Integrated geophysical-petrological inversion of surface wave and other data for the lithological structure of the Iberian crust

The Earth’s crust hosts most of the geo-resources of societal interests (e.g. minerals, geothermal energy etc.). Integrative approaches combining geophysical and petrological observations to study the mantle assuming thermodynamic equilibrium are relatively common nowadays. However, in contrast to the mantle, where thermodynamic equilibrium is prevalent, vast portions of the crust are thermodynamically metastable. This is because equilibration processes are essentially temperature activated and the temperature in the crust is usually too low to trigger them. Consequently, the mineralogical assemblage of crustal rocks is mostly decoupled from the in situ pressure and temperature conditions, reflecting instead the conditions present at the moment of rock formation. Here we present a new methodology for integrated geophysical-lithological multi-data modelling of the crust. Our primary constraining data are fundamental mode Rayleigh wave surface wave dispersion curves determined by interstation cross-correlation measurements and teleseisms, as well as surface elevation (isostasy) and heat flow. Additional prior information is provided by P-wave velocities coming from controlled source and body wave tomography data. The inversion is framed within an integrated geophysical-petrological setting where mantle seismic velocities and densities are computed thermodynamically as a function of the in situ temperature and compositional conditions. In the crust we invert for a three-layered crust defined by Vs, density and Vp/Vs ratios (or Poisson coefficients) linked according to statistical correlations from global petrophysical data sets. The new methodology is applied to the Iberian Peninsula and adjacent margins where we jointly invert for both the crustal and lithospheric mantle structure. Our results show that the Iberian upper-middle crust is characterized by a clear dichotomy between the high Vs and felsic lithologies (Vp/Vs<1.76) in the Iberian Massif, and the low Vs and mafic lithologies (Vp/Vs>1.81) in the Betic, Pyrenees and Cantabrian Alpine mountain chains. The pattern changes in the lower crust where we obtain felsic lithologies  in  the Central system, NE Betics and N Mediterranean margin, and mafic lithologies in the Ossa-Morena, South-Portuguese, Galician, and Asturian-Leones terranes in the Variscan Iberian Massif. Overall we find a good correlation with previous geophysical studies (receiver functions, controlled source seismics) and the petrology of the main magmatic episodes since the Neoproterozoic (575 Ma).