Oroclinal bending, distributed thrust and strike-slip faulting, and the accomodation of Arabia-Eurasia convergence in NE Iran since the Oligocene

Regional shortening is accommodated across NE Iran in response to the collision of Arabia with Eurasia. We examine how N-S shortening is accommodated on major thrust systems bounding the eastern branch of the Alborz (east of 57°E), Sabzevar and Kuh-e-Sorkh mountain ranges, which lie south of the Kopeh Dagh mountains in NE Iran. Although these ranges have experienced relatively few large earthquakes over the last 50 years, they have been subject to a number of devastating historical events at Neyshabur, Esfarayen, and Sabzevar. A significant change in the tectonics of the eastern Alborz occurs directly south of the Central Kopeh Dagh, near 57°E. To the east, shortening occurs on major thrust faults which bound the southern margin of the range, resulting in significant crustal thickening, forming peaks up to 3,000~m high. Faulting does not continue eastward into Afghanistan, which is thought to belong to stable Eurasia. The rate of shortening across thrust faults bounding the south side of the eastern Alborz north of Neyshabur is determined using optically stimulated luminescence dating of displaced river deposits, and is likely to be 0.5-- 0.8~mm/yr (maximum upper limit of 2~mm/yr). Shortening across the Sabzevar range ~150~km west of Neyshabur has previously been determined at 0.4--0.6~mm/yr (maximum upper limit of 1~mm/yr). Basinward migration of thrust faulting is common across NE Iran, especially in the Esfarayen region near 57°E, where the northward deflection of the East Alborz range reaches a maximum of 200±20~km (from its presumed linear E-W strike at the beginning of the Oligocene). The continued northward deflection of the eastern Alborz has probably resulted in the progressive clockwise rotation of thrust faults out of their optimum orientation, promoting the formation of new thrusts within the surrounding basins which strike oblique to the range front (and remain perpendicular to the shortening direction). West of 57°E, the tectonics of the Alborz are affected by the westward motion of the South Caspian region, which results in the partitioning of shortening onto separate thrust and left-lateral strike-slip faults north and south of the range, respectively. At the longitude of ~59°E, published GPS velocities indicate that ~50% of the overall shortening across NE Iran is accommodated in the Kopeh Dagh. Of the 50% regional shortening which remains to be accommodated south of the Kopeh Dagh, thrust faulting across the eastern Alborz and Kuh-e- Sorkh ranges probably each account for ~25%. West of 59°E, additional shortening across the Sabzevar range is probably related to the westward increase in regional shortening across NE Iran as indicated by GPS data. At present day rates the total 200±20~km N-S shortening across the eastern Alborz and Kopeh Dagh mountains since the beginning of inversion of the Kopeh Dagh basin would be accommodated in 30±8~Ma. This age is consistent with geological estimates of post Early-to-Middle Oligocene (<30~Ma) for onset of Kopeh Dagh inversion.