Moho depth and lithospheric thickness of the Arabian and Eurasian collision zone from potential field data


1 Assistant Professor, Department of Earth Physics, Institute of Geophysics, University of Tehran, Iran

2 Professor, Department of Earth Physics, Institute of Geophysics, University of Tehran, Iran

3 Professor, Faculty of Earth Sciences, Paris University, France


The targeted area of this research includes E Anatoly, NW Zagros, and Caucasus. These structures are known as a complex and active area and in the early stage of continent-continent collision, which give us unique possibility to monitor such collision in real time. Therefore, it is very important to study this active area to have a better knowledge about its tectonic behavior and lithospheric structure. Key parameters that we are looking for in this research are Moho depth, lithosphere-asthenosphere boundary (LAB) and average crustal density.
There are methods, which can give us some information about lithospheric structure such as the seismological method, seismic (controlled source) method, magnetotelluric, volcanology etc. The method used here is a direct, linearized, iterative inversion procedure in order to determine lateral variations in crustal thickness, average crustal density and lithospheric thickness via potential field data. The area of interest is subdivided into rectangular columns of constant size in E-W (X) and N-S (Y) directions. In depth (Z), each column is subdivided into four layers: seawater if present (with known thickness, i.e. bathymetry, and a density of 1030 kg/m3), crust, lithospheric mantle, and asthenosphere. For our research, the definition of the LAB is an isotherm and we try to calculate the temperature distribution in the lithosphere. During the inversion process, a cost function has to be minimized defined as C=Ed+lEp+mEs. The factor l allows controlling the overall importance of parameter variability (Ep) with respect to data adjustment (Ed), whereas m is a factor controlling the importance of smoothing, which can be different for each parameter set.
The method uses potential field data (free air gravity, geoid, and topography) which are globally available by satellite measurement and are freely accessible on the internet. The potential field data are sensitive to the lateral density variations, which happen across these two boundaries but at different depth. Free air gravity data are 2.5×2.5 arc-minute grid, which was taken from the database of Bureau Gravimétrique International (BGI). Geoid height variations correspond to the EGM2008 model. In order to avoid the effects of sublithospheric density variations on the geoid, we have removed the long-wavelength geoid signature corresponding to spherical harmonics until degree and order 10, tapered between 8 and 12. Topography data are taken from the 1-minute TOPEX global data sets. All data were interpolated on a regular 10x10 km grid.
Inverting potential field and topography data suffers from non-uniqueness since these data are not sensitive to vertical density variations, which may produce instabilities of the solution. Stabilization of the inversion process may be obtained through parameter damping and smoothing as well as the use of a priori information like crustal thicknesses from seismic profiles.
The 3D results show an important crustal root under Caucasus and relatively thick Moho for the eastern part of Anatolia and NW Zagros and a thin crust under the southern part of the Black Sea, which is thickening northward. Regarding LAB, the 3D results show thin lithosphere under the E-Anatolia, NW Zagros and the western part of Caucasus. The LAB thickens northward towards the Eurasia and in the western part of Anatolia.


Main Subjects

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