# Stress state reconstruction of oblique collision and evolution of deformation partitioning in W-Zagros (Iran, Kermanshah)

P. Navabpour, J. Angelier, E. Barrier

► To cite this version:

P. Navabpour, J. Angelier, E. Barrier. Stress state reconstruction of oblique collision and evolution of deformation partitioning in W-Zagros (Iran, Kermanshah). Geophysical Journal International, 2008, 175 (2), pp.755-782. 10.1111/j.1365-246X.2008.03916.x. hal-00407880

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# Stress state reconstruction of oblique collision and evolution of deformation partitioning in W-Zagros (Iran, Kermanshah)

Payman Navabpour,<fnref n="1" />,<fnref n="2" /> Jacques Angelier<fnref n="2" /> and Eric Barrier<fnref n="3" />

Accepted 2008 July 15. Received 2008 June 24; in original form 2007 October 26

## SUMMARY

The W-Zagros of Iran is characterized by a deformation partitioning pattern with a right-lateral strike-slip motion along the Main Recent Fault and a shortening component across the Zagros Simple Fold Belt under the late Cenozoic N–S oblique collision between the Arabian and Iranian continental plates. In this study, we examined the brittle structures that developed during the mountain building process in the High Zagros Belt of Kermanshah Province, which is structurally located between the two partitioned components of deformation, to decipher the history of polyphase deformation and variations in stress fields within the sedimentary cover since the onset of collision. The earthquake focal mechanisms of W-Zagros were analysed to evaluate the recent stress fields within the basement. Analytic inversion techniques enabled us to determine and separate different brittle tectonic regimes in terms of stress tensors. Brittle tectonic analyses were carried out to reconstruct possible geometrical relationships between different structures and to establish relative chronologies of the corresponding stress tensor, with reference to folding. Our stress reconstruction indicates a major change in stress state from a Miocene–Pliocene syn-folding compressional regime to a Pliocene–Recent post-folding strike-slip system, with a significant anticlockwise reorientation of the horizontal $σ_1$ stress axis from NE–SW to N–S. The results of the recent stress states indicate that the stress trends are similar throughout the basement and sedimentary cover. The differences between stress and strain axes suggest different possible degrees in partitioning of seismic and aseismic deformation, as revealed by earthquake focal mechanisms in the basement and geodetic surveys at the surface of the sedimentary cover. The integrated palaeostress results on the regional tectonic framework suggest that the late Cenozoic brittle tectonic evolution of the W-Zagros was achieved through the transition from thin- to thick-skinned deformation under the oblique plate convergence, highlighting a southwestward shift of the deformation partitioning during the final Pliocene collisional reorganization.

**Key words:** Continental tectonics; Seismotectonics; Dynamics of faulting; Asia.

## 1 INTRODUCTION

The W-Zagros is characterized by an orogenic-scale deformation partitioning (Berberian 1995) with a belt-parallel right-lateral slip along the Main Recent Fault (MRF; Tchalenko & Braud 1974) and a belt-perpendicular shortening across the Zagros Simple Fold Belt (ZSFB), which are separated by the crush zone of the High Zagros Belt (HZB) along the High Zagros Fault (HZF; Fig. 1). Oblique plate convergence and continental deformation partitioning style have already been investigated around the Earth in terms of different aspects involving stress fields, geodetic strain rates and plate kinematics, as for instance in California and New Zealand (Teyssier et al. 1995; Teyssier & Tikoff 1998), Himalaya

(McCaffrey & Nabelek 1998) and Java–Sumatra (McCaffrey 1992, 2002). The recent deformation partitioning style of the W-Zagros has extensively been studied in terms of seismicity and geodetic strain rates within the frame of oblique plate convergence (Talebian & Jackson 2002, 2004; Vernant et al. 2004; Vernant & Chéry 2006; Walpersdorf et al. 2006). Authemayou et al. (2006) has studied the Pliocene–Recent stress state that governs the partitioned deformation in the southeastern part of the MRF in central Zagros. However, no previous work attempted to investigate the relationships between the successive tectonic events of the W-Zagros deformation partitioning in terms of evolving stress fields through time since the inception of collision and onset of compressive tectonics.

<footnote n="1">¹Geological Survey of Iran, Meraj av., Azadi sq., PO Box 13185-1494, Tehran, Iran. E-mail: navabpour@geoazur obs-vlfr fr</footnote>

<footnote n="2">²Géoscience Azur, Port de la Darse, BP 48 - 06235 Villefranche-sur-Mer Cedex, France</footnote>

<footnote n="3">³Université Pierre et Marie Curie, Case 129, 75252 Paris Cedex 05, France</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Index map of the studied area within the Zagros fold-and-thrust belt. Topography from: http://edc.usgs.gov/products/elevation/gtopo30/gtopo30.html. 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 and BFZ, Balarud Fault Zone. Black arrow indicates the GPS convergence vector from Vernant *et al.* (2004). Inset (a): location map in the Middle East.</figcaption></figure>

Brittle tectonic analyses and palaeostress reconstructions have already been used to investigate successive tectonic events and structural evolution of continental deformation zones undergoing shortening, as for instance in the Hsüehshan Range of Taiwan (Angelier *et al.* 1990) and the NW-Caucasus (Saintot & Ange-lier 2002). Our recent brittle tectonic and palaeostress reconstruc-tion in the HZB of interior Fars revealed an evolving stress field (Navabpour *et al.* 2007a), characterizing a change in stress state from compressional to strike-slip regime that has accompanied a gradual anticlockwise reorientation of the maximum compressive stress trend from NE–SW to N–S since the onset of collision. In this paper, we aim at using a similar approach to investigate the brittle tectonic evolution of the HZB of W-Zagros, in the Kermanshah Province, and to reconstruct the corresponding palaeostress fields. We further aim at recognizing similarities and differences between successive tectonic events along the inner zone of the Zagros colli-sion, as compared with the HZB of interior Fars. We then examine the recent stress fields within the sedimentary cover and the base-ment based on the results of our stress tensor inversion of both the fault slip data and the earthquake focal mechanisms in relation to the available data of seismic and geodetic strain rates within the frame of oblique plate convergence. Finally, the palaeostress fields of the HZB of Kermanshah are evaluated within the past and re-cent deformation partitioning styles of W-Iran by integrating the results on the regional tectonic framework. A possible model of

the brittle tectonic and deformation partitioning evolution is thus proposed.

The brittle tectonic history of the HZB is not limited to the Cenozoic compression, of course. We observed syn-depositional evidence of Mesozoic normal faulting in this region, as has already been reported from the HZB of interior Fars (Navabpour et al. 2006). However, the present paper focuses on the Cenozoic history of oblique convergence and we do not discuss the Mesozoic extensional structures despite their importance for understanding the structural evolution of the Zagros sedimentary basin, because they reflect a quite different earlier history of the Arabian passive margin prior to the continental collision.

## 2 GEOLOGICAL SETTING

The studied area belongs to the HZB of W-Zagros in the Kerman-shah Province. The HZB is the most uplifted and eroded part of the Zagros mountain belt with an imbricate structure of the Mesozoic shelf limestones and deep-sea radiolarian cherts of the southern Neo-Tethyan margin (Fig. 2). Towards the southwest, in the ZSFB of Lurestan, shallow water carbonates and clastic deposits of the Arabian platform cropped out. To the northeast of the studied area, the metamorphic rocks of the Iranian continental block were over-thrust onto the HZB through a narrow thrust sheet of ophiolites,
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Geological map of the studied area, showing the position of the sites where brittle structures were collected. The map is mainly redrawn and simplified based on the geological map of GSI (1978). Map according to main difference in lithology from the Palaeozoic and metamorphic rocks of the SSMZ to the Mesozoic rocks of the HZB and the Tertiary rocks of the ZSFB. Deformation partitioning is evidenced by right-lateral strike-slip structure of the MRF to the north and reverse structures parallel to the HZF to the south (see cross section; redrawn after NIOC 1977b with corrections after Braud 1987 and Agard et al. 2005). Stratigraphic details in Fig. 3. Abbreviations as in Fig. 1.</figcaption></figure>
which is considered as the remnant of the Neo-Tethyan oceanic crust (Ricou 1968). In the following, a summary of stratigraphy and tectonic setting of the studied area is presented as auxiliary information to our brittle tectonic analyses.

## 2.1 Stratigraphy

In the W-Zagros, more than 10 km thick Palaeozoic–Cenozoic sedimentary layers (Motiei 1993) constitute the belt (Figs 2 and 3). Geological evidence suggests that Iran was a part of the Afro-Arabian continental platform, at least from the late Precambrian to the late Palaeozoic times, with epicontinental detrital deposits (Stocklin 1968; Nabavi 1976). The Permian–Triassic extension then produced the oceanic crust of the Neo-Tethyan basin to the northeast of the present HZB (Berberian & King 1981; Ricou 1994). During the Mesozoic era, the Zagros basin steadily subsided along the inherited faults and a thick sedimentary sequence of mainly marine carbonates was deposited on the Arabian passive margin (Berberian & King 1981). During this time, the present HZB of Kermanshah was a marginal basin (Ricou et al. 1977), with a thick sequence of deep-sea black marls, red radiolarian cherts and siliceous limestones (Braud 1970). This basin was separated from the Neo-Tethyan basin by the thick platform of Bisitoun shelf limestones. The relatively stable oceanic environment was interrupted by a southwestward obduction of the ophiolites and radiolarite nappe over the southwestern Neo-Tethyan margin in the late Cretaceous period (Ricou 1971; Lanphere & Pamic 1983) and the Maastrichtian–Palaeocene Amiran flysch was deposited (Braud 1990). The Tertiary units unconformably overlie the Mesozoic units (Braud 1971, 1987; Berthier et al. 1974; Agard et al. 2005); their typical sequence includes the Palaeocene underwater basaltic lavas and marls, Eocene pelagic

limestone and flysch deposits, late Oligocene–early Miocene conglomerates, Aquitanian–Burdigalian epicontinental limestones and Pliocene conglomerates.

The Cenozoic sequence of the ZSFB is characterized by a regional change in sedimentary facies, from an open marine to a continental environment (Fig. 3). After the late Cretaceous movements, the Palaeocene–Eocene neritic limestones, detrital red beds and white dolomites of the Taleh Zang, Kashkan and Shahbazan Formations were deposited, respectively. The lower Miocene red beds unconformably covered these units, which are mainly reported from the synclines of south Kermanshah (Motiei 1993; Braud 1990). Transition occurs with the more continuous, deeper basin sequence of the Palaeocene–Eocene Pabdeh neritic marls and shales and the Oligocene–Miocene shallow marine shelf limestones of the Asmari Formation to the southwest. The Oligocene unconformity, which is only observed to the northeast, did not affect the deeper basin to the southwest (Sepehr & Cosgrove 2004). The shallow marine evaporites and marls of the Gachsaran Formation followed the deposition of the Asmari Formation in the middle Miocene epoch. Contributions of folding, erosion and syn-tectonic sedimentation during the late Miocene epoch created a predominantly estuarine environment, in which the clastic red beds of the Agha Jari Formation were deposited with localized progressive unconformities to the southwest (Berberian & King 1981; Hessami et al. 2001). The latest major phase of the Zagros sedimentation was characterized by deposition of the Pliocene continental Bakhtyari molasse-type conglomerates (Stocklin 1968). These conglomerates overlie older formations throughout the belt, conformably in the Zagros Front and unconformably in the rest of the belt to the northeast (Hessami et al. 2001). A recent magneto-stratigraphic study on the syn-tectonic Agha Jari and Bakhtyari deposits indicated that folding

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Stratigraphic chart of the Kermanshah region modified and simplified from Braud (1970, 1990), Beydoun *et al.* (1992) and Motiei (1993). Position of the Agha Jari and Bakhtyari Formations based on Homke *et al.* (2004). Position of the Asmari Formation based on Ehrenberg *et al.* (2007). Absolute ages are from Palmer & Geissman (1999). Summary of tectonic history is added for complete account. Thick black lines show the main stages of folding in the HZB and ZSFB during the entire folding period of the Zagros (dashed line). Abbreviations as in Fig. 1.</figcaption></figure>
in the ZSFB of Lurestan was active since the late Miocene times to about the Pliocene–Pleistocene boundary, and then moved to the Zagros Front in the Quaternary period (Homke et al. 2004).

## 2.2 Structures and tectonic setting

In Iran, two major parallel domains of the Sanandaj–Sirjan Metamorphic Zone (SSMZ) and the Urumieh–Dokhtar Magmatic Arc (UDMA) (Fig. 1) characterize the Neo-Tethyan oceanic subduction beneath the Iranian continental active margin (Berberian & King 1981) that was terminated by the continental collision between the Arabian and Iranian plates in the middle–late Cenozoic times (Agard et al. 2005, 2006). After the collision, the former Arabian passive margin underwent a process of continental shortening highlighted by thin-skinned folding and thrusting of the sedimentary cover and thick-skinned reverse faulting of the basement (see Sherkati et al. 2005; Molinaro et al. 2005a). Estimates on balanced cross sections indicate that shortening related to the folding and thrusting is about 57 km across the W-Zagros (McQuarrie 2004).

In the studied area, the SSMZ is overthrust onto the HZB through the Main Zagros Thrust (MZT) (Braud 1971) as the suture zone between the two plates (Berberian & King 1981; Paul et al. 2003), indicating a southwestward tectonic transport with a N055° trend (Agard et al. 2005). The MZT is partly cut by the MRF, which is the place of some major earthquakes (Tchalenko & Braud 1974). It is suggested that the right-lateral displacement along the MRF is about 50 km and possibly up to 70 km since at least 3–5 Ma, creating some seismically active local pull-apart structures with a belt-parallel direction of extension (Talebian & Jackson 2002). The HZB of Kermanshah indicates a NW–SE trend of imbricate structures that are thrust through the HZF over the large open asymmetrical anticlines of the ZSFB, showing steeper SW-limbs and little surface faulting (Berberian 1995). Most of the Zagros seismicity, however, concentrates within the ZSFB; focal mechanisms suggest the presence of NE-dipping active blind reverse faults (Berberian 1995; Tatar et al. 2004). The inferred reverse faults were suggested to be inherited from the older extensional structures related to the Neo-Tethyan opening that were inverted during the collision (Jackson 1980; Berberian & King 1981; Jackson & McKenzie 1984; Sepehr & Cosgrove 2004; Navabpour et al. 2006). Towards the southwest, the Mountain Front Fault (MFF) and the Zagros Front reverse faults separate the ZSFB from the Dezful Embayment and Persian Gulf–Mesopotamian Lowland as the Zagros foreland (Berberian 1995).

According to the kinematic reconstruction by McQuarrie et al. (2003), the direction of Arabia–Eurasia convergence has changed from a NE–SW direction since 56 Ma to a N–S direction in about 19 Ma. New GPS measurements by Vernant et al. (2004) on the overall Arabia–Eurasia convergence revealed a present-day relative motion of 18 mm/yr trending N352° at coordinate 46.4°E-33.6°N in southern Lurestan (Fig. 1). The N–S oblique plate convergence is considered responsible for the recent deformation partitioning style of the W-Zagros (Berberian 1995; Talebian & Jackson 2002) (see cross section in Fig. 2). However, an older strong right-lateral shear deformation associated with thrust faulting has been considered in the SSMZ as an evidence of a deformation partitioning on the former Iranian active margin earlier in the history of oblique plate convergence (Mohajjel & Fergusson 2000; Mohajjel et al. 2003).

## 3 RECENT STRESS FIELDS

In order to reconstruct the palaeostress history of the studied area, we first attempted to understand the present-day stress fields that

govern over the W-Zagros. The recent GPS data of the Arabian Plate motion in the reference frames of both the Eurasia and Central Iran suggest an overall N–S plate convergence (Vernant et al. 2004; Hesami et al. 2006) that is thought to be responsible for the present-day deformation partitioning in W-Zagros (Talebian & Jackson 2004). A recent N–S stress direction of compression has already been evidenced along the collision zone along the southeastern segment of the MRF (Authemayou et al. 2006) and in the HZB of interior Fars (Navabpour et al. 2007a) based on fault slip data of post-Pliocene age. An evaluation of the recent stress state in our study area can be obtained from the earthquake focal mechanisms of W-Zagros (Table 1). In the first approximation, as a significant-albeit inaccurate-evaluation, a simple observation of *P* axes of the earthquake focal mechanisms indicate a variety of trends from northeast to northwest (Fig. 4, rose diagram). Because we did not intend to present a comprehensive analysis of the earthquake focal mechanisms at the regional scale (see Zamani 2007), the seismic source parameters related to the earthquakes of the Dezful Embayment and Central Iran are not included here.

In the ZSFB of Lurestan, most of the focal mechanisms show deep reverse faulting with nodal planes that strike parallel to the fold axes of the sedimentary cover (Fig. 4 and Table 1) (Jackson & McKenzie 1984; Ni & Barazangi 1986; Harvard CMT solutions 1976–2007). Most of the rakes of the seismic slip vectors are close to 90°, so that the slip vectors trend approximately perpendicular to the fold axes (Talebian & Jackson 2004). Four exceptions however exist, with two reverse mechanisms (1972 June 14 and 1998 August 05) indicating an average N309° trend for the *P* axis, and two mechanisms (1989 April 02 and 2002 June 18) being strike-slip in type. On the other hand, several significant earthquakes occurred along the MRF (Ambraseys & Moinfar 1973; Tchalenko & Braud 1974) most of them reveal strike-slip focal mechanisms (Shirakova 1967; Ambraseys & Moinfar 1974; Jackson & McKenzie 1984; Ni & Barazangi 1986; Harvard CMT solutions 1976–2007). These focal mechanisms are presented as deeper equivalent events related to the right-lateral offset of the MRF that is observed in the field (Talebian & Jackson 2002), considering the fault-parallel nodal planes and the corresponding seismic slip vectors (Talebian & Jackson 2004). Three exceptions were noted among these mechanisms, one of which (1998 August 21, with a *T* axis trending N291°) indicates normal faulting that is known to be the result of a local pull-apart structure along the MRF (Talebian & Jackson 2002) while the other two mechanisms (1957 December 13 and 2002 December 24) indicate reverse faulting with unusual strikes.

We analysed the seismic data to obtain the recent stress fields responsible for the present-day seismicity throughout the W-Zagros. The Right Dihedra method (Angelier & Mechler 1977) provides constraints on stress state indicated by focal mechanisms in a simple straightforward manner. Where two or more fault slips have occurred in the same stress regime, the $\sigma_1$ and $\sigma_3$ stress axes must belong to the *P* and *T* dihedra, respectively. Thus, application of this method to the consistent focal mechanisms of the MRF and ZSFB (ignoring the exceptions mentioned above) provided a preliminary visual evaluation of the prevailing stress state (Figs 4a and b, left-hand side). Note that this method defines compatibility domains but does not provide a unique solution concerning the attitudes of stress axes and the shape of the stress tensor (more detail in Table 2, upper half). The relatively low number of focal mechanisms and the lack of geometrical variety in their distribution preclude obtention of high-constrained stress fields. The compatibility areas are thus large, ranging from 100 per cent pressure (white) to 100 per cent
<table><caption>**Table 1.** Earthquake source parameters of W-Zagros.</caption><thead><tr><th rowspan="2">Date</th><th rowspan="2">Time</th><th colspan="3">Coordinates</th><th rowspan="2">D.</th><th rowspan="2">Mw</th><th colspan="3">Nodal plane 1</th><th colspan="3">Nodal plane 2</th><th rowspan="2">F</th><th colspan="2">P</th><th colspan="2">N</th><th colspan="2">T</th><th rowspan="2">Ref.</th></tr><tr><th>Long. E</th><th>Lat. N</th><th>Str.</th><th>Dip</th><th>Rake</th><th>Str.</th><th>Dip</th><th>Rake</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th></tr></thead><tbody><tr><td>1957.12.13</td><td>01:45</td><td>47,80</td><td>34,55</td><td></td><td>6.7</td><td>006</td><td>52</td><td>125</td><td>137</td><td>50</td><td>54</td><td>R</td><td>072</td><td>01</td><td>162</td><td>27</td><td>340</td><td>63</td><td>M</td></tr><tr><td>1958.08.16</td><td>19:13</td><td>48,17</td><td>34,30</td><td></td><td>6.6</td><td>325</td><td>70</td><td>170</td><td>058</td><td>81</td><td>20</td><td>S</td><td>190</td><td>07</td><td>082</td><td>68</td><td>283</td><td>21</td><td>S</td></tr><tr><td>1963.03.24</td><td>12:44</td><td>48,00</td><td>34,43</td><td>8</td><td>5.8</td><td>314</td><td>52</td><td>-164</td><td>214</td><td>77</td><td>-39</td><td>S</td><td>167</td><td>36</td><td>019</td><td>49</td><td>269</td><td>16</td><td>N</td></tr><tr><td>1967.01.11</td><td>11:20</td><td>45,66</td><td>34,07</td><td>10</td><td>5.6</td><td>140</td><td>50</td><td>81</td><td>334</td><td>41</td><td>101</td><td>R</td><td>236</td><td>05</td><td>146</td><td>07</td><td>358</td><td>82</td><td>N</td></tr><tr><td>1970.10.25</td><td>11:22</td><td>45,19</td><td>36,75</td><td></td><td>5.5</td><td>319</td><td>50</td><td>-154</td><td>212</td><td>70</td><td>-43</td><td>S</td><td>167</td><td>44</td><td>012</td><td>44</td><td>270</td><td>13</td><td>J</td></tr><tr><td>1972.06.12</td><td>13:34</td><td>46,27</td><td>34,04</td><td>11</td><td>5.0</td><td>114</td><td>56</td><td>83</td><td>306</td><td>35</td><td>100</td><td>R</td><td>209</td><td>10</td><td>118</td><td>06</td><td>359</td><td>78</td><td>N</td></tr><tr><td>1972.06.14</td><td>04:34</td><td>46,11</td><td>33,04</td><td></td><td>5.3</td><td>198</td><td>40</td><td>51</td><td>065</td><td>60</td><td>118</td><td>R</td><td>135</td><td>11</td><td>230</td><td>24</td><td>022</td><td>64</td><td>N</td></tr><tr><td>1976.11.07</td><td>11:08</td><td>47,90</td><td>33,21</td><td></td><td>4.8</td><td>138</td><td>58</td><td>121</td><td>269</td><td>43</td><td>50</td><td>R</td><td>206</td><td>09</td><td>301</td><td>26</td><td>100</td><td>63</td><td>J</td></tr><tr><td>1987.05.29</td><td>06:27</td><td>48,21</td><td>34,05</td><td>15</td><td>5.3</td><td>218</td><td>80</td><td>2</td><td>128</td><td>88</td><td>170</td><td>S</td><td>174</td><td>05</td><td>294</td><td>80</td><td>083</td><td>08</td><td>H</td></tr><tr><td>1988.01.26</td><td>09:34</td><td>46,85</td><td>32,33</td><td>20</td><td>5.6</td><td>306</td><td>20</td><td>79</td><td>137</td><td>70</td><td>94</td><td>R</td><td>224</td><td>25</td><td>316</td><td>04</td><td>054</td><td>65</td><td>H</td></tr><tr><td>1989.04.02</td><td>21:24</td><td>47,62</td><td>32,89</td><td>15</td><td>5.4</td><td>256</td><td>40</td><td>-7</td><td>351</td><td>85</td><td>-130</td><td>S</td><td>227</td><td>37</td><td>355</td><td>40</td><td>112</td><td>29</td><td>H</td></tr><tr><td>1990.08.03</td><td>11:57</td><td>48,24</td><td>32,80</td><td>15</td><td>5.3</td><td>96</td><td>33</td><td>53</td><td>318</td><td>64</td><td>111</td><td>R</td><td>033</td><td>17</td><td>128</td><td>19</td><td>264</td><td>64</td><td>H</td></tr><tr><td>1998.08.05</td><td>14:27</td><td>46,00</td><td>33,50</td><td>33</td><td>5.6</td><td>183</td><td>20</td><td>52</td><td>043</td><td>74</td><td>103</td><td>R</td><td>123</td><td>28</td><td>220</td><td>12</td><td>331</td><td>59</td><td>H</td></tr><tr><td>1998.08.21</td><td>05:13</td><td>48,03</td><td>34,38</td><td>9</td><td>5.0</td><td>025</td><td>39</td><td>-84</td><td>197</td><td>51</td><td>-95</td><td>N</td><td>078</td><td>83</td><td>201</td><td>04</td><td>291</td><td>06</td><td>H</td></tr><tr><td>1998.10.04</td><td>00:42</td><td>47,22</td><td>33,23</td><td>15</td><td>5.3</td><td>126</td><td>31</td><td>121</td><td>270</td><td>64</td><td>73</td><td>R</td><td>013</td><td>18</td><td>278</td><td>15</td><td>149</td><td>66</td><td>H</td></tr><tr><td>1998.10.05</td><td>02:20</td><td>47,22</td><td>33,20</td><td>37</td><td>5.4</td><td>131</td><td>41</td><td>103</td><td>294</td><td>50</td><td>79</td><td>R</td><td>032</td><td>05</td><td>301</td><td>09</td><td>149</td><td>80</td><td>H</td></tr><tr><td>2001.03.23</td><td>05:24</td><td>46,29</td><td>32,78</td><td>15</td><td>5.4</td><td>301</td><td>42</td><td>74</td><td>142</td><td>50</td><td>104</td><td>R</td><td>222</td><td>04</td><td>313</td><td>11</td><td>110</td><td>79</td><td>H</td></tr><tr><td>2001.09.01</td><td>22:38</td><td>47,58</td><td>32,81</td><td>15</td><td>5.0</td><td>121</td><td>31</td><td>101</td><td>288</td><td>60</td><td>83</td><td>R</td><td>023</td><td>14</td><td>291</td><td>06</td><td>180</td><td>74</td><td>H</td></tr><tr><td>2002.04.24</td><td>19:48</td><td>47,55</td><td>34,39</td><td>33</td><td>5.4</td><td>036</td><td>74</td><td>16</td><td>302</td><td>75</td><td>163</td><td>S</td><td>349</td><td>01</td><td>082</td><td>68</td><td>259</td><td>22</td><td>H</td></tr><tr><td>2002.06.18</td><td>03:19</td><td>45,17</td><td>33,74</td><td>33</td><td>5.3</td><td>242</td><td>74</td><td>5</td><td>151</td><td>85</td><td>164</td><td>S</td><td>198</td><td>07</td><td>313</td><td>73</td><td>106</td><td>15</td><td>H</td></tr><tr><td>2002.12.24</td><td>17:03</td><td>47,66</td><td>34,54</td><td>26</td><td>5.2</td><td>284</td><td>45</td><td>105</td><td>082</td><td>47</td><td>75</td><td>R</td><td>183</td><td>01</td><td>092</td><td>11</td><td>280</td><td>79</td><td>H</td></tr><tr><td>2004.10.16</td><td>10:04</td><td>45,63</td><td>33,56</td><td>14</td><td>4.9</td><td>282</td><td>42</td><td>19</td><td>177</td><td>77</td><td>130</td><td>R</td><td>237</td><td>22</td><td>346</td><td>39</td><td>126</td><td>43</td><td>H</td></tr><tr><td>2004.11.21</td><td>21:37</td><td>47,75</td><td>33,23</td><td>12</td><td>5.2</td><td>310</td><td>26</td><td>112</td><td>106</td><td>66</td><td>80</td><td>R</td><td>203</td><td>21</td><td>110</td><td>09</td><td>357</td><td>67</td><td>H</td></tr><tr><td>2004.11.22</td><td>04:01</td><td>47,82</td><td>33,21</td><td>12</td><td>5.0</td><td>308</td><td>29</td><td>111</td><td>105</td><td>63</td><td>79</td><td>R</td><td>203</td><td>17</td><td>110</td><td>10</td><td>351</td><td>70</td><td>H</td></tr><tr><td>2004.11.27</td><td>16:12</td><td>47,80</td><td>33,18</td><td>20</td><td>4.9</td><td>330</td><td>38</td><td>145</td><td>089</td><td>69</td><td>57</td><td>R</td><td>203</td><td>18</td><td>102</td><td>30</td><td>319</td><td>54</td><td>H</td></tr><tr><td>2005.01.25</td><td>11:39</td><td>45,69</td><td>33,40</td><td>12</td><td>5.0</td><td>350</td><td>27</td><td>129</td><td>127</td><td>70</td><td>72</td><td>R</td><td>231</td><td>23</td><td>134</td><td>16</td><td>011</td><td>61</td><td>H</td></tr><tr><td>2005.05.03</td><td>07:21</td><td>48,64</td><td>33,73</td><td>21</td><td>5.0</td><td>220</td><td>71</td><td>-17</td><td>316</td><td>74</td><td>-161</td><td>S</td><td>178</td><td>25</td><td>353</td><td>65</td><td>087</td><td>02</td><td>H</td></tr><tr><td>2006.03.30</td><td>19:36</td><td>48,78</td><td>33,65</td><td>19</td><td>5.2</td><td>321</td><td>70</td><td>-167</td><td>226</td><td>77</td><td>-20</td><td>S</td><td>182</td><td>23</td><td>016</td><td>66</td><td>275</td><td>05</td><td>H</td></tr><tr><td>2006.03.31</td><td>01:17</td><td>48,73</td><td>33,74</td><td>17</td><td>6.1</td><td>313</td><td>78</td><td>-174</td><td>222</td><td>84</td><td>-12</td><td>S</td><td>177</td><td>13</td><td>016</td><td>77</td><td>268</td><td>04</td><td>H</td></tr><tr><td>2006.03.31</td><td>11:54</td><td>48,75</td><td>33,73</td><td>25</td><td>5.1</td><td>319</td><td>67</td><td>-168</td><td>224</td><td>79</td><td>-24</td><td>S</td><td>180</td><td>24</td><td>021</td><td>64</td><td>274</td><td>08</td><td>H</td></tr><tr><td>2006.06.06</td><td>17:03</td><td>46,04</td><td>35,97</td><td>25</td><td>4.9</td><td>313</td><td>41</td><td>-168</td><td>214</td><td>82</td><td>-49</td><td>S</td><td>160</td><td>39</td><td>027</td><td>40</td><td>273</td><td>26</td><td>H</td></tr></tbody></table>

Locations of earthquakes before 1970 are based on their macroseismic areas (Ambraseys & Melville 1982). D, centroid depth (km), Nodal planes are shown by their strike and dip with the rake of slip vector. F, fault type: S, strike-slip; R, reverse; N, normal. P, N, and T are pressure, neutral, and tension axes that are indicated by their direction and plunge. References: M, McKenzie (1972); N, Ni & Barazangi (1986); S, Shirakova (1967); J, Jackson & McKenzie (1984); H, Harvard CMT Solution for 1976–2007 (http://www.seismology.harvard.edu). Site location in Fig. 4.

tension (black). The preferred axes of stress, $\sigma_{1pref}$ and $\sigma_{3pref}$ that, respectively, reflect maximum probability for pressure and tension areas, reveal a N–S compression of strike-slip type (vertical $\sigma_2$) along the MRF and a NE–SW compression of reverse type (vertical $\sigma_3$) across the ZSFB.

In order to determine the best-fitting stress tensors, we adopted the Direct Inversion method of Angelier (2002) for the nodal planes of double couple earthquake focal mechanisms. The application of this method to the total set of the focal mechanisms provides the optimum orientation of the three principal stress axes, $\sigma_1$, $\sigma_2$ and $\sigma_3$ (Figs 4a and b, right-hand side), as well as the ratio of principal stress magnitude differences, $\Phi$ (Table 2, lower half). This determination refines the data homogeneity and the results supports the Right Dihedra estimates, indicating a N176° trend for the $\sigma_1$ stress axis along the MRF and a N035° trend for the $\sigma_1$ stress axis across the ZSFB. In both cases, the obtained value of $\Phi$, close to 0.5, reveals a typical triaxial stress field, knowing however that the poor geometrical variety in double couple focal mechanisms leaves space for large variations in $\Phi$. As pointed out elsewhere (Angelier 2002), in the stress tensor inversion method adopted here the mechanical consistency is obtained by maximizing the values of a posteriori shear estimators, $\omega_{ave}$ and $\tau_{ave}^*$, while minimizing the shear-slip angle, $\alpha_{ave}$ (Table 2, lower half). It is worth noting that this method does not require any choice between the nodal planes of

each earthquake focal mechanism. Any a posteriori choice between nodal planes would result in a smaller shear-slip angle, and hence a better fit. We refer to Angelier (2002) for more detail about the inversion parameters.

In the W-Zagros, the absence of a dense local seismological network resulted in poorly constrained focal depth of the earthquakes. Reactivated basement normal faults at depths between 10 and 20 km were thought for a long time to be responsible for the major earthquakes in the Zagros (Jackson 1980; Berberian & King 1981; Jackson & Fitch 1981; Jackson & McKenzie 1984; Berberian 1995). Estimates of focal depths in W-Zagros range between 8 and 37 km (Table 1) and the recent reassessments based on teleseismic waveform modelling revealed a concentration of earthquakes at depths between 10 and 15 km (Maggi et al. 2000; Talebian & Jackson 2004; Engdahl et al. 2006). Basement structures likely played a role in the deformation of the Zagros by localizing major thrust faults in the sedimentary cover and some topographic steps (Jackson & Fitch 1981; Berberian 1995; Sherkati & Letouzey 2004; Mouthereau et al. 2006). The minimum depth to the basement was roughly estimated by aeromagnetic surveys throughout the Zagros to be 8–9 km (Morris 1977). This minimum depth was adopted in recent attempts at balancing cross sections across the Zagros (Molinaro et al. 2005a; Sherkati et al. 2005, 2006). Therefore, all the earthquakes used in our stress tensor inversion (Fig. 4) indicate
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Seismotectonic map of W-Zagros with rose distribution of the *P* axes of double couple earthquake focal mechanisms. Circles of the rose diagram show percentage of the *P* axes with 5 per cent intervals. Detail of focal mechanisms in Table 1. Fault traces are simplified from Tchalenko & Braud (1974) and NIOC (1977a). Note that the seismic events of the Dezful Embayment, DE, and Central Iran are not included. Inset (a): stress solutions for the focal mechanisms along the MRF, indicating a N–S stress trend. Inset (b): stress solutions for the focal mechanisms across the ZSFB of Lurestan, indicating a NE–SW stress trend. Stereoplots are Schmidt’s (equal area) projections of the lower hemisphere, including the position of magnetic north (marked by small M). Stars with 5, 4 and 3 branches mark the principal σ₁, σ₂ and σ₃ stress axes, respectively, with their 60, 75 and 90 per cent confidence ellipses. Pairs of grey arrows show directions of compression and extension with azimuthal confidence in grey curves. Number of analysed data, *n*, is written at top left-hand corner of each diagram. Detail of stress inversion and graphical illustrations in the text and Table 2. Location map and other descriptions as in Fig. 1.</figcaption></figure>

<table><caption>**Table 2.** Results of seismic stress analysis of 31 focal mechanisms in W-Zagros.</caption><thead><tr><th>Set</th><th><i>N</i></th><th>σ<sub>1pref</sub></th><th>σ<sub>2pref</sub></th><th>σ<sub>3pref</sub></th><th>Φ<sub>est</sub></th><th><i>T</i><sub>max</sub></th><th><i>P</i><sub>max</sub></th></tr></thead><tbody><tr><td>a</td><td>10</td><td>175/17</td><td>041/67</td><td>270/16</td><td>0.51</td><td>100</td><td>100</td></tr><tr><td>b</td><td>14</td><td>218/05</td><td>310/07</td><td>091/82</td><td>0.58</td><td>100</td><td>100</td></tr></tbody></table><table><thead><tr><th>Set</th><th><i>N</i></th><th>σ<sub>1</sub></th><th>σ<sub>2</sub></th><th>σ<sub>3</sub></th><th>Φ</th><th>ω<sub>ave</sub></th><th>τ<sup>*</sup><sub>ave</sub></th><th>α<sub>ave</sub></th></tr></thead><tbody><tr><td>a</td><td>13</td><td>176/22</td><td>025/66</td><td>270/11</td><td>0.53</td><td>93 ± 05</td><td>95 ± 04</td><td>9 ± 08</td></tr><tr><td>b</td><td>18</td><td>215/07</td><td>305/03</td><td>059/82</td><td>0.45</td><td>81 ± 12</td><td>88 ± 10</td><td>18 ± 13</td></tr></tbody></table>

Sets ‘a’ and ‘b’ refer to epicentres located in the inner Zagros along the MRF and MZT (mostly strike-slip; Fig. 4a) and in the outer Zagros across the ZSFB (mostly reverse; Fig. 4b), respectively. Upper half: Right Dihedra results (Angelier & Mechler, 1977). *N*, number of data. σ₁pref > σ₂pref > σ₃pref, preferred principal stress axes given as trend/plunge. Φ<sub>est</sub>, estimated ratio of principal stress magnitude differences (Φ) defined as $Φ = (σ₂ - σ₃)/(σ₁ - σ₃)$ (Angelier, 1975, 1989). T<sub>max</sub> and P<sub>max</sub>, maximum P and T ratios indicating the largest proportion of pressure and tension dihedra (in per cent). Lower half: Direct Inversion results (Angelier, 2002). σ₁ > σ₂ > σ₃, optimum stress axes given as trend/plunge. ω<sub>ave</sub>, average of a posteriori estimator (from −100 per cent, total misfit, to 100 per cent, perfect fit). τ<sup>*</sup><sub>ave</sub>, auxiliary estimator for the average shear stress as percentage of maximum shear stress. α<sub>ave</sub>, average angle between slip and shear stress.
basement deformation under two different stress regimes along the MRF and across the ZSFB of W-Zagros.

## 4 PALAEOSTRESS RECONSTRUCTIONS

The tectonic events that have affected the HZB during and after the collision have produced a wide range of regional scale and mesoscale fold and brittle structures, which revealed evolving stress fields through time. We used systematic brittle tectonic analyses of fault slip data, including both the stress tensor Direct Inversion (Angelier 1984, 1989, 1990) and the Right Dihedra analysis (Angelier & Mechler 1977), to decipher the succession of deformational events that resulted in the present-day structures.

Among the fault slip data, many faults (~57 per cent) belong to simple conjugate-type patterns. Such associations facilitated quick preliminary interpretation in the field. However, many other brittle structures (~43 per cent) consist of tension fractures, bedding plane slips and mono-set faults that do not follow the conjugate-type model. A widespread difficulty in interpreting the measurements within the frame of polyphase brittle deformation was related to the heterogeneity of data sets in the studied area. An example of such a complexity can be illustrated as the case of site 52 (Fig. 5). Data separation at this site revealed different strike-slip, reverse and normal faults, including some tension cracks. The strike-slip faults and tension cracks were also separated into two subsets. Stress analysis of the separated data sets indicates that different stress fields could have been responsible for these brittle structures. Thus, computing an average stress field using these heterogeneous data will yield to undesired result, especially if the data subsets actually correspond to different tectonic episodes. We consequently separated the data

sets collected at different sites throughout the studied area into mechanically homogeneous subsets in order to reconstruct the stress fields that prevailed during distinct tectonic events. This data separation is supported by extensive field observation of geometrical and crosscutting relationships between different brittle structures and folded strata, as well as the stratigraphic age of the affected rocks, as is described hereinafter.

Systematic analysis of the collected brittle tectonic data from more than sixty sites revealed a variety of strike-slip, compressional and extensional stress regimes with different stress fields throughout the studied area (Fig. 6). The stress regimes reflect stress axes that are obtained from predominating strike-slip, reverse and normal faults, respectively. A summary of parameters of the palaeostress reconstructions is provided (Table 3), in which the results corresponding to each stress regime are presented separately together with their geographical and geological situation, stratigraphic age of the affected rocks and structural setting of the studied sites. Bedding attitudes provide an important key to reconstruct the pretilting configuration where appropriate. For this reason, the bedding attitude of each site is given together with the identified relative chronology/geometry of the related brittle structure with respect to the local tilted strata. Calculated stress fields are defined by direction and plunge of the principal stress axes, number of data, method of analysis, ratio of stress magnitude differences and the reduced misfit angle. In addition, a quality estimator is attached to each determination, because local results of the stress calculations are not equal. This quality estimator provides a multivariable evaluation between different stress solutions that depends on strictly quantitative (such as the number of acceptable data and the average misfit level) and semi-quantitative and qualitative aspects (such as

<figure><img src="image_6.png" /><figcaption>**Figure 5.** An example of heterogeneous data collected at site 52. Data separation at this site reveals four subsets of different strike-slip, reverse and normal faults and two subsets of tension cracks; each of them indicates an individual stress field. In the stereoplots: dashed great circles are bedding planes, solid great circles show faults and tension cracks, striae are indicated by solid dots and slip arrows, isolated solid and open dots are poles to bedding and faults, respectively, solid squares are poles to tension cracks. Other symbols as in Fig. 4.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Stress map of the studied area, indicating different stress states in the HZB of Kermanshah. Inset (a): rose distribution of $\sigma_1$ stress axes ($n = 36$) related to the strike-slip stress regime. Inset (b): rose distribution of $\sigma_1$ stress axes ($n = 28$) related to the compressional stress regime. Inset (c): rose distribution of $\sigma_3$ stress axes ($n = 31$) related to the extensional stress regime. Circles of the rose diagrams show percentage of the stress axes with 5 per cent intervals. Geological detail in Figs 2 and 3. Detail of stress tensor parameters in Table 3.</figcaption></figure>

the confidence level and accuracy of fault slip measurements). The reader is referred to Angelier (1984, 1990) for more detail about the inversion parameters.

## 4.1 Brittle structures and folding

Our study in the HZB of W-Zagros required careful consideration of the relationships between brittle structures and folds. Various cri-

teria were used to establish the relative chronological relationships among the brittle structures, including crosscutting relationships of fault planes and successive striae observed on a fault surface. We have recently described the methodological aspects concerning the palaeostress reconstructions and the relative chronology/geometry between folds and faults in the paper devoted to the HZB of interior Fars (Navabpour et al. 2007a). In the present paper, we use the same approach with new data to reconstruct the evolution of the brittle
<table><caption>**Table 3.** Summary of geological position of brittle structures and reconstructed stress regimes.</caption><thead><tr><th rowspan="2">Stress regime</th><th rowspan="2">Location</th><th rowspan="2">Coordinates</th><th rowspan="2">Stratigraphic age</th><th rowspan="2">Structural position</th><th rowspan="2">Bedding</th><th rowspan="2">Site #</th><th rowspan="2">BS</th><th rowspan="2">M</th><th rowspan="2">N</th><th colspan="2">σ<sub>1</sub></th><th colspan="2">σ<sub>2</sub></th><th colspan="2">σ<sub>3</sub></th><th colspan="2">Φ</th><th colspan="2">α</th><th rowspan="2">Q</th></tr><tr><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th></tr></thead><tbody><tr><td rowspan="15">Strike-slip stress regimes (Fig. 6a)</td><td>W. Kermanshah</td><td>46,96</td><td>34,29</td><td>L.Jr.-E.Kr.</td><td>NE-Limb</td><td>120/53 N</td><td>01</td><td>S+</td><td>8</td><td>197</td><td>20</td><td>357</td><td>69</td><td>105</td><td>07</td><td>0,51</td><td>0</td><td>0</td><td>0</td><td>0</td></tr><tr><td>E. Kermanshah</td><td>47,38</td><td>34,28</td><td>L.Tr.-Jurassic</td><td>SW-Limb</td><td>116/61 S</td><td>04</td><td>S*</td><td>D1</td><td>5</td><td>321</td><td>09</td><td>108</td><td>79</td><td>230</td><td>06</td><td>0,54</td><td>13</td><td>A</td><td>C</td></tr><tr><td>E. Kermanshah</td><td>47,38</td><td>34,28</td><td>L.Tr.-Jurassic</td><td>SW-Limb</td><td>116/61 S</td><td>04</td><td>S+</td><td>RD</td><td>3</td><td>204</td><td>12</td><td>323</td><td>67</td><td>110</td><td>20</td><td>0,50</td><td>C</td><td>C</td><td>C</td></tr><tr><td>NW. Harsin</td><td>47,42</td><td>34,34</td><td>Jurassic</td><td>N-Limb</td><td>091/39 N</td><td>10</td><td>S+</td><td>D1</td><td>12</td><td>178</td><td>00</td><td>071</td><td>89</td><td>268</td><td>01</td><td>0,48</td><td>28</td><td>B</td><td>B</td></tr><tr><td>N. Bisitoun</td><td>47,44</td><td>34,57</td><td>Alb.-Cenom.</td><td>N-Limb</td><td>004/16 W</td><td>15</td><td>S</td><td>RD</td><td>3</td><td>260</td><td>35</td><td>012</td><td>29</td><td>132</td><td>42</td><td>0,52</td><td>C</td><td>C</td><td>C</td></tr><tr><td>SW. Kermanshah</td><td>46,63</td><td>34,15</td><td>Late Eocene</td><td>NE-Limb</td><td>116/14 N</td><td>20</td><td>S</td><td>RD</td><td>3</td><td>023</td><td>01</td><td>290</td><td>67</td><td>113</td><td>23</td><td>0,53</td><td>C</td><td>C</td><td>C</td></tr><tr><td>W. Kermanshah</td><td>46,91</td><td>34,29</td><td>Alb.-Cenom.</td><td>Crest</td><td>083/11 N</td><td>21</td><td>S</td><td>RD</td><td>2</td><td>003</td><td>02</td><td>234</td><td>87</td><td>093</td><td>02</td><td>0,50</td><td>C</td><td>C</td><td>C</td></tr><tr><td>S. Kermanshah</td><td>47,02</td><td>34,24</td><td>Campanian</td><td>NE-Limb</td><td>134/23 N</td><td>22</td><td>S*</td><td>D1</td><td>7</td><td>057</td><td>14</td><td>247</td><td>75</td><td>148</td><td>02</td><td>0,50</td><td>10</td><td>A</td><td>A</td></tr><tr><td>S. Kermanshah</td><td>47,02</td><td>34,24</td><td>Campanian</td><td>NE-Limb</td><td>134/23 N</td><td>22</td><td>S+</td><td>D1</td><td>6</td><td>189</td><td>18</td><td>065</td><td>59</td><td>287</td><td>24</td><td>0,43</td><td>20</td><td>B</td><td>B</td></tr><tr><td>S. Kermanshah</td><td>47,00</td><td>34,25</td><td>Campanian</td><td>NE-Limb</td><td>148/15 N</td><td>23</td><td>S*</td><td>D1</td><td>23</td><td>237</td><td>01</td><td>343</td><td>87</td><td>147</td><td>03</td><td>0,50</td><td>12</td><td>A</td><td>A</td></tr><tr><td>S. Kermanshah</td><td>47,07</td><td>34,25</td><td>Maastrichtian</td><td>NE-Limb</td><td>112/36 N</td><td>25</td><td>S</td><td>D1</td><td>12</td><td>234</td><td>07</td><td>108</td><td>78</td><td>325</td><td>10</td><td>0,58</td><td>16</td><td>B</td><td>B</td></tr><tr><td>NW. Kermanshah</td><td>46,91</td><td>34,48</td><td>Alb.-Cenom.</td><td>SE-Tip</td><td>027/25E</td><td>26</td><td>S</td><td>D1</td><td>27</td><td>159</td><td>03</td><td>351</td><td>87</td><td>249</td><td>01</td><td>0,50</td><td>7</td><td>A</td><td>A</td></tr><tr><td>NW. Kermanshah</td><td>46,91</td><td>34,48</td><td>Alb.-Cenom.</td><td>SE-Tip</td><td>027/25E</td><td>26</td><td>S</td><td>D1</td><td>8</td><td>078</td><td>08</td><td>235</td><td>81</td><td>348</td><td>03</td><td>0,56</td><td>17</td><td>B</td><td>B</td></tr><tr><td>NW. Kermanshah</td><td>46,76</td><td>35,78</td><td>Oligo-Aquit.</td><td>Crest</td><td>Horizontal</td><td>28</td><td>S</td><td>RD</td><td>3</td><td>197</td><td>17</td><td>342</td><td>69</td><td>103</td><td>11</td><td>0,48</td><td>C</td><td>C</td><td>C</td></tr><tr><td>NW. Kermanshah</td><td>46,89</td><td>34,71</td><td>Neoco.-Cenom.</td><td>NW-Tip</td><td>089/19 N</td><td>30</td><td>S</td><td>D1</td><td>13</td><td>243</td><td>02</td><td>146</td><td>74</td><td>334</td><td>15</td><td>0,54</td><td>15</td><td>A</td><td>A</td></tr><tr><td>N. Harsin</td><td>47,61</td><td>34,36</td><td>Oligo-Aquit.</td><td>NE-Limb</td><td>133/20 N</td><td>31</td><td>S</td><td>D1</td><td>4</td><td>354</td><td>03</td><td>173</td><td>87</td><td>264</td><td>00</td><td>0,52</td><td>6</td><td>A</td><td>A</td></tr><tr><td>NE. Harsin</td><td>47,64</td><td>34,37</td><td>Aquitanian</td><td>Syncline</td><td>100/N &amp; S</td><td>32</td><td>S*</td><td>D1</td><td>27</td><td>221</td><td>16</td><td>102</td><td>59</td><td>319</td><td>25</td><td>0,48</td><td>12</td><td>A</td><td>A</td></tr><tr><td>W. Harsin</td><td>47,44</td><td>34,35</td><td>L.Tr.-Jurassic</td><td>N-Limb</td><td>054/43 N</td><td>33</td><td>S+</td><td>D1</td><td>16</td><td>324</td><td>07</td><td>193</td><td>78</td><td>055</td><td>09</td><td>0,49</td><td>21</td><td>B</td><td>B</td></tr><tr><td>N. Harsin</td><td>47,58</td><td>34,45</td><td>E. Cretaceous</td><td>S-Limb</td><td>092/43 S</td><td>34</td><td>S~</td><td>RD</td><td>16</td><td>002</td><td>28</td><td>212</td><td>59</td><td>093</td><td>13</td><td>0,48</td><td>C</td><td>C</td><td>C</td></tr><tr><td>W. Kamyaran</td><td>46,66</td><td>34,73</td><td>E.-M. Cret.</td><td>SW-Limb</td><td>166/29 W</td><td>36</td><td>S*</td><td>D1</td><td>10</td><td>029</td><td>15</td><td>207</td><td>75</td><td>299</td><td>01</td><td>0,43</td><td>19</td><td>B</td><td>B</td></tr><tr><td>W. Kamyaran</td><td>46,66</td><td>34,73</td><td>E.-M. Cret.</td><td>SW-Limb</td><td>166/29 W</td><td>36</td><td>S+</td><td>D1</td><td>6</td><td>178</td><td>10</td><td>342</td><td>79</td><td>088</td><td>03</td><td>0,49</td><td>11</td><td>A</td><td>A</td></tr><tr><td>S. Kermanshah</td><td>47,21</td><td>34,02</td><td>Late Eocene</td><td>Crest</td><td>034/11E</td><td>37</td><td>S*</td><td>RD</td><td>1</td><td>032</td><td>00</td><td>302</td><td>89</td><td>122</td><td>01</td><td>0,50</td><td>C</td><td>C</td><td>C</td></tr><tr><td>S. Kermanshah</td><td>47,20</td><td>33,96</td><td>Late Eocene</td><td>SW-Limb</td><td>135/59 W</td><td>38</td><td>S*</td><td>D1</td><td>15</td><td>038</td><td>08</td><td>128</td><td>86</td><td>308</td><td>04</td><td>0,50</td><td>5</td><td>A</td><td>A</td></tr><tr><td>E. Kermanshah</td><td>47,29</td><td>34,19</td><td>L.Jr.-E.Kr.</td><td>SW-Limb</td><td>156/30 W</td><td>43</td><td>S~</td><td>RD</td><td>6</td><td>022</td><td>03</td><td>118</td><td>68</td><td>290</td><td>22</td><td>0,47</td><td>C</td><td>C</td><td>C</td></tr><tr><td>E. Kermanshah</td><td>47,33</td><td>34,34</td><td>L.Tr.-Jurassic</td><td>SW-Limb</td><td>135/28 S</td><td>45</td><td>S~</td><td>D1</td><td>10</td><td>024</td><td>18</td><td>214</td><td>71</td><td>115</td><td>03</td><td>0,38</td><td>20</td><td>B</td><td>B</td></tr><tr><td>SE. Kermanshah</td><td>47,36</td><td>34,18</td><td>L.Jr.-E.Kr.</td><td>NE-Limb</td><td>116/28 N</td><td>48</td><td>S~</td><td>RD</td><td>1</td><td>011</td><td>20</td><td>209</td><td>70</td><td>103</td><td>06</td><td>0,50</td><td>C</td><td>C</td><td>C</td></tr><tr><td>S. Kermanshah</td><td>46,80</td><td>34,09</td><td>Eocene</td><td>N-Limb</td><td>107/10 N</td><td>50</td><td>S</td><td>D1</td><td>21</td><td>020</td><td>08</td><td>169</td><td>81</td><td>289</td><td>05</td><td>0,51</td><td>6</td><td>A</td><td>A</td></tr><tr><td>W. Kermanshah</td><td>46,85</td><td>34,41</td><td>Cenom.-Camp.</td><td>NE-Limb</td><td>148/14 N</td><td>51</td><td>S</td><td>D1</td><td>13</td><td>023</td><td>16</td><td>207</td><td>74</td><td>113</td><td>01</td><td>0,44</td><td>10</td><td>A</td><td>A</td></tr><tr><td>W. Kermanshah</td><td>46,83</td><td>34,42</td><td>Cenom.-Camp.</td><td>NE-Limb</td><td>137/13 N</td><td>52</td><td>S</td><td>D1</td><td>8</td><td>236</td><td>06</td><td>126</td><td>73</td><td>327</td><td>16</td><td>0,49</td><td>13</td><td>A</td><td>A</td></tr><tr><td>W. Kermanshah</td><td>46,83</td><td>34,42</td><td>Cenom.-Camp.</td><td>NE-Limb</td><td>137/13 N</td><td>52</td><td>S</td><td>D1</td><td>10</td><td>007</td><td>04</td><td>250</td><td>82</td><td>098</td><td>07</td><td>0,53</td><td>10</td><td>A</td><td>A</td></tr><tr><td>E. Harsin</td><td>47,80</td><td>34,21</td><td>Miocene</td><td>SW-Limb</td><td>137/20 S</td><td>54</td><td>S</td><td>D1</td><td>18</td><td>168</td><td>22</td><td>334</td><td>67</td><td>076</td><td>05</td><td>0,48</td><td>12</td><td>A</td><td>A</td></tr><tr><td>N. Kermanshah</td><td>47,19</td><td>34,64</td><td>Eocene</td><td>N-Limb</td><td>051/18 N</td><td>55</td><td>S</td><td>D1</td><td>9</td><td>006</td><td>08</td><td>189</td><td>82</td><td>096</td><td>00</td><td>0,53</td><td>17</td><td>B</td><td>B</td></tr><tr><td>NW. Kermanshah</td><td>47,02</td><td>34,55</td><td>E.-M. Cret.</td><td>SW-Limb</td><td>132/49 S</td><td>56</td><td>S+</td><td>D1</td><td>7</td><td>141</td><td>10</td><td>022</td><td>69</td><td>234</td><td>18</td><td>0,48</td><td>16</td><td>B</td><td>B</td></tr><tr><td>SW. Kermanshah</td><td>46,67</td><td>34,21</td><td>Eocene</td><td>SW-Limb</td><td>116/45 S</td><td>57</td><td>S+</td><td>D1</td><td>19</td><td>182</td><td>20</td><td>029</td><td>68</td><td>275</td><td>09</td><td>0,54</td><td>10</td><td>A</td><td>A</td></tr><tr><td>S. Kermanshah</td><td>46,92</td><td>34,11</td><td>Middle Eocene</td><td>SW-Limb</td><td>144/29 W</td><td>58</td><td>S+</td><td>D1</td><td>20</td><td>184</td><td>23</td><td>057</td><td>64</td><td>279</td><td>13</td><td>0,51</td><td>6</td><td>A</td><td>A</td></tr><tr><td>S. Harsin</td><td>47,55</td><td>34,07</td><td>L.Jr.-E.Kr.</td><td>SW-Limb</td><td>118/64 S</td><td>61</td><td>S+</td><td>D1</td><td>5</td><td>184</td><td>19</td><td>358</td><td>71</td><td>093</td><td>02</td><td>0,49</td><td>10</td><td>A</td><td>A</td></tr><tr><td>W. Kermanshah</td><td>46,96</td><td>34,29</td><td>L.Jr.-E.Kr.</td><td>NE-Limb</td><td>120/53 N</td><td>01</td><td>R~</td><td>RD</td><td>8</td><td>038</td><td>10</td><td>307</td><td>04</td><td>196</td><td>79</td><td>0,51</td><td>C</td><td>C</td><td>C</td></tr><tr><td>W. Kermanshah</td><td>46,93</td><td>34,29</td><td>Campanian</td><td>NE-Limb</td><td>117/29 N</td><td>02</td><td>R</td><td>D1</td><td>6</td><td>188</td><td>00</td><td>098</td><td>36</td><td>279</td><td>54</td><td>0,47</td><td>15</td><td>B</td><td>B</td></tr><tr><td>E. Kermanshah</td><td>47,38</td><td>34,28</td><td>L.Tr.-Jurassic</td><td>SW-Limb</td><td>116/61 S</td><td>04</td><td>R+</td><td>D1</td><td>15</td><td>006</td><td>06</td><td>276</td><td>02</td><td>171</td><td>83</td><td>0,51</td><td>6</td><td>A</td><td>A</td></tr><tr><td>NW. Harsin</td><td>47,47</td><td>34,34</td><td>L.Tr.-Jurassic</td><td>SW-Limb</td><td>?</td><td>05</td><td>R</td><td>D1</td><td>12</td><td>160</td><td>02</td><td>251</td><td>33</td><td>066</td><td>56</td><td>0,54</td><td>9</td><td>A</td><td>A</td></tr><tr><td>W. Harsin</td><td>47,40</td><td>34,26</td><td>L.Tr.-Jurassic</td><td>SW-Limb</td><td>130/71 S</td><td>17</td><td>R*</td><td>D1</td><td>18</td><td>221</td><td>15</td><td>130</td><td>04</td><td>025</td><td>75</td><td>0,49</td><td>12</td><td>A</td><td>A</td></tr><tr><td>W. Kermanshah</td><td>46,91</td><td>34,29</td><td>Alb.-Cenom.</td><td>Crest</td><td>083/11 N</td><td>21</td><td>R</td><td>RD</td><td>3</td><td>013</td><td>16</td><td>103</td><td>01</td><td>195</td><td>74</td><td>0,48</td><td>C</td><td>C</td><td>C</td></tr><tr><td>S. Kermanshah</td><td>47,02</td><td>34,24</td><td>Campanian</td><td>NE-Limb</td><td>134/23 N</td><td>22</td><td>R~</td><td>D1</td><td>36</td><td>213</td><td>17</td><td>123</td><td>00</td><td>032</td><td>73</td><td>0,49</td><td>15</td><td>A</td><td>A</td></tr><tr><td>S. Kermanshah</td><td>47,02</td><td>34,24</td><td>Campanian</td><td>NE-Limb</td><td>134/23 N</td><td>22</td><td>R+</td><td>D1</td><td>8</td><td>001</td><td>04</td><td>092</td><td>15</td><td>258</td><td>74</td><td>0,51</td><td>14</td><td>B</td><td>B</td></tr></tbody></table>
<table><caption>Table 3. (Continued.)</caption><thead><tr><th rowspan="2">Stress regime</th><th rowspan="2">Location</th><th rowspan="2">Coordinates<br>Long. E Lat. N</th><th rowspan="2">Stratigraphic age</th><th rowspan="2">Structural position</th><th rowspan="2">Bedding<br>Strike/Dip</th><th rowspan="2">Site #</th><th rowspan="2">BS</th><th rowspan="2">M</th><th rowspan="2">N</th><th rowspan="2">σ<sub>1</sub></th><th colspan="2">σ<sub>2</sub></th><th colspan="2">σ<sub>3</sub></th><th colspan="2">Φ</th><th rowspan="2">α</th><th rowspan="2">Q</th></tr><tr><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th></tr></thead><tbody><tr><td rowspan="12">Extensional stress regimes (Fig. 6c)</td><td>S. Kermanshah</td><td>47.00</td><td>34.25</td><td>Campanian</td><td>NE-Limb</td><td>148/15 N</td><td>23</td><td>R+</td><td>RD</td><td>3</td><td>348</td><td>19</td><td>092</td><td>35</td><td>235</td><td>49</td><td>0.49</td><td>C</td></tr><tr><td>S. Kermanshah</td><td>47.02</td><td>34.24</td><td>Campanian</td><td>Crest</td><td>Horizontal</td><td>24</td><td>R</td><td>RD</td><td>3</td><td>200</td><td>24</td><td>103</td><td>14</td><td>347</td><td>62</td><td>0.50</td><td>C</td></tr><tr><td>NW. Kermanshah</td><td>46.91</td><td>34.48</td><td>Alb.-Cenom.</td><td>SE-Tip</td><td>027/25E</td><td>26</td><td>R+</td><td>RD</td><td>1</td><td>173</td><td>14</td><td>265</td><td>08</td><td>024</td><td>74</td><td>0.50</td><td>C</td></tr><tr><td>NE. Harsin</td><td>47.64</td><td>34.37</td><td>Aquitanian</td><td>Syncline</td><td>100/N &amp; S</td><td>32</td><td>R+</td><td>RD</td><td>19</td><td>000</td><td>04</td><td>270</td><td>12</td><td>109</td><td>77</td><td>0.48</td><td>C</td></tr><tr><td>W. Harsin</td><td>47.44</td><td>34.35</td><td>L.Tr.-Jurassic</td><td>N-Limb</td><td>054/43 N</td><td>33</td><td>R+</td><td>RD</td><td>3</td><td>131</td><td>09</td><td>223</td><td>08</td><td>353</td><td>78</td><td>0.54</td><td>C</td></tr><tr><td>N. Harsin</td><td>47.58</td><td>34.45</td><td>E. Cretaceous</td><td>S-Limb</td><td>092/45 S</td><td>34</td><td>R~</td><td>RD</td><td>15</td><td>026</td><td>06</td><td>119</td><td>24</td><td>283</td><td>65</td><td>0.48</td><td>15 B</td></tr><tr><td>S. Kamyaran</td><td>46.93</td><td>34.64</td><td>E.-M. Cret.</td><td>SW-Limb</td><td>164/07 W</td><td>35</td><td>R~</td><td>RD</td><td>6</td><td>043</td><td>23</td><td>311</td><td>04</td><td>211</td><td>67</td><td>0.50</td><td>C</td></tr><tr><td>S. Kamyaran</td><td>46.93</td><td>34.64</td><td>E.-M. Cret.</td><td>SW-Limb</td><td>164/07 W</td><td>35</td><td>R+</td><td>RD</td><td>4</td><td>176</td><td>26</td><td>267</td><td>03</td><td>003</td><td>64</td><td>0.50</td><td>C</td></tr><tr><td>S. Kermanshah</td><td>47.20</td><td>33.96</td><td>Late Eocene</td><td>SW-Limb</td><td>135/59 S</td><td>38</td><td>R+</td><td>RD</td><td>8</td><td>196</td><td>06</td><td>287</td><td>06</td><td>059</td><td>82</td><td>0.51</td><td>C</td></tr><tr><td>E. Kermanshah</td><td>47.30</td><td>34.23</td><td>L.Jr.-E.Kr.</td><td>NE-Limb</td><td>115/55 N</td><td>40</td><td>R~</td><td>RD</td><td>4</td><td>036</td><td>07</td><td>127</td><td>03</td><td>242</td><td>83</td><td>0.51</td><td>C</td></tr><tr><td>E. Kermanshah</td><td>47.30</td><td>34.22</td><td>L.Jr.-E.Kr.</td><td>NE-Limb</td><td>149/63 N</td><td>41</td><td>R*</td><td>RD</td><td>3</td><td>250</td><td>07</td><td>156</td><td>29</td><td>351</td><td>60</td><td>0.55</td><td>C</td></tr><tr><td>E. Kermanshah</td><td>47.29</td><td>34.19</td><td>L.Jr.-E.Kr.</td><td>SW-Limb</td><td>156/30 W</td><td>43</td><td>R</td><td>DI</td><td>12</td><td>203</td><td>31</td><td>297</td><td>06</td><td>037</td><td>58</td><td>0.45</td><td>11 A</td></tr><tr><td>E. Kermanshah</td><td>47.33</td><td>34.34</td><td>L.Tr.-Jurassic</td><td>SW-Limb</td><td>135/28 S</td><td>45</td><td>R</td><td>DI</td><td>8</td><td>131</td><td>28</td><td>030</td><td>20</td><td>270</td><td>55</td><td>0.53</td><td>23 B</td></tr><tr><td>W. Kermanshah</td><td>46.85</td><td>34.41</td><td>Cenom.-Camp.</td><td>NE-Limb</td><td>148/14 N</td><td>51</td><td>R~</td><td>RD</td><td>1</td><td>230</td><td>31</td><td>322</td><td>05</td><td>060</td><td>59</td><td>0.50</td><td>C</td></tr><tr><td>W. Kermanshah</td><td>46.83</td><td>34.42</td><td>Cenom.-Camp.</td><td>NE-Limb</td><td>137/13 S</td><td>52</td><td>R~</td><td>DI</td><td>10</td><td>214</td><td>26</td><td>121</td><td>06</td><td>019</td><td>64</td><td>0.55</td><td>11 A</td></tr><tr><td>E. Harsin</td><td>47.80</td><td>34.21</td><td>Miocene</td><td>SW-Limb</td><td>137/20 S</td><td>54</td><td>R</td><td>RD</td><td>3</td><td>219</td><td>01</td><td>129</td><td>24</td><td>311</td><td>66</td><td>0.55</td><td>C</td></tr><tr><td>N. Kermanshah</td><td>47.19</td><td>34.64</td><td>Eocene</td><td>N-Limb</td><td>051/18 N</td><td>55</td><td>R</td><td>DI</td><td>4</td><td>360</td><td>18</td><td>267</td><td>08</td><td>154</td><td>70</td><td>0.50</td><td>9 A</td></tr><tr><td>SW. Kermanshah</td><td>46.67</td><td>34.21</td><td>Eocene</td><td>SW-Limb</td><td>116/45 S</td><td>57</td><td>R~</td><td>RD</td><td>7</td><td>213</td><td>01</td><td>123</td><td>05</td><td>311</td><td>85</td><td>0.50</td><td>C</td></tr><tr><td>S. Harsin</td><td>47.46</td><td>34.13</td><td>L.Jr.-E.Kr.</td><td>SW-Limb</td><td>137/81 N</td><td>60</td><td>R+</td><td>RD</td><td>4</td><td>210</td><td>10</td><td>120</td><td>04</td><td>006</td><td>79</td><td>0.51</td><td>C</td></tr><tr><td>S. Harsin</td><td>47.55</td><td>34.07</td><td>L.Jr.-E.Kr.</td><td>SW-Limb</td><td>118/64 S</td><td>61</td><td>R*</td><td>RD</td><td>5</td><td>236</td><td>21</td><td>142</td><td>11</td><td>027</td><td>66</td><td>0.50</td><td>C</td></tr><tr><td>NW. Harsin</td><td>47.46</td><td>34.31</td><td>L.Tr.-Jurassic</td><td>N-Limb</td><td>101/33 N</td><td>06</td><td>N</td><td>DI</td><td>17</td><td>261</td><td>73</td><td>013</td><td>07</td><td>105</td><td>16</td><td>0.38</td><td>12 A</td></tr><tr><td>NW. Harsin</td><td>47.43</td><td>34.35</td><td>L.Jr.-E.Kr.</td><td>Crest</td><td>Horizontal</td><td>09</td><td>N</td><td>DI</td><td>16</td><td>244</td><td>75</td><td>025</td><td>12</td><td>117</td><td>09</td><td>0.46</td><td>21 B</td></tr><tr><td>NW. Harsin</td><td>47.42</td><td>34.34</td><td>Jurassic</td><td>N-Limb</td><td>091/39 N</td><td>10</td><td>N</td><td>RD</td><td>11</td><td>033</td><td>80</td><td>269</td><td>06</td><td>179</td><td>08</td><td>0.48</td><td>C</td></tr><tr><td>NW. Harsin</td><td>47.42</td><td>34.34</td><td>Jurassic</td><td>N-Limb</td><td>091/39 N</td><td>10</td><td>N</td><td>DI</td><td>6</td><td>243</td><td>86</td><td>359</td><td>02</td><td>089</td><td>04</td><td>0.43</td><td>16 B</td></tr><tr><td>N. Bisitoun</td><td>47.44</td><td>34.57</td><td>Alb.-Cenom.</td><td>N-Limb</td><td>004/16 W</td><td>15</td><td>N</td><td>DI</td><td>12</td><td>137</td><td>55</td><td>037</td><td>06</td><td>303</td><td>34</td><td>0.47</td><td>5 A</td></tr><tr><td>SW. Kermanshah</td><td>46.63</td><td>34.15</td><td>Late Eocene</td><td>NE-Limb</td><td>116/14 N</td><td>20</td><td>N</td><td>DI</td><td>30</td><td>251</td><td>63</td><td>122</td><td>17</td><td>026</td><td>20</td><td>0.50</td><td>12 A</td></tr><tr><td>W. Kermanshah</td><td>46.9</td><td>34.29</td><td>Alb.-Cenom.</td><td>Crest</td><td>083/11 N</td><td>21</td><td>N</td><td>DI</td><td>5</td><td>252</td><td>70</td><td>012</td><td>10</td><td>105</td><td>17</td><td>0.49</td><td>8 A</td></tr><tr><td>NW. Kermanshah</td><td>46.91</td><td>34.48</td><td>Alb.-Cenom.</td><td>SE-Tip</td><td>027/25E</td><td>26</td><td>N</td><td>DI</td><td>12</td><td>231</td><td>74</td><td>135</td><td>02</td><td>045</td><td>16</td><td>0.45</td><td>15 A</td></tr><tr><td>NW. Kermanshah</td><td>46.76</td><td>35.78</td><td>Oligo.-Aquit.</td><td>Crest</td><td>Horizontal</td><td>28</td><td>T</td><td>RD</td><td>29</td><td>051</td><td>73</td><td>229</td><td>17</td><td>319</td><td>01</td><td>0.68</td><td>C</td></tr><tr><td>NW. Kermanshah</td><td>46.76</td><td>35.78</td><td>Oligo.-Aquit.</td><td>Crest</td><td>Horizontal</td><td>28</td><td>N</td><td>DI</td><td>35</td><td>206</td><td>74</td><td>316</td><td>06</td><td>047</td><td>15</td><td>0.50</td><td>10 A</td></tr><tr><td>NW. Kermanshah</td><td>46.89</td><td>34.71</td><td>Neoco.-Cenom.</td><td>NW-Tip</td><td>089/19 N</td><td>30</td><td>T*</td><td>RD</td><td>20</td><td>104</td><td>68</td><td>244</td><td>17</td><td>338</td><td>14</td><td>0.66</td><td>C</td></tr><tr><td>N. Harsin</td><td>47.61</td><td>34.36</td><td>Oligo.-Aquit.</td><td>Crest</td><td>133/20 N</td><td>31</td><td>T+</td><td>RD</td><td>7</td><td>353</td><td>42</td><td>179</td><td>48</td><td>085</td><td>03</td><td>0.66</td><td>C</td></tr><tr><td>N. Harsin</td><td>47.61</td><td>34.36</td><td>Oligo.-Aquit.</td><td>Crest</td><td>133/20 N</td><td>31</td><td>N</td><td>DI</td><td>12</td><td>130</td><td>76</td><td>305</td><td>14</td><td>036</td><td>01</td><td>0.45</td><td>20 B</td></tr><tr><td>S. Kamyaran</td><td>46.93</td><td>34.64</td><td>E.-M. Cret.</td><td>SW-Limb</td><td>164/07 W</td><td>35</td><td>T~</td><td>RD</td><td>12</td><td>230</td><td>05</td><td>043</td><td>85</td><td>140</td><td>01</td><td>0.66</td><td>C</td></tr><tr><td>S. Kamyaran</td><td>46.93</td><td>34.64</td><td>E.-M. Cret.</td><td>SW-Limb</td><td>164/07 W</td><td>35</td><td>N</td><td>DI</td><td>7</td><td>041</td><td>74</td><td>177</td><td>12</td><td>270</td><td>11</td><td>0.47</td><td>12 A</td></tr><tr><td>W. Kamyaran</td><td>46.66</td><td>34.73</td><td>E.-M. Cret.</td><td>SW-Limb</td><td>166/29 W</td><td>36</td><td>T+</td><td>RD</td><td>7</td><td>183</td><td>04</td><td>002</td><td>86</td><td>093</td><td>00</td><td>0.65</td><td>C</td></tr><tr><td>W. Kamyaran</td><td>46.66</td><td>34.73</td><td>E.-M. Cret.</td><td>SW-Limb</td><td>166/29 W</td><td>36</td><td>N</td><td>DI</td><td>9</td><td>190</td><td>86</td><td>307</td><td>02</td><td>037</td><td>04</td><td>0.46</td><td>12 A</td></tr><tr><td>S. Kermanshah</td><td>47.21</td><td>34.02</td><td>Late Eocene</td><td>Crest</td><td>034/11E</td><td>37</td><td>T</td><td>RD</td><td>14</td><td>061</td><td>85</td><td>216</td><td>05</td><td>306</td><td>02</td><td>0.68</td><td>C</td></tr><tr><td>S. Kermanshah</td><td>47.20</td><td>33.96</td><td>Late Eocene</td><td>SW-Limb</td><td>135/59 S</td><td>38</td><td>T~</td><td>RD</td><td>4</td><td>044</td><td>39</td><td>214</td><td>50</td><td>310</td><td>05</td><td>0.64</td><td>C</td></tr><tr><td>E. Kermanshah</td><td>47.33</td><td>34.34</td><td>L.Tr.-Jurassic</td><td>SW-Limb</td><td>135/28 S</td><td>45</td><td>N</td><td>DI</td><td>10</td><td>163</td><td>75</td><td>023</td><td>12</td><td>291</td><td>09</td><td>0.51</td><td>8 A</td></tr><tr><td>SW. Kermanshah</td><td>46.65</td><td>34.15</td><td>Maastrichtian</td><td>Crest</td><td>123/15 N</td><td>49</td><td>N</td><td>DI</td><td>22</td><td>199</td><td>72</td><td>317</td><td>08</td><td>049</td><td>15</td><td>0.48</td><td>10 A</td></tr></tbody></table>
<table><caption>**Table 3.** (Continued)</caption><thead><tr><th rowspan="2">Stress regime</th><th rowspan="2">Location</th><th colspan="2">Coordinates</th><th rowspan="2">Stratigraphic age</th><th rowspan="2">Structural position</th><th rowspan="2">Bedding</th><th rowspan="2">Site #</th><th rowspan="2">BS</th><th rowspan="2">M</th><th rowspan="2">N</th><th colspan="2">σ₁</th><th colspan="2">σ₂</th><th colspan="2">σ₃</th><th colspan="2">Φ</th><th colspan="2">α</th><th colspan="2">Q</th></tr><tr><th>Long. E</th><th>Lat. N</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th><th>Dir.</th><th>Plung.</th></tr></thead><tbody><tr><td></td><td>S. Kermanshah</td><td>46.80</td><td>34.09</td><td>Eocene</td><td>N-Limb</td><td>107/10 N</td><td>50</td><td>T</td><td>RD</td><td>7</td><td>192</td><td>66</td><td>015</td><td>24</td><td>284</td><td>01</td><td>0.69</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>W. Kermanshah</td><td>46.85</td><td>34.41</td><td>Cenom.-Camp.</td><td>NE-Limb</td><td>148/14 N</td><td>51</td><td>T~</td><td>RD</td><td>18</td><td>200</td><td>78</td><td>021</td><td>12</td><td>291</td><td>00</td><td>0.74</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>W. Kermanshah</td><td>46.83</td><td>34.42</td><td>Cenom.-Camp.</td><td>NE-Limb</td><td>137/13 N</td><td>52</td><td>T~</td><td>RD</td><td>8</td><td>256</td><td>77</td><td>041</td><td>11</td><td>133</td><td>07</td><td>0.73</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>W. Kermanshah</td><td>46.83</td><td>34.42</td><td>Cenom.-Camp.</td><td>NE-Limb</td><td>137/13 N</td><td>52</td><td>N</td><td>DI</td><td>8</td><td>262</td><td>87</td><td>356</td><td>00</td><td>086</td><td>03</td><td>0.46</td><td>15</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>E. Harsin</td><td>47.80</td><td>34.21</td><td>Pliocene</td><td>?</td><td>Horizontal</td><td>53</td><td>T</td><td>RD</td><td>34</td><td>093</td><td>90</td><td>002</td><td>00</td><td>272</td><td>01</td><td>0.80</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>N. Kermanshah</td><td>47.19</td><td>34.64</td><td>Eocene</td><td>N-Limb</td><td>051/18 N</td><td>55</td><td>N</td><td>DI</td><td>9</td><td>030</td><td>78</td><td>228</td><td>11</td><td>137</td><td>04</td><td>0.54</td><td>16</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>SW. Kermanshah</td><td>46.67</td><td>34.21</td><td>Eocene</td><td>SW-Limb</td><td>116/45 S</td><td>57</td><td>T~</td><td>RD</td><td>10</td><td>014</td><td>71</td><td>196</td><td>19</td><td>105</td><td>01</td><td>0.70</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>S. Kermanshah</td><td>46.92</td><td>34.11</td><td>Middle Eocene</td><td>SW-Limb</td><td>144/29 W</td><td>58</td><td>N+</td><td>DI</td><td>5</td><td>201</td><td>84</td><td>007</td><td>06</td><td>097</td><td>02</td><td>0.50</td><td>7</td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>S. Harsin</td><td>47.64</td><td>34.17</td><td>Pliocene</td><td>?</td><td>Horizontal</td><td>59</td><td>T</td><td>RD</td><td>37</td><td>037</td><td>89</td><td>187</td><td>01</td><td>277</td><td>01</td><td>0.71</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>N. Bisitun</td><td>47.37</td><td>34.62</td><td>Eocene</td><td>NE-Limb</td><td>113/28 N</td><td>62</td><td>N</td><td>DI</td><td>12</td><td>341</td><td>83</td><td>210</td><td>05</td><td>120</td><td>05</td><td>0.47</td><td>9</td><td></td><td></td><td></td><td></td></tr></tbody></table>

BS, brittle Structures: S, strike-slip faults; R, reverse faults; N, normal faults; T, tension cracks (where: *, is for back-tilted strata; ~, is for syn-tilt data; +, is for post-tilt data). M, method of stress determination: RD, Right Dihedra; DI, Direct Inversion; N, number of fault slip data; Φ, ratio of stress magnitude differences [Φ = (σ₂ - σ₃)/(σ₁ - σ₃)]; α, average angle between observed slip and computed shear in degrees (acceptable for α < 25°); Q, quality of stress calculation (A, high; B, moderate; C, low). Structural positions are based on local main fold or fault structures. Bedding attitudes and stress axes are given after magnetic north correction. Site location in Figs 2 and 6.

structures and folds in the HZB of Kermanshah. Thus, the methodological presentation mainly deals with some new aspects like the succession of reverse and normal faults and the reader is referred to the earlier paper for methodological details of the succession of strike-slip faults and tension cracks.

To this end, we have provided a simple sketch that illustrates typical reverse and normal faults being produced coeval with a folding process (Fig. 7). Where the strata have remained horizontal (e.g. at the top of an anticline), no chronology of the reverse faulting can be established with respect to folding (Fig. 7a). This situation is reflected in the stereoplot of site 21. In addition, bedding-parallel reverse slips occur during folding and may be interpreted as syn-fold brittle structures (e.g. sites 01 and 57 in Fig. 7b; see also Fig. 8b). The difference between pre- and post-fold faulting becomes clear where the strata are significantly tilted. For pre-fold faulting, the old tilted reverse faults may resemble normal faults in steep fold flanks (e.g. sites 41 and 61 in Fig. 7c; see also Fig. 10a). In contrast, post-fold reverse faults with dip angles less than 45° cut across the folded strata regardless to the bedding attitudes (e.g. sites 38 and 60 in Fig. 7d; see also Fig. 9a). Note that the pre- and post-fold reverse faults are simplified to a fold-parallel strike situation in the sketches. The strike of the reverse faults would be oblique to the local fold axis (LFA) if the faults and fold developed within different stress fields. Such an obliquity can be detected among the pre- and post-fold reverse faults in the stereoplots of sites 22, 38, 60 and 61 (Figs 7c, d, 9 and 10).

Normal faults and tension cracks usually develop in extensional tectonic settings, but may also result from a folding process within a compressive stress field. In general, tension cracks are perpendicular to the local σ₃ stress axis and may develop into normal faults as deformation proceeds. In a strike-slip stress regime with horizontal σ₁ and σ₃ stress axes, these structures constitute transverse normal faults across the folds causing a lateral dilation parallel to an orogenic belt (see Hancock & Bevan 1987). Example of such geometry can be seen at site 09 with steep normal faults striking perpendicular to the general NW-SE structural trend of the belt (Fig. 7e; see also Fig. 13b). If the direction of compression remains constant during the folding, such normal faults strike perpendicular to the LFA (e.g. site 62 in Fig. 13), but it may not be the case where the direction of compression has changed. Some normal faults thus strike oblique to the LFA, indicating a post-fold brittle geometry (e.g. site 58 in Fig. 14a). In addition, as folding proceeds, another type of tension cracks and normal faults develop as the result of local extension at the crest of anticlines (e.g. site 49 in Fig. 7f; see also Fig. 13a). In the case of a fold that trends perpendicular to the regional direction of compression, such brittle structures strike parallel to the LFA and indicate a local anticlinal extension parallel to the direction of compression (e.g. site 31 in Fig. 13). Such a relationship between the anticlinal normal faults and an active fold-and-thrust structure has already been discussed in the El-Asnam (Algeria) earthquake of 1980 October 10 (King & Vita-Finzi 1981; Yielding et al. 1981) and can be considered as a syn-fold brittle geometry.

### 4.1.1 Reverse and strike-slip faults

In order to reconstruct the possible relationships between the reverse and strike-slip faults and folds, for instance, we consider the collected fault slip data at sites 01 and 57 (Fig. 8). At these sites, data separation revealed two different strike-slip and reverse fault systems. The strike-slip faults, which present nearly vertical planes with approximately horizontal striations (Fig. 8a), cut through the
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Schematic illustration of typical reverse and normal faults being produced coeval with a folding process. (a) Situation of a reverse fault where no chronology can be made with reference to folding. (b) Syn-folding bedding-parallel reverse slips. (c) Pre-folding reverse faults that resemble normal faults in steep fold flanks. (d) Post-folding reverse faults that cut across previously folded strata. (e) Transverse normal faults across a fold within a compressive stress field. (f) Anticlinal normal faults related to the outer arc of a fold. Pairs of open arrows show direction of extension. For each structural state, pairs of grey arrows show positions of compressional axes responsible for faulting. The real examples are provided as the included stereoplots of different sites. The north direction is added to fit the field view of the illustrations. Symbols of stereoplots as in Fig. 5. Field views in Figs 8–10 and 13.</figcaption></figure>

folded strata regardless to the dip of the bedding. Such a situation reveals post-fold faulting. The corresponding stress fields calculated for these fault slip data indicate an approximately N–S trend for the $σ_1$ stress axes. The second fault system is characterized by the bedding-parallel reverse slips as a syn-fold faulting. The slickensides strike parallel to the LFA and dip to both NE and SW. Such deformation was specially observed at site 22 (Fig. 8b; fault slip data in Fig. 9). At this site, the bedding-parallel reverse slips are concentrated near the hinge zone of a mesoscale tight syncline, with a combination of fault striations and related tension cracks filled by calcite (Fig. 8c). The corresponding stress fields calculated for these fault slip data indicate a general NE–SW trend for the $σ_1$ stress axes.

Different types of relative geometries between the reverse and strike-slip faults and folded strata were identified among the separated data at sites 22 and 38 (Fig. 9). The reverse faults were

observed to be in their post-fold undistorted configuration (Fig. 9a). They have slickensides with approximately E–W strikes that dip to south (at site 22) and north (at site 38). The corresponding stress fields indicate an approximately N–S trend for the $σ_1$ stress axes. The strike-slip faults are perpendicular to bedding with inclined striations (Fig. 9b) parallel to the tilted strata, which indicate a pre-fold brittle geometry. Back-tilting the strata to the horizontal unfolded position reveals vertical conjugate strike-slip faults with horizontal striae, indicating a pre-fold NE–SW trend for the corresponding $σ_1$ stress axes. Our data also revealed normal faults in the steeply tilted strata at sites 17 and 61 (Fig. 10) that can be considered as the pre-fold reverse fault geometry (Fig. 10a). Some of the data related to this geometry were observed as high-angle reverse faults in steep fold flanks (Figs 10a and b). Back-tilting the strata to the horizontal unfolded position reconstructs the pre-fold configuration
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Data separation for syn- and post-fold faulting as observed at sites 01 and 57. (a) Horizontal sinistral striae, S, associated with calcite steps and pressure solution needles, indicating a strike-slip fault that is not tilted with respect to the surrounding folded structures. (b) Reverse slips parallel to the bedding plane, B, indicating syn-fold faulting. (c) Tension cracks associated with the bedding-parallel reverse slips, indicating sense of shear. LFA, local fold axis. Note that the syn-fold reverse fault slip data show a NE–SW stress trend perpendicular to the LFA, while the post-fold strike-slip data show an approximate N–S stress trend. Other symbols as in Figs 4 and 5.</figcaption></figure>

of the reverse fault systems that strike NW–SE. The corresponding $σ_1$ stress axes are NE–SW similar to that of the pre-fold strike-slip faults. Data separation at site 61 also revealed a post-fold strike-slip fault system with a corresponding N–S trend for the $σ_1$ stress axis similar to the cases already described for sites 01 and 57 (see Fig. 8).

A small part of the collected fault slip data reflects a NW–SE direction of compression, almost parallel to the structural trend of the belt (Fig. 11). Because of the specific orientation of this stress direction, it is difficult to establish a clear relative geometry between the related brittle structures and folded strata. As for instance, at site 04, the fault slip data can either be interpreted as reverse faults in a post-fold brittle geometry or be considered as right-lateral strike-slip faults in a pre-fold brittle geometry (Fig. 11, left-hand side). The later interpretation is preferably adopted for these fault slip data because of a higher level of consistency observed between the data when they are back-tilted to the pre-fold attitude, suggesting

subvertical E–W strike-slip faults with horizontal striae. In contrast, the comparison between bedding and fault attitudes reveals a post-fold brittle geometry for the strike-slip faults at sites 33 and 56 (Fig. 11, right-hand side). Such a NW–SE stress trend has already been reported from the Fars Province of E-Zagros as a more likely post-fold brittle tectonic event (Lacombe *et al.* 2006; Navabpour *et al.* 2007a). Nevertheless, the regional significance of such a belt-parallel compressive stress trend remains a matter of debate.

### 4.1.2 Normal faults and tension cracks

Reconstruction of the normal faults and tension cracks that can be related to the middle-late Cenozoic compressive deformation of the region has been carried out considering their identified relative chronologies with respect to the reverse and strike-slip faults, their relative geometries in relation to the folds, and the stratigraphic age of the affected rocks. For instance, the fault slip data at site
<figure><img src="image_10.png" /><figcaption>**Figure 9.** Data separation for pre- and post-fold faulting as observed at sites 22 and 38. (a) A reverse fault, F, cut through the previously folded strata, indicating post-fold faulting. (b) Tilted dextral striae associated with calcite steps, indicating a pre-fold strike-slip fault that is tilted by folding. Note that the pre-fold strike-slip data show an approximate NE–SW stress trend perpendicular to the LFA, while the post-fold reverse fault slip data show an approximate N–S stress trend. Symbols as in Figs 4, 5 and 8.</figcaption></figure>

10 reveals successive fault striae that show reactivation of a conjugate strike-slip fault system as younger normal faults (Fig. 12). The stress tensor analysis for the total set of normal faults resulted in an unstable solution with a low ratio of stress magnitude differences ($Φ = 0.09$; i.e. $σ_2 = σ_3$), which indicates two possible directions for the $σ_3$ stress axis. Further data separation and refining the analysed data into more stable solutions revealed two different normal fault subsets, indicating two different N–S and E–W directions of extension. These directions of extension are, respectively, parallel and perpendicular to the N–S direction of compression that is obtained from the conjugate strike-slip faults and local folded strata at the same site.

An example of anticlinal normal faults that strike perpendicular to the direction of compression was observed at site 36 (Fig. 13). Data separation at this site revealed two different strike-slip and normal fault subsets that show a local NE–SW extension parallel to the direction of compression. At sites 28 and 31, the Aquitanian limestones are affected by anticlinal fold-parallel normal faults (Fig. 13a; see also Table 3), indicating a post-Aquitanian age for this brittle deformation. An example of transverse normal faults that strike parallel to the direction of compression was obtained from the fault slip data of site 45 (Fig. 13). Data separation at this site revealed two different strike-slip and normal fault subsets that show a local NW–SE lateral extension perpendicular to the direction of compression.
<figure><img src="image_11.png" /><figcaption>**Figure 10.** Reconstruction of pre-fold reverse faulting as observed at sites 17 and 61. (a) Normal faults in steeply tilted strata that show pre-fold reverse faulting after back-tilting. (b) A high-angle reverse fault in steeply tilted strata that show pre-fold reverse faulting after back-tilting. Note that the pre-fold reverse fault slip data show an approximate NE–SW stress trend perpendicular to the LFA, while the post-fold strike-slip data show a N–S stress trend. Symbols as in Figs 4, 5 and 8.</figcaption></figure>

At sites 55 and 62, the limestones and pyroclastic rocks of Eocene age are affected by the fold-perpendicular transverse normal faults (Fig. 13b; see also Table 3), indicating a post-Eocene age for this brittle deformation. At site 58 (Fig. 14a), the separated fault slip data show normal faults with a corresponding approximately E–W direction of extension, consistent with the stress field of the strike-slip faults that show a N–S direction of compression at the same site and other sites as well. The strikes of normal faults are almost N–S, oblique to the LFA, which suggest a post-fold brittle geometry and hence a post-fold stress field. Numerous vertical tension cracks were observed within the Pliocene Bakhtyari conglomerates that strike close to N–S (Fig. 14b). These brittle structures also indicate an approximately E–W trend for the $\sigma_3$ stress axis, suggesting an approximately N–S trend for the corresponding $\sigma_1$ stress axis.

## 4.2 Main stress trends

In order to realize the regional averages of the main stress and folding trends that governed the late Cenozoic history of compression in the HZB of Kermanshah, we provided statistical rose distributions of the reconstructed stress axes taken as a whole in different classified diagrams (Fig. 15). Folded structures often obtain the direction of maximum shortening. In a regional scale, the orthogonal direction to the fold axes indicates the first-order stress/shortening trend (FST), assuming that the stress and strain were coaxial. Numerous measurements of the folded strata throughout the studied area helped us to calculate an average N038° trend for the FST. This direction shows a more stable trend with respect to the various $\sigma_1$ stress axes of compressional and strike-slip stress regimes that were
<figure><img src="image_12.png" /><figcaption>**Figure 11.** Stereoplots showing the NW–SE stress trend as observed at sites 04, 33 and 56. At site 04, the fault slip data can be interpreted either as post-fold reverse faults or as pre-fold strike-slip faults, in both cases indicating a NW–SE stress trend. Note to the clear post-fold geometry of the fault slip data with respect to the bedding attitudes at sites 33 and 56. Symbols as in Figs 4 and 5.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Figure 12.** Example of fault slip reactivation at site 10. Successive striae at this site show reactivation of conjugate strike-slip faults as normal faults with a low ratio of Φ. This situation resolves two normal fault subsets with two different N–S and E–W extension trends as a function of compression and folding. Symbols as in Figs 4 and 5.</figcaption></figure>

obtained from the reverse and strike-slip fault slip data (Fig. 15, first row).

Data separation based on the relative chronology/geometry be-
tween the brittle structures and folded strata helped us to recon-
struct the pre-, syn- and post-fold attitudes of the corresponding
stress trends (Fig. 15, second row). According to our data, most of
the pre-fold faulting has occurred under a strike-slip stress regime
(see Table 3) and about 90 per cent of the back-tilted fault slip data
indicate an average N051° trend for the σ₁ stress axis. The syn-
fold brittle deformation has mainly occurred under a compressional
stress regime (see Table 3) and about 94 per cent of the correspond-
ing fault slip data shows an average N031° trend for the σ₁ stress

axis. This stress trend is very close to the FST, confirming that the
stress and strain were coaxial during the folding process. Therefore,
most of the folding and corresponding brittle structures were re-
sulted from a N031°–N038° compressive stress trend. This regular
pattern also suggests that no considerable horizontal block rotation
have taken place during the folding process. For the post-fold defor-
mation, the analysed fault slip data show that faulting has occurred
under both the strike-slip and compressional stress regimes (see
Table 3) and about 91 per cent of the corresponding brittle struc-
tures indicate an average N005° trend for the σ₁ stress axis.

In addition, the normal fault slip data and tension cracks of the ex-
tensional stress regimes revealed two different trends of extension,
<figure><img src="image_14.png" /><figcaption>**Figure 13.** Data separation and normal fault geometry in relation to folding and conjugate strike-slip faults at different sites. (a) Aquitanian limestones affected by anticlinal normal faults, indicating a local NE–SW extension. (b) Eocene volcanic rocks affected by transverse normal faults, indicating a NW–SE lateral extension. Note to the relationships between the directions of extension, compression and attitudes of the LFA. Symbols as in Figs 4, 5 and 8.</figcaption></figure>

characterized by the belt-perpendicular anticlinal extension and the belt-parallel dilation (Fig. 15, third row). The fault slip data related to the anticlinal extension reveals an average N034° trend for the $σ_3$ stress axis throughout the studied area, evidencing a syn-fold extensional stress trend almost parallel to both the FST and the syn-fold $σ_1$ stress axis. The fault slip data related to the belt dilation show a general direction of extension ranging from NW–SE to E–W with an average N112° trend for the $σ_3$ stress axis. This feature suggests a firm perpendicularity between the belt dilation and the general direction of compression that ranges from NE–SW to N–S with an average N024° trend of $σ_1$ stress axis.

## 4.3 Relative chronology and dating the events

In order to place an age estimate on the relative chronology of the main stress trends, particular attention was paid to the stratigraphic ages of the affected rocks and unconformities revealed by geological mapping (GSI 1978, 1996, 1997, 1999a,b, 2006). However, tectonic events cannot be accurately dated by means of stratigraphic observations, especially for the Cenozoic events in the HZB where Mesozoic outcrops are widespread. Recent studies based on the field observations of unconformities and balanced cross sections suggested that the folding was initiated in the early Miocene times and propagated southwestward (Sherkati *et al.* 2005), the main stage

of which took place in SE-HZB in the middle-late Miocene times (Molinaro *et al.* 2005a; see Fig. 3). A magneto-stratigraphic study by Homke *et al.* (2004) also indicated that the main stage of folding across the ZSFB of Lurestan was ceased in the late Pliocene times. The presence of the angular unconformity between the folded Aquitanian limestones and the Cretaceous rocks to the east of Bisitoun (Fig. 16, right-hand side) indicates that deformation and folding had started in the HZB of Kermanshah prior to the early Miocene times, related either to the ophiolite obduction or to the onset of collision.

Because of the lack of data on the ophiolite units, we cannot characterize the brittle deformation and corresponding stress field that possibly occurred during the late Cretaceous southwestward ophiolite obduction over the southern Neo-Tethyan passive margin. In any case, the crosscutting relationship between the fault slip data as well as the relative chronology/geometry between brittle structures and folds (see Figs 8–10) helped us to establish binary chronologies between the different corresponding stress fields. The chronological matrix analysis (Angelier 1991) of these binary data allows constraining the best possible succession of the different tectonic events by concentrating the number of binary chronologies in the upper right half of the matrix (Fig. 16, left-hand side; more detail in Navabpour *et al.* 2007a). This succession is obtained based on 42 relative chronologies between five events (including the main folding event, FL) identified at 23 sites. The best possible succession accounts for 39 (93 per cent) consistent binary chronologies
<figure><img src="image_15.png" /><figcaption>**Figure 14.** Post-fold geometries of normal faults and tension cracks. (a) Post-fold geometry of normal faults in relation to bedding attitudes and LFA, compatible with the strike-slip data, indicating an E–W extension under a N–S compression. (b) Vertical tension cracks, T, in the Pliocene conglomerates at sites 53 and 59, suggesting a N–S compression. Circles of the rose diagram show percentage of the tension cracks with 5 per cent intervals. Other symbols as in Figs 4, 5 and 8.</figcaption></figure>

and includes only 3 (7 per cent) inconsistent ones. This succession indicates a general anticlockwise change in the direction of compression from NE–SW to N–S, evolving from pre- and syn-fold to post-fold faulting (see also Fig. 15, second row). Although, the relative chronology of the NW–SE stress trend is not reliably constrained with respect to the N–S stress trend, the matrix solution shows that the NW–SE stress trend can be interpreted as a post-fold event.

According to the stratigraphic ages of the affected rocks, we could not trace the pre-fold NE–SW stress trend in the rocks younger than the Aquitanian limestones, while the post-fold N–S stress trend affected the Pliocene conglomerates of the Bakhtyari Formation to the southeast of Harsin (see Table 3). The folding and related syn-fold brittle structures indicate an approximate NNE–SSW trend of compression. Therefore, we conclude that the deformation had initiated in this region under the NE–SW stress trend prior to the early Miocene times and then continued during the Miocene epoch under the NNE–SSW compression. The N–S stress trend should have occurred in the post-Pliocene times, consistent with the recent stress trend obtained from the earthquake focal mechanisms along the MRF and reported elsewhere in the HZB (Authemayou *et al.* 2006; Navabpour *et al.* 2007a). Although the general aspects of the stress re-orientation resemble those identified in the HZB of interior Fars (see Navabpour *et al.* 2007a), the distinct NNE–SSW post-fold direction of compression in a strike-slip stress regime that was thought to have existed during the Pliocene epoch in that region could not be observed in the HZB of Kermanshah.

# 5 DISCUSSION

The brittle tectonic reconstruction presented in the previous sections revealed different fault systems with corresponding stress fields that developed during the mountain building process in the HZB of Kermanshah, with special reference to the folding process.

The interaction between folding and faulting, which resulted in the present-day structural complexity, allowed easier understanding of the tectonic relative chronology because the brittle events could be dated with reference to the folded strata. Similar aspects of such structural relationships have been recently discussed in the HZB of interior Fars (Navabpour *et al.* 2007a). In this section, we discuss the results of our stress tensor inversion in relation to the regional tectonic framework and the structural evolution of W-Zagros in terms of its deformation partitioning style under the oblique plate convergence.

## 5.1 Stress reorientation and change in plate convergence

According to our previous work in the HZB of interior Fars in the E-Zagros (Navabpour *et al.* 2007a), the palaeostress reconstruction brings new constraints about the succession of the late Cenozoic compressive tectonic events that have affected the HZB during and after the collision. Despite the common characteristics of brittle tectonic evolution and stress reorientation, some differences exist between the HZB of interior Fars and the HZB of Kermanshah. These differences are discussed hereinafter, with special emphasis on the deformation partitioning style of the W-Zagros and its evolution through time. A rather consistent chronological picture emerges from the tectonic analysis presented in this study, indicating that a NE stress event occurred first with a N051° trend of $σ_1$ axis and prevailed over the area of unfolded strata in the Oligocene–Early Miocene times (Fig. 17a). It has already been shown that the MZT was the place of southwestward thrusting and tectonic transport of the SSMZ over the HZB (e.g. Braud 1971) at least since the early Miocene times (Gidon *et al.* 1974b). This thrust structure characterizes an average N055° trend of the southwestward reverse slip vectors in the Kermanshah region (GSI 1997; Agard *et al.* 2005), which is consistent with our pre-fold direction of compression. Subsequently, the syn-fold reverse faulting that characterizes
<figure><img src="image_16.png" /><figcaption>**Figure 15.** Statistical presentation of different stress trends in relation to the folded strata. Top panel: rose distribution of the calculated $\sigma_1$ stress axes in compressional and strike-slip stress regimes in relation to the first-order stress/shortening trend, FST, which is obtained from the folded strata. Middle panel: rose distribution of $\sigma_1$ stress axes, for which the relative chronology/geometry is reconstructed with respect to the folded strata. Note to the different pre- and post-fold orientation of $\sigma_1$ stress axes with respect to the FST. Pair of black arrows indicates direction of compression, with average numerical value at bottom right-hand corner of each diagram. Bottom panel: rose distribution of $\sigma_3$ stress axes in local extensional stress regimes in relation to both the FST and the direction of compression. Number of data, *n*, is shown at top left-hand corner of each diagram. Circles of the rose diagrams show percentage of the stress axes with 5 per cent intervals. See Table 3 for more detail. Other symbols as in Figs 5 and 7.</figcaption></figure>

<figure><img src="image_17.png" /><figcaption>**Figure 16.** Relative chronology and dating of events. Left-hand side: chronological matrix analysis (based on Angelier 1991). Rows and columns refer to the first and second chronologies, respectively. NE, NNE, NW and N-S refer to the main stress trends with the same $\sigma_1$ direction. FL refers to folding. The white arrow with question mark indicates the uncertainty in relative position of the NW stress with respect to the N-S one. Right-hand side: angular unconformity (dashed line) between the Cretaceous strata, Kr, and the folded Aquitanian limestones, Aq, in the east of Bisitoun.</figcaption></figure>
<figure><img src="image_18.png" /><figcaption>**Figure 17.** Brittle tectonic evolution of the HZB within the regional tectonic framework since the Oligocene epoch, indicating the palaeostress trends (a and b) together with the recent stress and strain trends within the sedimentary cover (c) and the basement (d). The reader is referred to the text for numerical values of stress and strain trends. (1) Arabian motion direction with respect to Eurasia from McQuarrie *et al.* (2003); (2) Arabia–Central Iran GPS convergence direction from Vernant *et al.* (2004); (3) palaeostress trend reconstructed from brittle tectonic analysis (this study); (4) regional extension inferred from normal faults and tension cracks (this study); (5) seismic stress axis issued from inversion of earthquake double couple focal mechanisms from Zamani (2007) and this study; (6) palaeostress trend based on brittle tectonic analysis from Navabpour *et al.* (2007a); (7 and 8) seismic and geodetic strain axes, respectively, from Masson *et al.* (2005); (9) tectonic transport direction from Agard *et al.* (2005) and (10) first-order stress/shortening trajectories. Abbreviations as in Fig. 1.</figcaption></figure>

a N031°–N038° trend for the corresponding $σ_1$ stress axis followed the pre-fold brittle deformation and prevailed during the main stage of folding within the sedimentary cover in the Miocene epoch (Fig. 17b), mainly under a compressional stress regime (i.e. vertical $σ_3$). The main stage of folding process then continued and migrated progressively towards the southwest (i.e. the ZSFB of Lurestan) during the late Miocene–Pliocene times (see Hessami *et al.* 2001; Sherkati *et al.* 2005; Molinaro *et al.* 2005a). The brittle deformation has then continued in the innermost zone (i.e. the HZB of Kermanshah) mainly under a strike-slip stress regime (i.e. vertical $σ_2$), characterizing a N005° trend for the corresponding $σ_1$ stress axis during the Pliocene–Recent times (Fig. 17c).

Although recent deformation resulted in gently tilted Pliocene conglomerates, most of the fault slip data of the HZB related to the

Pliocene–Recent times shows the post-fold brittle geometries. This situation confirms that most of the fold-and-thrust structure of the HZB has already been achieved under the Miocene compressional stress regime. Accordingly, the last activity of the MZT is thought to have occurred in the early Pliocene times, as the upper Pliocene deposits have sealed the corresponding structures (Gidon *et al.* 1974a,b). Following this deformation, the MRF is thought to have been initiated during the Pliocene epoch under a N–S direction of compression (Talebian & Jackson 2002; Authemayou *et al.* 2006). The results of our brittle tectonic analysis and stress tensor inversion suggest that the stress reorientation could have been a mechanical factor associated with triggering the right-lateral slip along the MRF. We cannot attribute the change in stress direction to a possible block rotation, because of the parallelism between the syn-fold stress trend
and the FST (see Fig. 15). In addition, the constant trend of folds and consistency of each stress field throughout the studied area suggests that no differential block rotations have occurred. Furthermore, this change in stress direction resembles that reconstructed in the HZB of interior Fars (Navabpour et al. 2007a) and concurs to reveal a general stress reorientation from NE–SW to N–S trend throughout the HZB, most of which occurred during the Pliocene epoch.

Because seismological and geodetic data are absent for the earlier deformation of the Zagros collision, except for the present-day, an evaluation of the stress reorientation in the sedimentary cover may thus be obtained by the kinematic reconstruction of relative Arabia–Eurasia plate convergence since the Oligocene times, as already discussed for the HZB of interior Fars (Navabpour et al. 2007a). According to this reconstruction (McQuarrie et al. 2003), the path of Arabia with respect to Eurasia underwent a change in direction from N030° (56–33 Ma) to N025° (33–19 Ma), N009° (19–10 Ma) and N005° (last 10 Ma) in average regional values. For the present-day region of W-Zagros, the corresponding trends of plate convergence were N035°, N025°, N002° and N358°, respectively. Despite uncertainties in kinematic reconstructions, these results indicate a significant anticlockwise change (~37°) in the path of Arabia with respect to Eurasia since the early Oligocene times, most of which (~23°) has occurred in about 19 Ma (Figs 17a and b). We have already discussed a possible rough relationship between the gradual change of both the stress orientation and the plate convergence trend in the E-Zagros since the late Oligocene times (Navabpour et al. 2007a). In the HZB of W-Zagros, however, considering the timing of acquisition of the final N–S direction for both the convergence and stress trends (compare Figs 17b and c), a significant time interval reveals between the change in plate convergence trend in ~19 Ma and the stress reorientation in ~5 Ma that is somewhat different from the E-Zagros. This time interval was contemporaneous with the Miocene thin-skinned deformation of the sedimentary cover, as argued by Sherkati et al. (2005) and Molinaro et al. (2005a), during which the folding and reverse faulting has occurred in the HZB under an overall NE–SW compressive stress trend within the frame of oblique plate convergence.

The time of stress reorientation could thus correspond to the Pliocene age proposed for the transition from thin-skinned to thick-skinned deformation and initiation of the basement reverse fault activities recognized in the E-Zagros (see Molinaro et al. 2005a). Our brittle tectonic analysis also revealed a significant change in stress state throughout the HZB from the Miocene–Pliocene compression to the Pliocene–Recent strike-slip system (Navabpour et al. 2007a; this study). Such a change in stress state has already been reported from southeasternmost HZB (Regard et al. 2004), central HZB (Authemayou et al. 2006) and eastern Anatolia (Kocyigit et al. 2001), being in all cases attributed to the Pliocene epoch coeval with the reorganization of the Arabia–Eurasia collision at 5 ± 2 Ma (Westaway 1994; Allen et al. 2004). Molinaro et al. (2005b) argued that a slab break-off process and the subsequent thermal uplift might have induced the major regional change from thin-to thick-skinned deformation. Faccenna et al. (2006) have shown that a slab break-off process might result in collisional reorganization and change in kinematic regime in Turkey, through an increase in mechanical coupling between the two plates (Authemayou et al. 2006). Regardless of such general geodynamic and kinematic influences, we consider that this plate reorganization may account for both the transition from thin- to thick-skinned collision and the stress reorientation from the syn-fold NE–SW belt-perpendicular trend to the post-fold N–S convergence-parallel one close to the collisional suture zone.

## 5.2 Recent stress fields and cover vs. basement deformation

As is mentioned elsewhere, the present-day deformation style of the W-Zagros is characterized by the deformation partitioning, with the right-lateral strike-slip motion along the MRF and the belt-perpendicular shortening across the ZSFB of Lurestan (see Fig. 4; Berberian 1995; Talebian & Jackson 2002, 2004). With the MRF, the HZF is the only fault that shows significant present-day activity in the HZB; microseismic surveys (Yamini-Fard 2003; Yamini-Fard et al. 2006) suggested active partitioning between these two faults. Based on our post-fold fault slip data, we could characterize that both the strike-slip and reverse faulting highlight the deformation of the sedimentary cover under the recent N–S stress trend across the HZB of Kermanshah (see Table 3). This deformational style may represent the HZB of Kermanshah as a transitional zone between the strike-slip component of the MRF and the reverse slips of the ZSFB. Although some arguments suggest complete partitioning in W-Zagros based on consideration of seismic slip vectors (Talebian & Jackson 2004), recent rheological lithospheric modelling suggested that the MRF can only accommodate ~25 per cent of the total strike-slip motion between the Arabian and Iranian plates (Vernant & Chéry 2006), the remaining ~75 per cent of the strike-slip motion being accommodated elsewhere across the belt.

Our results of the seismic stress tensor inversion highlighted different stress states across the W-Zagros, characterizing a N176° trend of $σ_1$ stress axis along the MRF in a strike-slip regime and a N035° trend of $σ_1$ stress axis across the ZSFB in a compressional regime (Figs 4 and 17d). These directions of seismic stress tensors are consistent with directions of compressive seismic strain axes of Masson et al. (2005), that is, N001° in the MRF vicinity and N039° in the ZSFB vicinity (Fig. 17d), as calculated based on the moment tensor summation of Kostrov (1974). Not to mention what is stress and strain (e.g. Marrett & Peacock 1999; Peacock & Marrett 2000), we refer to both our stress axes and Masson's strain axes, which are calculated based on the earthquake source parameters, as the compressive seismic axes hereinafter, noting that the significance of these axes in terms of stress versus strain can be a matter of debate. The seismic behaviour of the MRF reflects the relative motion between the basements of the Arabian and Iranian plates (Talebian & Jackson 2004), knowing that slip along large interplate structures also provides insights in terms of plate kinematics (e.g. Yamini-Fard et al. 2007). Therefore, the corresponding seismic compressive axis would be consistent with the plate convergence vector (i.e. N357° in the Central Iran reference frame; see Vernant et al. 2004) (Fig. 17d). The seismic behaviour of the ZSFB, on the other hand, indicates internal deformation of the Arabian plate along favourably oriented reverse faults of the basement (Gillard & Wyss 1995; Berberian 1995; Talebian & Jackson 2004). Thus, the corresponding seismic compressive axis would be perpendicular to the fault strikes. From a general point of view, such a relationship between directions of compression and plate motion is discussed elsewhere (Tikoff & Wojtal 1999).

Despite the widespread seismicity, it has already been argued that most of the deformation of the Zagros is accommodating in an aseismic way (Jackson et al. 1995). The analyses of earthquake moment tensor and GPS data suggested a seismic versus geodetic strain rate less than 5 per cent in the Zagros Mountains (Masson et al. 2005). The deformation may also be different at the surface of the sedimentary cover where it is decoupled from the basement through the main basal Infracambrian salt detachment (Colmans-Sadd 1978; Berberian 1995) and other intermediate multilevel
detachment layers (Blanc et al. 2003; Sherkati & Letouzey 2004; Bosold et al. 2005; Sepehr et al. 2006). The estimates of balanced cross sections indicated that the basement faults contribute to about 20 per cent of the total shortening observed in the sedimentary cover (Sherkati et al. 2005; Molinaro et al. 2005a). The concentration of geodetic shortening across the MFF in the Fars Arc and Dezful Embayment confirms decoupling between deformation of the sedimentary cover and the basement (Walpersdorf et al. 2006), as compared to the more distributed seismicity across the ZSFB.

The deformation in the sedimentary cover of W-Zagros is characterized by a N027°–N031° trends of maximum shortening (Fig. 17c), derived from the geodetic strain-rate tensors of Masson et al. (2005). More detail work of Walpersdorf et al. (2006) for the GPS sites located southwest of the MRF indicate NW-trending motion vectors with similar velocities in the Central Iran reference frame and SW-trending motion vectors with southwestward-decreasing velocities in the Arabia reference frame. This GPS vector pattern indicates deformation partitioning of the sedimentary cover, with a right-lateral interplate motion along the MRF and a belt-perpendicular intra-plate shortening across the ZSFB, similar to that of the basement documented by the seismic slip vectors in the W-Zagros (see Talebian & Jackson 2004). Such a deformation consistency between cover and basement may be explained by the absence of the thick Infracambrian salt detachment in the Lurestan and Kermanshah regions (Sharland et al. 2001), where the mechanical characteristics of the equivalent sedimentary facies may have been insufficient to behave as a tectonically active detachment layer (Talbot & Alavi 1996). Our recent N005° compressive stress trend obtained from the post-fold fault slip data of the sedimentary cover in the HZB is approximately parallel to the seismic compressive axes of the basement along the MRF (i.e. N176° and N001°; Figs 17c and d). This situation suggests that at least in the HZB of W-Zagros the recent compressive stress orientations are similar in both the sedimentary cover and the basement. However, a significant difference of 22°–26° can be observed between the compressive stress and strain axes within the sedimentary cover (Fig. 17c), as compared with the N027°–N031° geodetic shortening axes of Masson et al. (2005).

It has already been discussed that the strike-slip continental deformation partitioning depends on the kinematic relationship between the angle of plate convergence vector (α) and instantaneous strain axis of the contraction component (θ) with respect to the strike of the deformation zone (Fig. 18; Teyssier et al. 1995). As pointed out elsewhere, instantaneous strain axis can be correlated with stress axis through the assumption of steady-state deformation, because coseismic slip usually reflects small displacement. The Zagros structural grain trends N130° while the plate convergence vector between Arabia and Central Iran trends approximately N–S (see Vernant et al. 2004), so that α = 50°. Adopting the N035°–N039° trends of the seismic compressive axes across the ZSFB as the contraction component indicates that θ = 85°–89°. Therefore, a tentative evaluation based on the kinematic relationships of Teyssier et al. (1995) suggests a partitioning of 79–97 per cent within the basement (Fig. 18). In contrast, adopting the N027°–N031° geodetic axes of shortening across the W-Zagros as the contraction component implies θ = 77°–81°, which indicates a partitioning of 42–61 per cent within the sedimentary cover. Such an evaluation, if correct, may highlight the difference between seismic deformation of the basement and geodetic deformation of the sedimentary cover that occur with different strain rates. In the E-Zagros, however, the stress tensor inversion of fault slip data and earthquake focal mechanisms revealed recent to present-day stress axes that trend

N002° and N009°, respectively within the sedimentary cover of the HZB of interior Fars (Navabpour et al. 2007a) and in the basement of the ZSFB of Fars Arc (Zamani 2007). A comparison between these stress axes and the geodetic and seismic strain axes of Masson et al. (2005), that is, N017° and N012°, respectively (Figs 17c and d), suggests that the stress and strain trends are similar and coaxial throughout the sedimentary cover and the basement. Accordingly, considering the N009°–N012° trends of the seismic compressive axes and the N017° trend of the geodetic shortening axis as the contraction components indicates the angles of 59°–62° and 67° for θ, respectively for the basement and the sedimentary cover of the E-Zagros. These values of θ imply that no strike-slip partitioning occurs at the rear of the Fars region in both the basement and the sedimentary cover (Fig. 18).

In addition to the differences of partitioning between basement and sedimentary cover and between W- and E-Zagros mentioned above, another distinguishable difference is apparently revealed between the cover and basement deformation throughout the Zagros Mountains. This difference is characterized by the distributed local extensional brittle structures (Authemayou et al. 2006; Lacombe et al. 2006; Navabpour et al. 2007a; this study), indicating a belt-parallel dilation of the sedimentary cover, as already discussed elsewhere in other orogenic belts (Hancock & Bevan 1987). Although some of these structures indicate GPS-based active extension (Hesami et al. 2006) with coseismic fault slips (Bachmanov et al. 2004), however, no similar seismic event has yet been evidenced within the basement.

## 5.3 Plate kinematics and deformation partitioning evolution

According to the recent reconstructions of folded structures and balanced cross sections, a late Cenozoic belt-perpendicular shortening of about 57–85 km was adopted across the W-Zagros (McQuarrie 2004). In addition, about 50–70 km of right-lateral offset is considered along the MRF, based on the geological markers of rock units (Gidon et al. 1974b) and a possible drainage pattern restoration (Talebian & Jackson 2002). Considering a Pliocene–Recent age for the Zagros folding (Falcon 1969, 1974) and attributing the

<figure><img src="image_19.png" /></figure>

**Figure 18.** Kinematic relationships between the angles of plate convergence vector (α) and instantaneous strain axis of the contraction component (θ) with respect to the strike of the deformation zone, characterizing the portion of strike-slip partitioning (from Teyssier et al. 1995), with configurations of W- and E-Zagros added. In W-Zagros, partitioning is about 42–61 per cent, based on the geodetic shortening axis of the sedimentary cover, and about 79–97 per cent, based on the seismic compressive axes within the basement. In E-Zagros no partitioning occurs.
same age to the MRF, Talebian & Jackson (2002) discussed that these deformations should have accommodated a major portion of the convergence between the Arabian and Eurasian plates. They thus proposed a N–S shortening rate of 14–23 mm yr⁻¹ across the W-Zagros and a cumulative slip rate of 10–17 mm yr⁻¹ along the MRF since 5–3 Ma. Bachmanov *et al.* (2004) has also obtained a cumulative slip rate of ~10 mm yr⁻¹ along the MRF, based on offset of a river valley incized into the surface of an intermountain depression probably post-glacial in age. However, the N–S shortening rate mentioned above is not consistent with the present N–S shortening rate of 4 ± 2 mm yr⁻¹ obtained from GPS data of Vernant *et al.* (2004) for the W-Zagros. The inconsistency between the cumulative slip rate of the MRF and the present GPS belt-parallel slip rate along this fault (i.e. 3 mm yr⁻¹; Vernant *et al.* 2004; Reilinger *et al.* 2006) is also a matter of debate (Authemayou *et al.* 2006; Walpersdorf *et al.* 2006).

Agard *et al.* (2005) have shown that at least 50–70 km of convergence was accommodated only by thrusting of the SSMZ over the HZB through the MZT and associated thrusts after the onset of collision. The shortening rate related to this over-thrusting can thus be estimated about 4.2–5.8 mm yr⁻¹, taking into account the proposed period of activity of the MZT from the early Miocene until the early Pliocene times (Gidon *et al.* 1974b) that is about 12 Ma. In addition, as mentioned earlier, the recent studies on timing of the folding process indicate that the folding was initiated in the region of the HZB in the early Miocene times (Sherkati *et al.* 2005) and was ceased at the MFF in the late Pliocene times (Homke *et al.* 2004) (see Fig. 3). This indicates that the 57–85 km of shortening of McQuarrie (2004) reconstructed across the W-Zagros yields a rate of 4.8–7.1 mm yr⁻¹ during approximately the same period of activity of the MZT. This information, if correct, suggests a total belt-perpendicular cumulative shortening rate of at least 9–13 mm yr⁻¹ between the SSMZ and the Zagros Front across the W-Zagros with a similar belt-parallel component of dextral shear (probably to the north of the MZT) during the Miocene–Pliocene oblique convergence, not to consider the shortening portion of the vanished passive margin. McQuarrie *et al.* (2003) adopted an average margin-parallel dextral motion rate of 15 mm yr⁻¹ between the Neo-Tethyan subducting plate and the Iranian active margin (i.e. SSMZ) for the last 20 Ma during the collisional process, based on plate kinematic reconstructions. These relatively high rates of deformation suggest that during the earlier stages of collision a major portion of the Arabia–Eurasia oblique convergence was accommodating across the Arabian and Iranian margins, so that extrapolating the present-day low GPS slip rates to the past (as expressed by Walpersdorf *et al.* 2006) does not substantiate the Miocene deformation rates.

Partitioning mechanisms have been previously described within subduction zones associated with oblique convergence (Fitch 1972; McCaffrey 1992). Relative plate motion in such convergent margins is often partitioned into reverse dip-slip displacements across the subduction zone and trench-parallel strike-slip displacements in the overriding plate. To the north of the MZT, Mohajjel & Fergusson (2000) have shown a widespread domain of intensely folded and foliated metamorphic units of the SSMZ that are cut by thrusts, indicating a N052° southwestward tectonic vergence (Fig. 19a, left-hand side). They have also shown that this deformation is associated with mylonitic rocks that contain sub-horizontal N143° stretching lineations, indicating a major dextral shear component farther to the northeast (Fig. 19a, right-hand side). It was thus suggested that deformation partitioning has already occurred across the SSMZ of southwest Iranian active margin in a slightly oblique convergent setting (Mohajjel & Fergusson 2000). We could characterize that stress

<figure><img src="image_20.png" /><figcaption>**Figure 19.** Deformation partitioning across the SSMZ. (a) Stereoplots of reverse and strike-slip stretching lineations from Mohajjel & Fergusson (2000), with average equivalent fault slip data added as the grey great circles and slip vectors. (b) Right Dihedra stress solution of the stretching lineations, indicating two different N048° and N005° directions of $\sigma_1$ stress axis for the partitioned reverse and strike-slip deformations, respectively. Description of stereoplots as in Fig. 4.</figcaption></figure>

reconstruction of the stretching lineations of SSMZ indicates two different N048° and N005° trends for $\sigma_1$ stress axes, respectively related to the thrusting over the future MZT (Fig. 19b, left-hand side) and the dextral shear across the SSMZ (Fig. 19b, right-hand side). This stress pattern resembles that of the Java trench and Sumatra arc (see McCaffrey 2002) where the maximum shortening axes of geodetic strain indicate the deformation partitioning style of the overriding active plate margin in an oblique subduction zone. Agard *et al.* (2005) argued that the existence of large strike-parallel shear deformation early in the convergence history could account for the absence of exhumed high-pressure rocks along the future W-Zagros suture zone (contrary to Turkey and Oman), because of weak coupling between the two plates. These authors have shown that during the first stages of continental collision the SSMZ was emplaced over the HZB through the MZT with a N055° southwestward tectonic vergence. A high right-lateral shear deformation affecting the Eocene volcanic rocks of both the HZB and SSMZ along the future MRF has also been considered by Agard *et al.* (2005), because of the oblique convergence prior to the initiation of the MRF. The information mentioned above suggests a deformation partitioning that existed across the SSMZ before and during the collision (Fig. 20a).

Our palaeostress reconstruction confirms that a major part of the fold and thrust structure of the HZB have already been completed under an almost NE–SW direction of compression during the first stages of continental collision. We infer that during the Pliocene transition to the thick-skinned collision, the high belt-parallel shear component of the oblique plate convergence possibly triggered the right-lateral slip along the MRF as the suture line between the two Arabian and Iranian plates, which may partly account for the relatively high cumulative slip rate observed along this fault. The Pliocene stress reorientation across the HZB–MRF can be consistent with the initiation of the MRF and its recent N–S seismic stress trend, characterizing a southwestward migration of the deformation partitioning across the W-Zagros (Fig. 20b; Navabpour *et al.*
<figure><img src="image_21.png" /><figcaption>**Figure 20.** A schematic illustration of deformation partitioning in W-Iran. (a) Partitioning across the SSMZ of the Iranian active margin before and during the collision, based on the stretching lineations (grey arrows) from Mohajjel & Fergusson (2000) and Agard *et al.* (2005). (b) Partitioning across the W-Zagros of the former Arabian margin during and after the collision, based on the seismic slip vectors (black arrows) from the CMT data. Black and grey lines mark active and inactive structures, respectively. The MZT and MRF mark the suture between the Arabian and Iranian plates. A shift of partitioning from the SSMZ to the Zagros foreland is evidenced based on our stress trajectory determination (thick grey line). Big open arrows indicate average plate convergence trend from McQuarrie *et al.* (2003). Illustrations are not to scale. Abbreviations as in Fig. 1.</figcaption></figure>

2007b). The shift of partitioning towards the foreland of W-Zagros may thus provide good grounds to reconcile the contrasting stress fields of the Miocene–Pliocene and Pliocene–Recent times within the frame of the approximately constant N–S oblique plate convergence trend during the late Cenozoic era. The increase in coupling between the two plates after the Pliocene thick-skinned collision and slab break-off process may provide a mechanical explanation for the low present-day GPS rates of deformation observed in the W-Zagros (Authemayou *et al.* 2006). The rheological lithospheric modelling of the Zagros oblique convergence provided good mechanical constraints to understand the low GPS slip rates related to the present-day configuration of the deformation partitioning (Vernant & Chéry 2006). Nevertheless, as the inferred numerical stress direction obtained in this modelling (i.e. ~N022°–N025°) is different from those presented and discussed in our study, further analyses would be essential to simulate the present-day deformation partitioning of the W-Zagros within the different lithospheric layers. It is worth noting that a significant proportion of the present-day Arabia–Eurasia oblique convergence is accommodating across Azerbaijan and Caucasus (Vernant *et al.* 2004), which is thought to have been initiated during the Pliocene collisional reorganization (Allen *et al.* 2002, 2003) because of a deformation shift away from the thicker Iranian–Turkish plateau (Allen *et al.* 2004).

## 6 CONCLUSION

Our palaeostress reconstruction for the late Cenozoic oblique collision of the W-Zagros revealed a major change in stress state, evolving from a Miocene–Pliocene syn-folding compressional regime to a Pliocene–Recent post-folding strike-slip one in the HZB of Kermanshah between the MRF and ZSFB. This change in stress state accompanied a significant anticlockwise stress reorientation of the compressive stress from NE–SW to N–S trend, somewhat different from the more gradual stress reorientation already observed in the HZB of interior Fars. The Pliocene age of the change in stress regime and stress reorientation best fits with the initiation of the MRF during this period, which is documented elsewhere by field observation of geological features.

The results of stress tensor inversion of the earthquake focal mechanisms highlighted the partitioning in the W-Zagros with a N–S compression parallel to the plate convergence trend along the MRF and a NE–SW compression perpendicular to the fold axes across the ZSFB. By integrating the recent compressive stress axes obtained from fault slip and seismic analyses on the shortening strain axes obtained from geodetic calculations, the relationship between stress and deformation of the basement and sedimentary cover is evaluated. This evaluation revealed different trends for the seismic compressive axis and the geodetic shortening axis, which suggests different degrees of partitioning within the basement and the sedimentary cover. However, despite the differences observed between deformation of the basement and sedimentary cover, especially in terms of seismic stress state and geodetic strain field, the recent compressive stress directions seem to be similar throughout the basement and the sedimentary cover.

The brittle tectonic analysis described in this paper helped understanding the stress state related to different episodes of deformation distributed across the belt under an oblique plate convergence. The palaeostress results suggested that the stress reorientation has coincided with the transition from thin- to thick-skinned collision. This information reconstructed the situation of the HZB of W-Zagros within an evolving deformation partitioning that existed earlier in the tectonic history of oblique convergence on the Iranian active margin and shifted to the southwest on the Arabian margin during the continental collision. Considering the relatively constant N–S direction of plate convergence, the proposed shift of deformation partitioning towards the Zagros foreland, which is consistent with the Pliocene collisional plate reorganization, accounts for both the change in stress regime and the stress reorientation observed across the HZB of W-Zagros during the late Cenozoic oblique collision.

## ACKNOWLEDGMENTS

We thank the Geological Survey of Iran (GSI) for providing vehicles and financial support during the extensive fieldwork in Iran. Trips to Iran were supported by the Middle East Basin Evolution (MEBE) project. J. Braud, J. Omrani and A. Shahidi provided helpful points
of view in the field and H. McQuillan proofread the manuscript; their help is gratefully acknowledged. Valuable comments by the reviewers and M. Talebian resulted in significant improvement of the manuscript. The senior author thanks the MEBE project and Cultural Service of the French Embassy in Tehran for thesis grants, enabling him to study in the University of Nice – Sophia Antipolis.

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