Seismic Signature of the Continental Crust: What Thermodynamics Says. An Example From the Italian Peninsula

Abstract

Unraveling the temperature distribution and composition of Earth's crust is key for understanding its origin, evolution, and mechanical behavior. Models of compressional (VP) and shear wave (VS) velocity are obtained from seismological studies and can be interpreted in terms of temperature and composition, using relationship defined through laboratory experiments. These empirical evidences often do not properly account for the effects driven by temperature, pressure, water content, and phase change of minerals. In this study, we use thermodynamic modeling to properly investigate the role of these variables in affecting seismic properties, as a tool to guide (joint) inversion and interpretation of geophysical data. We find that mineralogical phase transitions can be more seismically relevant than a change in chemical composition. In particular, the α‐β quartz transition would cause a jump in acoustic impedance and VP/VS ratio >8%, occurring in the 15–25 km depth range, depending on the thermal gradient. Moreover, in the case of a cold lower crust, the consumption of plagioclase in favor of high‐velocity minerals might represent another relevant seismic discontinuity. Different chemical compositions proposed for the Italian crust would be seismically indistinguishable, since they give overlapping seismic properties. Values of VS < 3.6 km s−1 would imply a strong contribution of sediments and/or partial melt. The VS/density ratio shows a narrow variability, suggesting that densities at depth can be directly derived in first approximation from VS.

Inferring Crustal Temperatures Beneath Italy From Joint Inversion of Receiver Functions and Surface Waves

“…The − quartz transition is dominantly temperature dependent and relies on the assumption of sufficient modal quartz in the bulk rock. In Diaferia and Cammarano (2017) we demonstrated that a quartz content below <20% vol is already sufficient to produce an appreciable increase in V P ∕V S at the transition, suggesting that especially within the upper and middle crust the detection of the quartz transition might be seismically relevant. However, caution is necessary for geologically complex areas with expected structural and lithological heterogeneity that can undermine the reliability of RFs (from which V P ∕V S are derived) due to the presence of dipping interfaces and anisotropy.…”

“…Following Piana Agostinetti & Malinverno (2010Malinverno ( , 2018 and Gao and Lekic (2018), we decide to treat V P ∕V S as a variable that can be randomly perturbed along the chain. As a matter of fact, both laboratory experiments (Peng & Redfern, 2013;Ohno et al, 2006) and thermodynamic modeling (Diaferia & Cammarano, 2017) suggest that V P ∕V S can strongly vary at crustal depths as a result of the phase transition of quartz from its to form. Therefore, given the scope of this study, we find that fixing V P ∕V S at an arbitrary value will hamper an unbiased interpretation.…”

“…Moreover, these are associated with high V P ∕V S ratio. Laboratory experiments on single minerals and rock aggregates (Peng & Redfern, 2013;Shen et al, 1993) and thermodynamic modeling (Diaferia & Cammarano, 2017) suggest that the − quartz transition is the major, seismically relevant phase change that can occur within the crust. The transition temperature is T q = 575 • C at ambient pressure and varies with a gradient of 0.0256 • C/bar (e.g., T q = 702 • C at 20 km and 777 • C at 30 km), according to the laboratory experiments of Shen et al (1993).…”

Constraints on the Structure of the Shallow Crust in Central Italy from Geophysical Log Data

“…In this work, we analysed only the sonic log data, although we are conscious that the variability of seismic wave velocity and density is a function also of temperature, pressure, chemical composition and water content 53,54 . Of all these factors, temperature may be the dominant influence on Vp and Vs, especially in areas of significant magmatism.…”

LitMod2D_2.0: An Improved Integrated Geophysical‐Petrological Modeling Tool for the Physical Interpretation of Upper Mantle Anomalies

“…Within the crust, P wave velocities are calculated using empirical V P ‐density relationships from Brocher (2005), whereas S wave velocities are calculated assuming a constant V P / V S = 1.73 ratio (Figure 10b). Indeed, an option including crustal material files is kept for future or if user have a thermodynamic database for the crustal chemical composition (e.g., Diaferia & Cammarano, 2017). Below 400 km depth (base of the model), velocities from ak135 model are used.…”