Where did Gustav Steinmann see the trinity? Back to the roots of an Alpine ophiolite concept

Abstract

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, notably radiolarian cherts and pelagic limestones. In his view, the consanguineous association of ultramafic and mafic material was typical for suboceanic environments from where these magmas had ascended during folding of the oceanic sediments. Eventually, the importance of Steinmann's discovery was recognized, and the association of serpentinites, pillow lavas and radiolarites became known as the Steinmann trinity and, finally, a synonym for ophiolitic associations in general. Ironically, it appears that Steinmann never saw a completely developed ophiolite like those in Oman or Troodos. In the type-area of the trinity, in the Penninic zone of the Alps where Steinmann worked, 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, radiolarian cherts, and pelagic limestones stratigraphically overlie serpentinized mantle rocks of subcontinental origin that were exhumed along concave-downward detachment faults and exposed on the sea floor. The gabbros intruded the partially serpentinized peridotites at a shallow depth. Undeformed basaltic dikes cut across gabbros deformed at high temperatures, and pillow lavas directly overlie the exhumed peridotites and gabbros. Both types of mafic rocks are characterized by eNd values typical for an asthenospheric mid-ocean ridge-type source. 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 the development of a slow spreading ridge. This situation is conspicuously similar to that of the early Cretaceous ocean-continent transition along the west-Iberian passive margin where subcontinental mantle was exhumed to the sea floor prior to the onset of sea floor spreading.