Alpine Definition of Ophiolites and Oceanic Lithosphere and the Evolution of Geological Thought

The term ‘ophiolite' was first introduced by the French geologist Brongniart (1813) to define different varieties of mafic-ultramafic rocks, mainly serpentinite, gabbro and basalt. Brongniart (1813, 1821) and some Italian geologists (Pantanelli 1880; Lotti 1886) recognized the spatial association of ultramafic rocks, gabbros, diabase-spilite, and chert in the Northern Apennines.

At the time when Steinmann (1905) described the close association of serpentinites, diabase, and radiolarites in Eastern Switzerland and then in the Northern Apennines, ophiolites were considered as intrusions of mafic-ultramafic magmas emplaced during folding of oceanic sediments deposited in the axial part of geosynclines. Following Bailey and MacCallien's (1950, 1953) definition of the Steinmann trinity, the ophiolitic tripartite was redefined as the association of serpentinite/peridotite, gabbro, and dolerite/spilite.

Ultramafic rocks in ophiolites were originally interpreted as intrusive-extrusive rocks, reminiscent of the “alpine-type” peridotites interpreted as plutons intruded into folded geosynclinal sediments (Benson, 1926). Subsequently, Hess (1954) and de Roever (1957) suggested that ultramafics in ophiolites represented mantle rocks. Structural studies of the Troodos and Oman ophiolites and seismic refraction studies in modern oceans demonstrated the stratified nature of the oceanic lithosphere.

The advent of plate tectonics led to the establishment of an idealized four-layer pseudostratigraphy of ophiolites, including oceanic sedimentary cover (layer 1), extrusive sequence and sheeted dike complex (layer 2), gabbroic complex (layer 3), and upper mantle (layer 4), matching the geophysical structure of the oceanic lithosphere (Anonymous, 1972).

The ophiolite concept and the Steinmann trinity as originally established in the Alps and Apennines consisted of an incomplete pseudostratigraphy in comparison to the Penrose-type oceanic lithosphere. However, ophiolite studies and marine investigations in recent years have shown that the structure and composition of the oceanic lithosphere vary significantly depending on the geodynamic environments of formation and different magmatic and tectonic processes involved in these diverse settings.