Thermo-petrological constraints on seismic velocities of the Adria lower crust

The Adria microplate represents the main geodynamic driver in the central Mediterranean, and its interaction with the surrounding plates controls the distribution of stress, strain and seismicity across the adjacent domains. In this context, the geometry and thermal structure of the lithosphere play a key role in partitioning the deformation across the Tyrrhenian–Apennines–Adriatic system. However, these properties remain poorly constrained by direct observations. Here, we address this problem through thermo-petrological forward modelling constrained by geophysical data aimed at quantifying lateral variations in lower-crustal seismic velocities.

The modelling was performed along a profile across the central Apennines, constructed using a structural and density model of the crust and upper mantle. The profile was sampled at multiple points to derive geothermal and lithostatic gradients from heat-flow and density data, thereby constraining pressure-temperature conditions along the section. Moho depth and its associated uncertainties were incorporated into the pressure-temperature estimates.

We adopted pyroxenite, peridotite, and metagabbro samples from well-exposed natural analogues as proxies for the lower crust and upper mantle of the Adria lithosphere. For each lithology, stable mineral assemblages, phase proportions, elastic properties and seismic velocities were computed as a function of pressure and temperature using the thermodynamic and elastic modelling code Perple_X (Connolly, 2005).

Calculations were performed using a mantle-oriented thermodynamic database and complemented by a sensitivity test based on an alternative parametrization optimized for crustal petrology, to quantify how differences in thermodynamic databases affect phase assemblages and the resulting seismic velocities.

Modelled P- and S- wave velocities were compared with independent laboratory measurements on representative rocks and with regional seismic tomography to assess the consistency between mineral assemblages, seismic velocities and independent constraints, indicating that the adopted thermo-petrological structure provides a realistic representation of the Adria lower crust and upper mantle.

References

Connolly JAD (2005). Computation of phase equilibria by linear programming: A tool for geodynamic modeling and its application to subduction zone decarbonation. Earth and Planetary Science Letters 236:524-541.