Role of orthopyroxene in rheological weakening of the lithosphere via dynamic recrystallization

Abstract

For plate tectonics to operate on a terrestrial planet, the surface layer (the lithosphere) must have a modest strength (Earth, ≤200 MPa), but a standard strength profile based on olivine far exceeds this threshold value. Consequently, it is essential to identify mechanisms that reduce the strength of the lithosphere on Earth. Here we report results of high-strain laboratory deformation experiments on a representative olivine-orthopyroxene composition that show the addition of orthopyroxene substantially reduces the strength in the ductile regime within a certain temperature window. The reduction in strength is associated with the formation of small orthopyroxene and olivine grains. Our samples show heterogeneous microstructures similar to those observed in natural peridotites in shear zones: fine-grained regions containing both orthopyroxene and olivine that form interconnected bands where a large fraction of strain is accommodated. A model is developed to apply these results to geological conditions. Such a model, combined with our experimental observations, suggests that orthopyroxene may play a key role in the plastic deformation of the lithosphere in a critical temperature range, leading to long-term weakening associated with strain localization in the lithosphere.rheology | two-phase | Zener pinning | deformation mechanisms | electron probe microanalysis P late tectonic style of convection is characterized by regions of localized deformation, such as subduction zones, and the origin of these regions needs to be investigated to understand why plate tectonics operates on Earth. Localized deformation at plate boundaries involves not only brittle fracture at shallow portions but also localized deformation in the ductile shear zones (1-4). Although the basic processes of localized deformation in the brittle regime are well understood (5, 6), mechanisms of localized deformation in the ductile regime remain elusive. The strength profile of the lithosphere in a standard model (7) predicts stresses that are much higher than the critical strength below which plate tectonic style of convection would occur (8,9).Among the possible mechanisms of shear localization (10), grain size reduction is most often recognized in ductile shear zones and has been given particular attention in previous studies (11)(12)(13)(14)(15). Grain size reduction can occur during high-temperature deformation by dynamic recrystallization during dislocation creep (16,17). In many cases, small grains are formed along preexisting grain boundaries (18). If the degree of grain-size reduction is large enough and these fine-grained regions are connected, then regions of small grain size will be deformed by grain-size-sensitive creep processes-diffusion creep or dislocation creep accommodated by grain-boundary sliding (Dis-GBS) introduced by Hirth and Kohlstedt (1995) (19). Deformation in both regimes results in the weakening of a rock, although less so in the Dis-GBS regime. In addition, for this weakening to cause substantial shear localization, the ...

Macroscopic strength of oceanic lithosphere revealed by ubiquitous fracture-zone instabilities

“…Our new estimate of the activation energy thus implies that the rheology of oceanic lithosphere is not well represented by that of pure olivine aggregates. In fact, a recent laboratory experiment suggests that the presence of a secondary phase such as orthopyroxene is important for rheological weakening (Farla et al, 2013), and our estimate of ∼100 ± 50 kJ mol −1 (or 225 ± 112 kJ mol −1 for dislocation creep) may represent the 'effective' activation energy for such polymineralic aggregates. Whereas the results of our geoid inversion themselves do not distinguish between diffusion and dislocation creep, we suspect that dislocation creep would be more appropriate to explain the inferred occurrence of SSC around the fracture zones.…”

“…So far proposed mechanisms include the feedback between shear localization and grain size evolution (Kameyama et al, 1997;Braun et al, 1999;Landuyt et al, 2008), the pre-existing zone of weakness such an oceanic fracture zone (Toth and Gurnis, 1998;Hall et al, 2003;Gurnis et al, 2004), the higher water content of oceanic lithosphere (e.g. Regenauer-Lieb et al, 2001), hydration by thermal cracking (Korenaga, 2007), and rheological weakening by a secondary orthopyroxene phase (Farla et al, 2013).…”

Evidence of phase nucleation during olivine diffusion creep: A new perspective for mantle strain localisation

“…In these cracks, phase mixing cannot result from a two-stage-process involving first the formation of the small grains, and then a mixing process by, e.g., grain boundary sliding (GBS) as proposed by Farla et al (2013) and Linckens et al (2014). The nature of the local mixture also excludes dynamic recrystallization.…”

“…Many studies have pointed out the presence of minor phases, such as micas or pyroxenes, as a condition for small grain sizes during grain-size-sensitive creep (Hiraga et al, 2010;Herwegh et al, 2011;Farla et al, 2013;Linckens et al, 2014;Platt, 2015). Most of them attributed the reduction of grain size to a combination of dynamic recrystallization and grain boundary sliding (GBS), resulting in phase mixing.…”

Characterization of olivine fabrics and mylonite in the presence of fluid and implications for seismic anisotropy and shear localization

“…Previous studies showed that the secondary phase like pyroxenes can cause Zener pinning and slows down the kinetics of grain growth, resulting in the weakening of material and strain localization in the mylonite (Linckens et al 2011;Farla et al 2013). The small-grained area in the Hundskjeften peridotite does not contain much opx (less than 5%) but it has high cpx content (33% and 45%), which might have contributed to reduce grain growth in the mylonite by Zener pinning of cpx, which is believed to result in the shear localization in the peridotite.…”