Strain hardening in garnet during deep crustal earthquakes

Abstract text (incl. figure legends and references)

Mechanical models of the lithosphere assume a brittle upper layer and a ductile middle to lower layer separated by a steady-state transition occurring at a depth of ~15 km. Consequently, earthquakes typically occur in the upper continental crust¹. Seismicity indicators such as pseudotachylytes in deeply exhumed rocks, however, evince earthquakes also occur in the lower crust (>50 km)². Under these high-temperature conditions, rocks are expected to behave ductily therefore post-seismic ductile creep commonly obscures microstructures associated with seismic events in most crust-forming minerals³. Thus, the mechanics of seismic failure in the deeper crust remain largely unknown. Garnet, however, is a high-strength mineral and preserves structures that can be used to understand the rheological behaviour of the lower crust⁴. Herein, we study garnet porphyroclasts from an eclogite facies mylonite in the Musgrave province (central Australia) to investigate the mechanisms by which garnet is deformed under dry, lower crustal conditions. Our correlated microscopy approach combines 2D and 3D structural and geochemical analytical techniques including electron backscatter diffraction (EBSD), high-contrast backscatter electron (BSE) imaging, and atom probe tomography (APT). EBSD mapping and BSE imaging reveal bands of small, strain-free grains with scattered orientations, outlined by polygonal to lobate high-angle grain boundaries crosscutting the porphyroclasts in the vicinity of fractures. APT analysis of a high-angle grain boundary shows Fe enrichment in the form of planar and nearly equally spaced arrays of Fe-rich nanoclusters. The combined results demonstrate Fe segregation along grain boundaries of recrystallized garnet, resulting in the nucleation of Fe-rich clusters that can act as barriers for migrating dislocations and lead to strain-hardening and facilitate brittle fracture. Our findings potentially contribute to the mechanisms of mechanical failure in the lower continental crust that lead to deep seismicity.

¹Green, H., Houston, H., 1995, The mechanics of deep earthquakes: Annual Review of Earth and Planetary Sciences, v. 23, p. 169–213.

²Austrheim, H., and Boundy, T., 1994, Pseudotachylytes generated during seismic faulting and eclogitization of the deep crust: Science, v. 265, p. 82–83.

³Kirkpatrick, J., and Rowe, C., 2013, Disappearing ink: How pseudotachylytes are lost from the rock record: Journal of Structural Geology, v. 52, p. 183–198.

⁴Mancktelow, N., Camacho, A., and Pennacchioni, G., 2022, Time-lapse record of an earthquake in the dry felsic lower continental crust preserved in a pseudotachylyte-bearing fault: Journal of Geophysical Research: Solid Earth, v. 127, p. 1–32.