Elastic thickness of the Iranian lithosphere from gravity and seismic data

Abstract

We estimate the (effective) elastic thickness of the Iranian lithosphere (and adjoining tectonic plates) by using the approach that combines the Vening Meinesz-Moritz's (VMM) regional isostatic principle with the isostatic flexural model formulated based on solving a flexural differential equation for a thin elastic shell. To model the response on a load more realistically, we also consider the lithospheric density structure. The resulting expression describes a functional relation that links gravity field and mechanical properties of the lithosphere. The Young modulus and the Poisson ratio are computed from seismic velocity data in prior of estimating the lithospheric elastic thickness. The presented results reveal that the estimated elastic thickness closely resembles a regional tectonic configuration associated with the extensional tectonism along the Red Sea-Gulf Rift System, the continental collision of the Arabian and Eurasian plates, and the subduction along the Makran Subduction Zone. Seismically and volcanically active convergent tectonic margins of the Zagros and Kopeh Dagh Fold and Thrust Belts further extending along the Makran Accretionary Complex are characterised by a low lithospheric strength, with the elastic thickness typically less than ∼30km. These small values of the elastic thickness are in a striking contrast to much larger values within most of the Central Iranian Blocks. According to our estimate, local maxima there reach ∼70km in the Tabas micro-block. The elastic thickness of the Turan and Arabian Platforms reaches maxima of ∼100km. These results generally support the hypothesis that tectonically active zones and orogens have a relatively low strength, resulting in a significant response of the lithosphere on various tectonic loads, compared to a significant strength of old cratonic formations. Interestingly, however, we observe a striking contrast between a low strength of the Arabian Shield compared to a high strength of the Arabian Platform. A possible explanation of this finding could be given by a different thermal regime of the Arabian lithosphere, controlled mainly by a mantle upwelling and a consequent extensional tectonism along the Red Sea-Gulf Rift System.