How Shear Zones Get Started: Mechanical Heterogeneity, Bridge Zones, and Rock Weakening

He Feng, Georgia Southern University
Christopher Gerbi, University of Maine
Scott Johnson, University of Maine
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Abstract

The lithosphere is mechanically heterogeneous. From mineral grains to tectonic plates, variations in strength, phase distribution, grain geometry, and weak domain spacing can cause local stress and pressure fields to deviate substantially from bulk loading conditions. As a result, even when macroscopic differential stress is modest, local conditions may promote damage, reaction, fluid migration, and weakening. A key unresolved question is how such perturbations evolve into persistent weakening networks that initiate strain localization in the viscous regime.
Numerical modeling of two phase viscous systems shows that significant stress amplification can develop within the stronger matrix between weak domains. The primary controls are weak phase spacing, rheological contrast, and loading geometry. Although extreme idealized geometries can generate amplification factors approaching an order of magnitude, more geologically common configurations produce amplifications around a factor of two. Rather than passive byproducts of phase heterogeneity, these perturbations provide a driving force for modifying the intervening matrix through microfracturing, grain size reduction, fluid migration, dissolution precipitation, neocrystallization, and reaction assisted weakening.
Natural examples from greenschist, amphibolite, and granulite facies rocks preserve the microstructural products of these coupled processes. In low strain rocks near the margins of larger shear zones, we identify fine grained, commonly polyphase domains that we term rheological bridge zones. These bridge zones link adjacent weak domains, commonly quartz or biotite rich regions, through stronger feldspar rich material. Electron microscopy, cathodoluminescence, electron probe microanalysis, and crystallographic orientation data indicate that bridge zones are not simply cataclastic domains. Instead, they record local damage, chemical mobility, phase mixing, and neocrystallization within the stronger material separating neighboring weak domains. Thus, natural bridge zones provide microstructural evidence for stress driven weakening predicted by the numerical models.
The numerical and natural results suggest a feedback among mechanical heterogeneity, stress amplification, and microstructural weakening. Local stress concentrations develop between weak domains, promote chemical and mechanical modification of the intervening strong matrix, and produce bridge zones that reduce bulk strength. Numerical modeling indicates that adding under 3 percent bridge-zone-like weak material can cause substantial bulk weakening, highlighting the disproportionate effect of small-volume microstructural change. Rheological bridge zones provide a mechanism by which discontinuous weak domains can evolve into interconnected weakening networks and ultimately into localized shear zones in the viscous crust. Because the same mechanical principle applies wherever weak domains interact through a stronger matrix, this bridging process is scale independent, linking microscale rheological change to localization across geological scales.

Session

Experiments of all sorts