We present a new 2D crustal-scale model of the northwestern Iranian plateau based on gravity-magnetic modeling along the 500 km long China-Iran Geological and Geophysical Survey in the Iranian plateau (CIGSIP) seismic profile across major tectonic provinces of Iran from the Arabian plate into the South Caspian Basin (SCB). The seismic P-wave receiver function (RF) model along the profile is used to constrain major crustal boundaries in the density model. Our 2D crustal model shows significant variation in the sedimentary thickness, Moho depth, and the depth and extent of intra-crustal interfaces. The Main Recent Fault (MRF) between the Arabian crust and the overriding central Iran crust dips at approximately 13 degrees towards the northeast to a depth of about 40 km. The geometry of the MRF suggests about 150 km of underthrusting of the Arabian plate beneath central Iran. Our results indicate the presence of a high-density lower crustal layer beneath Zagros. We identify a new crustal-scale suture beneath the Tarom valley between the South Caspian Basin crust and Central Iran and the Alborz. This suture is associated with sharp variation in Moho depth, topography, and magnetic anomalies, and is underlain by a 20 km thick high-density crustal root at 35-55 km depth. The high-density lower crust in Alborz and Zagros may be related to partial eclogitization of crustal roots below about 40 km depth. The gravity and magnetic models indicate a highly extended continental crust for the SCB crust along the profile. Low observed magnetic susceptibility of the Kermanshah ophiolites likely indicates that the ophiolite rocks only form a thin layer that has been thrust over the sedimentary cover.