Evolution of the Josephine Peridotite Shear Zones: 2. Influences on Olivine CPO Evolution

Abstract

Seismic anisotropy arises in the upper mantle due to the alignment of olivine crystal lattices and is often used to interpret mantle flow direction. Experiments on the evolution of olivine crystal‐preferred orientation (CPO) have found that the texture that develops is dependent on many factors, including water content, differential stress, preexisting CPO, and deformation kinematics. To evaluate the role of these factors in naturally deformed samples, we present microstructural transects across three shear zones in the Josephine Peridotite. Samples from these shear zones exhibit a mixture of A‐type textures, which have been associated with dry conditions and primary activation of the olivine [100](010) slip system, and of E‐type textures, which have been associated with wetter conditions and primary activation of the [100](001) slip system. CPOs with characteristics of both A‐type and E‐type textures are also present. CPO type does not evolve systematically as a function of either strain or water content. We used a micromechanical model to evaluate the roles of preexisting texture and kinematics on olivine CPO evolution. We find that the preexisting texture controls CPO evolution at strains up to 5 during simple shear. Kinematics involving a combination of simple shear and pure shear can explain the olivine CPOs at higher strain. Hence, preexisting CPOs and deformation kinematics should be considered in the interpretation of CPOs measured in naturally deformed rocks and of large‐scale patterns in upper‐mantle seismic anisotropy.

Evolution of the Josephine Peridotite Shear Zones: 1. Compositional Variation and Shear Initiation

“…Thus we suggest that the shear zones formed at a minimum temperature of ~1000 °C using the Al/Cr‐in‐opx thermometer. This temperature estimate is consistent with the high‐temperature microstructures and inferred deformation mechanisms of the Fresno Bench shear zones (Kumamoto et al, in revision; Nevitt et al, ; Skemer et al, , ; Warren et al, ). The lower temperature range (900 °C ± 26 °C) given by the olivine‐spinel thermometer (Li et al, ) is also consistent with these processes.…”

“…There is no evidence for shear heating (Figure S2), and the spatial variation of pyroxene abundance does not have a direct influence on strain localization in these shear zones (for a detailed discussion, see Hansen & Warren, ). However, in all three shear zones, a preexisting CPO in olivine evolves to become better aligned with the shear direction with increasing strain (Kumamoto et al, in revision; Skemer et al, ; Warren et al, ). In a series of deformation experiments on dry, pretextured olivine aggregates, Hansen et al () showed that viscosity could decrease by up to an order of magnitude for an aggregate with a shear‐aligned CPO compared to an aggregate with a CPO aligned poorly relative to the shear direction.…”

“…The presence of hydrous melt resulted in a positive feedback loop between increased melt fraction, decreased viscosity, and increased porosity (e.g., Kelemen & Dick, ; Stevenson, ) that eventually produced lithological melt channels. As strain localized in the less viscous centers of the shear zones, development of a shear‐aligned CPO promoted further localization (Kumamoto et al, in revision; Skemer et al, , ; Warren et al, ). In SZA and SZG, grain size reduction also contributed to localization.…”

“…These three subparallel shear zones are among the best exposed in the Fresno Bench outcrop. The microstructures of all three shear zones are closely examined in a companion study (Kumamoto et al, in revision), as well as previous work (Hansen & Warren, ; Recanati et al, ; Skemer et al, ; Warren et al, ). Our data set indicates that the relationship between water and shear localization is more complex than previously estimated.…”

Pre‐Alpine Fault Fabrics in Mantle Xenoliths From East Otago, South Island, New Zealand

“…On this basis we infer that orthorhombic type CPO is most likely to be A‐type ((010)[100]; Jung & Karato, 2001; Tommasi et al., 1999) and the axial [010] and axial [100] subtle variations on the A‐type. This is consistent with two studies from kinematically controlled samples from shear zones (Hansen et al., 2013; Kumamoto et al., 2019) where CPOs with [100] parallel to lineation correspond to misorientation axes that range between [001] an [010]; no subgrain trace analyses are presented in these papers.…”