IRAN project : interseismic loading in fast deformation zone

***Study of interseismic loading in rapid deformation zone

Summary
Iran is located between the Arabian Plate to the south and the Turan Platform, considered part of stable Eurasia to the north. From a structural point of view, there are three main units :
– a southern unit, corresponding to the arabic plate and including the Zagros chain.
– a central unit, corresponding to an assembly of micro blocks that have accreted to the southern margin of Eurasia. This unit includes in particular the Sanandaj-Sirjan metamorphic area north of Zagros, the Central Iran and Lut blocks to the east and the Alborz Range.
– a northern unit, corresponding to the southern margin of the Eurasian continent at the end of the Paleozoic, including the Kopet Dag chain and the Turan platform in Turkmenistan.

Iran is an active recent intra-continental collision zone. The kinematics, regular since 60 m.a., of the convergence between Arabia and Eurasia is relatively well constrained by the analysis of magnetic anomalies at sea and the agreement with GPS measurements is good. The processes and accidents that accommodate the shortening between Arabia and Eurasia are numerous and spread throughout the country, from the Persian Gulf to the central part of the Caspian Sea.
There are two main types of mechanisms that accommodate this convergence : 1) intracontinental shortening in the form of mountain ranges, and 2) large decreases.

Intracontinental shortening is concentrated in the Zagros to the south and in the Caucasus, Alborz and Kopet Dag ranges to the north. The major areas of disengagement separate the different blocks such as Central Iran, Lut or the southern (oceanic lithosphere) part of the Caspian Sea. The distribution of earthquakes clearly shows the existence of these blocks, whose boundaries are often marked by ophiolites, which are witnesses of ancient accretion processes. Finally, paleomagnetic data suggest that the convergence between Arabia and Eurasia has also been accompanied by significant rotations of some of the lithospheric blocks in Central Iran.

The discontinuities inherited from ancient tectonic episodes (sutures, major lithospheric faults) and the probable changes in the nature of the lithosphere between blocks lead to an additional complexity of the current deformation field. For example, the Alborz range is limited on either side of the Caspian Sea by decreases that transfer convergence towards the Caucasus in the west and Kopet Dag in the east.

In summary, Iran is a major example of an active juvenile continental collision in which inherited structures, as well as the likely heterogeneity of the lithosphere, most likely exercise significant control over the location and style of deformations. The entire collision zone is located in Iranian territory, from stable Arabia in the south to stable Eurasia in the north. Iran’s relatively small size makes it a laboratory for observing most of the tectonic processes that accompany intracontinental convergence.

Global analysis of GPS measurements in different networks covering the Zagros

To quantify the current deformation located in the Zagros and constrain tectonic mechanisms, dense GPS networks have been installed. The Zagros Central network makes it possible to identify the rate of shortening in a context of pure compression, and to identify the distribution of deformation on the different tectonic structures (Main Zagros Reverse fault, High Zagros fault, Sarvestan fault, Main Front fault). The Nord Zagros network covers an area where shortening is transferred in dexterous decay with Main Recent fault as the main active structure. Between these two study areas is located the Kazerun fault, crossing the Zagros with a North-South orientation, which could play a major role in the transfer between the two regimes in shortening and decreasing. A special GPS network is dedicated to its study and serves to establish the links between the two main deformation mechanisms in the Zagros. In addition to GPS measurements, a recent and current deformation study (thesis at CEREGE supervised by O. Bellier) is focused on the Kazerun fault. The comparison of the respective results will allow a thorough tectonic interpretation of the current deformation field quantified by GPS.

The joint tectonic interpretation of the deformation fields of Zagros North, Kazerun and Central Zagros obtained by the re-measurements in 2003 and 2004, supplemented by observations from the Makran networks (R. Bayer, IT 1999, 2001) and Global Iran (F. Masson, IT 1999, 2001), will constrain the current deformation mechanisms in the Zagros chain as a whole.