Subduction-related intermediate-depth and deep seismicity in Italy: insights from thermal and rheological modelling

Geological data suggest that, after a period of oceanic lithosphere subduction, about 170 km of continental lithosphere were subducted under the Northern Apennines since about 23 Myear at rates of ca. 1 cm/year, whereas more than 700 km of Mesozoic oceanic lithosphere (Ionian lithosphere) have been subducted under the Calabrian arc at rates of about 3 cm/year. However, a well-developed Wadati-Benioff zone can be only recognised below the Calabrian arc down to depths of 450-500 km, whereas intermediate-depth seismicity reaches maximum depths of about 90 km under the Northern Apennines. We model the thermal state of these two subduction zones and apply temperature-dependent non-linear rheological laws to evaluate the down-dip extent of brittle regions in the two subduction zones. We show that differences in subduction rate and in slab composition (continental vs. oceanic) produce a far deeper (down to 290-380 km) brittle field in the colder Calabrian slab than in the warmer Northern Apenninic slab (70-120 km), thus explaining differences in the maximum depth of seismicity in both regions. Concerning deep earthquakes in the Calabrian subduction zone, models predict that metastable olivine persists down to depths of 430 km. This could possibly explain the maximum depth of earthquakes in the area. The small extent of the metastable olivine wedge produces a small density anomaly that cannot explain the pervasive down-dip compression observed in the Ionian slab. (c) 2004 Elsevier B.V. All rights reserved.