Ophiolites and Ancient Continental Margins

Abstract

A consideration of continental margins would not be complete without a discussion of the large tracts of ophiolites, the assemblage of mafic to ultramafic rocks so common along recent and ancient continental margins. Present-day plate tectonic models show ophiolite as oceanic crust being generated at mid-ocean ridges, from whence it slowly migrates toward continental margins—there to be subducted into the mantle. Formation of volcanic arcs and development of calc-alkaline pluton-ism have been ascribed to this consumption of oceanic crust (Dickinson, 1970). Under some circumstances the slabs of oceanic crust and mantle overlie the continental crust, and have apparently overridden [obducted] the continental edges (Coleman, 1971b), but an acceptable mechanism for emplacement of slabs of dense oceanic crust on top of lighter continental crust along such continental margins has not been published. Perhaps the continental edge can shear off large sheet-like masses (flakes) of oceanic crust, similar to shavings from a wood plane, the flakes then being driven onto the continental plate during subduction (Oxburgh, 1972). Some geologists (Lockwood, 1971; Maxwell, 1969) have suggested gravity sliding during uplift of the adjacent ocean crust as an emplacement mechanism. Others (Moores, 1970; Dewey and Bird, 1971; Roeder, 1973) have proposed “aborted” subduction.