A new seismic model of Ivrea-type lower continental crust accounting for realistic structural complexity, spatial variability, and generic anisotropy

The Ivrea-Verbano Zone (IVZ) is widely regarded as an archetypal outcrop of Phanerozoic lower continental crust due to its comprehensive exposure, near-vertical structural orientation, and far-reaching absence of deformation and alteration. As such, geological evidence from the IVZ and associated laboratory measurements of pertinent rock physical properties have been widely used to construct canonical seismic models of the lower continental crust. These endeavors were either based on deterministic or stochastic approaches. While deterministic models allow us to directly relate the seismic response to field observations, they are inherently hampered by the fact that geological and rock physical information of adequate quality is only available for part of the IVZ. These problems can be alleviated through stochastic approaches to model building. Efforts of this kind were so far based on standard covariance-type statistics and, hence, were unable to fully capture the prevailing structural complexity and spatial variability. Quite importantly, studies accounting for seismic anisotropy were so far largely limited to deterministic or stochastic 1D layered models. To address these challenges, we use a multi-point statistics (MPS) approach, which we train on detailed geological maps from the central IVZ. In addition to the spatial information associated with the categorized lithologies, we also use the orientation of the foliation as part of the underlying training information. Each grid point of the resulting MPS simulations is then assigned anisotropic P- and S-waves seismic velocities associated with the categorized lithology at the given location. The values of the seismic velocities are randomly chosen from corresponding distributions based on available laboratory measurements. The generic nature of the seismic anisotropy prevailing the IVZ is accounted for by aligning the fast axes of the P- and S-wave velocities with the orientation of the foliation. To emulate the short-range coherence of the seismic velocity variations observed in sonic logs from two recent boreholes drilled into the upper and lower parts of the IVZ in the framework of the ICDP DIVE program, we subject these so far locally uncorrelated models to an accordingly parameterized autoregressive moving average (ARMA) process. Finally, we evaluate and assess the seismic response of this new generation of lower crustal models by simulating near-vertical and wide-angle synthetic seismic reflection profiles using a finite-difference solution of the generically anisotropic viscoelastic equations.