Magnetic spectral analysis in Portugal and its adjacent seas

Abstract

Magnetic spectral analysis, which has often been applied to estimate Curie point depths, was used to delineate thermal and crustal structures of Portugal and its adjacent seas. The magnetic data set was the grid file of 5 km interval. Assuming random samplings from magnetic prisms, we obtained power spectra of two-dimensional square areas by the double Fourier transform and estimated statistical average depths. The cell-size for the area we used is 320 km. The average centroid depth, z 0, was obtained automatically from the gradient of natural log of [(power spectrum)/(radial frequency)] in the range between 0.005 and 0.02 km −1 in frequency (50–200 km in wavelength). The average depth to the top of the magnetic layer, z t, was estimated from the gradient of the power spectrum between 0.02 and 0.04 km −1 in frequency (25–50 km in wavelength). The average depth to the bottom of the magnetic layer is calculated from 2 z 0− z t. The results show that the bottom depths of land areas range between 20 and 55 km below sea level. The statistical errors of centroid depth are within 8 km. Marine linear magnetic anomalies widely cover the offshore areas, where random samples of prisms is inconsistent. We take one-dimensional analysis for the areas. The south of Portugal marks shallow depths of 20–30 km, while deep depth areas greater than 30 km spread widely in the north. One-dimensional analysis of the offshore area indicates the depth of marine magnetic layer of oceanic crust. The wide variation of depth is interpreted to be a reflection of the crustal and the thermal structures. Since the Moho must correspond to the base of the magnetic layer, the results could be the depths to the Moho instead of the Curie point isotherm in the northern Portugal. South Portuguese Zone is in the area ranging from 20 to 30 km in depth. These depths are shallower than the Moho. Two-dimensional forward modeling confirms that there is a throw of the bottom of the magnetic layer across the Ficalho over thrust from 35 to 40 km in the Ossa Morena Zone to 20–30 km in the South Portuguese Zone. There is a geothermal anomaly of high heat flow density values more than three times the normal average value. The Curie point isotherm estimated by the anomaly is shallower than the depth to the bottom of the magnetic layer. One interpretation is that the bottom of the magnetic layer corresponds to a Moho depth that could be 28–30 km in the Ossa Morena Zone and to a Curie point isotherm that could be 20–30 km in the South Portuguese Zone. The high heat flow density values of this area could be produced by hydrothermal convection along tectonic lines such as the Ferreira–Ficalho over thrust.