Development of the crust model for Iranian plateau using Bouguer anomaly extracted from EGM2008 geopotential model

Bouguer anomaly is the difference between the real gravity and the gravity of the reference potential, which does not contain the effect of the topographical masses above the geoid. The crust thickness variation is well represented in the Bouguer anomaly.
 





^*نگارنده رابط:       تلفن: 46858955-021          دورنگار: 46858955-021                       E-mail: arash_mot@yahoo.co.uk
 





Since direct gravity observation in Iranian plateau is too difficult due to coverage of mountainous areas and deserts, consequently the anomaly extracted from EGM2008 geopotential model can be an appropriate data source. Maximum accessible spatial resolution equal to 0.1 degree, equivalent to10 kilometers is applied to reconstruct the crust model for Iranian plateau.
Since the average crust thickness in the region is 40 kilometers, the dominant expected 40 kilometer wavelength is predictable. To enhance the wavelengths more than 40 kilometers and attenuate the wavelengths less than that, a moving average 5×5 low pass filter is brought into played.   The filtered data in a 0.1 degree grid data is applied to construct a crust model in equivalent resolution grid. International crust model with 2 degree resolution is applied as priory information. Bouguer anomaly dataset consists of 30351 points ranged from -249.0mgal to +68mgal with average value of -86.0mgal and standard deviation equal to 48.4mgal.
According to Bouguer infinite slab forward equation ( ), a linear equation is fitted between Bouguer anomaly and crust thickness. The gradient of fitted line assessed to be -12.499 which is equivalent to the density factor equal to 300 kg/cubic meters. The estimated amount is less than the pre-assumed amount (400 kg/cubic meters).
The crust thickness ranges from maximum of 57.4km in north of Zagros and minimum of 16.1km in east of the Oman Gulf. The average value is 40.5km with the standard deviation of 5.24km.
The constructed model is statistically compared to (Dehghani and Makris (1983) model. The correlation factor between the two datasets is equal to 0.752 and the average difference between them is -1.90 kilometers with standard deviation equal to 3.6 kilometers. This model is also compared with the crust 2.0 modal.
 
Table1. Statistical comparison between the recent models, Dehghani and crust 2.0





 


Correlation factor


Covariance factor


Average of difference


Standard error of difference


Average deviation of difference


standard deviation of difference




Dehghani-Recent Model


0.752


16.59


1.899


0.314


2.772


3.603




CRUST 2.0-Recent Model


0.747


11.76


1.051


0.248


2.044


2.835




Dehghani-CRUST 2.0


0.704


14.89


0.848


0.339


2.841


3.879





 
The result shows that the observed and the recent model is better matched to the crust 2.0 model (Table 1).
More than 10 kilometers a beneath the central Alborz is observed. The comparison between; the crust thickness, estimated in 11 seismological local stations beneath the central Alborz by (Sodoudi et al., 2009) using receiver function, with the result of this method confirmed the accuracy of the recent model. The mentioned difference could be due to the lack of direct gravity observations in the region and has disproved the Dehghani’s hypothesis that Alborz Mountains have no isostatic root. (Dehghani and Makis 1983). Such difference can also be seen in other regions like North West of Alborz, Central Iran and Makran. With the lack of direct field observation in the regions.