Curie temperature depths in the Alps and the Po Plain (northern Italy): Comparison with heat flow and seismic tomography data

Abstract

We report on the spectral analysis of the aeromagnetic residuals of the Alps and the Po Plain (northernItaly) to derive the Curie point depth (CPD), assumed to represent the 550◦C isotherm depth. We ana-lysed both the aeromagnetic residuals of northern Italy gathered by Agip (now Eni) and the recent EMAG2compilation. We used the centroid method on 44 and 96 (respectively) 100 × 100 km2windows consid-ering both a random and a fractal magnetization distribution, but found that, at least for the Alps, thefractal model yields unrealistically shallow CPDs. Analyses considering a random magnetization modelgive CPDs varying between 12 and 39 km (22 to 24 km on average considering the two data sets) in the PoPlain, representing the Adriatic-African foreland area of the Alps, in substantial agreement with recentlyreported heat flow values of 60–70 mW/m2. In the Alps, the Eni data set yields shallow CPDs rangingbetween 6 and 23 km (13 km on average). EMAG2 analysis basically confirms the “hot” Alpine crust, butreduces it to three 50–100 km wide patches elongated along the chain, where CPDs vary between 10 and15 km. Such “hot” Alpine domains occur just north of maximum (50–55 km) crustal thickness zones ofthe Alps and correspond to low seismic wave velocity anomalies recently documented in the 20–22 to35–38 km depth interval, whereas no relation is apparent with local geology. Assuming an average crustalthermal conductivity of 2.5 W/m◦C and a steady-state conductive model, CPDs from the hot zones of theAlps translate into heat flow values of 110–120 mW/m2, and to a basal heat flow from the mantle exceed-ing 100 mW/m2that is significantly greater than that expected in a mountain range. Thus we concludethat the steady-state conductive model does not apply for the Alps and granitic melts occur at ∼15–40 kmdepths, consistently with seismic tomography evidence.