Tectonothermal processes and mechanical strength in a recent orogenic belt: Northern Apennines

This paper investigates the seismic cutoff temperature at the maximum depth of seismicity across a young orogenic belt, the northern Apennines, by attempting to take account of the different tectonothermal processes which have occurred in the region since the last 20 Myr. The analysis of seismicity over a period of 20 years (1981–2002) in combination with the thermal regime shows that the western part of the northern Apennines (peri-Tyrrhenian zone) is characterized by extensional tectonics, few earthquakes, generally shallow and of low magnitude, and high surface heat flux. The entire crust of the eastern part of the orogenic belt (external zone) is affected by seismic activity of moderate magnitude and low heat flux. Earthquakes increase both in number and energy released in the inner portion of the belt (internal zone), where events with larger focal depth indicate subduction. The cutoff potential temperature at the largest depth of seismicity is 550 °C for the internal zone. In the external zone of the orogen, the temperature at the maximum depth of seismicity is about 550 °C. This result agrees with recent work on the thermal structure and seismicity of several continental areas. The limiting temperature in the peri-Tyrrhenian zone can, instead, exceed 750 °C. A possible explanation is that the deepest earthquakes might be associated to magma movement. The overall earthquake spatial distribution across the orogen appears related to the long-term rheology, which varies roughly in the same direction of tectonic transport of the chain. This consistency appears also from the b value of the frequency-magnitude relation, which can be interpreted in relation to the rheological properties of the medium. The b value seems to be inversely correlated to the expected mechanical strength of the crust and increases eastward in the direction of tectonic transport. The largest number of shocks and magnitude of the internal zone happen where a sharp lateral change of strength should occur.