Crustal and lithospheric model of the Alborz, Central Iran and South Caspian Basin, using geoid and topographical data sets

Geological events and the curiosity of human mind to comprehend these phenomenacompel the researchers to investigate their structures and tectonic evolution. Some keyparameters to better understand these subjects are Moho Depth (the boundary betweencrust and mantle) and also the Lithosphere-Asthenosphere Boundary (LAB). There are methods available which can give us some knowledge about these key parameters such as seismic, magnetotelluric, volcanologic and etc. Each one has advantages and disadvantages. In the seismological method, a period of about six months is needed to be sure that a reasonable quantity and quality of events has been detected to record enough data during the research. The high expense of instruments and lack of access roads in high topography has limited this method to adequately capture the data researchers seek. These problems also exist in the seismic base method. The seismic method is generally expensive. Moreover, this method is nearly blind in lithospheric depth such as LAB.We tried to introduce another method that used potential field data. Our data weretopography and geoid undulation mainly observed by satellites. The method for this studyutilizes some basic concepts such as local isostasy as wel as some basic physical andmathematical rules, relations and equations. Our topography data were from the newlyreleased topography database for all over the world, ETOPO1. The spatial resolution ofthe data were 1 Arc-minute. The geoid undulation was calculated by a spherical harmonicup to order 2159 and degree 2190 from Earth Gravitational Model's 2008 (EGM2008). Toavoid the effects of anomalies deeper than LAB, wavelengths greater than 4000 km wereremoved. There were some advantages to this method such as: the higher speed of thecalculation so that the examination of a big region was possibile at a fraction of the costfor other methods. Modeling was done on a very substantial area in the Northern part ofthe Iranian plateau that included the Northern part of Central Iran, the Alborz Mountainsand the South Caspian Basin.The results showed the evidence of thickening of the crust up to ~55 km underneath the Alborz Mountains. However, many previous researchers concluded no roots there. The other outcome of utilizing this method was thinning the crust and lithosphere underneath the Northern part of Central Iran (36-38 km for the crust and 140 km for the LAB) relative to the surrounding area. Our data reflected a solid correlation with some previous work and geological evidence. Subduction of the south Caspian basin (probable oceanic crust) underneath the Eurasia plate is completely visible; however, this activity was not recognized in Kopet-Dagh.