We investigate the shallow crustal structure in NW Iran by utilizing the vertical-component of P waves derived from the autocorrelation of ambient seismic noise time series recorded by a temporary broadband seismic network that covers the southwestern Caspian coastal region, the western Alborz and Tarom mountains, and the Zanjan-Abhar region which lies on the northern margin of central Iran. The autocorrelation time series at each seismic station contain reflection pulses from seismic discontinuities. We use Bayesian statistics to invert the autocorrelation series into 1-D P-wave velocities for each of the stations. For this purpose, we employ the Markov Chain Monte Carlo method along with the Delayed Rejection Adaptive Metropolis algorithm (DRAM) to draw samples from a probability distribution of model parameters. A seismic image based on the station velocity models along a profile in NW Iran is presented. The image reveals the shallow crustal discontinuities and sedimentary cover structure of the study area and we interpret them in terms of the tectonic setting and geodynamic evolution of NW Iran during the Cenozoic. Our results suggest that the Tarom Valley adjacent to the Alborz range is covered with low-velocity sediments with a thickness of about 5 km. At the same time a thinning of higher-velocity layers beneath the shallow sediments is observed. This structure is consistent with the reported extension of the crust in the Tarom Valley during the Neogene, and confirms that the formation of the valley has a tectonic origin. In the southwestern margin of the South Caspian Basin, the thickness of the youngest sedimentary layers ranges from 8 to 12 km (i.e., Pliocene-Quaternary deposits), sitting on top of older sequences of 10 to 12 km in thickness. The sediment cover progressively thickens from the foothills of the Alborz towards the Caspian coast. These results imply significant sediment accumulation in the Caspian basin and a pronounced structural contrast with the neighboring region of Alborz and Talesh mountains. In the southern part of the study area, the Soltanieh and Tarom mountains both exhibit uplift of the upper crustal layers down to 10-15 km depth range. The uplift of these layers indicate the dominance of crustal compressional regime in the latter part of the Cenozoic in the Alborz and the northern margins of central Iran. The velocity model shows the underthrusting of central Iran under the Alborz along the Manjil Fault. At the same time no clear indication of underthrusting of the Caspian block southward under the northern Iran is discernible.