Strain localization during high temperature creep of marble: The effect of inclusions

Abstract

The deformation of rocks in the Earth's middle and lower crust is often localized in ductile shear zones. To better understand the initiation and propagation of high-temperature shear zones induced by the presence of structural and material heterogeneities, we performed deformation experiments in the dislocation creep regime on Carrara marble samples containing weak (limestone) or strong (novaculite) second phase inclusions. The samples were mostly deformed in torsion at a bulk shear strain rate of ≈ 1.9x10 -4 s -1 to bulk shear strains γ between 0.02 and 2.9 using a Paterson-type gas deformation apparatus at 900°C temperature and 400 MPa confining pressure. At low strain, twisted specimens with weak inclusions show minor strain hardening that is replaced by strain weakening at γ > 0.1 -0.2.Peak shear stress at the imposed conditions is about 20 MPa, which is ≈8% lower than the strength of intact samples. Strain progressively localized within the matrix with increasing bulk strain, but decayed rapidly with increasing distance from the inclusion tip.Microstructural analysis shows twinning and recrystallization within this process zone, with a strong crystallographic preferred orientation, dominated by {r} and (c) slip in <a>.Recrystallization-induced weakening starts at local shear strain of about 1 in the process zone, corresponding to a bulk shear strain of about 0.1. In contrast, torsion of a sample containing strong inclusions deformed at similar stress as inclusion-free samples but do not show localization. The experiments demonstrate that the presence of weak heterogeneities initiates localized creep at local stress concentrations around the inclusion tips. Recrystallizationinduced grain size reduction may only locally promote grain boundary diffusion creep. Accordingly, the bulk strength of the twisted aggregate is close to or slightly below the lower (isostress) strength bound, determined from the flow strength and volume fraction of matrix and inclusions.

Strain Localization and Weakening Processes in Viscously Deforming Rocks: Numerical Modeling Based on Laboratory Torsion Experiments

“…Straight vertical scratches on the jacket surface serve as passive strain markers. As shown in Rybacki et al (), at experimental P‐T conditions the fine grained limestone is substantially softer than Carrara marble and therefore acts as a weak material heterogeneity within a homogeneous stronger matrix. Two different loading conditions, constant twist rate (equivalent to a shear strain rate of 1.9 × 10 −4 s −1 at the outer periphery) and constant torque (~18.8 MPa) were tested.…”

“…Sample preparation is following the procedures described in Rybacki et al (): Cylinders of Carrara marble (10 mm in length, 15‐mm outer diameter) were cut from a single block of marble, and an internal borehole (6.1 mm of inner diameter) was cored and subsequently filled with cylinders of solid gold to provide a homogeneous distribution of stress over the entire sample through the full duration of the experiments (Paterson & Olgaard, ). Circular segments of Solnhofen limestone (arc length ~11.8 mm), a very fine grained (average grain size <10 μm) rock, were produced by polishing ~750‐μm thick sections that were subsequently inserted in the external surface of the Carrara marble cylinders (see Figure a).…”

“…Strong stress and strain gradients close to matrix/inclusion interfaces were also observed in a two‐phase study using anorthite‐diopside aggregates and are similar to deformation microstructures observed in ultramylonites (Dimanov & Dresen, ). The effect of material heterogeneities on the rheological response of otherwise homogeneous Earth materials has been recently addressed by Rybacki et al (), who analyzed the effect of material heterogeneities on the onset of localized viscous deformation. These studies revealed a rich interplay of various factors and processes, yet it is difficult to study individual processes in isolation and to provide a time‐dependent view of strain localization.…”

Major softening at brittle‐ductile transition due to interplay between chemical and deformation processes: An insight from evolution of shear bands in the South Armorican Shear Zone

“…Microcracks have become frequently recognized as precursors of ductile shear zones affecting midcrustal granitoids [e.g., Pennacchioni and Mancktelow, 2007;Fusseis and Handy, 2008;Mancktelow and Pennacchioni, 2013] but also lower crustal rocks [Menegon et al, 2013;Okudaira et al, 2015]. The microcracking is typically explained by stress concentrations where the effective viscosity contrasts between weak and strong phases are high, e.g., at the tips of phyllosilicates [Holyoke and Tullis, 2006] or in material with the same composition but very different grain size [Rybacki et al, 2014]. Similarly, the observed microcracks from SASZ are often associated with large white mica I grains elongated parallel to S and occurring within or in the vicinity of quartz aggregates (Figure 4a).…”

Paleostress evolution during the exhumation of high-P marbles, Samaná Complex, northern Hispaniola

“…Las microestructuras relacionadas con el creep de disolución-precipitación implican la presencia y actividad de fluidos a lo largo de las superficies de foliación S2 en un estadio inmediatamente posterior al de la cataclasis. Además, las inclusiones fluidas de los mármoles formadas al comienzo del proceso de exhumación pudieron favorecer la localización y desarrollo de cizallas, como experimentalmente han demostrado Rybacki et al, (2014). Resulta probable que el efecto de las inclusiones en los mármoles consistiera en: (1) reducir la resistencia mecánica de los mármoles y calco-esquistos favoreciendo la cataclasis; y (2) potenciar la localización y desarrollo de cizallas S2, de manera que la mayor parte de las inclusiones pudieron desaparecer durante el desarrollo de la foliación y los fluidos emigrar a través de las superficies de foliación-disolución.…”