50-8 Why do some shear zones shut down or jump around? Shear zone strength cycling due to geometrically necessary strain rate acceleration

Session: Latest Research Advances in Structural Geology and Tectonics

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Large shear zones are dynamic systems that change significantly through time. The drivers of this dynamic change — for example, the reasons why shear zones shut down or jump around over time — are not well established. The modeling presented here suggests a simple, geometric mechanism for dynamic shear zone strength. A series of numerical incremental strain and strain rate models explore how strain rates and differential stresses evolve when shear zones narrow due to a pure shear component of deformation. The numerical models apply a volume preserving monoclinic transpression with fixed relative velocity across the shear zone. Volume preservation implies that the shear zone boundaries stay fixed to material points, as is common for shear zones that localize along lithologic boundaries. The models demonstrate that strain rates can increase by at least an order of magnitude when the shear zone is attenuated to 10% of its original width. This change occurs quickly, on the 1-10 m.y. time scale for plate boundary shear zone systems that begin with a 40 km width. For reasonable crustal conditions and assuming a quartz dislocation creep flow law, this increased strain rate translates to an increase of several 10s of MPa of differential stress. The models illustrate that non-steady state shear zones strengthen through time solely due to the geometrically necessary increase in strain rate without any changes in the deformation mechanism, mineralogy, temperature, or pressure. Three implications of the models are discussed: 1) the narrowing and increased strain rate of shear zones may lead to episodes of enhanced fault localization in the overlying frictional crust; 2) the strengthening of shear zones may lead to sudden reorganizations or “jumping around” of shear zone systems; and 3) the strain rate hardening phenomenon of non-simple shear zones may play a role in the eventual shut down of plate boundaries in convergent settings.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9219

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