Ophiolites and Regional Tectonics

Abstract

Ophiolites are large-scale thrust sheets of oceanic crust and upper mantle rocks that have been emplaced (obducted) onto continental crust. The formal definition of an ophiolite complex dates back to the Penrose meeting in 1972 where it was decided that a typical ‘ophiolite’ comprises a mantle sequence ultramafic complex of peridotites at the base overlain by a crustal sequence made up of gabbros and a sheeted dyke complex with a mafic volcanic sequence, often pillow lavas, at the top. This Penrose ophiolite ‘stratigraphy’ became an analogue for the seismic velocity structure of the oceans, and was soon proven by an intense period of drilling during the Ocean Drilling Project (ODP). The later International Ocean Drilling Project (IODP) has spent millions of dollars drilling shallow wells in all the World’s oceans, but these cores only ever penetrate the upper few kilometres of the crust down to the dyke-gabbro contact, and nowhere has a complete section through the oceanic crust been drilled. Ophiolite complexes therefore, are of immense importance because they reveal the petrological composition and structural architecture of the otherwise inaccessible oceanic crust and upper mantle.