Yamato and Brun (Nature Geoscience 10, 46-50, 2017) claimed that metamorphic data from global (ultra)high-pressure ((U)HP) rocks exhibit an unusual linear relation, between peak pressure and pressure drop, which challenges current interpretation of P-T-t paths but supports their model invoking excessive overpressures. If their model holds, most research on (U)HP rocks since their discovery would require serious reconsideration. Here, I demonstrate that their model requires critical assumptions that are neither justified by the principles of rock mechanics in the context of realistic geologic settings nor consistent with microstructures of (U)HP rocks. Furthermore, contrary to their claim, the global (U)HP data can be readily explained in the current framework but are inconsistent with their model prediction.

Bauville and Yamato (2021, G-cubed, https://doi.org/10.1029/2020GC009280) propose model-based methods to convert metamorphic pressures to depths based on the claim that pressure data from global (ultra)high-pressure rocks challenge the lithostatic assumption and support their model which invokes excessive overpressures. It is argued here that the opposite is true: Natural pressure data are fully consistent with the lithostatic assumption. They reflect selection of (ultra)high-pressure rocks by accessibility and preservation. The data are however inconsistent with the model predictions of Yamato and Brun (2017, Nature Geoscience 10, 46-50) and Bauville and Yamato (2021). Furthermore, their model requires critical assumptions that are not justified by the principles of rock mechanics and unsupported by microstructures from (U)HP rocks.

We quantitatively investigated the flow laws of wet quartzite in steady-state dislocation creep regimes by considering both the dependence of the activation enthalpy on pressure and dependence of the stress exponent on slip systems. From a critically-selected set of creep experiments of wet quartzite with microstructures and c-axis fabrics suggesting steady-state dislocation creeps, we obtained two endmember flow laws corresponding respectively to dominant prism slip and dominant basal slip systems. To characterize the dislocation creep of wet quartzite by a continuous combination of prism and basal slips commonly observed in nature and experiments, we developed a self-consistent micromechanics-based homogenization approach. Our results reconciled the large discrepancies in flow law parameters for wet quartzite determined from different creep experiments and are broadly consistent with microstructures and c-axis fabrics from nature and experiments as well as theoretical considerations.

Quartz c-axis fabrics in natural mylonites can vary to such an extent that they apparently give opposite senses of shear in a single thin section. Many hypotheses have been invoked to explain this. Here, we couple our self-consistent multiscale approach for flow partitioning with the visco-plastic self-consistent model for crystallographic fabric simulation to investigate quartz c-axis fabric development. Quartz aggregates are regarded as microscale Eshelby inhomogeneities embedded in a macroscale medium whose effective rheology is represented by a hypothetical homogeneous equivalent medium which is rheologically isotropic or has a planar anisotropy. We reproduced the observed quartz c-axis fabrics. We found that, although the microscale flow fields are distinct from one another and from the macroscale flow, the microscale vorticity in every inhomogeneity has the same sense as the macroscale vorticity. This implies that one can use the average of the microscale vorticity axes determined through the crystallographic vorticity axis analysis to obtain the macroscale vorticity axis. However, quartz c-axis fabrics cannot be used to determine the vorticity number where flow partitioning is significant.

Folds with axial traces parallel to the extension direction are a common feature in continental detachment systems and metamorphic core complexes. Yet, how they form has been puzzling for many decades. Here, we show that the key to solving the conundrum lies in revising the long-held single-scale view toward natural deformation and application of kinematic models. We demonstrate that extensional folding can result naturally from the partitioned stress field in competent layers in plate-scale extension and transtension deformations. Competent layers that develop extension folds should be regarded as rheological inclusions in the lithosphere rather than infinitely extending plates clamped at system boundaries and subjected to system boundary conditions.