F. Masson - Lab. de Géophysique, Tectonique et Sédimentologie, Université Montpellier II, CNRS, Montpellier, France.
F. Nilforoushan - National Cartographic Center, Tehran, Iran.
P. Vernant - International Institute of Earthquake Engineering and Seismology, Tehran, Iran.
D. Hatzfeld - Lab. de Géophysique Interne et Tectonophysique, Université J. Fourier, CNRS, Grenoble, France.

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 : ~0.8cm/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 Fourth International Conference of Earthquake Engineering and Seismology 12-14 May 2003 Tehran, Islamic Republic of Iran 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-lateralmovement (~0.7 cm/year) occurs in the Tabriz region.