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Author's personal copy

NW Iran-eastern Turkey present-day kinematics: Results from the Iranian permanentGPS network

Yahya Djamour a, Philippe Vernant b,!, Hamid Reza Nankali c, Farrokh Tavakoli ca Geomatics College, National Cartographic Center (NCC), Meraj Av., Azadi square, P.O. Box 13185-1684, Tehran, I. R. Iranb Lab. Geosiences Montpellier, University Montpellier 2-CNRS, 34095 Montpellier, Francec National Cartographic Center (NCC), Meraj Av., Azadi square, P.O. Box 13185-1684, Tehran, I. R. Iran

a b s t r a c ta r t i c l e i n f o

Article history:Received 25 February 2011Received in revised form 22 April 2011Accepted 24 April 2011Available online 25 May 2011

Editor: Dr. P. Shearer

Keywords:GPSNW IranWest Alborzactive tectonicNorth Tabriz fault

A network of continuous GPS stations has been installed in NW Iran since 2005 to complement the survey GPSnetwork already existing in the region. We present the 1999–2009 GPS-derived velocity !eld for this regionbased on the continuous and survey-mode observations. The results con!rm a right lateral slip of 7±1 mm/yrfor the North Tabriz fault, in agreement with previous studies. This rate is consistent with earthquakes ofmagnitude 7–7.3 and recurrence times of 250–300 yr. The higher spatial coverage of the new network showsthat deformation is localized in the vicinity of the Chalderan, south Gailatu-Siah Cheshmeh-Khoy fault and theNorth Tabriz fault. However the eastern end of the North Tabriz fault appears to cross Mount Bozgush ratherthan following its southern foothills. This new velocity !eld does not indicate the 8 mm/yr of NNE–SSWextension suggested earlier for the region, but rather shows lower extension of 1–2±1 mm/yr across theeastern segment of the North Tabriz fault and the Talesh. To the west, the Chalderan and the western NorthTabriz fault segment act like pure strike slip faults without signi!cant extension or compression. The densernetwork in the Rudbar earthquake region (Ms 7.3, 1992) shows no signi!cant motion across the fault,suggesting that the recurrence time of earthquakes like the Rudbar event must be very long. The lack ofsubstantial compressive strain and the sharp azimuth change of the velocity vectors in the transition zonefrom Arabia to Lesser Caucasus motion imply that processes other than “extrusion”, possibly related to oldsubduction or delamination, contribute to active deformation.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction and seismotectonic setting

The regions of northwestern Iran, eastern Turkey and Caucasus areone of the most intriguing regions of the Arabia–Eurasia collision. It isa pure intracontinental collision zone with the highest elevation inwestern Asia. This is in some points similar to the Pamir–Karakoramcollision zone. The Turkish–Iranian plateau (Fig. 1) has an averageelevation of approximately 2 km. It results from the accretion ofcontinental fragments to the southern Eurasian margin by the lateCretaceous or early Tertiary (e.g., Sengör, 1990). The onset of theintra-continental tectonics is thought to have occurred at about 12 Ma(e.g., McQuarrie et al., 2003). The Arabia–Eurasia direction ofconvergence is due north in this region with a present day rate ofapproximately 17 mm/yr (e.g., Reilinger et al., 2006), consistent withaverages of over 11 Ma (McQuarrie et al., 2003). Seismic activityalso widespread in the area remains localized on the main mappedfaults (Fig. 1), and volcanic activity has been ongoing since 6–8 Ma(Innocenti et al., 1976; Pearce et al., 1990). This region is known for a

spatial separation of subparallel thrusts and strike-slip faults. Thispartitioning is shown by the focal mechanisms and slip vectors(Jackson, 1992) or the GPS velocity !eld (Reilinger et al., 2006). TheCaucasus accommodates NE–SW shortening perpendicular to thestrike of the belt while the remainder is taken up on strike slipsfaults in the Turkish–Iranian plateau (e.g., North Tabriz and Chalderanfaults, Fig. 1). This pattern is surprising since the strike slip fault isusually on the overriding block of the thrust fault, as it is for examplefor the Main Recent fault in the Zagros (Talebian and Jackson, 2002).Numerical modeling studies (Vernant and Chery, 2006) suggest thatArabia–Eurasia collision induced forces alone cannot explain thisunexpected present day velocity !eld, and that other forces (such asfor example slab pull beneath the Caucasus) are needed to explain thekinematic of the region.

Earlier GPS studies (Masson et al., 2006; Masson et al., 2007;Nilforoushan et al., 2003; Vernant et al., 2004) suggest that the Tabrizfault (Fig. 1) accommodates about 8 mm/yr of right lateral strikeslip motion, a rate that seems to be in agreement with the historicalseismicity of the region (Berberian and Yeats, 1999). Masson et al.(2006) further suggest that the Talesh is experiencing a NNE–SSWextension at a rate of 8 mm/yr. These observations are consistent withthe results of (Reilinger et al., 2006) which suggest some extension

Earth and Planetary Science Letters 307 (2011) 27–34

! Corresponding author.E-mail address: [email protected] (P. Vernant).

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on the North Tabriz fault (NTF) based on block models, however theMasson et al. (2006) rate is 10 times higher. This extension issurprising since 400 km north of this region, the Main Caucasusthrust, subparallel to the NTF accommodates ~10 mm/yr shortening.Moreover, E–W extension is noted by Copley and Jackson (2006)between the Turkish–Iranian plateau and the Central Iranian block onthe Serow normal faults (Fig. 1).

To the east of our study area, the south Caspian basin is a relativelya seismic block, expected to be relatively rigid (Priestley et al., 1994),and covered by one of the thickest sedimentary basins anywherein the world (Brunet et al., 2003). The basement of this basin maysubduct below the Eurasian margin along the central Caspian Seaas attested by earthquake focal mechanisms (Jackson et al., 2002).The kinematics of the south Caspian block are still unclear, but bycomparing long term displacements along faults (Allen et al., 2003;Jackson et al., 2002) and recent offsets (Ritz et al., 2006) or GPS results(Djamour et al., 2010), it seems that the kinematics of the Caspian andsurrounding areas may have changed sometime around 1 Ma.

We present new GPS data to examine the relations between thefaulting and velocity !eld in northwest Iran and eastern Turkey.Several surveys of existing GPS sites (SGPS) were conducted since1999 in this region, and 8more SGPS sites have been established sincethe last velocity !eld was released (Masson et al., 2006). The newSGPS sites are in the Rudbar–Tarom region where one of the mostdestructive recent earthquakes in Iran occurred (Ms 7.3 in June 1990,e.g. Tatar and Hatzfeld, 2009; (Berberian and Walker, 2010)). In 2005and 2006 a network of 25 continuous GPS (CGPS) Ashtech iCGRS-Z12receivers were installed by the National Cartographic Center of Iran(Djamour et al., 2006). In this paper, we !rst process the data from

these 51 sites to determine the regional velocity !eld, and combineit with the published velocity !eld from surrounding countries(Reilinger et al., 2006). We analyze this velocity !eld to estimatethe deformation of the region and use block models to constrain sliprates on the main active faults.

2. Data processing and velocity !eld

The GPS network extends from the Caspian Sea to the Iran/Turkeyborder (Fig. 2). The average distance between the two sites is about20 km in the vicinity of the Tabriz fault and 30 to 70 km elsewhere.All continuous GPS benchmarks are setup on geodetically designedpillars deeply rooted in stable ground. Most campaign sites areanchored in bedrock, have a screw and small mast, forced centeringsetup, and have been surveyed for 48 h at least 3 times from 2000 to2008. All campaigns and continuous GPS sites were measured withAshtech Z12 and Trimble 4000SSI receivers equipped with choke-ringantennas. Not all the CGPS sites were installed at the same time, KKDYand BZGN recorded for 1.6 yr, BZGN, ZARI, GGSH, KHJE, KLBR, HSTD,KRMD have more than 2 yr, but less than 2.5 yr of recording, MMKN,VLDN, YKKZ, BRMN, ARDH recorded data for 2.5 to 3 yr and POLD,TASJ, NZSF, BSOF, AMND, TABZ, SKOH, MNDB, AHAR, TKCE and RSHThave at least 3 yr of continuous recording.

We used the GAMIT/GLOBK software package (Herring et al., 2009c;Herring et al., 2009b; Herring et al., 2009a) to compute the coordinatesand velocities of the sites using a three-step strategy (Dong et al., 1998;Feigl et al., 1993). GPS data of 14 IGS stations were introduced in theprocess to tie our local network to the ITRF reference frame. Our localquasi-observations were combined with the global quasi-observations

Fig. 1. Summary structural map adapted from Jackson et al. (2002), Karakhanian et al. (2004) and Copley and Jackson (2006). Seismicity is from National Earthquake InformationCenter catalog of crustal earthquakes with magnitudes from 3 to 6.5 (1976–2009). Black vectors are from Reilinger et al. (2006) and grey vectors are fromMasson et al. (2006). Boz,Mount Bozgush; Cha, Chalderan fault; CIB, Central Iranian block; EAF, East Anatolian fault; GSKF, Gailatu–Siah Cheshmeh–Khoy fault; NAF, North Anatolian fault; Nak, Nakhichevanfault; NTF, North Tabriz fault; Mak, Maku fault; Rud, Rudbar earthquake region; Ser, Serow normal fault system; TIP, Turkish–Iranian plateau. Inset: Arabia–Eurasia collision zone,and focal mechanisms from the Global CMT (http://www.globalcmt.org/) in black and Jackson et al. (2002) in grey.

28 Y. Djamour et al. / Earth and Planetary Science Letters 307 (2011) 27–34


provided by MIT (http://www-gpsg.mit.edu/~simon/gtgk/index.htm)from 1996 to day 245 of the year 2008. According to Reilinger et al.(2006), we account for the correlated errors in the time series bycalculating a unique noise model for each continuous GPS station.The algorithm used to model the data noise spectrum assumes thateach time series can be adequately modeled using a !rst-order GaussMarkov (FOGM) process noise (Gelb, 1974). The FOGM is estimatedfrom individual stations time series by averaging the residuals overincreasingly longer intervals that range from a minimum of 7 days toa maximum of 1/10th of the total time series span. For each intervalwe compute the Chi-square per degree of freedom. To the opposite of awhite process noise, the Chi-square/dof values of nonwhite noisespectra increase with increasing averaging time. The interval-averagedChi-square/dof values are then !t to the FOGM model where acorrelation time and long-term variance are estimated. This estimatedFOGMmodel is then used to predict the site velocity uncertainty basedon the time span of the time series. The equivalent random-walk noisevalues obtain for the NW Iran CGPS sites range from 0.3 mm/!yr to2.2 mm/!yr in horizontal and 0.8 mm/!yr to 4.1 mm/!yr in vertical.The upper value of 2.2 mm/!yr in horizontal is for one station (MNDB),and 1.2 mm/!yr for NZSF, all the others being less or equal to0.95 mm/!yr. Accordingly, we choose to use this value to estimate the

SGPS horizontal uncertainties. For the vertical randomwalk noise value,4 mm/!yr corresponds to the CGPS site RSHT, all the others are less orequal to 3 mm/!yr, therefore we use 3 mm/!yr. These values areconsistent with the values estimated by Djamour et al. (2010) for theAlborz range. Finally, velocities and their 1! con!dence uncertaintieswere estimated in ITRF 2005 and then the Eurasian reference framewas de!ned by minimizing the horizontal velocities of 23 IGS stationslocated in Europe and Central Asia (ARTU, BOR1, BRUS, GRAS, GRAZ,IRKT, JOZE, KOSG, KSTU, MADR, METS, NYAL, ONSA, POTS, TIXI, TOUL,TROM, VILL, WTZR, YAKT, ZECK, ZIMM, ZWEN). The WRMS value forthe velocity residuals of these 23 sites is 0.1 mm/yr. There is goodagreement between the SGPS and CGPS velocities for nearby Iraniansites since the differences are lower than 1 mm/yr. The GPS velocitiesand their uncertainties are shown on Fig. 2 and given in Table 1 of thesupplementary material in a Eurasia-!xed reference frame. Wecombined our velocity !eld with the velocity !eld of Reilinger et al.(2006). Only 3 sites are common to the two solutions, the CGPS NSSPin Armenia and 2 SGPS sites in NW Iran (MIAN and BIJA). The RMSof the difference between our velocity solution and the Reilinger et al.(2006) solution is 0.69 mm/yr, within the average 2! velocityuncertainty. Therefore we assume that there is no signi!cant differencebetween the reference frames of these 2 velocity solutions.

Fig. 2.Map showing GPS velocities and 95% con!dence ellipses relative to Eurasia determined in this study (black vectors are SGPS sites, grey vectors are CGPS sites, all velocities aregiven in Table 1 of the supplementary material) and in Reilinger et al. (2006, white vectors). The location and width of the velocity pro!les plotted in Fig. 3 (pro!les 1 and 2) andFig. 8 (pro!le 3) are indicated.

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3. Discussion

The new velocity solution con!rms the almost due north conver-gence of Arabia relative to Eurasia. North of the Turkish–Iranianplateau and the Central Iranian block, the velocities have a highereastern component. On the base of our GPS results, we address threeaspects of the NW Iran tectonics. First, we check if the new resultscon!rm the right lateral strike-slip rate of the North Tabriz fault(NTF). We use a block model with different geometries of the blocksbased on mapped faults to study how the deformation extends westof the NTF into Turkey and east of the NTF where the fault directionseems to change and lead to the growth of the Mount Bozgush. Itshould be noted that our block model does not mean that nodeformation occurs within the blocks. Several historical earthquakeshave been reported within the blocks derived from the velocity !led(e.g., Allen et al., 2011; Karakhanian et al., 2004; Le Dortz et al., 2009)and instrumental seismicity occurs from time to time within a block(Fig. 1). However the deformation within the blocks is not signi!cantgiven the uncertainties on our GPS velocity solution. Thus, we areunfortunately unable to estimate the internal deformation of theblocks and we can only give an upper bound that is usually around1–2 mm/yr over the length of the block, and therefore the faultswithinthe block have slip rate lower than this upper bound. Second, in lightof the denser network, we re-evaluate the compression/extensionacross the NTF and Talesh. To do so we use velocity pro!les acrossthe North Tabriz fault and the Talesh together with our best blockmodel solution. Then we use a pro!le across the western Alborz toevaluate the strain of this regionwhere theMs 7.3, Rudbar earthquakeoccurred in 1990.

3.1. Slip and geometry of the north Tabriz fault

The fault parallel velocity component for sites located along pro!le2 (Figs. 2 and 3) crossing the NTF central segment shows anarctangent like shape as expected for interseismic displacement fora strike slip fault (Savage and Burford, 1973). Furthermore, this pro!lesuggests that the strike slip component on the central segment ofthe NTF is likely accommodated by only one fault since most ofthe transient interseismic elastic deformation is concentrated over aregion of a 100 km width centered on the NTF. The right lateral strike

slip rate deduced from this pro!le using the Savage and Burford(1973) strain accumulation model is 8±2 mm/yr for a locking depthgreater than 15.5 km in agreement with previous studies (Massonet al., 2006; Masson et al., 2007; Vernant et al., 2004). Using reducedChi-square statistics we are not able to de!ne a realistic range for thelocking depth on this part of the fault (see supplementary material),it is probably related to the fact that the fault trace is curved andthat a simple 1D pro!le approach isn't suf!cient. Substantial new GPSsites are now available in the western region where the NTF mergeswith the Nakhichevan fault (NF), Maku fault (MF), Gailatu–SiahCheshmeh–Khoy fault (GSKF) and the Chalderan fault (CF) (Fig. 1).Fig. 3 shows the fault parallel velocity along a pro!le across thesefaults (pro!le 1). This pro!le also shows a classic arctangent-like shapewith the elastic interseismic deformation restricted to a 100–120 kmbandwidth centered on the subparallel NTF, south GSKF segment, andthe Chalderan fault trace, suggesting that most of the deformation isaccommodated by this fault system. The best elastic strain accumu-lation model suggests a rate of 8.3 mm/yr (7–10.3 mm/yr) and alocking depth of 14 km. The reducedChi-square shows that the lockingdepth can range from 6 to 26 km (see supplementary material).Therefore it is not relevant to try to compare this estimate with thedepth extent of the seismicity of this region.

To investigate further the lateral extent of the NTF and to estimatethe fault slip rate of themain active faults, we use a blockmodel. As nosigni!cant earthquake has occurred during the time interval of themeasurements, and no postseismic transients are expected in thisregion, our GPS measurements depict the inter-seismic deformationof NW Iran. We use DEFNODE (McCaffrey, 2002) to solve for relativeblock motions by minimizing the GPS residual motions within theblocks in a least square sense. The elastic strain accumulation on blockbounding faults follows Okada's formulation (Okada, 1985; Okada,1992). The model allows no permanent deformation of the blocks orslip on unconnected faults, requiring that all the faults used be on ablock boundary.

We de!ne a Lesser Caucasus–Talesh (LCT) block, a Turkish–Iranianplateau (TIP) block, and a Central Iranian block corresponding to theSanandaj–Sirjan zone (Vernant et al., 2004). The block geometry isde!ned using the main active faults mapped in the region. We varythe southern boundary of the LCTB so it corresponds to either on oneof the following faults: GSKF, Maku or Chalderan. We do the same for

Fig. 3.West (pro!le 1) and East (pro!le 2) velocity pro!les across the North Tabriz fault and 2! uncertainties relative to Eurasia (see Fig. 2 for pro!le location andwidth). The dottedline shows the interseismic elastic deformation based on the Savage and Burford (1973) model (see text for details). The value 0 along the fault normal distance axis gives thelocation of the North Tabriz fault.



the eastern extent of the NTF and check if our network is denseenough to evaluate whether the eastward extent of the NTF is likely tobe located north or south of Mount Bozgush. As the Central Iranianblock (CIB) encompasses a region larger than our study, we have alsocombined some of the data of Masson et al. (2007) and Walpersdorfet al., (2006) to better constrain CIB motion.

To the east, the NTF is located along the southern foothills ofMount Bozgush, but a fault is also mapped to the north. Fig. 4 showsthree different geometries for the eastern segment of the NTF. Theresiduals north of Bozgush are always signi!cant and point consis-tently toward the same direction, which suggests that this is not alocal effect at one site, but rather a regional effect at the scale of MountBozgush. With the active fault being south of Mount Bozgush(Fig. 4A), the residuals show a fault parallel component that decreasesif we use a fault trace following the ridge of Bozgush (Fig. 4B) andalmost disappear if we assume that the active fault is located north ofMount Bozgush (Fig. 4C). This suggests that the right lateral strike slipmotion extending east of theNTF is located north of Bozgush. Howeversigni!cant residuals with a direction normal to the fault trace remain.They suggest that the northern components of the modeled velocitiesare too slow. Given the direction change of the fault and the relativeblock motion one would expect to see N–S shortening on the MountBozgush, however, the residuals in this region suggest no shortening.A N–S pro!le across Bozgush con!rms these results (Fig. 5), the faultnormal velocity does not show any signi!cant compression, but thestrike slip motion has slowed down to 5–6 mm/yr. Possibly localphenomenon in the region of Bozgush prevents the compression tooccur and transfer it further north between ORTA and TAZA (Fig. 5).Therefore it seems likely that most of the right lateral strike slipdeformation on the NTF is transferred north of Bozgush, as suggestedby one focal mechanism in the region (Jackson et al., 2002), and thatthe present day deformation of Mount Bozgush is not as simple as anuplifted structure created by a horsetail like fault or a change in thefault orientation. A denser GPS network in the region is needed tobetter assess the local deformation in the Mount Bozgush region.

West of the NTF, the fault pattern is more complicated, severalWNW–ESE faults (Chalderan, GSK, Maku and Nakhichevan, Fig. 1)merge on the NTF, and up to now the GPS observations were tooscarce to estimate if one of these faults is more active than the others.We run several models with different geometries using the geologicalmapping of the Chalderan, GSK, Maku and Nakhichevan faults. Theslip rate on the Nakhichevan fault seems to be below the value that wecan signi!cantly determine with our network geometry and velocityuncertainties. Hence, the slip rate must be below ~1 mm/yr and we

choose not to include this fault in our models. Fig. 6 shows theresiduals for two different geometries of the TIP–LCT block boundaryfor the region west of the NTF. We use either the Chalderan (Fig. 6A)or theMaku (Fig. 6B) fault as the main active fault. The residuals showthat the Maku fault is far from being the main active fault boundingthe TIP block to the north. The Chalderan fault is the most likely mainactive fault of the region, transferring the right lateral motion furtherwest into Turkey. This is in some way in agreement with Jackson(1992) model where the faults became inactive as they are draggedfurther north by the Arabia/Eurasia convergence. However, someresiduals remain close from the GSK fault suggesting that this faultis probably active, as attested by the historical seismicity with anearthquake in 1679 (M=6.9, Karakhanian et al., 2004). Using theGSKF as the main boundary between the TIP and the LCT blocks leadsto high residuals. It suggests that this fault is not the main active faultof the region, but the geometry and scarcity of GPS sites in thewesternpart of the network do not allow us to test for a block geometry withthe Chalderan and the GSK fault in the same solution.

Fig. 4.Map showing block model residual velocities (observed minus modeled) and 95% con!dence ellipses for 3 different geometries of the eastern extent of the North Tabriz faultin the Mount Bozgush region (A — south, B — along the ridge, C — north of the mount). Light block boundaries show fault with estimated slip rate. Dashed grey line shows blockboundaries with no estimation of slip rate. The thin black line shows the active fault trace (see Fig. 1).

Fig. 5. North–South velocity pro!le across Mount Bozgush located at longitude 47.5°Eand 2! uncertainties relative to Eurasia.



Our results show that themain active fault system that extends theNTF toward the west is the Chalderan fault that links with the NTFusing the southern part of the GSKF (Fig. 1). This is in agreement withthe lack of historical earthquakes on theMaku and Nakhichevan faults(Karakhanian et al., 2004), and the long recurrence time interval on theGSKF suggested also by historical seismicity (N1500 yr, Karakhanianet al., 2004). Hence, the right lateral motion of the NTF is transferredwest into Turkey mainly on the Chalderan fault. Our study suggests arate of ~8 mm/yr, although this is the higher bound of the slip ratesince a part (b2 mm/yr) is probably taken up by the GSKF.

Based on our GPS network geometry, the optimum block modelgeometry and the associated residuals are shown by Fig. 7A. Eight sitesare located on the TIP block, 38 on the LCTB and 33 on the CIB withWRMS values of 1.6, 1.2 and 1.3 mm/yr respectively. The slip ratesestimated from the block model (Fig. 7B) correspond to a case whereno off-fault deformation occurs and only one fault accommodates thedifferential motion between two blocks. Therefore the uncertaintieson the slip rates depend on the uncertainties on the Euler vectors andcan be considered of ~1 mm/yr (see Reilinger et al. (2006) for furtherdiscussion on the uncertainties estimation with block models). TheEuler vectors are given in Table 2 of the supplementary material.

3.2. Extension and shortening in NW Iran

The along pro!le velocity component for the two pro!les crossingthe NTF (Fig. 3) shows that the velocity component normal to theChalderan-NTF varies from no signi!cant compression nor extensionin the region close to the Iran–Turkey border to a slight extensionin the Talesh. This is con!rmed by our best model that shows noextension or compression near the Iran–Turkey border, but indictsabout 1 mm/yr of extension along the central part of the NTF. Based onour new results, the extension in the Talesh is lower than theestimation of Masson et al. (2006). Indeed, the authors suggested thatabout 4 mm/yr of extension occurred a little north of the NTF and then4 mm/yr more extension south of the site DAMO. Evidence for thelatter 4 mm/yr comes from only one site (DAMO), this site has beenresurveyed since then, and it seems that it is affected by local effects

Fig. 6. Map showing block model residual velocities (observed minus modeled) and95% con!dence ellipses for 2 different geometries of the western extent of the NorthTabriz fault (A — active fault goes through the southern part of the GSKF and then theChalderan fault, B — active fault follows the Maku fault). Light block boundaries haveslip rates estimate, dashed grey lines shows block boundaries with no estimation of sliprates. The thin grey lines show active fault traces (see Fig. 1).

Fig. 7. A — Map showing block model residual velocities (observed minus modeled) and 95% con!dence ellipses for our preferred block model described in the text. CIB, CentralIranian block; LCT, Lesser Caucasus–Talesh block; TIP, Turkish–Iranian plateau block. B—Map showing fault slip rates (mm/yr) deduced from the block model shown in Fig. 7A. Topnumbers (no parentheses) are strike-slip rates, positive being right-lateral. Numbers in parentheses are fault-normal slip rates, negative being closing. Light lines show blockboundaries with estimated slip rates, dashed lines show block boundaries with no estimation of slip rates.



since the north component of its velocity has slowed down since 2006(see Fig. 1 of the supplementary material). Therefore this site shouldnot be used to estimate the N–S extension across the Talesh. Lookingat the time series of the other sites that Masson et al. (2006) used toestimate 4 mm/yr (ARBI, PIRM and TAZA), there is no evidence ofchange of the velocities of these sites with our new measurements.The residuals of these sites relative to a rigid block motion have asystematically low north component (~0.8 mm/yr), indicating that1 mm/yr of extension on the central part of the NTF is an average andthat extension is below 2 mm/yr, as illustrated in Pro!le 2 (Fig. 3). Themain conclusion of our results being that the Chalderan-North Tabrizfault system is mainly a strike-slip fault and that the extensionalcomponent increases toward the east, although the extension rate of1–2 mm/yr remains barely signi!cant.

3.3. The serow normal fault system

Copley and Jackson (2006) described the Serow normal faultsystem as an almost NNW–SSE striking, normal fault system with aright-lateral strike slip component. This fault system corresponds tothe boundary between the Turkish–Iranian plateau and the CentralIranian blocks in our blockmodel. Our kinematicmodeling gives a lowextension rate for this fault system (~0.6 mm/yr) and an even lowerright lateral strike slip component (from 0.1 to 0.5 mm/yr dependingon the azimuth of the fault). These values agree with a 250° to 290°geological slip vector direction (Copley and Jackson, 2006) and depictvery low deformation between the TIP and the Central Iranian blocks.However, given the uncertainties on the Euler vector estimates, theseslip rates estimations are not signi!cant, even though the residualsshow a consistent orientation.

3.4. Transition from right-lateral to left-lateral strike slip, the Rudbarregion

In the Rudbar earthquake region (M7.3, 1990, (Berberian et al.,1992)), we have 4 more GPS sites than prior studies (Masson et al.,2006) in the region (DAND, GHO1, KRMD, MARG and RSHT) providingimproved estimates of fault related deformation. We plot the sitelocations and the velocity components along a pro!le normal to the

western end of the Alborz rangewhere the Rudbar earthquake occurred(Fig. 1). Masson et al. (2006) based on the motion of the site ATTAsuggested a progressive evolution from compression in central Alborzto extension in Talesh. Our results con!rm that very low deformationoccurs from the Central Iranian block to the northern foothills of theeastern end of the Alborz range. Based on the velocity pro!le across therange (Fig. 8) the deformation is about 0±1mm/yr. However, theprogressive evolution from shortening in Central Alborz to extension inTalesh as suggested by Masson et al. (2006) is not supported since ourre-evaluation of the extension rate in the Talesh gives a lower rate, andshortening occurs offshore north of the eastern Alborz (Djamour et al.,2010). But, it is clear that this region is a transition zone from the right-lateral strike slip of theNorth Tabriz fault to the left-lateralmotionof thefault system south of the Alborz range.

4. Conclusion

A denser GPS network with 25 new continuous GPS sites con!rmsa right-lateral strike-slip rate on the order of 7 mm/yr for the NorthTabriz fault in agreement with previous studies (Masson et al., 2006;Vernant et al., 2004) and shows that right lateral motion extendswest into Turkey along the Chalderan fault. As pointed out by previousstudies (Masson et al., 2006; Vernant et al., 2004), the empiricalrelationships linking themaximumor the average displacements withearthquake magnitude (Wells and Coppersmith, 1994) suggest thatthis rate is consistent with a 250–300 yrs recurrence time interval andmagnitude 7–7.3 earthquakes consistent with historical seismicityof the North Tabriz fault in 858, 1042, 1273, 1304, 1641, 1721, 1780and 1786 (Berberian and Yeats, 1999). The longer recurrence timeproposed by (Hessami et al., 2003) would lead to magnitudes (7.4 to7.7) that are inconsistent with the historical seismicity. The lastlarge earthquakes to strike the Tabriz city were in 1780 and 1786 withestimated magnitudes of 7.3 and 6.3 respectively (Berberian andYeats, 1999), indicating the potential for a signi!cant earthquake onthe NTF in the next 50 yr. On the contrary, deformation in the regionof transition between the Talesh and the Alborz mountains is veryslow, and the recurrence time for large earthquakes like the Rudbarone must be very long.

Our new measurements do not con!rm the large extension rategiven by Masson et al. (2006), but rather suggest a lower extensionalrate of 1–2 mm/yr for the central part of the North Tabriz fault, thewestern segment and the Chalderan fault being pure strike slip withno signi!cant extension nor compression. The eastern segment ofthe Tabriz fault seems to extend north of Mount Bozgush ratherthan south. Although this Mountain seems to be related to a pop upassociated with the directional change of the NTF, there is no evidenceof present day compression; on the contrary it seems that thisstructure is undergoing N–S extension. Unfortunately, our network istoo sparse in the Bozgush region to better assess the processes causingthe N–S extension of the Range. Although the N–S extension acrossthe Talesh is lower than what Masson et al. (2006) have suggested,our network con!rms the sharp azimuth change of the velocitiesnorth of the NTF. This implies that the Arabian indenter is no longerthe only force driving deformation of the region, but that other forcesare needed. According to Vernant and Chéry (2006) the gravitationalpotential energy (GPE) related to the topography induces low effects(b1 mm/yr) on the velocity !eld. Therefore GPE must be ruled outand the other forces needed to explain the velocity !eld of the regionare possibly related to subduction of an old remnant of oceaniccrust beneath the Caucasus (Vernant and Chery, 2006) or to activedelamination in the Lesser Caucasus region (Sosson et al., 2010).

Acknowledgements

This work was realized in the frame of a co-operative researchagreement between INSU-CNRS (DYETI, RNCC), MAE (French Foreign

Fig. 8. Velocity pro!le in the transition zone between Talesh and Alborz and 2!uncertainties relative to Eurasia (see Fig. 2 for pro!le location and width). The value0 along the fault normal distance axis gives the location of the surface rupture of theRudbar earthquake.



Of!ce Ministry) and National Cartographic Center (NCC, Tehran). TheCGPS network has been installed and is maintained by the NCC. Weare grateful to R. Reilinger for the discussions on an early version ofthis work and to two anonymous reviewers for their constructivecomments on this manuscript and their help to improve our English.We thank all the teams who went out in the !eld to collect thedata, and those international colleagues who contribute data to theIGS Tracking Network.

Appendix A. Supplementary data

Supplementary data to this article can be found online atdoi:10.1016/j.epsl.2011.04.029.

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