Giant megathrust earthquakes typically occur above the upper-plate Moho or the 350°C isotherm in warm subduction zones. However, large earthquakes also occur within the mantle wedge corner at 35-55 km depth at cold subduction zones, such as the Kermadec, Japan Trench, and Chilean margins. Here we investigate the frictional behavior of lawsonite blueschist metagreywacke, potentially found in cold subduction channels, to better understand the control of metamorphic grade on such first-order variations in seismogenic depth. We perform velocity-step experiments on metagreywacke gouge from room temperature to 500°C and effective normal stresses from 50 MPa to 320 MPa, capturing the unstable friction regime and the brittle-to-flow transition. The constitutive behavior of metagreywacke indicates a potential seismogenic behavior at high temperatures below the Moho. Large megathrust earthquakes in the mantle wedge corner may develop in the lawsonite blueschist metasediment channel of cold subduction slabs.

A piezometric relationship is derived between subgrain size and stress in plastically deformed crystalline materials, based on the assumption that subgrain boundaries become stabilized once their strain energy per unit area exceeds that in the adjacent lattice. The subgrain diameter at this point depends on the density of geometrically necessary dislocations (GND), many of which are housed in the subgrain walls, and on their spacing in the walls. Subgrains grow by merging, which results in a linear relationship between size and misorientation. The piezometric relationship for any given misorientation has a stress exponent close to unity. The subgrain piezometer can be calibrated using literature-based estimates of the size of dynamically recrystallized grains produced by subgrain rotation (SGR) for different minerals.Grain-size piezometers reflect processes of nucleation and post-nucleation modification during dynamic recrystallization. New grains formed by these different processes occupy different regions in grain-size/stress space. These regions are bounded by the subgrain piezometer and the Dmin line, which marks the smallest size that newly formed grains can have without being eliminated by surface-energy driven grain-boundary migration. The piezometers are statistically defined by the scatter of grain-sizes produced by these different processes. Piezometers defined in this way for quartz, olivine, and calcite correspond well within uncertainties to experimentally determined piezometers. No experimental piezometer exists for feldspars; a piezometer based on the concepts advanced in this paper is suggested that can be tested by experiment.

Quartz and ice both exhibit distinctive microstructures when deformed at low stress and high homologous temperature, known as grain-boundary migration (GBM) microstructures. These are difficult to reproduce experimentally in silicate minerals, and no correlation has been established between quantifiable aspects of the microstructure and deformational conditions. We carried out direct shear experiments to investigate the effect of stress and temperature (T) on GBM microstructures in ice, which is crystallographically analogous to quartz. Differential stress was 0.7-6.0 MPa, confining pressure 4-9 MPa, and T -3 degrees to -25 degrees C. There is a clear transition at - -12 degrees C from granular microstructures produced by rotation recrystallization at high stress and low T, to GBM microstructures at low stress and high T, accompanied by an increase in strength of the crystallographic preferred orientation (CPO). Samples with GBM microstructure show lobate grain-boundaries and "island grains" where several distinct and separate areas have the same crystallographic orientation, representing lobes of a single grain isolated on the cut surfaces. The size of lobes and island grains define an array with respect to stress with a slope of 0.9, similar to but offset from the slope of published stress/grain-size data. This suggests the possibility of determining paleostress from GBM microstructures in both ice and quartz.

It is widely recognized that major brittle faults in the upper crust transition downwards into ductile shear zones that then widen with depth. However, the controls on shear zone width at any specific depth, and the mechanisms that may cause the width to change over time, are less well understood. This study therefore reconstructs the geometry and rheology of the crustal-scale, normal-sense Simplon Shear Zone between the depths of similar to 3 and similar to 26 km, by combining field- and microstructural mapping, quartz paleopiezometry, and published estimates for the pressure, temperature, and timing of deformation at different crustal levels. The geometry is complex, with multiple strands actively deforming at any one depth, although overall the cumulative width does increase downwards. A scaling law is developed relating shear zone width to rheology and displacement rate, assuming simple shear and neglecting stretching parallel to the shear plane, and rheological parameters for granitoids with a quartz, feldspar, or mica dominated microstructure are quantified. Localization during exhumation was primarily accompanied by hydrous retrogression of feldspar to white mica, and the associated formation of interconnected weak mica layers. This weakened the shear zone, allowing the degree of strain localization to increase (and the shear zone width to decrease) by a factor of similar to 4-9.

The Eastern Belt of the Franciscan Complex in the northern California Coast Ranges consists of coherent thrust sheets predominately made up of ocean floor sediments subducted in the Early Cretaceous and then accreted to the overriding plate at depths of 25-40 km. Progressive packet accretion resulted in the juxtaposition of a series of thrust sheets of differing metamorphic grades. This study utilizes laser Raman analysis of carbonaceous material to determine peak metamorphic temperatures across the Eastern Belt and phengite barometry to determine peak metamorphic pressures. Locating faults that separate packets in the field is difficult, but they can be accurately located based on differences in peak metamorphic temperature revealed by Raman analysis. The Taliaferro Metamorphic Complex in the west reached 323-336 degrees C at a minimum pressure of similar to 11 kbar; the surrounding Yolla Bolly Unit 215-290 degrees C; the Valentine Springs Unit 282-288 degrees C at 7:8 +/- 0:7 kbar; the South Fork Mountain Schist 314-349 degrees C at 8.6-9.5 kbar, a thin slice in the eastern portion of the SFMS, identified here for the first time, was metamorphosed at similar to 365 degrees C and 9:7 +/- 0:7 kbar; and a slice attributed to the Galice Formation of the Western Klamath Mountains at 281 +/- 13 degrees C. Temperatures in the Yolla Bolly Unit and Galice slice were too low for the application of phengite barometry. Microfossil fragments in the South Fork Mountain Schist are smaller and less abundant than in the underlying Valentine Springs Unit, providing an additional method of identifying the boundary between the two units. Faults that record a temperature difference across them were active after peak metamorphism while faults that do not were active prior to peak metamorphism, allowing for the location of packet bounding faults at the time of accretion. The South Fork Mountain Schist consists of two accreted packets with thicknesses of 300m and 3.5 km. The existence of imbricate thrust faults both with and without differences in peak metamorphic temperature across them provides evidence for synconvergent exhumation.

Present exposure of the ductile Caledonian retrowedge in northwestern Scotland records the evolution of a shear zone that was exhuming while actively deforming, providing a natural laboratory to study strain localization in a progressively cooling system. Examination of rocks from two detailed transects across this region consistently show a transition from microstructures that are dominated by interconnected phyllosilicate networks in a quartz-rich matrix with feldspar porphyroclasts, to interconnected fine-grained regions of mixed quartz + phyllosilicate + feldspar. These polyphase regions are demonstrably weaker than surrounding quartz layers and likely deform by grain-size sensitive mechanisms including diffusion-accommodated grain boundary sliding. Microstructures characterized by a quartz-rich matrix and interconnected phyllosilicates undergo quartz recrystallization by high temperature grain boundary migration and are dominated by prism a slip. In contrast, fine-grained polyphase microstructures record quartz recrystallization dominated by subgrain rotation and activation of rhomb a and basal a slip systems. We propose transient hardening occurs in quartz-dominated regions as quartz with a strong Y-axis maximum undergoes the switch from prism a easy slip to basal a easy slip during cooling, and thus partitions strain into interconnected phyllosilicate layers. In response, interconnected phyllosilicate layers undergo mechanical comminution, becoming increasingly mixed by grain-size sensitive creep processes to form polyphase layers as they accommodate an increased proportion of strain. This transition from quartz-rich matrix with phyllosilicate interconnected weak layers to fine-grained, polyphase weak layers could be of first-order importance in strain localization within polyphase mylonitic and ultramylonitic rocks.

The upward-tapering channel model proposed by Marques et al. (2018) for the Himalayas has a "base" that forms part of the subducting footwall and therefore does not close the channel. This configuration does not produce return flow, and no dynamic overpressure develops in the channel. The geometrical and kinematic configuration they actually use for their calculations differs from this and is both geologically and mechanically improbable. In addition, the fixed upper boundary condition in their models is mechanically unrealistic and inconsistent with geological and geophysical constraints from the Himalayan orogen. In reality, the dynamic pressures calculated from their model, which exceed lithostatic pressure by as much as 1.5 GPa, would cause elastic flexure or permanent deformation of the upper plate. I estimate that a flexural upwarp of 50 km of the upper plate would be required to balance forces, which would lead to geologically unrealistic topographic and gravity anomalies. The magnitude of the dynamic overpressure that could be confined is in fact limited by the shear strength of the upper plate in the Himalayas, which is likely to be <120MPa.