Tectonic Implications of GPS Measurements In Iran

Fourth International Conference of Earthquake Engineering and Seismology 12-14 May 2003 Tehran, Islamic Republic of Iran Tectonic implications of GPS measurements in Iran. F. Masson1, F. Nilforoushan2, P. Vernant1, M. Abassi3, D. Hatzfeld4, C. Vigny5, F. Tavakoli2, R. Bayer1, J. Chéry1, E. Doerflinger1 and J. Martinod4. 1 Lab. de Géophysique, Tectonique et Sédimentologie, Université Montpellier II, CNRS, Montpellier, France. (fmasson@dstu.univ-montp2.fr) 2 National Cartographic Center, Tehran, Iran. 3 International Institute of Earthquake Engineering and Seismology, Tehran, Iran. 4 Lab. de Géophysique Interne et Tectonophysique, Université J. Fourier, CNRS, Grenoble, France. 5 Lab. de Géologie de l’Ecole Normale Supérieure, Paris, CNRS, France. ABSTRACT The present tectonic of Iran results from the north-south convergence between relatively undeformed shield areas to the south-west (Arabia) and north-east (Eurasia). The global plate motion model Nuvel-1A predicts a convergence rate of about 3.0 cm/year. The deformation of Iran involves intracontinental shortening except along its south-eastern margin (Makran) where the Oman oceanic lithosphere subducts northward under south-east Iran. Within Iran, most of the deformation is accommodated in the major belts (Zagros, Alborz, Kopet-Dag) and along large strike-slip faults which surround blocks with moderated relief and relatively aseismic (Central Iranian Plateau, Lut block, Southern Caspian Sea). We report on the first results of a French-Iranian GPS project designed to quantify the inter- and intra-plate deformation in Iran and constrain the Arabian-Eurasian plate motions. The main purpose is to understand how the deformation is distributed in the whole country. To answer to this question, a GPS network of 25 sites in Iran and 2 sites in Oman has been installed and measured in September 1999 and October 2001. We analyzed the data using the GAMIT/GLOBK software. We included data from 19 IGS stations. For both campaigns the average repeatabilities for the North and East components of the Iranian baselines are about 2 mm. The results indicate a northward motion of the Arabian plate relative to Eurasia of 2.3 cm/year. The velocity of the subduction of the Gulf of Oman beneath the Makran is ~1.8 cm/year in the east and decreases toward the west. The transition from subduction (Makran) to collision (Zagros) is very sharp. In the eastern part of Iran, most of the shortening is accommodated in Makran while in the western part the shortening is more distributed from south (Zagros : ~0.8 cm/year) to north (Alborz : ~0.8 cm/year). The large faults around the Strait of Hormuz and the stable Lut block accommodate most of the subduction-collision transition (~1.4 cm/year). The Helmand block is fixed relative to Eurasia. There is a small shortening (~0.2 cm/year) in the Persian Gulf. The Kopet-Dag accomodates about 0.4 cm/year of shortening. Large rigth-lateral movement (~0.7 cm/year) occurs in the Tabriz region. 1. INTRODUCTION The Arabia-Eurasia collision zone in Iran is an outstanding example of large-scale continental deformation. The present tectonic of Iran results from the north-south convergence between relatively undeformed shield areas to the south-west (Arabia) and north-east (Eurasia) (Jackson and McKenzie 1984). The NUVEL1A model (Demets et al. 1994) predicts a roughly N-S, 3040 mm/year convergence rate. The deformation of Iran involves a juvenile continental collision except along its south-eastern margin where the Oman oceanic lithosphere subducts northward under south-east Iran. Within Iran, most of the deformation is accommodated in major belts and along large strike-slip faults which surround blocks with moderate relief and seismicity. Continental collision of the western part is widely distributed between Zagros, Caucasus, Alborz and Kopet-Dag mountain belts. In the eastern part of Iran, in front of the subduction, most of the shortening is concentrated in the Gulf of Oman. Based on new GPS data we have obtained a velocity model for the Arabia-Eurasia collision in Iran. 2 - GPS MEASUREMENTS AND DATA PROCESSING A GPS network of 28 sites have been surveyed by Iranian and French institutions in September 1999 and October 2001 (Figure 1). Measurements were made at these sites during at least four 24-hour sessions. Data analysis was done using GAMIT (version 10.05, King and Bock 2002) and GLOBK (version 10.0, Herring 2002) softwares in a three-step approach (Feigl et al. (1993), McClusky et al. (2000)). 16 GPS stations belonging to IGS were used to constrain the motion of our sites with respect to Eurasia. Baseline repeatabilities reach mean values of 1.3 mm for each horizontal component and 3.4 mm for the vertical component. Velocities in an Eurasia fixed reference frame are shown in figure 2, and the 95% uncertainty attached to the values is on the order of 2 mm/year. Figure 1: GPS site of ROBA 3 - THE VELOCITY FIELD We find a north-south shortening between Arabia and Eurasia of ~25 mm/year at the longitude of the Gulf of Oman, less than previously estimated in NUVEL1A (DeMets et al. 1994). This discrepancy is probably due to an oversimplification of NUVEL1A for the Middle-East/Africa region (Jestin et al., 1994). Within Iran, the prominent feature of the velocity model is the sharp transition from the western continental collision domain to the eastern subduction domain. The velocity field west to 57°E is gently evolving from south to north on 1500 km. No significant shortening ( ~0.5 mm/year on BAHR-ALIS baseline) occurs in the Persian gulf. The main feature of the continental collision is the mighty Zagros range which extends for about 1500 km along the NW-SE trending boundary between the Arabian plate and the Central Iranian Plateau. Despite its size and its proximity to the Arabian collider, the Zagros absorbs northsouth shortening at only 3 mm/year in its western part at 46°E and 8 mm/year in its eastern part at 56°E. The Central Iranian Plateau coherently moves to the north at a velocity of 13 ± 2 mm/year as sampled by GPS sites of MIAN, BIJA, SHAH and ARDA. Although shown in Figure 2, the motion of KSHA is probably affected by a local motion and is not used when analysing the tectonic deformation. The mountain belts surrounding the Central Iran Plateau to the North are deformed in a more complicated way than the Zagros. This area includes the Caucasus and NW Iran, the Alborz, the Central Iranian Desert and the Kopet-Dag. The transition between NW Iran and the lesser Caucasus is marked by a large variation of the orientation of the velocity field, which is 12 mm/year at 350°N in NW Iran and 14 mm/year at 26°N for DAMO. This azimuth change is compatible with the large set of velocity vectors inside the lesser Caucasus (McClusky et al 2000). The high (3600-4800 m) and narrow (60-120 km) Alborz range is located between the Central Iranian Desert and the south Caspian basin. From ARDA (Central Iranian Desert) to SEMN (south of the Alborz) and from SEMN to KORD (north of the Alborz), the NS shortening is respectively 4 and 4 mm/year. Assuming that most of the shortening in the Central Iranian Desert is located in the frontal thrusts south of Tehran, then we can estimate that the total shortening through the Alborz is about 8 mm/year. However, we do not account for a possible motion of the south Caspian basin, which may move to the S-W with respect to Eurasia (Jackson et al. 2002). Deformation of eastern Iran is dominated by the collision-subduction transition. We interpret the GPS velocity field as a right-lateral movement of 11 mm/year at 26°N between Zagros (LAMB) and Makran (JASK) . To the north, another large velocity variation is observed between the Central Iranian Desert and the stable Helmand block. From KERM to ZABO at 30°N the N-S velocity variation is 14 mm/year. Because of the deformation of the central iranian desert and of the northern edge of the Lut block, little deformation remains in the Kopet Dag belt where shortening does not exceed 3 mm/year (KASH-SHIR baseline). Sites located north of the Kopet-Dag (SHIR and YAZT) do not move relative to Eurasia. The lack of northward motion in the Helmand block is related to the concentration of the deformation inside the Gulf of Oman. This domain is governed by the low-angle oceanic subduction of the easternmost Arabian plate beneath the Makran accretionary prism. Based on the GPS velocity field, the velocity of the subduction of the Arabian plate beneath the Makran is 18 mm/year in the east (from MUSC to CHAB) and decreases toward the west (15 mm/year from MUSC to JASK). At the longitude of the Gulf of Oman, all the deformation related to the Arabia-Eurasia shortening is therefore located at the front of the subduction and in the Makran. Figure 2 : GPS displacement vectors superimposed on topographic map of the studied area. GPS stations are indicated in white, major geological structures in black. Black arrows (this study) and blue arrows (McClusky et al. 2000) indicate the GPS velocities calculated in a Eurasia-fixed reference frame. 4 - TECTONIC IMPLICATIONS Our GPS-based velocity field is not dense enough to capture the localised motion that occurs along major intracontinental faults of Iran. However, if we assume that the position of the main active faults are known by their geomorphological signature and by historical seismicity (Berberian 1995, Berberian and Yeats 1999), we can use the far field velocities given by GPS as a proxy of the long term velocity of the faults (Thatcher 1995). We made this exercise for the large strike-slip faults crossing Iran, assuming that all the shearing motion is concentrated on these faults. Doing that, we obtain an upper bound of the long term slip rate that may be compared to geological slip rates. We obtain a right-lateral strike-slip movement for the MRF of 2 mm/year in the Northern Zagros. By contrast, the differential motion between the Central Iranian Plateau (MIAN and BIJA) and the north of the Iranian Azerbadjan (DAMO) is large, and the residual strike-slip motion in a direction parallel to the Tabriz fault is on the order of 7 mm/year. This high slip rate is coherent with the intense historical seismicity of the Tabriz area (Ambrasey and Jackson 1998, Berberian and Yeats 1999). The interpretation of the velocity field in term of fault slip rate in Alborz is complex because of the geometrical change occurring around the south Caspian basin. The 0°N orientation of the velocity field of the central Iranian plateau, together with the 110°N trend of main faults of western Alborz suggests that this belt may suffer a right-lateral deformation if the south Caspian basin does not move with respect to Eurasia. This motion is not compatible with the 1990, Mw 7.3 large earthquake of Rudbar, which ruptured on three main fault segments oriented from 95° to 120° with a left-lateral horizontal component (Berberian et al. 1992). A solution to this conflict may come from a differential motion of the south Caspian basin with respect to Eurasia. This microplate is surrounded to the North by the Apsheron-Balkhan Sill which is highlighted by a dense and deep seismicity (Priestley et al. 1994). Moreover, the western part of the south Caspian basin seems to subduct beneath the Talesh province as suggested by thrust events with a eastward shortening direction (Jackson et al 2002). A westward velocity of at least 5 mm/year of the basin near the Talesh shoreline may explain why western Alborz has a left-lateral present-day horizontal motion. The large lateral velocity variation of 14 mm/year from the Central Iranian Plateau to the Helmand bloc is probably accommodated by several N-S fault systems (Minab-Zendan and Gowk fault systems at 58°E and Sistan suture at 60°E). All these faults produced large rightlateral ruptures in the past (Berberian and Yeats 1999). Because the Lut block has not been sampled yet with GPS sites, the relative motion of these fault systems is unknown. Also, the Lut block may rotate clockwise, which would decrease the fault slip rate of the surrounding faults. 5. CONCLUSIONS Because the GPS measurements were done in a global terrestrial frame (ITRF 2000, Altamimi 2002) we can relate the Arabia-Eurasia collision in Iran to the observations that have been made to the west and east. West of Iran, the Arabia-Eurasia collision is accommodated by the East Turkey-Caucasus system and the westward motion of the Anatolian plate at a rate of 24 mm/year (McClusky et al 2000) relative to Eurasia along the North Anatolian Fault (NAF). The Tabriz region suffers a large right-lateral motion which leads us to suggest that the eastward continuation of the NAF joins the Tabriz fault system, and ends into Talesh. East of Iran, the Helmand block indents the southern margin of Eurasia and separates the ArabiaEurasia and the India-Eurasia collisions. 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