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1. Imaging and Segmenting Grains and Subgrains Using Backscattered Electron Techniques

   https://doi.org/10.1093/mam/ozae092  

   Bennett, Thomas J; Taleff, Eric M (September 2024, Microscopy and Microanalysis)

   Abstract We present two new methods of processing data from backscattered electron signals in a scanning electron microscope to image grains and subgrains. The first combines data from multiple backscattered electron images acquired at different specimen geometries to (1) better reveal grain boundaries in recrystallized microstructures and (2) distinguish between recrystallized and unrecrystallized regions in partially recrystallized microstructures. The second utilizes spherical harmonic transform indexing of electron backscatter diffraction patterns to produce high angular resolution orientation data that enable the characterization of subgrains. Subgrains are produced during high-temperature plastic deformation and have boundary misorientation angles ranging from a few degrees down to a few hundredths of a degree. We also present an algorithm to automatically segment grains from combined backscattered electron image data or grains and subgrains from high angular resolution electron backscatter diffraction data. Together, these new techniques enable rapid measurements of individual grains and subgrains from large populations. 

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2. Natural and experimental constraints on a flow law for dislocation-dominated creep in wet quartz

   https://doi.org/https://doi.org/10.1029/2020JB021302  

   Lusk, A.D.; Platt, J.P.; Platt, J.A. (April 2021, Journal of geophysical research)

   null (Ed.)

   We present a flow law for dislocation-dominated creep in wet quartz derived from compiled experimental and field-based rheological data. By integrating the field-based data, including independently calculated strain rates, deformation temperatures, pressures, and differential stresses, we add constraints for dislocation-dominated creep at conditions unattainable in quartz deformation experiments. A Markov Chain Monte Carlo (MCMC) statistical analysis computes internally consistent parameters for the generalized flow law: urn:x-wiley:21699313:media:jgrb54871:jgrb54871-math-0001 = Aσnurn:x-wiley:21699313:media:jgrb54871:jgrb54871-math-0002e−(Q+VP)/RT. From this initial analysis, we identify different effective stress exponents for quartz deformed at confining pressures above and below ∼700 MPa. To minimize the possible effect of confining pressure, compiled data are separated into “low-pressure” (<560 MPa) and “high-pressure” (700–1,600 MPa) groups and reanalyzed using the MCMC approach. The “low-pressure” data set, which is most applicable at midcrustal to lower-crustal confining pressures, yields the following parameters: log(A) = −9.30 ± 0.66 MPa−n−r s−1; n = 3.5 ± 0.2; r = 0.49 ± 0.13; Q = 118 ± 5 kJ mol−1; and V = 2.59 ± 2.45 cm3 mol−1. The “high-pressure” data set produces a different set of parameters: log(A) = −7.90 ± 0.34 MPa−n−r s−1; n = 2.0 ± 0.1; r = 0.49 ± 0.13; Q = 77 ± 8 kJ mol−1; and V = 2.59 ± 2.45 cm3 mol−1. Predicted quartz rheology is compared to other flow laws for dislocation creep; the calibrations presented in this study predict faster strain rates under geological conditions by more than 1 order of magnitude. The change in n at high confining pressure may result from an increase in the activity of grain size sensitive creep. 

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3. UNDERSTANDING GRADIENTS IN THE DIFFERENTIAL STRESS DRIVING FLOW: IMPLICATIONS FOR THE CRUSTAL ESCAPE FLOW MODEL IN THE SOUTHERN APPALACHIAN INNER PIEDMONT

   https://doi.org/10.1130/abs/2021AM-371086  

   Clark, Gillian; Spencer, Brandon; Thigpen, Ryan; Merschat, Arthur J.; Casale, Gabriele; Levine, Jamie (October 2021, Abstracts Geological Society of America)

   The southern Appalachian Inner Piedmont (IP) has been interpreted to represent a relict crustal escape flow system that was active during the Neoacadian (370-340 Ma) orogeny. Critical to the support of this hypothesis is the identification of both the high-temperature, rheologically weak “channel,” with crustal flow driven by relatively low differential stress, and the rheologically strong “buttress,” where deformation was driven by relatively high differential stress. Paleopiezometric analyses from southern Appalachian quartz mylonites, quartzites, and quartz-bearing pelitic rocks of the eastern Blue Ridge (EBR; buttress) and IP (channel) allow gradients of differential stress driving flow to be examined. Samples located along a transect in the EBR northwest of the Brevard fault zone (BFZ) are used to define gradients in differential stress and deformation temperatures in the proposed buttress. Preliminary results for these samples suggest that deformation temperature increased, and differential stress decreased during deformation approaching the BFZ from the northwest. Mechanisms of quartz recrystallization shift from minor grain boundary bulging and dominant subgrain rotation (SGR) in samples located away from the BFZ to dominant SGR and minor grain boundary migration (GBM) in samples in the immediate BFZ footwall. Recrystallized grain sizes in the EBR are, in almost all cases, less than 150 microns on the major axis of ellipses used to approximate grain size. In the proposed crustal channel, quartz-rich samples collected along a transect south of Rosman, NC in the Brevard and Brindle Creek thrust sheets of the IP show GBM and SGR as the dominant mechanisms of deformation, as well as a general increase in grain size into the “core” of the proposed crustal channel. Recrystallized grain sizes in the Piedmont away from the BFZ commonly exceed 500 microns. These preliminary results suggest increasing deformation temperatures and decreasing differential stresses from close to the BFZ into the Piedmont, which is consistent with increased cooling of the proposed channel proximal to the colder, stronger Blue Ridge. Ongoing piezometric analyses, combined with quartz c-axis thermometry and thermochronology, may provide additional evidence to better refine the hypothesized buttress-channel relationship. 

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4. Ductile Deformation of the Lithospheric Mantle

   https://doi.org/10.1146/annurev-earth-031621-063756  

   Warren, Jessica M.; Hansen, Lars N. (May 2023, Annual Review of Earth and Planetary Sciences)

   The strength of lithospheric plates is a central component of plate tectonics, governed by brittle processes in the shallow portion of the plate and ductile behavior in the deeper portion. We review experimental constraints on ductile deformation of olivine, the main mineral in the upper mantle and thus the lithosphere. Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding (GBS), and diffusion-accommodated grain-boundary sliding (diffusion creep). Deformation in most of the lithosphere is dominated by GBS, except in shear zones—in which diffusion creep dominates—and in the brittle-ductile transition—in which low-temperature plasticity may dominate. We find that observations from naturally deformed rocks are consistent with extrapolation of the experimentally constrained olivine flow laws to geological conditions but that geophysical observations predict a weaker lithosphere. The causes of this discrepancy are unresolved but likely reside in the uncertainty surrounding processes in the brittle-ductile transition, at which the lithosphere is strongest. ▪ Ductile deformation of the lithospheric mantle is constrained by experimental data for olivine. ▪ Olivine deforms by four major mechanisms: low-temperature plasticity, dislocation creep, dislocation-accommodated grain-boundary sliding, and diffusion creep. ▪ Observations of naturally deformed rocks are consistent with extrapolation of olivine flow laws from experimental conditions. ▪ Experiments predict stronger lithosphere than geophysical observations, likely due to gaps in constraints on deformation in the brittle-ductile transition. 

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5. Superplasticity in Amphibole and its Implications for the Rheology of the Lower Crust

   https://doi.org/10.1130/abs/2025AM-10533  

   Miranda, Elena; Schwartz, Joshua; Klepeis, Keith; Rhea, Gillian (January 2025, Geological Society of America)

   Superplastic behavior is observed in metals, ceramics, and some geologic materials where fine grained materials deform to large strains (>200%) without failure. While somewhat common in metals and ceramics, superplasticity is less studied in geologic materials, and tends to be restricted to olivine deformation experiments that are used to infer mantle rheology. Here we show unequivocal evidence for the operation of DisGBS-mediated superplasticity in naturally deformed amphibole, an abundant mineral in deep continental crust, which has not been previously shown to deform by high-temperature grain size sensitive processes. We sampled hornblendites from the base of Cordilleran-style continental arc crust exposed in Fiordland, New Zealand. The samples are from the Hawes Head ultramafic complex at the base of the Misty Pluton, which is composed of a hornblende diorite emplaced at 120-115 Ma at depths of 25-40 km. The hornblendites have an anastomosing mylonitic foliation which formed within the long-lived Straight River Shear Zone at the western edge of the Misty Pluton when it was reactivated in the Cretaceous (~126-110 Ma) during a pulse of arc magmatism. We place maximum P and T estimates on mylonitic fabric development in our samples based on amphibole crystallization pressures of 8.5 -10 kbar and temperatures of 940-1000ºC derived from electron microprobe analyses of an adjacent undeformed hornblendite. We use electron backscatter diffraction to investigate deformation mechanisms of naturally deformed amphibole in the hornblendite. Microstructural, pole figure, and misorientation data indicate that dynamic recrystallization (DRX) and grain size reduction led to the transition from dislocation creep to the onset of DisGBS-mediated superplasticity in the mylonites. Our data are consistent with a textural evolution in which easy slip on (100)[001] initially drives subgrain rotation DRX and CPO development in porphyroclasts during dislocation creep, producing dislocations that quickly climb out of the grains, leaving behind dislocations indicative of a harder slip system such as (010)[100] or {110}1/2<110>. Matrix grains show textural traits of DisGBS-mediated superplasticity including four grain junctions, aspect ratios >1.5, a paucity of subgrains, and the operation of a single hard slip system. This discovery of superplasticity in a natural example of amphibole-rich lower crust has profound implications for the ease of strain localization leading to the creation of crustal-scale shear zones that reactivate and drive plate tectonic boundary development. 

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