Slow-spreading ridge-axis tectonics: evidence from the Lizard complex, UK

Roberts, S., Andrews, J.R., Bull, J.M. and Sanderson, D.J. (1993) Slow-spreading ridge-axis tectonics: evidence from the Lizard complex, UK. Earth and Planetary Science Letters, 116 (1-4), 101-112. (doi:10.1016/0012-821X(93)90047-D).

Abstract

The east-coast section of the Lizard ophiolite complex, Cornwall, U.K., preserves a series of interrelated extensional faults and dyke sets which indicate that the formation of this section of obducted oceanic lithosphere resulted from a combination of contemporaneous structural and magmatic events. Three dolerite dyke sets are evident; the first two (sets 1 and 2) are cut by and rotated on a series of ridge-parallel, extensional faults and shear zones. Later dykes (set 3) cross-cut the extensional faults. These data indicate that both rotation and significant thinning, in the region of 40%, of this part of the oceanic lithosphere occurred prior to the final period of dyke injection. The thinning of the lithosphere was the result of extension along the normal faults.

All of the dolerite dykes show MORB characteristics, although the late dykes (set 3) are more primitive, tending to be aphyric to sparse olivine-phyric and show higher Cr, Ni and lower Zr values than the typically plagioclase phyric dolerite dykes (sets 1 and 2) intruded prior to the ductile shearing event. The more primitive dolerites post-date the tectonism and indicate the lack of a significant magma chamber following a period of amagmatic extension. The data are consistent with observations at present day slow-spreading ridge-axes and some mafic-ultramafic ophiolite sequences, and provide insight into the nature of ridge-related structures and the interrelationships between tectonism and geochemical evolution of the dykes, and record a mechanism (listric faulting) to account for the presence of peridotites and gabbros commonly dredged at slow-spreading centres.

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Contributors

Author: D.J. Sanderson

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