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 epsilon(Hf) (+ 14.4 to + 14.9) as well as bulk rock epsilon(Nd)
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.