SUMMARY A network of 27 GPS sites was implemented in Iran and northern Oman to measure displacements in this part of the Alpine–Himalayan mountain belt. We present and interpret the results of two surveys performed in 1999 September and 2001 October. GPS sites in Oman show northward motion of the Arabian Plate relative to Eurasia slower than the NUVEL‐1A estimates (e.g. 22 ± 2 mm yr−1 at N8°± 5°E instead of 30.5 mm yr−1 at N6°E at Bahrain longitude). We define a GPS Arabia–Eurasia Euler vector of 27.9°± 0.5°N, 19.5°± 1.4°E, 0.41°± 0.1° Myr−1. The Arabia–Eurasia convergence is accommodated differently in eastern and western Iran. East of 58°E, most of the shortening is accommodated by the Makran subduction zone (19.5 ± 2 mm yr−1) and less by the Kopet‐Dag (6.5 ± 2 mm yr−1). West of 58°E, the deformation is distributed in separate fold and thrust belts. At the longitude of Tehran, the Zagros and the Alborz mountain ranges accommodate 6.5 ± 2 mm yr−1 and 8 ± 2 mm yr−1 respectively. The right‐lateral displacement along the Main Recent Fault in the northern Zagros is about 3 ± 2 mm yr−1, smaller than what was generally expected. By contrast, large right‐lateral displacement takes place in northwestern Iran (up to 8 ± mm yr−1). The Central Iranian Block is characterized by coherent plate motion (internal deformation <2 mm yr−1). Sites east of 61°E show very low displacements relative to Eurasia. The kinematic contrast between eastern and western Iran is accommodated by strike‐slip motions along the Lut Block. To the south, the transition zone between Zagros and Makran is under transpression with right‐lateral displacements of 11 ± 2 mm yr−1.

Abstract. Two-dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere. The subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence. The lithosphere is assumed to be underlain by an inviscid asthenosphere. Different rheological laws have been considered for the lithosphere, including elasticity and elastoplasticity. The modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters: the density contrast between lithosphere and asthenosphere ( Ap = Pz -PA ) and the coefficient of friction along the subduction plane. Varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc-back arc system. Extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction, while negative Ap values and/or high friction leads to a compressional regime. The coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0.1. The results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions.

International audienceMeasurements on either side of the Kazerun fault system in the Zagros Mountain Belt, Iran, show that the accommodation of the convergence of the Arabian and Eurasian Plates differs across the region. In northwest Zagros, the deformation is partitioned as 3–6 mm yr−1 of shortening perpendicular to the axis of the mountain belt, and 4–6 mm yr−1 of dextral strike-slip motion on northwest–southeast trending faults. No individual strike-slip fault seems to slip at a rate higher than ~2 mm yr−1. In southeast Zagros, the deformation is pure shortening of 8 ± 2 mm yr−1 occurring perpendicular to the simple folded belt and restricted to the Persian Gulf shore. The fact that most of the deformation is located in front of the simple folded belt, close to the Persian Gulf, while seismicity is more widely spread across the mountain belt, confirms the decoupling of the surface sedimentary layers from the seismogenic basement. A comparison with the folding and topography corroborates a southwestward propagation of the surface deformation. The difference in deformation between the two regions suggests that right-lateral shear cumulates on the north–south trending Kazerun strike-slip fault system to 6 ± 2 mm yr−1

International audienceA combined analysis of the geodetic strain-rate field and the strain-rate field deduced from the seismicity allows us to define the style of deformation and to distinguish seismic from aseismic deformation. We perform this analysis in Iran where the present-day tectonics results from the north–south convergence between the plates of Arabia to the south and Eurasia to the north. The data consist of velocities measured with a GPS network of 28 benchmarks and of instrumental and historical earthquake catalogues. The axes of the seismic strain-rate tensor have similar orientations to those deduced from the GPS velocity field. This indicates that the seismicity can be used to improve GPS information on the style and the orientation of the deformation. Comparison of seismic and geodetic strain rates indicates that highly strained zones experience mainly aseismic deformation in southern Iran and seismic deformation in northern Iran. A large contrast is observed between the Zagros (less than 5 per cent seismic deformation) and the Alborz–Kopet-Dag regions (more than 30–100 per cent seismic deformation). The distribution of the seismic/geodetic ratio correlates with the distribution of large earthquakes: intensive, low-magnitude seismicity is observed in the Zagros whereas the largest earthquakes occur in northern Iran. The contrast of seismic deformation between the Zagros and peri-Caspian mountains is confirmed considering 300 or 1000 yr of seismicity rather than 100 or 200 yr

In 1997 and in 2000, we measured the distances between 14 geodetic benchmarks across the central Zagros mountain belt. The results show that about 10 mm/yr of shortening in the central Zagros is distributed across the mountain belt. This shortening corresponds to roughly 50% of the total convergence between Arabia and Eurasia and is consistent in direction. The Persian Gulf does not deform significantly. The Main Zagros Reverse Fault is not an active kinematic boundary. The internal deformation of the folded belt is rather homogeneous, at the scale of our survey, which does not allow us to detect any individual active blind fault. However, the strain pattern suggests that N‐S dextral strike slip faults may accommodate part of the deformation.

Collisional mountain belts grow as a consequence of continental plate convergence and eventually disappear under the combined effects of gravitational collapse and erosion. Using a decade of GPS data, we show that the western Alps are currently characterized by zero horizontal velocity boundary conditions, offering the opportunity to investigate orogen evolution at the time of cessation of plate convergence. We find no significant horizontal motion within the belt, but GPS and levelling measurements independently show a regional pattern of uplift reaching ~2.5 mm/yr in the northwestern Alps. Unless a low viscosity crustal root under the northwestern Alps locally enhances the vertical response to surface unloading, the summed effects of isostatic responses to erosion and glaciation explain at most 60% of the observed uplift rates. Rock-uplift rates corrected from transient glacial isostatic adjustment contributions likely exceed erosion rates in the northwestern Alps. In the absence of active convergence, the observed surface uplift must result from deep-seated processes.

International audienceThe Bandar Abbas-Strait of Hormuz zone is considered as a transition between the Zagros collision and the Makran oceanic subduction. We used GPS network measurements collected in 2000 and 2002 to better understand the distribution of the deformation between the collision zone and the Makran subduction. Analysing the GPS velocities, we show that transfer of the deformation is mainly accommodated along the NNW-SSE-trending reverse right-lateral Zendan-Minab-Palami (ZMP) fault system. The rate is estimated to 10 +/- 3 mm yr-1 near the faults. Assuming that the ZMP fault system transfers the motion between the Makran-Lut Block and the Arabian plate, we estimate to 15 mm yr-1 and 6 mm yr-1, respectively, the dextral strike-slip and shortening components of the long-term transpressive displacement. Our geodetic measurements suggest also a 10-15 km locking depth for the ZMP fault system. The radial velocity pattern and the orientation of compressive strain axes around the straight of Hormuz is probably the consequence of the subducting Musandam promontory. The N-S Jiroft-Sabzevaran (JS) fault system prolongates southwards the dextral shear motion of the Nayband-Gowk (NG) fault system at an apparent rate of 3.1 +/- 2.5 mm yr-1. The change from strong to weak coupling for underthrusting the Arabian plate beneath the Zagros (strong) and the Makran (weak) may explain the dextral motion along the ZMP, JS/NG and Neh-Zahedan fault systems which transfer the convergence from a broad zone in the western Iran (Zagros, Tabriz fault system, Alborz, Caucasus and Caspian sea surroundings) to Makran subduction

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