DEPTH OF CURIE TEMPERATURE ISOTHERM FROM AEROMAGNETIC SPECTRA IN IRAN: TECTONICS IMPLICATIONS

Using statistical spectral method corrected for the fractal nature of magnetization, we estimate the depth of Curie isotherm and depth to the top of magnetic layer for Iran. The aeromagnetic data acquired by the Geological Survey of Iran over 7.5 km line spacing is used after revised micro-leveling. To estimate the Curie depth, the depth at which rocks lose ferromagnetic properties, Iranian territory is divided into square blocks of 140x140 km in size overlapping by 80% in both horizontal directions. In each block, the radially averaged power spectra was computed, depths to the top and bottom magnetic layer are estimated for each block and assigned to its centre. The estimated Curie depth ranges between 12 and 24.3 km below the surface. The depth of top magnetic layer varies within a range of 1 to 13 km. The depth of Curie temperature is deeper across the Zagros Mountains and reaches to a maximum depth of 24.3 km in NW of the mountain. In the Alborz Mountains, Curie depth increases west to east towards the Kopeh-Dagh Mountains. In Orumiyeh-Dokhtar volcanic belt, it shallows and varies between 10 and 16 km. Makran subduction zone in SE Iran, the depth of Curie isotherm is about 23 km and increases west to east. In Central Iran it varies between 16 and 24 km, reaches to maximum value of 24 km within the Tabas region where there is a deep sedimentary basin. NW of Iran displays a significantly shallow Curie depth due to widespread volcanic activity.

Confinement of a major part of seismicity to depths above 25 km implies that the Curie depth closely defines the bottom depth of seismogenic layer in Iran.

The thickness of magnetic layer essentially is almost constant over the whole Iran and varies by around 10.5±1.5 km.

Assuming a constant vertical geothermal gradient in the upper crust and using our estimated depth to the top and bottom of magnetic layer and the map of surface mean temperature, we estimate thermal gradient map for Iran. It shows higher values of the thermal gradient over geothermal fields of Sabalan and Damavand Quaternary volcanoes. The thermal gradient map is cross examined with independent data acquired from Sabalan and Damavand geothermal sites.