belts results in different transpression rates. Scaled shortening rates
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(Table 2) suggest that the highest values are to be expected along the Zagros (c 22 mm/year), whereas lower rates are predicted for the EAT belt (c 14 mm/ year) and the Sistan fault belt (c 12 mm/year). Absolute movement of selected nodes at northern margin of the triangular-shaped domain of model block A also suggests shortening rates of c 11 mm/year along the Kopet Dagh. Overthrusting of the Central Iran over the Lut block is expected to occur at a slow rate of c 0.7 mm/year. These predicted deformation rates are in good agreement with the available rates determined from numerical and plate tectonics models as well as geological data (see Table 2). GPS measurements in the Zagros (Table 2) show slip rates that are about the half of the velocities predicted by the analysis of the analogue model (Hessami, 2002; Tatar et al., 2002). Other GPS data related to the other deformation belts (Vernant et al., 2002a, b, 2003; Hatzfeld et al., 2003) are only roughly comparable with the model deformation, as the exact location of the measurement points are not reported. In general, the GPS data show slip rates that are comparatively lower than the velocities predicted by the analysis of the analogue model (Table 2). These differences may result from the short (hu-
Table 2 Deformation rates from model and previous works Deformation belt
Predicted rates (mm/year) from the analogue model
Rates (mm/year) predicted from numerical and plate tectonics models or geological data
Rates (mm/year) determined from GPS measurements
Zagros (Z)
22
10 – 28 [1,2] 10 [3]
Elburz-AranTorud (EAT) Kopet Dagh
14.4
13 – 17 [9]
10 [4] 10 ( F 3) [5] 10 ( F 4) [6] 8 ( F 2) [7] 8 [8]
11
Sistan (St) Nayband (Na)
11.7 0.7
13 – 15 [9, 10] 6 ? [2] 10 – 20 [11] 0.5 – 0.7 [11]a
6 ( F 2) [7] 6 – 9 [7] –
[1] Jackson and McKenzie (1984); [2] Jackson and McKenzie (1988); [3] Falcon (1974); [4] Hatzfeld et al. (2003); [5] Hessami (2002); [6] Tatar et al. (2002); [7] Vernant et al. (2002a); [8] Vernant et al. (2002b, 2003) [9] Jackson et al. (2002); [10] Lyberis and Manby (1999); [11] Walker and Jackson (2002). a E – W convergent rate calculated considering the slip partitioning between strike-slip and reverse faulting.
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man) span of time covered by GPS data, whereas extrapolated model velocities represent averaged values over geological times. This implies that the modelderived velocity field is more appropriately compared to plate tectonics models or geological data covering a large time interval of deformation. The good correspondence of the experimental velocity field with that from previous models, supports the reliability of our approach.
6. Conclusions The analysis of an analogue experiment and the comparison of the results with the structural pattern and the seismicity of Iran suggested the following main conclusions: 1. The deformation pattern of the experiment, which was designed specifically for simulating deformation in the Iranian region, yielded comparable structural components to the natural prototype; 2. The first-order deformation belts in the model are consistent with the distribution of seismic activity as well as with the kinematics of available focal mechanism solutions; 3. The velocity field and the deformation rates along the first-order deformation belts predicted from the model are in satisfactory agreement with those estimated by other methods for the first-order seismogenetic belts in Iran; 4. The possibility to compare and integrate different geological-structural and seismological data with the results of modelling, indicates the potentiality of this approach in providing insights into the assessment of the seismotectonic model of a given region. Although analogue modelling necessarily simplifies the natural prototype, it offers the possibility of portraying the progressive deformational path generated by the firstorder structures since the beginning of shortening. By comparison, information from focal mechanism solutions covers only a few tens of years, which are instantaneous at the scale of the whole natural process. This modelling approach is able to provide reliable constraints to the kinematics of the main crustal blocks and to the transpressional rates estimated for the main
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deformation belts. This may provide information that is of paramount importance in other regions where no geodetic measurements (space-based or terrestrial) are currently available to monitor the regional kinematics.
Acknowledgements We thank the journal reviewers J. Che´ry and L. Ratschbacher and the Editor in Chief J.P. Burg for the stimulating and constructive comments which helped improve the manuscript. S.A.P.L. Cloetingh and D. Sokoutis acknowledge the financial support from ISES and The Netherlands Organization for Scientific Research (NWO).
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Eidgeno¨ssische Technische Hochschule (ETH), 2003. Catalog of Swiss Moment Tensor Solutions. Zu¨rich. http://seismo.ethz.ch/ info/mt.html. Falcon, N., 1974. Southern Iran: Zagros mountains. In: Spencer, A.M. (Ed.), Mesozoic – Cenozoic Orogenic belts. Spec. Publ.Geol. Soc. Lond., vol. 4, pp. 199 – 211. Geological Survey of Iran, 1973. Tectonic Map of Iran, Scale 1:2,500,000, Offset Press, Tehran. Hatzfeld, D., Tatar, M., Yaminifard, F., Ghafory-Ashtiany, M., Mokhtari, M., Nankali, H., Walpersdorf, A., 2003. Seismological and geodetic constraints on the Zagros (Iran). Geophys. Res. Abstr. 5, 4261. Harvard CMT, 2003. CMT Catalog, Centroid Moment Tensor Project. http://www.seismology.harvard.edu/projects/CMT/. Hearn, T.M., Ni, J.F., 1994. Pn velocities beneath continental collision zones: the Turkish – Iranian plateau. Geophys. J. Int. 117, 273 – 283. Hessami, K., 2002. Tectonic history and present-day deformation in the Zagros fold-thrust belt. PhD thesis, Uppsala University, Sweden. International Seismological Centre (ISC), 2003. On-line seismic catalog. http://www.isc.ac.uk/. Jackson, J., 2002. Strength of the continental lithosphere: time to abandon the jelly sandwich? GSA Today 12 (9), 4 – 10. Jackson, J., McKenzie, D.P., 1984. Active tectonics of the Alpine – Himalayan belt between western Turkey and Pakistan. Geophys. J. R. Astron. Soc. 77, 185 – 264. Jackson, J.A., McKenzie, D., 1988. The relationship between plate motions and seismic moment tensors, and the rates of active deformation in the Mediterranean and the Middle East. Geophys. J. R. Astron. Soc. 93, 45 – 73. Jackson, M.P.A., Cornelius, R.R., Craig, C.H., Gansser, A., Sto¨cklin, J., Talbot, C.J., 1990. Salt diapirs of the Great Kavir central Iran. Geol. Soc. Am., Mem. 177, 139. Jackson, J., Haines, J., Holt, W., 1995. The accommodation of Arabia – Eurasia plate convergence in Iran. J. Geophys. Res. 100, 15205 – 15219. Jackson, J., Priestly, K., Allen, M., Berberian, M., 2002. Active tectonics of the south Caspian basin. Geophys. J. Int. 148, 214 – 245. Jestin, F., Huchon, P., Gaulier, M., 1994. The Somalia plate and the East African rift system: present-day kinematics. Geophys. J. Int. 116, 637 – 654. Kostrov, V.V., 1974. Seismic moment and energy of earthquakes and seismic flow of rocks. Izv.-Acad. Sci. USSR, Phys. Solid Earth 1, 23 – 40. Koyi, H., Hessami, K., Teixell, A., 2000. Epicenter distribution and magnitude of earthquakes in fold-thrust belts: insights from sandbox modelling. Geophys. Res. Lett. 27, 273 – 276. Lyberis, N., Manby, G., 1999. Oblique to orthogonal convergence across the Turan block in the post-Miocene. Am. Assoc. Pet. Geol. Bull. 83, 1135 – 1160. Maggi, A., Jackson, J.A., McKenzie, D., Priestley, K., 2000. A reassessment of focal depth distribution in southern Iran, the Tien Shan and northern India: do earthquakes really occur in the continental mantle? Geophys. J. Int. 143, 629 – 661. Maggi, A., Priestley, K., McKenzie, D., 2002. Seismic structure of
M. Bonini et al. / Tectonophysics 376 (2003) 137–149 the Middle East. Eos. Trans. AGU 83 (47), S62B – 1185 (Fall Meet. Suppl., Abstract). McKenzie, D., 1972. Active tectonics of the Mediterranean region. Geophys. J. R. Astron. Soc. 30, 109 – 185. Pasyanos, M.E., Walter, W.R., 2002. Crust and upper-mantle structure of the North Africa, Europe and the Middle East from inversion of surface waves. Geophys. J. Int. 149, 463 – 481. Priestly, K., Baker, G., Jackson, J.A., 1994. Implication of earthquake focal mechanism data for active tectonics of the Caspian basin and surrounding regions. Geophys. J. Int. 118, 111 – 141. Ranalli, G., 1995. Rheology of the Earth, 2nd ed. Chapman & Hall, London. 413 pp. Shoja-Taheri, J., Niazi, M., 1981. Seismicity of the Iranian plateau and bordering regions. Bull. Seismol. Soc. Am. 71, 477 – 489. Sobouti, F., Arkani-Hamed, J., 1996. Numerical modelling of the deformation of the Iranian plateau. Geophys. J. Int. 126, 805 – 818. Sokoutis, D., Bonini, M., Medvedev, S., Boccaletti, M., Talbot, C.J., Koyi, H., 2000. Indentation of a continent with a built-in thickness change: experiment and nature. Tectonophysics 320, 243 – 270. Talbot, C.J., Alavi, M., 1996. The past of a future syntaxis across the Zagros. In: Alsop, G.I., Blundell, D.J., Davison, I. (Eds.), Salt Tectonics. Geol. Soc. Spec. Publ., vol. 100, pp. 89 – 109. Tatar, M., Hatzfeld, D., Martinod, J., Walpersdorf, A., GhaforyAshtiany, M., Che´ry, J., 2002. The present-day deformation of the central Zagros from GPS measurements. Geophys. Res. Lett. 29 (19), 1927 (doi: 10.1029/2002GL015427). Tchalenko, J.S., Braud, J., 1974. Seismicity and structure of the
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Zagros (Iran): the main recent fault between 33j and 35 jN. Philos. Trans. R. Soc. Lond., A 277, 1 – 25. Vannucci, G., Gasperini, P., 2003. A database of revised fault plane solutions for Italy and surrounding regions. Computers and Geosciences 29, 903 – 909. Vernant, P., Nilforoushan, F., Masson, F., Hatzfeld, D., Abassi, M., Vigny, C., Tavakoli, F., Bayer, R., Martinod, J., Ashtiani, M., Che´ry, J., 2002a. Implications of GPS measurements in Iran on the contemporary crustal deformation and plate kinematics in Middle East. Eos. Trans. 84 (47), S62B – 1183 (Fall Meet. Suppl., Abstract). Vernant, P., Nilforoushan, F., Masson, F., Hatzfeld, D., Abassi, M., Vigny, C., Tavakoli, F., Bayer, R., Che´ry, J., Martinod, J., 2002b. Tectonic implications of GPS measurements in Iran. Geophys. Res. Abstr., 4. Vernant, P., Nilforoushan, F., Bayer, R., Sedighi, M., Che´ry, J., Tavakoli, F., Masson, F., 2003. Present-day crustal deformation in central Alborz (Iran) inferred from GPS measurements. Geophys. Res. Abstr. 5, 11081. Walker, R., Jackson, J., 2002. Offset and evolution of the Gowk fault, S.E. Iran: a major intra-continental strike-slip system. J. Struct. Geol. 24, 1677 – 1698. Weijermars, R., 1986. Flow behaviour and physical chemistry of bouncing putties and related polymers in view of tectonic laboratory applications. Tectonophysics 124, 325 – 358. Wessel, P. and Smith, W.H.F., 1991. Free software helps map and display data, Eos Transactions of the AGU, 72, 441 (GMT— The Generic Mapping Tools. http://gmt.soest.hawaii.edu/, (2002).
Insights from scaled analogue modelling into the seismotectonics of the Iranian region Marco Bonini a,*, Giacomo Corti b, Dimitrios Sokoutis c, Gianfranco Vannucci d, Paolo Gasperini e, Sierd Cloetingh c a
C.N.R., Istituto di Geoscienze e Georisorse, Sezione di Firenze, via G. La Pira, 4, 50121 Florence, Italy b Dipartimento di Scienze della Terra, Universita` degli Studi di Firenze, via G. La Pira, 4, 50121 Florence, Italy c Netherlands Centre for Integrated Solid Earth Science, Faculty of Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlands d Istituto Nazionale di Geofisica e Vulcanologia, Viale Berti Pichat, 8, 40127 Bologna, Italy e Dipartimento di Fisica, Settore di Geofisica, Universita` degli Studi di Bologna, Viale Berti Pichat, 8, 40127 Bologna, Italy Received 13 February 2003; accepted 29 July 2003
Abstract We show how analogue modelling provides insights into complex crustal block kinematics and the seismotectonics of active areas undergoing contraction, such as the Iranian region. The seismotectonic model deriving from this analysis is consistent with partitioning of the N-directed Arabia indentation into a composite system of collision-oblique and collision-parallel seismogenetic belts. These features include (1) two main conjugate transpressive belts (the dextral NW – SE-trending Zagros belt and the NE – SW-trending sinistral Elburz-Aran-Torud (EAT) belt), and (2) a modest lateral escape of Central Iran towards the Lut block along the Nayband belt. This cinematic model fits the seismic activity and the current fault pattern of the region. Also, the velocity field scaled from the model to nature shows a good similarity with previous numerical modelling as well as with deformation rates along the first-order seismogenetic belts determined by plate tectonic models and GPS data. D 2003 Elsevier B.V. All rights reserved. Keywords: Continental collision; Iran; Seismotectonics; Analogue modelling; Moment tensor sum
1. Introduction and geological setting Seismotectonics in regions undergoing collision results in complex deformation patterns characterized by the interaction between strike-slip and reverse * Corresponding author. Tel.: +39-55-2757528; fax: +39-55290312. E-mail address: [email protected] (M. Bonini). 0040-1951/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2003.07.002
faulting. This is particularly evident in Iran where intraplate deformation generated by the N-directed Arabia indentation is partitioned into a composite system of collision-oblique and collision-parallel seismogenetic belts (McKenzie, 1972; Berberian, 1981; Jackson and McKenzie, 1984; DeMets et al., 1990; Jackson et al., 1990, 1995; Jestin et al., 1994). These belts (Fig. 1) delimit two adjacent continental blocks, the Central Iran block to the West and the Lut block to
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Fig. 1. Schematic tectonic map of the Iranian region (modified after Geological Survey of Iran, 1973) superposed onto a digital elevation model. Topography elevation was taken from ETOPO-2 global data set, using GMT mapping tool (Wessel and Smith, 1991).
the East (Jackson et al., 1995 and references therein). These crustal blocks behave rigidly within a deforming zone that is well constrained by the relatively undeformed cratons of Arabia to the SW, Eurasia to the North and by its promontory (Afghan block) to the East (Jackson and McKenzie, 1984, 1988). Main bulk shortening occurs across the most active seismic zone in Iran, the NW – SE-trending Zagros mountain chain
bounding the Central Iran block along its South – Western margin. This chain is markedly oblique to the convergence direction and it is characterized by an important orogen-parallel dextral strike-slip component of displacement (e.g., Tchalenko and Braud, 1974; Berberian, 1981, 1995; Jackson and McKenzie, 1984, 1988). Further North, the collision-related deformation extends well beyond the Zagros, concen-
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trating into the N –S-trending fault belts bounding the Lut block (Nayband and Sistan belts), as well as into the Elburz-Kopet Dagh mountain chain system, at the northern boundary of the deformed zone (Fig. 1). In plan view, the Elburz-Kopet Dagh chain attains a sigmoidal, E –W-oriented, shape and it is characterized by complex kinematics dominated by sinistral transpression along its NE-trending segment (Priestly et al., 1994; Jackson et al., 2002). The dextral (e.g., Zagros fold-and-thrust belt) and sinistral (e.g., Elburz mountains, Great Kavir Fault) conjugate belts have been interpreted to accomplish the lateral escape of Central Iran towards the Lut block (Boccaletti and Dainelli, 1982). Reconciling the complex kinematics of each distinct belt within a deforming region into a unique seismotectonic model is obviously rather difficult and requires the understanding of the overall deformative evolution of the system. For this purpose, we get insights into this complex active region by further developing the analysis of the analogue model reported in Sokoutis et al. (2000) that considered a model continent shortened parallel to a lateral thickness variation. In Sokoutis et al. (2000), the model structural pattern was qualitatively compared to that of the Iranian region, while here we attempt to quantify the model kinematics and to scale it to nature. Particularly, based on the mechanical consistency of the model with respect to nature, we show how the results of modelling can be extrapolated to the natural case in order to (1) identify the main crustal blocks and the deformation belts resulting from their interactions, (2) correlate the available seismic data to the natural deformation pattern (e.g., Koyi et al., 2000), and (3) predict both the kinematics and the velocities of main crustal blocks, as well as (4) the deformation rates along the first-order seismogenetic belts.
2. Analogue modelling The model was designed to take into account the initial main boundary conditions acting in the region, in particular (1) the N – S convergence, (2) the rigid boundaries of the deforming zone (Fig. 1) and (3) the lateral variation in crustal thickness between Central Iran block (thickness z 50 km; Hearn and
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Ni, 1994; Pasyanos and Walter, 2002; block A in Fig. 2) and the adjacent Lut block (thickness < 40 km; Dehghani and Makris, 1983; Sobouti and Arkani-Hamed, 1996; block B in Fig. 2). The experimental set-up reproduced a two-layer continental crust floating above a gypsum –glycerol mixture simulating the upper mantle and asthenosphere (Fig. 2a). The lower crust was reproduced by using a 0.8cm-thick layer of a near-Newtonian mixture of finely ground baryte into polydimethylsiloxane (PDMS; Weijermars, 1986), whereas the upper crust was simulated by using sand. The thickness of this latter varied from 0.4 cm (block B) to 0.7 cm (block A), creating a lateral crustal thickness variation trending parallel to the push direction (Fig. 2a). The model was shortened at a collision rate of 1.8 cm/h in order to scale the 3.5 cm/year convergent rate of Arabia with respect to Eurasia during the late Miocene –Present (DeMets et al., 1990). The dynamic and geometric scaling of the model are discussed in detail in Sokoutis et al. (2000) and reported in Table 1. 2.1. Limitations and simplifications of analogue modelling The analogue model by Sokoutis et al. (2000) involved important limitations, mainly related to the lithospheric rheology and to thermal effects during deformation. Particularly, by assuming an initial setup characterized by two main blocks (A and B) with different thickness, the model necessarily simplified the complex blocks puzzle composing the deforming zone between Arabia and Eurasia. Most notably, the model set-up emphasised the lateral thickness variation between Central Iran and the Lut block, whereas the rheological variation related to the Caspian block was not included. The model did not also include pre-existing discrete discontinuities between (or internal to) the crustal blocks that may have been reactivated during continental collision. During the experiment, the lithospheric rheology was simplified assuming a weak upper mantle. Although the most accepted view of continental rheology suggests the presence of a strong upper lithospheric mantle (e.g., Ranalli, 1995 and reference therein), in the case of Iran geophysical
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Fig. 2. (a) Experimental set-up (from Sokoutis et al., 2000) of a three-layer system, where quartz sand representing the upper crust and PDMS + barite mixture representing the lower crust floated above a gypsum – glycerol mixture simulating the upper mantle. (b) The initial model strength profiles of blocks A and B are compared with a lithospheric profile involving wet lower crust and upper mantle (from Jackson, 2002), as inferred for the Iranian region (Hearn and Ni, 1994; Maggi et al., 2000, 2002). (c) Top-view photograph of the model at the end of deformation (38% bulk shortening); light is from the left. The fixed distal wall represented Eurasia in nature, while the moving wall simulated the Arabia indentation. (d) Line drawing of the main structures; gray dotted lines indicate the deformed initial square passive grid.
investigations support the presence of a weak upper mantle, as both a strong seismic shear wave attenuation (between 50 and 150 km depth) beneath the plateau and a long wavelength gravity high are consistent with an increase in mantle temperature beneath Iran (Hearn and Ni, 1994; Maggi et al., 2000, 2002). As a consequence, the model strength
profile has been simplified as characterized by a strength peak in the upper crust and a ductile lower crust stronger than the mantle (Maggi et al., 2000; Jackson et al., 2002; Fig. 2b). This implies that the lithospheric strength resides within the crust, so that the thinner structural domain in the model configuring the Lut block (see following Section 3.2) is
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Table 1 Model and natural parameters used in the experiments (BC, brittle crust; DC, ductile crust) Parameter
Model
Nature
Model/nature ratio
Density BC (kg/cm3) Density DC (kg/cm3) Gravity, g (m/s2) Horizontal scale, l (m) Crustal thickness, h (m) Gravitational stress BC, r (Pa) Gravitational stress DC, r (Pa) Cohesion BC (Pa) Coefficient of friction BC, l Viscosity DC, g (Pa s) Velocity of displacement (m/s) Time
1300 1370 9.81 0.25 0.012 – 0.015 51 – 89 107 negligible 0.6 5 105 5 10 6 f8 h
2700 f 2900 9.81 9 105 4.2 – 5.3 104 3.7 108 – 6.6 108 8 108 6 107 0.6 – 0.85 6 1022 5.7 10 10 f 15 Ma
0.48 0.47 1 2.8 10 7 2.8 10 7 1.3 10 7 1.3 10 7 – f1 8 10 18 4.5 103 6.2 10 11
assumed to be weaker than the thicker domain corresponding to the Central Iran block in nature. Additionally, the experimental set-up did not consider thermal variations during the progressive deformation of model, reducing the complex thermomechanical process of continental collision to a purely mechanical process. Also, syn-shortening erosion and sedimentation were not considered during the experiment, although the thick sedimentary wedge in the Caspian Basin suggests that these processes may locally affect significantly the evolution of deformation (e.g., Brunet et al., 2003). Despite the abovementioned experimental simplifications, our study represents a first attempt to relate seismic activity to first-order structures in nature through analogue modelling, an approach that can be greatly refined in the future.
This undeformed domain slightly moved towards the thinner block B that showed comparatively higher internal deformation. Another main transpressional belt composed of en echelon thrusts formed across the distal corner of block B (Fig. 2c and d). All these structures were already formed by about 26% bulk shortening, such that during the latest stages of deformation the model merely deformed along pre-existing structures. In the following sections, we focus on the most deformed stages that are directly comparable to the kinematics of active deformation in Iran. We compare the model fault pattern with the first-order structures and seismic activity of the Iranian region attempting to integrate the available information in a comprehensive seismotectonic model. 3.2. Comparison of model results with the deformation pattern of the Iranian region
3. Experimental results and comparison with the Iranian region 3.1. Summary of modelling results Indentation of an experimental continent with a lateral variation in strength parallel to the convergence direction (Fig. 2c and d), resulted in the development of deformative belts oblique and parallel to the shortening direction (Sokoutis et al., 2000). Two main conjugate oblique belts of transpressional shear developed within block A, resulting in a main triangular-shaped domain characterized by no significant internal deformation (Fig. 2c and d).
The model deformation exhibits a marked similarity with that of the Iranian region, suggesting a close similarity of dynamic processes. The essential points of similarity between model results and geological and geophysical information on Iran are listed below: (1) The main dextral transpressional belt in the model is comparable to the NW –SE Zagros fold-and-thrust belt in nature, which bounds the Central Iran block to the SW. As in the model, this deformation belt in nature accommodates most of the shortening related to the Arabia
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indentation. Additionally, dextral displacement associated with hinterlandward thrusting along the model belt is matched by similar displacements in the Zagros belt (e.g., Tchalenko and Braud, 1974). An obvious difference between model and nature is related to the width of the deformation belts, which in the Zagros is wider than in the experiment. This difference may be related to the presence of a thick salt layer at the base of the sedimentary cover that strongly weakens the eastern Zagros and enhances the lateral differences in strength (Talbot and Alavi, 1996). Despite these differences, the model dextral transpressional belt is directly comparable to the overall kinematics exhibited by the Zagros fold-and-thrust belt. (2) The conjugate main sinistral transpressional belt in the experiment is correlative, in our interpretation, with a main ENE –WSW-oriented deformative belt bordering the Central Iran block to the NW (Fig. 1). This system of structures is well expressed in the NE – SWtrending segment of the Elburz mountains, separating the Caspian block from the Central Iran (Fig. 1). However, an evident difference between the analogue model and nature is that only minor expression of the NE –SW-trending deformative belt observed in the model is clearly recognisable in the region between the Elburz mountains and the Zagros belt. This difference is probably associated with the strong influence exerted by the southern boundary of the Caspian block on the structural pattern, by controlling reactivation of structures and strain localization along this main inherited rheological anisotropy. In any case, the presence of several active NE-oriented left-lateral transpressional splays along the Aran-Torud belt branching south-westwards from the NE-trending Elburz segment, as well as the occurrence of earthquakes with oblique (transpressional)
(3)
(4)
(5)
(6)
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motion (see Section 4), may be compatible with the presence of a main deep sinistral transpressional zone transferring deformation from the northwestern Zagros up to Kopet Dagh (Fig. 1). We refer to this inferred main deformation belt as Elburz-Aran-Torud belt (EAT). Both conjugate belts configure a triangularshaped block that is correlative with the triangular Central Iran block. On the basis of model block kinematics, we suggest that these main conjugate belts (Zagros and EAT) accomplish the overall N – NE-directed movement of both Central Iran block and Lut block (Jackson et al., 1995; see Section 5 below). The modest overthrusting of triangular block A over block B (Fig. 1) accords with the presence of N –S trending and E-verging thrust folds and provide evidence for shortening along the Nayband belt (Geological Survey of Iran, 1973), although other geological data suggest a dominant strike-slip kinematics for this belt (Walker and Jackson, 2002). The lack of significant transcurrence along the correspondent Nayband belt in the model was prevented by the experimental set-up that did not include a pre-existing discrete crustal discontinuity separating blocks A and B. The structures accommodating deformation at the right margin of the experimental block B strikingly fit those resulting from the dextral shearing of the Lut block against the rigid Afghan block, which acts as the fixed lateral walls of the Plexiglas box in the model (compare the Sistan belt and the structures propagating into the Northern Lut block in Fig. 1 with the deformed model in Fig. 2c and d). The lateral motion of main crustal blocks in the model is inhibited by the confinement of the deforming zone between rigid boundaries, determining that the uplifting and the thickening are the main deformation modes, similarly to what
Fig. 3. (a) Instrumental earthquakes (red open circles) from the on-line ISC Catalogue (2003). Focal solutions (in black) are taken from the literature (Vannucci and Gasperini, 2003 and reference therein) and Harvard CMT (2003) and ETH (2003) on-line Catalogues. The interpreted main deformational belts, indicated by the blue oblique pattern, are: ‘‘EAT’’, Elburz-Aran-Torud belt; ‘‘Mk’’, Makran; ‘‘Na’’, Nayband belt; ‘‘St’’, Sistan belt; ‘‘Zg’’, Zagros belt. The black dashed line indicates the trend of the Elburz-Kopet Dagh mountain chain system. (b) Sums of moment tensor components (Kostrov, 1974) (in blue) on a regular grid with mesh of 1j. Superimposed red lines indicate the best double couple. Topography (from ETOPO-2 global data set), plot of epicentres and focal mechanisms are obtained using GMT mapping tool (Wessel and Smith, 1991).
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has been suggested for the Iranian region (Sobouti and Arkani-Hamed, 1996).
4. Comparison with seismic data The deformation pattern is consistent with the seismic activity (Fig. 3a). This analysis has been carried out considering (1) the spatial distribution of instrumental earthquakes taken from the on-line International Seismological Centre (ISC, 2003) Catalogue, (2) the available focal mechanism solutions (Dziewonskii et al., 1981 and subsequent papers appeared quarterly on Phys. Earth and Planet Inter.; Braunmiller et al., 2002; Vannucci and Gasperini, 2003 and references therein) and (3) the seismic moment sum (Kostrov, 1974) over a regular grid. Earthquakes in the Iranian region show a marked nonuniform distribution, being mostly concentrated within the belts surrounding the more stable, relatively aseismic, Central Iran and Lut blocks (e.g., ShojaTaheri and Niazi, 1981; Jackson and McKenzie, 1984; Jackson and McKenzie, 1988; Jackson et al., 1995; Berberian and Yeats, 1999; Fig. 3a). Such a nonuniform distribution of epicentres is well explained by the pattern of deformation in the models showing localization of faulting into discrete transpressive belts surrounding more undeformed regions, although preexisting topography has been shown to be capable of inducing gravitational stresses with magnitudes similar to those of tectonic origin (Bada et al., 2001). Consistent with the transpressive kinematics inferred from the analogue model, focal mechanism solutions indicates that most events are compressive and strike-slip. In Fig. 3a, we show focal solutions reported by Harvard Centroid Moment Tensor-CMT (2003) and Eidgeno¨ssische Technische Hochschule (ETH, 2003) on-line moment tensors Catalogues, as well as by papery literature (Vannucci and Gasperini, 2003 and references therein). For these latter mechanisms, the correctness and the consistency of the original parameters has been checked by Vannucci and Gasperini (2003). This procedure also included the choice of a ‘‘best mechanism’’ when more than one solution is given in the literature and/or in the online catalogues. The comparison between nature and experiment is implemented by the analysis of complessive mecha-
nisms obtained as the sum of moment tensors of individual earthquakes (Kostrov, 1974) on a regular grid with mesh of 1j (Fig. 3b). Notwithstanding the different resolution between the moment tensor in nature and the analysis of the strain in the model, the overall kinematics exhibited by the main deformation belts show a nearly good correspondence (compare Fig. 3a and b with Fig. 2d). For example, focal mechanisms along the Zagros belt (‘‘Zg’’ in Fig. 3a) indicate mainly compressive deformation, with direction varying from N – S to NE –SW, associated with transcurrent faulting resulting in dextral transpression (Berberian, 1995). Focal mechanism solutions along the EAT zone (Fig. 3a) also exhibit an interaction between strike-slip and reverse faulting that result in an overall sinistral transpressional kinematics along the same deformative belts, as also reported by Priestly et al. (1994) for the northeastern EAT. Additionally, mostly compressional events in the Nayband belt (‘‘Na’’ in Fig. 3a) and the dextral component of movement highlighted by focal mechanism solutions along the Sistan belt (‘‘St’’ in Fig. 3a) fit well the model kinematics. More problematic is instead the comparison between model deformation and the focal mechanisms in the Makran zone (‘‘Mk’’ in Fig. 3a) where compressive, strike-slip and extensional solutions are related to subduction processes, which were not considered in the modelling.
5. Model relevance to prediction of deformation rates in Iran The striking correspondence between the actual deformation pattern outlined by seismic activity and the model structural pattern suggests that analogue modelling can be used to trace the first-order seismogenetic structures in nature. However, it is important to mention that experimental results cannot explain all the structural details that develop on a scale irrelevant to this approach. To achieve a comprehensive comparison of modelling results with nature, we reconstructed the velocity field on the analogue model surface by monitoring the displacement of selected nodes of the passive grid during deformation. The overall blocks kinematics is efficiently described by the velocity vectors in both absolute and relative reference frames (Fig. 4a and b).
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The absolute reference frame allows to compare the velocity field with (1) numerical modelling results, (2) cinematic analyses based on elaboration of seismicity and seismic moment and (3) GPS data. On the other hand, the relative reference frame describes the relative movement between points within the model, thus allows displaying the relative displacement between blocks and the local kinematics along individual deformation belts. This procedure consents the comparison of modelling results with field data, because geologists usually consider the deformations they study in relation to a locality or a region that has probably moved with respect to the absolute reference frame.
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In the absolute reference frame, each moving node (AA) of the passive grid is identified by Cartesian coordinates (xA, yA) with respect to the origin (OA) fixed at the left corner of the distal stable Plexiglas box wall (Fig. 4a). At the end of deformation, the displacement vector of the selected nodes is defined by the velocity (v) and direction of movement (a). In particular, considering the new coordinates (xVA, yVA), the velocity (v) can be calculated dividing the Euclidean distance between initial and final position of each grid node (the amount of displacement, d) by the total duration of experiment (8 h; Fig. 4b and c). Doing so we have assumed a constant deformation rate, calculated as the average value during the experiment. The
Fig. 4. (a) Initial position of the nodes used to calculate the velocity vectors in the absolute (OA; black squares) and relative (OR; white symbols) reference frames. (b) Final position of the selected nodes together with the absolute (black arrows) and relative (white arrows) model velocity vectors, reported in mm/hour. Note that the main structures in the model are labelled as the deformation belts in nature (Fig. 3a), to which they are hypothesized to correspond. (c) Trigonometric relations between initial and final positions of selected nodes used to estimate the velocity vector in terms of velocity modulus and direction. See text for details.
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direction of movement of the selected nodes is given by the clockwise angle a between the velocity vector and the axis Y (representing the North in nature; Fig. 4a and b). In the relative reference frame, the origin of the Cartesian reference frame (OR) has been conveniently fixed within one main crustal block and the corresponding moving point (AR) chosen within one adjacent block (Fig. 4a). During deformation both points are absolutely moving with respect to the stable Plexiglas box wall, but the relative displacement of point A can be still calculated by considering the initial (xR, yR) and the final (xVR, yVR) coordinates (Fig. 4b and c). In this case, to better define the displacement rates along the individual deformation belts, we have calculated the velocity of the relative nodes by averaging the relative movement over the time interval of activity of the single belt. Thus, the term t in equations reported in Fig. 4b is the time interval between the development of the considered belt and the cessation of the experiment. Both absolute and relative velocity vectors highlight the oblique motion of the main blocks in the central part of the model, with a modest relative convergence-orthogonal overthrusting of the triangular block A domain over block B (Fig. 4b). An intriguing hypothesis resulting from the mechanical consistency between experiment and nature is that the model velocity field can be extrapolated to natural conditions by considering a velocity scaling factor v* = vm/vn=(1.8 cm/hour)/(3.5 cm/year) = 4.5 103, where subscripts m and n refer to model and nature, respectively (see Table 1). Best fitting of the model deformation pattern to the Iranian region allows the positioning of the scaled experimental velocity vectors on the correspondent crustal blocks and deformation belts in nature (Fig. 5a). The scaled velocity vectors confirm that the overall kinematics can be described in terms of N25jE-directed movement of both Central Iran and Lut blocks with respect to the fixed Eurasia. This analogue modelling-derived velocity field (Fig. 5a) shows a good agreement with previous numerical models (Fig. 5b; Jackson et al., 1995; Sobouti and Arkani-Hamed, 1996). However, whereas the direction of vectors in the numerical models is rather constant across the various crustal blocks, in the analogue modelling abrupt changes at the block boundaries are observed, particularly across the Zagros
Fig. 5. Comparison between (a) the scaled model velocity field (in mm/year) rotated 5j clockwise to fit the geometry of the deforming Iranian region, and (b) the velocity of crustal blocks estimated from the spatial variation in the strain rates indicated by earthquakes (from Jackson et al., 1995). Symbols as in Fig. 4.
belt (cf. Fig. 5a and b). This difference is likely related to the development of discrete shear zones, allowing the differential movement of crustal blocks. Strain partitioning among deformational belts results in different transpression rates. Scaled shortening rates
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(Table 2) suggest that the highest values are to be expected along the Zagros (c 22 mm/year), whereas lower rates are predicted for the EAT belt (c 14 mm/ year) and the Sistan fault belt (c 12 mm/year). Absolute movement of selected nodes at northern margin of the triangular-shaped domain of model block A also suggests shortening rates of c 11 mm/year along the Kopet Dagh. Overthrusting of the Central Iran over the Lut block is expected to occur at a slow rate of c 0.7 mm/year. These predicted deformation rates are in good agreement with the available rates determined from numerical and plate tectonics models as well as geological data (see Table 2). GPS measurements in the Zagros (Table 2) show slip rates that are about the half of the velocities predicted by the analysis of the analogue model (Hessami, 2002; Tatar et al., 2002). Other GPS data related to the other deformation belts (Vernant et al., 2002a, b, 2003; Hatzfeld et al., 2003) are only roughly comparable with the model deformation, as the exact location of the measurement points are not reported. In general, the GPS data show slip rates that are comparatively lower than the velocities predicted by the analysis of the analogue model (Table 2). These differences may result from the short (hu-
Table 2 Deformation rates from model and previous works Deformation belt
Predicted rates (mm/year) from the analogue model
Rates (mm/year) predicted from numerical and plate tectonics models or geological data
Rates (mm/year) determined from GPS measurements
Zagros (Z)
22
10 – 28 [1,2] 10 [3]
Elburz-AranTorud (EAT) Kopet Dagh
14.4
13 – 17 [9]
10 [4] 10 ( F 3) [5] 10 ( F 4) [6] 8 ( F 2) [7] 8 [8]
11
Sistan (St) Nayband (Na)
11.7 0.7
13 – 15 [9, 10] 6 ? [2] 10 – 20 [11] 0.5 – 0.7 [11]a
6 ( F 2) [7] 6 – 9 [7] –
[1] Jackson and McKenzie (1984); [2] Jackson and McKenzie (1988); [3] Falcon (1974); [4] Hatzfeld et al. (2003); [5] Hessami (2002); [6] Tatar et al. (2002); [7] Vernant et al. (2002a); [8] Vernant et al. (2002b, 2003) [9] Jackson et al. (2002); [10] Lyberis and Manby (1999); [11] Walker and Jackson (2002). a E – W convergent rate calculated considering the slip partitioning between strike-slip and reverse faulting.
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man) span of time covered by GPS data, whereas extrapolated model velocities represent averaged values over geological times. This implies that the modelderived velocity field is more appropriately compared to plate tectonics models or geological data covering a large time interval of deformation. The good correspondence of the experimental velocity field with that from previous models, supports the reliability of our approach.
6. Conclusions The analysis of an analogue experiment and the comparison of the results with the structural pattern and the seismicity of Iran suggested the following main conclusions: 1. The deformation pattern of the experiment, which was designed specifically for simulating deformation in the Iranian region, yielded comparable structural components to the natural prototype; 2. The first-order deformation belts in the model are consistent with the distribution of seismic activity as well as with the kinematics of available focal mechanism solutions; 3. The velocity field and the deformation rates along the first-order deformation belts predicted from the model are in satisfactory agreement with those estimated by other methods for the first-order seismogenetic belts in Iran; 4. The possibility to compare and integrate different geological-structural and seismological data with the results of modelling, indicates the potentiality of this approach in providing insights into the assessment of the seismotectonic model of a given region. Although analogue modelling necessarily simplifies the natural prototype, it offers the possibility of portraying the progressive deformational path generated by the firstorder structures since the beginning of shortening. By comparison, information from focal mechanism solutions covers only a few tens of years, which are instantaneous at the scale of the whole natural process. This modelling approach is able to provide reliable constraints to the kinematics of the main crustal blocks and to the transpressional rates estimated for the main
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deformation belts. This may provide information that is of paramount importance in other regions where no geodetic measurements (space-based or terrestrial) are currently available to monitor the regional kinematics.
Acknowledgements We thank the journal reviewers J. Che´ry and L. Ratschbacher and the Editor in Chief J.P. Burg for the stimulating and constructive comments which helped improve the manuscript. S.A.P.L. Cloetingh and D. Sokoutis acknowledge the financial support from ISES and The Netherlands Organization for Scientific Research (NWO).
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