Quantifying Polyphase Lower Crustal Rheology: Constraints from Mount Hay Granulites (Central Australia)

Seth Kruckenberg, Boston College
Lauren Shea, Boston College
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Abstract

Characterizing the bulk rheology of polyphase materials remains a central challenge in understanding lower crustal dynamics. This study presents microstructural and fabric data from 42 lower crustal granulite samples collected from the Mount Hay block, central Australia, to empirically model aggregate viscosities. The samples span variable mafic and felsic domains composed primarily of anorthite, pyroxene, and quartz. EBSD-derived thin-section maps reveal prominent crystallographic preferred orientations (CPOs) indicative of dominant dislocation creep. We utilized grain-size piezometry and two-pyroxene geothermometry to establish deformation conditions, yielding differential stresses of 34–54 MPa (quartz), 53–103 MPa (plagioclase), and 10–41 MPa (orthopyroxene) at temperatures of 780–810 °C. Applying monophase flow laws, individual phase strain rates and effective viscosities were calculated. To scale these inputs to polyphase aggregates, we applied two distinct homogenization approaches: the Minimized Power Geometric model (Huet et al., 2014) and the Asymptotic Expansion Homogenization (AEH) framework (Cook, 2006). At a reference strain rate of 10⁻¹⁴ s⁻¹, the resulting bulk aggregate effective viscosities span 3.1 × 10²⁰ to 2.7 × 10²¹ Pa·s. Our results confirm that the aggregate viscosity of the heterogeneous Mount Hay lower crust varies primarily as a function of composition, plotting intermediate to monophase quartz and plagioclase endmembers, which aligns closely with broad-scale geophysical constraints.

Session

Large-scale tectonics