Pressure variations among rheologically heterogeneous elements in Earth's lithosphere: A micromechanics investigation

Extrinsic Anisotropy of Two‐Phase Newtonian Aggregates: Fabric Characterization and Parameterization

“…However, these limits still represent the upper and lower bounds of the mechanical properties of an aggregate. To overcome these limitations, alternatives methods in the field of micro‐mechanics (Mura, 1987; Nemat‐Nasser & Hori, 2013; J. Qu & Cherkaoui, 2006) based on Eshelby's work Eshelby (1957, 1959) have been developed and applied to geology in recent years (e.g., Dabrowski et al., 2012; Jiang, 2014; Jiang & Bhandari, 2018; Yang et al., 2019). These methods however work under the assumption of ellipsoidal inclusions.…”

“…Two-phase aggregates are mechanically heterogeneous materials where the stiffness tensor of the composite is no longer isotropic. It is not possible to infer all the components of the anisotropic viscous tensor of the composite directly from the 3D models, but only an effective viscosity (e.g., Jiang & Bhandari, 2018): second invariant of an arbitrary second-order tensor C, defined as…”

Comment on “Pressure‐to‐Depth Conversion Models for Metamorphic Rocks: Derivation and Applications” by Bauville and Yamato

“…There is no microstructural evidence from (U)HP rocks that supports this. Third, because (U)HP rocks are rheologically distinct bodies constrained at great depth in the lithosphere, the stress orientations and magnitudes in them are determined by their mechanical interaction with the surrounding lithosphere (Eshelby, 1957; Jiang, 2016; Jiang & Bhandari, 2018), and are unlikely Andersonian.…”

“…This interpretation assumes that P p and P r are approximately lithostatic (lithostatic assumption, hereafter). In reality, both P p and P r may deviate from the lithostatic values, but the magnitude of deviation is limited by the rock strength, which is likely less than hundreds of MPa for the time scale relevant for (U)HP metamorphism and far below the GPa level lithostatic pressure (e.g., Jiang & Bhandari, 2018).…”

“…This interpretation assumes that P p and P r are approximately lithostatic (lithostatic assumption, hereafter). In reality, both P p and P r may deviate from the lithostatic values, but the magnitude of deviation is limited by the rock strength, which is likely less than hundreds of MPa for the time scale relevant for (U)HP metamorphism and far below the GPa level lithostatic pressure (e.g., Jiang & Bhandari, 2018).The pressure data from global (U)HP rocks as compiled in Bauville and Yamato (2021) are replotted in the p P versus ΔP space (Figure 1a) and in the p P versus P r space (Figure 1c). Yamato and Brun (2017, p. 47) claim that the linear relation between p P and ΔP "…puts in question the classical interpretation (based on the lithostatic assumption) of P-T-t paths …" but supports their model that invokes excessive overpressures.…”