Seismological constraints on the crustal structure beneath the Zagros Mountain belt (Iran)

The Zagros Mountain belt of western Iran results from the collision of the Arabian and Central Iran continental blocks. The stage of the collision is unclear and the crustal structure of the Zagros is rather poorly known. In this study we investigate the velocity structure of the crust and upper mantle beneath the Ghir region located in the Central Zagros using data collected by a temporary local seismological network including a broad-band instrument. The structures of the sedimentary cover and the upper crystalline crust are estimated from the inversion of P and S traveltimes of local earthquakes recorded on a dense seismological network. The upper crust consists of an ∼11 km thick sedimentary layer (Vp∼ 4.70 km s−1) above a ∼8 km thick upper crystalline crust (Vp∼ 5.85 km s−1). The velocity of the lower crust and the depth of the Moho are found using receiver function analysis of teleseismic earthquakes. The lower crystalline crust is unusually slow (Vp∼ 6.5 km s−1) and ∼27 km thick. The upper bound for the total crustal thickness beneath the Ghir region is 46 ± 2 km. A comparison of the thickness of the crystalline crust of the Zagros with available information for the thickness of the crystalline crust of the Arabian Platform shows that, at present, the Zagros has a thinner crust. The current crustal thickness beneath the Zagros is comparable to the pre-collision crustal thickness of the Arabian Platform, suggesting that the Zagros is now in a very early stage of continental collision.