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The close association of serpentinites, basalts and radiolarites, later known as the Steinmann Trinity, was clearly described by Steinmann from the south Pennine Arosa zone and its southern prolongation, the Platta nappe of the eastern Swiss Alps. This classical 'ophiolite' is distinctly different from typical fast-spreading ridge associations and can be compared with the transitional crust occurring along non-volcanic passive continental margins in present-day oceans. It includes serpentinized peridotites that we interpret as subcontinental mantle rocks, which were exhumed along low-angle detachment faults and locally overlain by extensional allochthons of continental crust, minor gabbroic intrusions, tholeiitic pillow lavas and flows and a succession of oceanic sediments. The serpentinized peridotites record deformation at falling temperatures during extension leading to final exposure of the mantle rocks at the sea floor and their inclusion in tectonosedimentary breccias (ophicalcites). Field relationships, and mineral-chemical and radiometric data show that the gabbros intruded already serpentinized mantle rocks at shallow depth 161Ma ago. They are Mg gabbros, Fe gabbros and Fe-Ti gabbros, cut by dioritic pegmatoid veins and albitite dykes, which originated by differentiation from the same parental magma. All gabbros show the same metamorphic evolution, i.e. intrusion at relatively low pressure, oceanic hydration at elevated temperature and a subsequent static oceanic alteration. The pillow lavas stratigraphically overlie the exhumed mantle rocks and the tectono-sedimentary breccias related to the exhumation of both mantle rocks and gabbros. However, both gabbros and pillow basalts are characterized by eNd values typical for an asthenospheric mid-ocean ridge-type source of the melts. They may be the products of a steady process that combined extensional deformation with magma generation and emplacement. They appear to document the onset of sea-floor spreading across an exhumed subcontinental mantle during the earliest phases of a slow-spreading ridge.

We provide new geological and isotope geochemical constraints on the evolution from continental rifting to sea-floor spreading along a segment of the Jurassic Tethyan margin exposed in the Platta and Err nappes (eastern Central Alps). Field observations show that the ocean-continent transition zone is characterized by oceanward-dipping detachment faults leading to the exhumation of subcontinental mantle rocks subsequently intruded by gabbro bodies and dolerite dikes, and covered by pillow basalts and radiolarites. Zircons extracted from gabbros and albitite yield concordant U-Pb ages of 161 ± 1 Ma; their initial ɛHf (+ 14.4 to + 14.9) as well as bulk rock ɛNd values of from gabbros and basalts (+ 7.3 to + 9.5) point to a MORB-type depleted mantle source. These data suggest that the onset of magmatic activity coincides with the latest phase of mantle exhumation along low-angle detachment faults and may be controlled by upwelling asthenosphere beneath a zone of exhumed continental mantle

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