Abstract

The Springers Hill area (Lewis Hills, Bay of Islands Ophiolite) may represent oceanic lithosphere created in close proximity to the nontransform segment of an oceanic fracture zone. Upper mantle rocks exposed on Springers Hill were investigated to establish whether their development was affected by the thermal and rheological changes associated with oceanic fracture zones. Harzburgites of the Springers Hill area reveal complex structural patterns. On a small scale, foliations defined by orthopyroxene grains intersect foliations defined by spinel grains at various angles. Olivine petrofabric work demonstrates that only the spinel foliation is related to the preserved flow plane. The orthopyroxene foliation appears to be the result of pull-apart of formerly larger grains during high-temperature deformation. On the larger scale, orientation patterns of foliation, lineation, and dykes suggest that strike-slip movement occurred parallel and at high angle to the fracture- zone contact at various stages of a complex flow history. Given its location adjacent to a nontransform segment of oceanic lithosphere, the origin of the strike-slip movement parallel to the fracture zone must be clarified. It can be accounted for by movement of the older lithosphere past asthenosphere of the young spreading ridge during plate-driven flow.

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