Some attributes of wavefields scattered from Ivrea-type lower crust

Abstract

The Ivrea Zone in northern Italy is a well studied sliver of extended lower continental crust which was brought to the surface in the course of Alpine lithospheric shortening. Based on two 1:25,000 geological maps from the central Ivrea Zone and petrophysical parameters of the corresponding lithologies we have constructed geologically constrained, stochastic seismic models of Ivrea-type lower crust. A stochastic rather than a deterministic approach is applied due to the limited availability of. detailed geological maps and the high degree of small-scale structural and petrophysical complexity of the Ivrea Zone. The primary characteristic of the resulting model is its “layered” self-affine or fractal structure and its bimodal velocity distribution. While we are not suggesting that Ivrea-type structure is universal for extended lower continental crust, synthetic seismograms may help to constrain some pertinent quantitative aspects observed in deep seismic reflection data: 1. (1) Ivrea-type lower crust can explain the igh observed reflection coefficients and bright reflectivity in the lower crust; 2. (2) observed lower-crustal Q-factors can be largely explained by scattering losses and intrinsic Q-factors of reflective lower crust are likely to be larger than 1000, which is incompatible with the presence of free volatiles; 3. (3) in agreement with recent broad band studies of the lower crust, Ivrea-type lower crust does not create any tuning effects; and 4. (4) for Ivrea-type heterogeneity the lateral correlation of the reflected signal is largely independent of the lateral characteristic scale of the scattering structures, which suggests that multiple scattering may be important in the reflective lower crust.