CRUSTAL STRUCTURE IN NORTHWEST IRAN FROM RECEIVER FUNCTION STUDIES

We have determined the lateral variations of crustal structure in northwest Iran from a receiver function analysis of a temporary seismic network. The network is an array of 11 broadband stations on a 200 km east-west line that run from the western Caspian coast in the west towards the interior of NW Iran, roughly along the N38 parallel. The eastern end of the line covers the Talesh Mountains along the coast of the south Caspian basin. The aim of the study is to determine the moho depth variations, as well as shallower crustal discontinuities, in the region. The receiver functions (RF) were calculated in time domain by iterative deconvolution. A bandpass filter of 0.05 -0.8 Hz was applied before calculating the RFs. The width of the Gaussian filter used in time domain was 0.4. We migrated the RFs from time domain to depth domain. We did this by two schemes; the common midpoint (CMP) stacking, and the common conversion point (CCP) migration. The results show a gently eastward-sloping moho in the western part of the profile. In the western end, the moho is about 42 km deep. The moho deepens to about 50 km just in the west of the Talesh Mountains. Under the Talesh the situation is more complicated. There, an abrupt jump in moho to a depth of 28 km is observed. Furthermore, the moho under the Talesh has a steeper dip. It is believed that the Talesh Mountains are being thrust over the south Caspian from the west. Geophysical evidence also show that the Caspian crust is much thinner than its surrounding regions, and the Caspian coast line is a zone of transition in crustal thickness. For those stations situated in Talesh we observe some dependence of the receiver functions on back-azimuth. This can be suggestive of dipping discontinuities in, or at the base of the crust.