Ophiolitic mylonites in the Lizard complex: Ductile extension in the lower oceanic crust

Ophiolitic rocks in the Lizard complex, southwestern England, preserve fine examples of mylonitic schists within low-angle amphibolite-facies shear zones. Two prominent shear zones, with type localities at Carrick Luz and Porthoustock, both have a footwall in the mafic-ultramafic interface at the base of the oceanic crust. The Carrick Luz shear zone shows mylonitized gabbro above peridotite; mafic dikes both predate and postdate the shearing. The Porthoustock shear zone brings down higher oceanic crustal levels (gabbro-sheeted dike complex) against ultramafic rocks and has an extensional geometry. Shear-sense indicators show hanging-wall movement directions different from those predicted by previous northward-obduction models. Generation of these ophiolitic mylonites is interpreted instead as due to ductile extension at an oceanic spreading center.