High-temperature deformation in the Josephine Peridotite: linkage between crustal and mantle spreading structure in a supra-subduction zone ophiolite
MetadataShow full item record
The Josephine peridotite is a tectonized harzburgite that represents the upper mantle beneath a paleo-spreading ridge (Dick, 1976; Loney and Himmelberg,1976; Dick and Sinton, 1979). Structures in the peridotite such as layering domains, foliations, isoclinal folds, and lower shear strain LPOs related to foliations which cross cut modal layering, provide a framework for a model of upper mantle flow beneath a spreading ridge. Layers within the Josephine peridotite have ubiquitously shallow dips and can be divided into three distinct domains. Plastic foliations within the Josephine peridotite are divided into four types on the basis of geometry with respect to modal layering: (1) parallel to modal layering (2) in layer domain boundaries (3) cross cutting layering within layer domains and (4) adjacent to tabular dunites. The foliation types can be further distinguished from each other on the basis of microstructural observations, LPO pattern and lineation type. On the basis of lineation data and cross cutting relationships, vertical shortening and horizontal extension occurred first during upwelling. Shear on steeper dipping lower temperature foliation planes occurred probably during spreading. Group one foliations and lineations are shallowly dipping/plunging in comparison with groups two and three foliations/lineations. Layer domains are bounded by high-angle shear zones. Domain boundaries are also characterized by large dunite bodies and may have provided pathways for melt migration. At least two temperature regimes are documented in the olivine LPO. Imbricate olivine neoblasts are the result of high temperature (a1200°C), high shear strain (a1.4) deformation (Zhang et al., 2000). Samples with olivine ribbons have LPO patterns that support lower temperature (si 100°C), lower shear strain (sO.8) deformation (Zhang et al., 2000). The observations from this work support the interpretation that km-sized blocks of mantle harzburgite were vertically shortened and horizontally extended at temperatures a 1200°C. Boundaries behween blocks are defined by dunite shear zones that accommodated the movement of individual blocks in relation to each other and acted as zones of accumulation and transport of basalt through the upper mantle. Lithosphere-scale 'boudinage' of the upper mantle may have occurred beneath the ridge axis during plate separation. Lineation patterns broadly reflect north south extension in the mantle, consistent with extension directions in the crustal section.