The significance of the Brianconnais domain in the Alpine orogen is
reviewed in the light of data concerning its collision with the active
Adriatic margin and the passive Helvetic margin. The Brianconnais which
formerly belonged to the Iberian plate, was located on the northern
margin of the Alpine Tethys (Liguro-Piemont ocean) since its opening in
the early-Middle Jurassic. Together with the Iberian plate the
Brianconnais terrane was separated from the European plate in the Late
Jurassic-Early Cretaceous, following the northern Atlantic, Bay of
Biscay, Valais ocean opening. This was accompanied by the onset of
subduction along the northern margin of Adria and the closure of the
Alpine Tethys. Stratigraphic and metamorphic data regarding this
subduction and the geohistory of the Brianconnais allows the scenario of
subduction-obduction processes during the Late Cretaceous-early Tertiary
in the eastern and western Alps to be specified. HP-LT metamorphism
record a long-lasting history of oceanic subduction-accretion, followed
in the Middle Eocene by the incorporation of the Brianconnais as an
exotic terrane into the accretionary prism. Middle to Late Eocene
cooling ages of the Brianconnais basement and the presence of pelagic,
anorogenic sedimentation lasting until the Middle Eocene on the
Brianconnais preclude any sort of collision before that time between
this domain and the active Adria margin or the Helvetic margin. This is
confirmed by plate reconstructions constrained by magnetic anomalies in
the Atlantic domain. Only a small percentage of the former Brianconnais
domain was obducted, most of the crust and lithospheric roots were
subducted. This applies also to domains formerly belonging to the
southern Alpine Tethys margin (Austroalpine-inner Carpathian domain). It
is proposed that there was a single Palaeogene subduction zone
responsible for the Alpine orogen formation (from northern Spain to the
East Carpathians), with the exception of a short-lived Late Cretaceous
partial closure of the Valais ocean. Subduction in the western Tethyan
domain originated during the closure of the Meliata ocean during the
Jurassic incorporating the Austroalpine-Carpathian domain as terranes
during the Cretaceous. The subduction zone propagated into the northern
margin of Adria and then to the northern margin of the Iberian plate,
where it gave birth to the Pyrenean-Provencal orogenic belt. This
implies the absence of a separated Cretaceous subduction zone within the
Austro-Carpathian Penninic ocean. Collision of Iberia with Europe forced
the subduction to jump to the SE margin of Iberia in the Eocene,
creating the Apenninic orogenic wedge and inverting the vergence of
subduction from south- to north-directed. (C) 1998 Elsevier Science B.V.
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