WHERE DID GUSTAV STEINMANN SEE THE TRINITY?

In 1905 Gustav Steinmann noted the close association of serpentinites, "diabase" and radiolarite and considered this "greenstone" or ophiolitic association as characteristic for the axial part of the "geosyncline" and the deep ocean floor. Although Steinmann considered diabase, spilite and "variolite" as intrusive rocks distinctly younger than the associated sediments, he stressed their association with deep-sea sediments. In his view, the association of ultramafic and mafic material was typical for suboceanic environments from where these magmas ascended during folding of the oceanic sediments. Eventually, the importance of Steinmann's discovery was recognized amd the association of serpentinites, pillow lavas and radiolarites became known as the Steinmann Trinity. With time, the Steinmann trinity became a synonym for ophiolitic associations in the sense of the 1972 Penrose Conference.

Ironically, it appears that Steinmann never saw a "complete" ophiolite like in Oman or Troodos. In the "type-area" of the trinity, the Penninic zone of the Alps, the Jurassic "ophiolites" are dominated by serpentinites, pillow lavas and oceanic sediments, whereas gabbros appear to play a subordinate role and no relics of a sheeted dike complex are found. Instead, oceanic sediments are observed to stratigraphically overlie serpentinized mantle rocks of subcontinental origin which were exhumed along low-angle detachment faults and exposed on the sea floor. The gabbros intruded the already serpentinized peridotites at shallow depth. Undeformed basaltic dikes cut across gabbros deformed at high temperatures, and pillow lavas directly overlie the exhumed peridotites and gabbros. These mafic rocks show eNd values typical for an asthenospheric MOR-type source of the melts. They may be the products of a steady process which combined extensional deformation with magma generation and emplacement, and appear to record the onset of sea-floor spreading across an exhumed subcontinental mantle during the earliest phases of a slow spreading ridge. This situation is conspicuously similar to that of the early Cretaceous margin west of Iberia.