Subduction and obduction processes in the Swiss Alps

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. All rights reserved.

Salvage treatment for venous aneurysm complicating vascular access arteriovenous fistula: use of an exoprosthesis to reinforce the vein after aneurysmorrhaphy.

OBJECTIVES: We report a new salvage technique for treating venous aneurysms (VAs) complicating vascular access arteriovenous fistula (AVF) using externally reinforced venous aneurysmorrhaphy. DESIGN: A retrospective study over a 20-month period from a single centre. PATIENTS: Patients presenting to the vascular surgery department, Bordeaux University Hospital for revision of a vascular access AVF were included. METHODS: Reinforced venous aneurysmorrhaphy consisted in removal of redundant vessel wall followed by reinforcement using an external prosthetic graft. Patency, diameter and flow were assessed by duplex ultrasound at 1, 6 and 12 months after salvage. RESULTS: Thirty-eight eligible patients were identified. Five were excluded because VA was associated with central vein stenosis; the remaining 33 underwent salvage. Indications were rapidly expanding or painful VA in seven cases; VA with frequent bleeding or damaged overlying skin in eight; VA in close relation to a stenosis in two; and VA associated with high-flow rate in 16. Cannulation was attempted after 30 days. Mean follow-up time was 12 S.D. 5 months (range: 4-22). Two repaired AVFs failed. Primary 1-year patency was 93%. No aneurysm or infection occurred. Reduction of high flow was successful in 12 of 16 patients. The remaining four required re-operation. CONCLUSIONS: Reinforced venous aneurysmorrhaphy is effective in controlling venous dilation and achieving patency. Reduction of high-flow rates was not always achieved. Further study is needed to evaluate long-term efficacy of this treatment.

Le Briançonnais, terrain exotique dans les Alpes ?

New reconstructions of the Western Alps from late Early Jurassic till early Tertiary are proposed. These reconstructions use deep lithospheric data gathered through recent seismic surveys and tomographic studies carried out in the Alps. The present day position, under the Po plain, of the southern limit of the European plate (fig. 1), allows to define the former geometry of the Brianconnais peninsula. The Brianconnais domain is regarded as an exotic terrane formerly belonging to the European margin until Late Jurassic, then transported eastward during the drift of Iberia (fig. 5). Therefore, on a present day Western Alps cross section, a duplication of the European continental margin can be recognized (fig. 10). Stratigraphic and sedimentological data along a zone linking the Pyrenean fracture zone to the Brianconnais, can be related to a rifting event starting in Oxfordian time. This event is responsible for the Late Jurassic till mid-Cretaceous drift of Iberia opening, first the northern Atlantic, then the Gulf of Biscay. Simultaneously, the drift of the Brianconnais will open the Valais ocean and close the Piemontese ocean. The resulting oblique collision zone between the Brianconnais and the Apulian margin generates HP/LT metamorphism starting in Early Cretaceous. The eastward drift of the Brianconnais peninsula will eventually bring it in front of a more northerly segment of the former European margin. The thrusting of the Brianconnais unto that margin takes place in early Tertiary (fig. 9), following the subduction of the Valais ocean. The present nappe pile results not only from continent/continent frontal collision, but also from important lateral displacement of terranes, the most important one being the Brianconnais. The dilemma of `'en echelon'' oceanic domains in the Alps is an outcome of these translations. A solution is found when considering the opening of a Cretaceous Valais ocean across the European margin, running out eastward into the Piemontese ocean, where the drift is taken up along a former transform fault and compensated by subduction under the Apulian margin (fig. 8). In the Western Alps we are then dealing with two oceans, the Piemontese and the Valaisan and a duplicated European margin. In the Eastern Alps the single Piemontese ocean is cut by newly created oceanic crust. All these elements will be incorporated into the Penninic structural domain which does not represent a former unique paleogeographic area, it is a composite accretionary domain squeezed between Europe and Apulia.