206 Strain and Displacement: Patterns, Gradients, Partitioning, and Reconstructions

This session aims to bring together researchers from structure-related disciplines to facilitate discussions of approaches that inform our understanding of strain and displacement. We welcome contributions from observations, experiments, theory, and modeling that characterize deformation from micro to tectonic scales.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 217C

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

Paper number: 206-3
Program: Technical Sessions
Time: 2:20 PM – 2:40 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414442

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