Curie point depths of Bulgarian territory inferred from geomagnetic observations and its correlation with regional thermal structure and seismicity

The basal depth of the magnetic layer (Curie point depths) of Bulgarian territory was calculated using geomagnetic field observation and the variations of this border were mapped for the first time. The adopted computational method transforms the spatial data into the frequency domain and provides a relationship between radially averaged power spectrum of the magnetic anomalies and the depths to the magnetic sources. The magnetic layer bottom thus obtained has “normal” (28–32 km) values in the Moesian platform and central part of Bulgaria which are the relatively most stable regions of the investigated territory. In the Moesian platform CPD and Moho are very close but the magnetic layer's bottom is still marked as a thermal boundary. In the southern part, calculated depths gradually reach the minimal values of the derived Curie isotherm (up to 14–16 km) with relatively high gradient. According to the results from geothermal modeling and paleomagnetic investigations, magnetic layer's bottom of the Bulgarian territory corresponds to different Curie temperatures. They vary from 400–425 °C in the regions of Rila-Rhodope unit, up to 550–600 °C in the Moesian platform, West- and East-Balkan unit. The obtained results comprise useful information concerning regional estimates of the correlation between different geophysical and geological characteristics of the earth's crust of the Bulgarian territory.