The Role of Pre-existing Mechanical Anisotropy on Shear Zone Development within Oceanic Mantle Lithosphere: an Example from the Oman Ophiolite

Abstract

Structural and fabric analysis of the well-exposed Hilti mantle section, Oman ophiolite, suggests that shear zone development, which may have resulted from oceanic plate fragmentation, was influenced by pre-existing mantle fabric present at the paleoridge. Detailed structural mapping in the mantle section revealed a gently undulating structure with an eastEEEwest flow direction. A NWEEESE strike-slip shear zone cuts across this horizontal structure. The crystal preferred orientation (CPO) of olivine within the foliation is dominated by (010) axial patterns rather than more commonly observed (010)[100] patterns, suggesting that the horizontal flow close to the Moho involved non-coaxial flow. Olivine CPO within the shear zone formed at low temperature is characterized by (001)[100] patterns and a sinistral sense of shear. The olivine CPO becomes weaker with progressive mylonitization and accompanying grain size reduction, and ultimately develops into an ultra-mylonite with a random CPO pattern. The olivine [010]-axis is consistently sub-vertical, even where the horizontal foliation has been rotated to a sub-vertical orientation within the shear zone. These observations suggest that the primary mechanical anisotropy (mantle fabric) has been readily transformed into a secondary structure (shear zone) with minimum modification. This occurred as a result of a change of the olivine slip systems during oceanic detachment and related tectonics during cooling. We propose that primary olivine CPO fabrics may play a significant role in the subsequent structural development of the mantle. Thus, the structural behavior of oceanic mantle lithosphere during subduction and obduction may be strongly influenced by initial mechanical anisotropy developed at an oceanic spreading center.

From crystal to crustal: petrofabric-derived seismic modelling of regional tectonics

“…Thus, the ductile Liachar simple-shear zone exhibits significantly lower seismic anisotropy than the 'background' gneisses of the NPM. Similar behaviour, whereby a shear zone is less anisotropic than its wall rocks, has been observed by Michibayashi & Mainprice (2004) and Michibayashi et al (2006), being attributed in both cases to the presence and significance of pre-existing mechanical anisotropy on shear-zone development. If correct, this suggests that the 'background' gneisses of the NPM exhibit a significant deformation fabric in terms of the seismic properties.…”

Fabric Development in a Middle Devonian Intraoceanic Subduction Regime: The Careón Ophiolite (Northwest Spain)

“…On that ground, quantitative characterization of textures in ophiolites may facilitate recognition of deformational stages, enable correlation of ophiolitic units, and complement petrological and geochemical studies. The study of olivine fabrics from mantle sections in ophiolites has provided important clues to the formation of oceanic mantle and deformation of the oceanic lithosphere (e.g., Ceuleneer et al 1988;Nicolas et al 1994;Michibayashi and Mainprice 2004). However, crustal sections of ophiolites, dominated by mafic rocks, typically have been ignored in quantitative fabric analyses, mainly because of their polymineral character, including low-symmetry phases (triclinicmonoclinic) as major components of the fabric, which result in complex diffraction patterns when conventional diffraction techniques are applied to analyze the preferred orientation of minerals (Siegesmund et al 1994;Leiss et al 2002;Pehl and Wenk 2005).…”

High-strain zones: laboratory perspectives on strain softening during ductile deformation

“…The slower weakening may be caused by the LPO build up, where grains are preferentially oriented for easy slip and therefore render the aggregate softer. Additional support for the hypothesis of strain softening by this mechanism comes from a recent field study by Michibayashi & Mainprice (2004). They report that pre-existing fabric in olivines influenced development of a shear zone in the Oman ophiolite.…”