Earth and Planetary Science Letters 223 (2004) 177–185 www.elsevier.com/locate/epsl

Deciphering oblique shortening of central in using geodetic data

Ph. Vernanta,*, F. Nilforoushanb, J. Che´rya, R. Bayera, Y. Djamourb, F. Massona, H. Nankalib, J.-F. Ritza, M. Sedighib, F. Tavakolib

a Laboratoire Dynamique de la Lithosphe`re, CNRS-Universite´ de Montpellier II, CC 060, 4 place E. Bataillon, 34095 Montpellier Cedex 05, France b Geodetic Department, National Cartographic Center, PO Box 13185-1684, Meraj Av., , Iran

Received 27 January 2004; received in revised form 16 April 2004; accepted 19 April 2004

Abstract

The Alborz is a narrow (100 km) and elevated (3000 m) mountain belt which accommodates the differential motion between the Sanandaj–Sirjan zone in central Iran and the South Caspian basin. GPS measurements of 12 geodetic sites in Central Alborz between 2000 and 2002 allow to constrain the motion of the belt with respect to western . One site velocity on the Caspian shoreline suggests that the South Caspian basin moves northwest at a rate of 6 F 2 mm/year with respect to western Eurasia. North–South shortening across the Alborz occurs at 5 F 2 mm/year. To the South, deformation seems to extend beyond the piedmont area, probably due to active thrusting on the Pishva fault. We also observe a left-lateral shear of the overall belt at a rate of 4 F 2 mm/year, consistent with the geological motion observed along E–W active strike-slip faults inside the belt (e.g., the Mosha fault). D 2004 Elsevier B.V. All rights reserved.

Keywords: Alborz; oblique collision; GPS; fault motion; seismic hazard

1. Introduction and tectonic setting Surprisingly, no crustal root has been detected below the high topography [1], and the compensation mech- The Alborz is a narrow mountain of 100 km wide anism remains conjectural. Recent tectonic history is only which wraps around the South . The thought to be separated in two phases: a mean elevation in the Alborz drops sharply from 3000 North–South compression between the Central - m in the inner belt to À 28 m at the Caspian shoreline an Block and the South Caspian Basin; a and to the North. The topographic contrast is less pro- Quaternary North–East shortening oblique to the E– nounced to the south where the connection with the W structures of Central Alborz [2–4]. These authors lowlands of the Central Iranian desert is progressive. have proposed that shortening and left-lateral shear occur in a partitioned manner. Our goal is to discuss the agreement between this geological interpretation * Corresponding author. Tel.: +33-4-67-14-45-91; fax: +33-4- and new geodetic data. 67-52-39-08. Knowledge of the strain pattern in Iran is rapidly E-mail address: [email protected] (Ph. Vernant). improving due to repeated GPS surveys recently

0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.04.017 178 Ph. Vernant et al. / Earth and Planetary Science Letters 223 (2004) 177–185 made in Iran [5,6] and several key areas such as short-term rate is also in fairly good agreement with Central Zagros, the Zagros–Makran transition and the long-term rates of 20–31 mm/year provided by Alborz are under geodetic investigation [5–7]. At the plate reconstruction [10,11]. A outstanding feature longitude of the Persian Gulf, Arabian and Eurasian provided by these surveys is the quasi-rigid behav- plates collide at a rate of f 22 mm/year [6], iour of the Sanandaj–Sirjan zone in Central Iran that compatible with recent geodetic studies [8,9]. This moves to the north with respect to Eurasia (Fig. 1) at

Fig. 1. Velocity field and seismicity of central and NE Iran. Geodetic motion of Central Iranian Block with respect to western Eurasia is given by black arrows in the Sanandaj–Sirjan zone [6]. Dots indicate instrumental seismicity. Empty circles indicate historical seismicity from Ambrasey and Melville [23]. The box indicates the location of Fig. 2. T = Tehran; K = Kashan; I = Ispahan. Ph. Vernant et al. / Earth and Planetary Science Letters 223 (2004) 177–185 179 a velocity of 14 F 1 mm/year [6]. As already pro- This implies that the deformation of the Alborz and posed [12], the foregoing study demonstrates that no Kopet Dag mountains must be explained in the motion to the east can be detected for the Central framework of the northward shortening between the Iranian Block, suggesting that the Lut and Helmand Central Iranian Block and the Eurasian plate. We blocks do not allow an eastward displacement of the discuss this aspect on the basis of repeated geodetic Central Iranian Block. Therefore, the Helmand block surveys made in Central Alborz between 2000 and acts like an eastward-locked boundary condition. 2002.

Fig. 2. Velocity field of Central Alborz based on 2000 and 2002 GPS surveys (black arrows) and location of the profile of Fig. 3. Empty arrows on CHSH and MOBA represent the motion predicted when using the pole and rotation rate determined for the Central Iranian Block [6]. Main faults are drawn following a recent compilation [27]. Poorly documented faults are drawn with dashed lines. Earthquakes focal mechanisms are from Jackson et al. [3]. Inset: direction of geodetic strain rate (black arrows), direction of seismic strain (empty arrows) and geodetic rotation rate (counter-clockwise). 180 Ph. Vernant et al. / Earth and Planetary Science Letters 223 (2004) 177–185

2. Data and 3A). Four other sites are installed up to 100 km apart from the profile in order to evaluate possible A GPS network of 12 points has been installed and lateral velocity variations. Ten of the sites are an- measured in the framework of the Franco–Iranian chored in the bedrock; the others two were set up on cooperation. The network runs from the Caspian geodetically designed pillars encased in recent but shoreline in the north to the external thrusts of the consolidated deposits. All but one site have been Alborz in the central Iranian desert to the south (Fig. surveyed three times in September 2000, 2001 and 2). Eight of our GPS sites are distributed on a 200-km- 2002, with occupations lasting 48 or 72 h. In order to long profile 60 km east of Tehran. This profile has a constrain the motion of our local network relative to Nj10E orientation, perpendicular to the general trend the surrounding plates, data of 16 GPS stations of Alborz at 51jE longitude. Southern and northern belonging to Eurasian and Arabian plates have been sites are clearly outside the high elevation areas, included in our analysis together with our local data which are restricted to a zone 100 km wide (Figs. 2 (Table 1). Data analysis was done using GAMIT,

Fig. 3. (A) Topographic profile of Alborz along a 10jN direction between the sites MOBA and MAHM. Solid line represents the average value over a band of 15 km half-width. Dashed lines represent minimum and maximum elevations over the same area. Fault locations are indicated, but no information is given on their dip due to the large vertical exaggeration. (B) Longitudinal component of the velocity field along the profile with the 1j uncertainties. (C) Transversal component of the velocity field along the profile (N280j shear component). Heavy solid line represent the interseismic velocity field due to 4 mm/year of shear motion on a strike-slip locked fault (Mosha, vertical line at 105 km) with a locking depth of 15 km. Open symbols correspond to GPS sites far from the profile. The symbols above the profiles B and C indicate the location of the faults. Ph. Vernant et al. / Earth and Planetary Science Letters 223 (2004) 177–185 181

Table 1 GPS site velocities and 1j uncertainties Site Lon Lat Ve Se Vn Sn Cor N280j N010j Alborz sites MOBA 51.81 34.98 2.8 2.6 11.6 2.5 0.02 À 0.7 11.9 CHSH 50.99 35.09 0.3 1.5 9.9 1.5 0.01 1.5 9.8 PISH 51.89 35.22 1.0 1.7 9.2 1.7 0.02 0.6 9.2 TANG 52.04 35.49 0.0 1.7 8.6 1.6 0.02 1.5 8.5 TEHN 51.33 35.70 1.0 1.5 9.1 1.5 0.01 0.6 9.2 DAMA 52.06 35.70 À 1.6 1.7 8.0 1.6 0.02 3.0 7.6 AMIN 52.59 35.70 À 1.9 1.7 8.8 1.6 0.02 3.4 8.4 BOOM 51.81 35.73 À 0.4 1.0 9.9 1.0 0.01 2.1 9.7 ABAL 51.99 35.79 0.7 1.6 7.6 1.6 0.02 0.6 7.6 MEHR 52.16 35.87 À 0.4 1.7 7.1 1.7 0.02 1.7 6.9 HELI 52.31 36.21 À 3.2 1.8 9.2 1.6 0.03 4.8 8.5 MAHM 52.29 36.59 À 2.6 1.6 6.1 1.5 0.02 3.6 5.6

Eurasian sites used to define Eurasian fixed reference frame Site Lon Lat Ve Se Vn Sn cor BOR1a 17.07 52.27 0.26 0.20 0.73 0.73 0.000 BRUSa 4.36 50.78 À 0.07 À 0.76 0.72 0.72 0.000 GRASa 6.92 43.75 À 0.24 À 0.50 0.66 0.66 0.000 GRAZa 15.49 47.07 0.50 À 0.22 0.71 0.71 À 0.001 JOZEa 21.03 52.10 À 0.28 0.28 0.72 0.72 À 0.001 KIRUa 20.97 67.86 À 0.19 0.25 0.69 0.69 À 0.001 KOSGa 5.81 52.18 À 0.53 0.78 0.68 0.68 0.000 METSa 24.39 60.22 0.29 À 0.93 0.71 0.71 0.000 NYALa 11.86 78.93 0.02 À 0.98 0.63 0.63 À 0.001 ONSAa 11.93 57.39 À 0.75 0.04 0.69 0.68 0.000 POTSa 13.07 52.38 À 0.07 0.30 0.69 0.69 0.000 TIXIa 128.87 71.63 0.13 À 0.31 0.24 0.24 À 0.003 WSRTa 6.60 52.91 À 0.28 0.45 0.81 0.81 0.000 WTZRa 12.88 49.14 0.12 0.20 0.73 0.73 0.000 ZIMMa 7.46 46.88 0.52 0.18 0.70 0.70 0.000 ZWENa 36.76 55.70 0.50 0.09 0.65 0.64 0.000 Latitude (Lat) and Longitude (Lon) are given in degrees north and east, respectively. East (Ve) and North velocities components and their uncertainties (Se and Sn) are given in mm/year. cor: correlation coefficient between the east and north uncertainties. The Eurasian frame is determined by minimizing the adjustments to the horizontal velocities of the 16 stations given at the end of the table. a Permanent stations available by ftp at: www.lox.ucsd.edu. version 10.05 [13] and GLOBK, version 10.0 [14]. ment instability affects our data, we added a random The accuracy of our measurements can be expressed walk component equal to 2 mm/year1/2 [28].We in different ways. The baseline repeatability (which estimate a long-term error of about 2 mm/year for expresses the short time correlation between the data) the velocities. In order to interpret our results in the displays values of about 1 mm/year. However, these framework of Arabia–Eurasia collision, we express small values cannot be directly used to estimate the the velocities with respect to a stable (zero velocity) error on the long-term velocity. Indeed, statistics made Eurasia. To do so, we minimize the site velocity of 16 with GPS time series recorded by permanent stations Eurasian sites spanning locations between 0jE and generally show a long time correlation (often called 130jE of longitude east and between 39jN and 80jN colored noise, [15,16]) and seasonal variations [17], of latitude north (Table 1). Except for these two sites, implying that formal errors given by baseline repeat- RMS velocities in Eurasia are 0.4 mm/year, which is ability lead to underestimate velocity uncertainty. an a posteriori validation of the rigidity hypothesis of Assuming that a colored noise and possible monu- the western Eurasian plate. Velocities in Central 182 Ph. Vernant et al. / Earth and Planetary Science Letters 223 (2004) 177–185

Alborz display a clear north–south evolution (Fig. 2 (misfit < 1 mm/year) with the exception of HELI. and Table 1). To the south, CHSM and MOBA sites The difference in velocity between sites at the ends move at a mean north velocity of 10.7 F 2 mm/year. of the profile is 5 F 2mm/year,thisisslightly Sites velocities progressively decrease with increasing different from the 8 F 2 mm/year proposed by Vernant latitude. To the north, HELI and MAHM sites move et al. [6]. The difference is due to the lack of GPS sites NNW at 9.2 F 2 and 6.1 F 2 mm/year, respectively. A in the Alborz in the previous study (Fig. 1), and an clear velocity orientation change also occurs, with a erroneous velocity, probably due to a local problem on southernmost sites motion at N10j and a progressive a station located north of Tehran (this station have not orientation change leading to an N f 340j motion for been plotted in the Fig. 1). This new rate of 5 F 2 mm/ the northernmost sites. year appears compatible with what is expected for the shortening during the Pliocene–Quaternary period [4] which is thought to be 30 km over 5 My (i.e., a long- 3. Discussion term rate of 6 mm/year). Due to the errors occurring in each of the surveys, it is not possible to detect small- On the basis of our GPS results across Alborz, we scale strain variations along the profile. However, the address two aspects of the Iranian tectonics: first, the three profile points based south of the high relief internal deformation and the possible partitioning of (TANG, PISH and MOBA) suggest that the shorten- relative motion in the Central Alborz; second, the ing across the external thrusts of Parchin and Pishva relative motion of Alborz with respect to Eurasia and could be f 3 mm/year. Although less regular than for to the Central Iranian block. To do so, we assume that the shortening velocity, an overall decay of the shear the northern and southern most sites (CHSH, MOBA velocity component of f 4 mm/year over the profile and MAHM) are far enough of the active faults to be is also visible. Formally, this parallel velocity varia- nearly free of interseismic effects. These stations were tion can be interpreted equally like a rigid rotation of settled at more than 25–30 km from the actives faults, the whole profile or like a simple shear motion. therefore, assuming an elastic model [18], the inter- Because of the lack of a network at the scale of seismic effects should be less than 25–30% of the Alborz, it may be difficult to discriminate between velocity of the nearest fault. these two possible motions. In the case of a rigid rotation, this would imply a counter-clockwise rota- 3.1. Central Alborz deformation tion rate of f 0.86j/My of Central Alborz. Although such a rotation rate is relatively low and therefore We observe a clear variation in amplitude and possible, we note that no significant geodetic rotation orientation of the GPS velocity field. However, infer- is observed in the areas adjacent to Alborz [6]. To the ring strain of Central Alborz with these data is not north, two GPS sites on the Caspian shorelines move straightforward. Indeed, Alborz is not a perfectly at the same velocity, suggesting no rotation of the linear belt, and structural trends change from N110j South Caspian Basin with respect to Eurasia. To the in Western Alborz to N80j in Eastern Alborz [4].A South, the Sanandaj–Sirjan GPS sites move to the marked hinge occurs near 52j30V of longitude, as North with a similar velocity, preventing a rotation shown by the switch between the strike of Mosha fault larger than 0.1j/My. In this context, a rigid rotation of and the strike of the Firuzkuh fault (Fig. 2), and by a Central Alborz appears unlikely. The alternate possi- sharp change in strike direction within the Khazar bility is that Alborz behaves like a transpressional fault. In the following, we analyse the data by con- orogen in order to accommodate the differential sidering separately the direction of normal (i.e., short- motion between the South Caspian basin and the ening) and parallel (i.e., shear) to the main trend Central Iranian block. In order to quantify the geo- which is roughly N100jE. The projection of each detic strain, we compute the linear trend of the point position and its normal and parallel velocity is velocity field which minimizes the residual velocity given in Fig. 3B and C. The normal component on east and north component for the 12 GPS sites. We evolves gently along the profile, and a linear trend then compute the spatial derivatives of this linear field over the 180-km-long profile fits all points well that constitute the average velocity gradient over Ph. Vernant et al. / Earth and Planetary Science Letters 223 (2004) 177–185 183

Central Alborz. Computing the symmetric and anti- parallel velocity component is accommodated by the symmetric part of this tensor allows to define respec- Mosha fault, implying partitioning. It seems therefore tively the deformation rate and the rotation rate. The likely that most of the range-parallel velocity compo- direction of maximum shortening appears to be f 30 nent is accommodated by the Mosha fault in central jN, with no significant extension in the perpendicular Alborz. This would imply that other active faults are direction (Fig. 2, inset). The main seismic strain axis pure thrust faults. provided by the moment tensor summation [19] of seven earthquakes that occurred in Central Alborz 3.2. Central Iranian block between 1957 and 1992 (Fig. 2) appears closely compatible with this direction. This is also consistent The North–South shortening rate in central Alborz with the geological strain state proposed by some appears to be 5 F 2 mm/year, which is significantly authors [3,4]. In their view, the simple shear motion different to the value of 6–10 mm/year deduced from a is accommodated by some of the E–W trending GPS network made at the scale of Iran [6]. The reason faults. Among them, the Mosha fault is the most is that southernmost sites of our network in the Central prominent left-lateral strike-slip fault in the Central Iranian depression have a northward and eastward Alborz. Its Pliocene–Quaternary offset has been sug- velocity of respectively 9.9 and 0.3 mm/year (CHSM) gested to be 35 km, which corresponds to a slip rate of and 11.6 and 2.8 mm/year (MOBA). Their direction of 7 mm/year over a period of 5 My [4]. Preliminary motion is in good agreement with the overall motion of paleoseismologic work near the Tar Lake 15 km NE the Central Iranian block which moves northward at 14 to the DAMA site indicates a minimum left-lateral mm/year. However, a difference of f 3 mm/year of component of 2.7 F 0.5 mm/year for the Holocene the North component indicates that some shortening period [20]. This fault behaviour appears compatible may occur between the Central Iranian Block and the with the variation of the East velocity component foreland basin of Alborz. Because of the lack of GPS along the whole profile. At a smaller scale, we do sites south of our network, the precise location of the not observe a left-lateral motion between the sites suspected shortening is unknown. Nevertheless, there within 15 km of the Mosha fault trace (sites BOOM, a few historical reports or instrumental records of DAMA, ABAL, MEHR, see Fig. 3C). On the con- seismicity south of the Pishva fault [23,24] in the trary, relative motion between sites north (ABAL) and Alborz foreland. Also, no quaternary deformation south (BOOM and DAMA) of the Mosha fault has been described south of the Pishva fault that is suggest right lateral displacement. However, this mo- probably the most external thrust of central Alborz. tion is inside the 1j error, and therefore, we rather Therefore, the location of the residual motion between believe that the Mosha fault is now locked in an the Alborz and the Central Iranian Block may be interseismic stage since the last 1830 earthquake [21]. located as far south as the northern side of the This may explain why no significant different motion Sanandaj Sirjan zone (Fig. 1). There, near the city of is occurring in the fault vicinity [22]. Based on GPS Kashan, potentially active faults have been reported, velocity field, it is tempting to propose that no together with the occurrence of destructive historical localized deformation occurs in the vicinity of the earthquakes in 1755, 1778 and 1844 [23]. Therefore, Mosha fault since a linear trend fit the GPS measure- both seismotectonic and geodetic evidences suggest ments. We rather believe that this lack of localized that slow deformation occurs on the northern edge of motion is due to the insufficient resolution of our GPS the Sanandaj Sirjan zone. measurements. Indeed, predicted velocities assuming an elastic deformation due to 4 mm/year of a deep 3.3. South Caspian basin strike-slip motion having a locking depth of the fault of 15 km [18] could not be distinguished from the Near the Caspian shoreline, the northernmost site linear trend due to the error bars of the measurements of our profile (MAHM) has a northward and westward (Fig. 3C). Taken together, the joint analysis of the velocity of, respectively, 6.1 and 2.6 mm/year with paleoseismologic evidences [20] and the interseismic respect to western Eurasia. Because this velocity is GPS velocity field suggests that most of the range- equivalent to that suggested by another GPS study for 184 Ph. Vernant et al. / Earth and Planetary Science Letters 223 (2004) 177–185 the Caspian shoreline [5,6], we assume in the follow- basin (6 mm/year) and south of Alborz (3 mm/year), ing that the velocity of MAHM reflects the motion of possibly on the edge of the Central Iranian block. the southermost Caspian basin. Whether it represents Internal deformation of Alborz appears to be trans- the motion of the overall South Caspian basin is pressional, which is consistent with the activity of questionable. This basin is probably a remnant oce- both thrust faults (North Alborz and Khazar to the anic lithosphere covered by recently folded sediments North, Parchin, Pishva, Kahrizak to the South) and [25]. Different lines of evidences suggest that this left-lateral faults (Taleqhan and Mosha faults). very thick sedimentary basin is moving. To the north, a trend of intermediate seismicity crosses the Caspian Sea between the northern and the Kopet Acknowledgements Dagh. This seismic zone (Apsheron Sill) has been interpreted as a northward subduction of the South This work has been supported by the ‘‘Inte´rieur de Caspian Basin under Eurasia [3]. To the west, the la Terre’’ INSU-CNRS program and by the internal South Caspian Basin is flanked by the Talesh moun- funding of the NCC of Iran. We thank all the tains which are the geographical continuation of observers and drivers that made the GPS surveys Alborz toward West. In this area, thrusting on almost successful. We thank also the French Embassy of Iran flat faults with slip vectors of East–West direction for its support. This paper benefited from constructive indicates a shortening component for the relative reviews of Bertrand Meyer and an anonymous motion between the South Caspian Basin and the reviewer. [VC] Talesh [3]. Because there is no seismicity within the South Caspian Basin, these authors have proposed a rigid kinematical solution for the motion of this basin References that corresponds to a NW lateral motion along the Kopet Dagh–Apsheron Sill border to the North. [1] G.A. Dehghani, J. Makris, The gravity field and crustal struc- Using the Arabia–Eurasia convergence rates of Sella ture of Iran, Neues Jahrb. Geol. Pala¨ontol. Abh. 168 (1984) 215–229. et al. [9], Allen et al. [26] have revised the velocity [2] J.A. Jackson, D.P. McKenzie, Active tectonics of the Alpine– triangle of Jackson et al. [3]. 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