The coherence function and lithospheric elastic thickness of the Zagros fold and thrust belt

Abstract

SUMMARY This study derives the spatial variation of the elastic thickness (Te) and its implications for understanding the structure, geodynamic and seismicity of the lithosphere for the Zagros fold and thrust belt region of the Arabia–Eurasia collision zone. Te is calculated using the coherence function in the fan wavelet domain based on recent terrestrial Bouguer gravity and topography data as input signals. Utilizing the load deconvolution method and Brent's method of 1-D minimization, the final Te for the survey region is estimated for each grid node of the studied area. To illustrate the mass distribution in the studied area, the subsurface loading fraction (F) is calculated simultaneously with Te in the inversion. The crust thickness and density from three different global crustal models are tested and the results obtained for these input models do not yield substantially different Te patterns. The final results are in accord with the global Te models as well as previous rheological, geodynamical and flexural studies, however, this study establishes much more detailed regional information. The calculations yield a mean value of Te of 61 km for the Zagros, with a mean estimated error of about 5 km. The high-Te values (>70 km) are observed in the southeast of the studied area (some parts of the Sanandaj–Sirjan zone, Urumieh–Dokhtar magmatic arc and most of the Central Iranian blocks); while over most of the northwest of the studied area, the value of Te is about 58 km. The Te results are consistent with the lithospheric structure of the study area and also support the idea of the crust–mantle decoupling. Further, there is a positive and negative correlation between the surface wave velocity and surface heat flow, respectively. The mean value estimated for the internal loading friction (F) of 0.4 means in most of the studied areas we may consider that the surface loading is dominant, or at least the ratio of the surface and subsurface loading can be assumed equal. Based on earthquake distribution in the period 1900–2020, seismicity is more likely to occur in areas with a relatively low value of Te.