Abstract
We present results from a joint inversion of new seismic and recently compiled gravity data to constrain the structure of a prominent geophysical anomaly in the European Alps: the Ivrea Geophysical Body (IGB). We investigate the IGB structure along the West-East oriented Val Sesia profile at higher resolution than previous studies. We deployed 10 broadband seismic stations at 5 km spacing for 27 months, producing a new database of ∼1000 high-quality seismic receiver functions (RFs). The compiled gravity data yields 1 gravity point every 1–2 km along the profile. We set up an inversion scheme, in which RFs and gravity anomalies jointly constrain the shape and the physical properties of the IGB. We model the IGB’s top surface as a single density and shear-wave velocity discontinuity, whose geometry is defined by four, spatially variable nodes between far-field constraints. An iterative algorithm was implemented to efficiently explore the model space, directing the search toward better fitting areas. For each new candidate model, we use the velocity-model structures for both ray-tracing and observed-RFs migration, and for computation and migration of synthetic RFs: the two migrated images are then compared via cross-correlation. Similarly, forward gravity modeling for a 2D density distribution is implemented. The joint inversion performance is the product of the seismic and gravity misfits. The inversion results show the IGB protruding at shallow depths with a horizontal width of ∼30 km in the western part of the profile. Its shallowest segment reaches either 3–7 or 1–3 km depth below sea-level. The latter location fits better the outcropping lower crustal rocks at the western edge of the Ivrea-Verbano Zone. A prominent, steep eastward-deepening feature near the middle of the profile, coincident with the Pogallo Fault Zone, is interpreted as inherited crustal thickness variation. The found density and velocity contrasts of the IGB agree with physical properties of the main rock units observed in the field. Finally, by frequency-dependent analysis of RFs, we constrain the sharpness of the shallowest portion of the IGB velocity discontinuity as a vertical gradient of thickness between 0.8 km and 0.4 km.
Highlights
The geologically defined Ivrea-Verbano Zone (IVZ) and the related but much longer Ivrea Geophysical Body (IGB) belong to the most outstanding features of the whole Alpine domain
We focus on a central cross-section of this most recent 3D IGB model (Figure 1B) and we integrate the gravity dataset with new high-resolution broad-band passive seismic data, recorded during the IvreaArray passive seismic experiment (Hetényi et al, 2017)
The inversion results can be interpreted in light of the existing multidisciplinary investigations on the IVZ formation history and the surrounding crustal structures
Summary
The geologically defined Ivrea-Verbano Zone (IVZ) and the related but much longer Ivrea Geophysical Body (IGB) belong to the most outstanding features of the whole Alpine domain. The reader is referred to (Bodin et al, 2012) for further discussion on RF inversion approaches For this joint gravity and seismic data inversion, we implemented an iterative algorithm to explore the ensemble of all possible IGB models (i.e., the model space) and to evaluate their performance in terms of their fit to the observations. We model the IGB’s upper boundary as a single 2D discontinuity with a few segments, associated with sharp density (δρ) and shear-wave seismic velocity (δvS) contrasts (Figure 5) representing the IGB bulk physical properties with respect to the surrounding crust While the former parameters are allowed to vary independently during the inversion, a homogeneous crustal background is assigned to the model and characterized by absolute values of vS = 3.5 km/s and density = 2700 kg/m3, which is a consistent choice of velocity and density values, according to the vS-density relationship from Brocher (2005) (as further discussed in section “Results”). We used the a priori knowledge on the IGB to assign reasonable boundaries to all inversion parameters (as discussed in section “Model parameterization”) and we use a performance-driven pseudo-random walk to guide our exploration toward the best-fitting areas of the model space, to retrieve an ensemble of acceptable IGB models, which reproduce and explain the observed datasets
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