Oblique Collision in the Northern Zagros Observed by GPS

The total Arabia-Eurasia collision with 22mm/yr of shortening is absorbed on the Iranian territory mainly in two mountain belts, the Zagros and the Alborz, and on a few large strike-slip faults. Our study focuses on the northern part of the Zagros (west of the NS trending Kazerun fault system), that has been covered by an 18 site GPS network measured in 2001 and 2003. In northern Zagros, shortening is oblique to the mountain belt and partitioning of the deformation on strike-slip and thrust faults could be expected. An example is the pure strike-slip earthquake of magnitude 7.4 on the Main Recent Fault, the major strike-slip fault in northern Zagros, close to Dorud in 1909. Our precise velocities of bedrock sites with forced antenna centring show that 3-6 mm/yr of shortening is occurring in northern Zagros perpendicular to the mountain belt axis, with a total dextral strike-slip component of 4-6mm/yr. A part of this strike-slip motion is found on the Main Recent Fault. The detailed GPS velocity field indicates that the deformation is not completely distributed but can be attributed to some individual faults. However, there is no clear evidence of partitioning. This result will be compared with GPS studies of the central Zagros, where the major deformation mechanism is axis perpendicular shortening with 8mm/yr..