Explanatory notes to the map : metamorphic structure of the Alps transition from the Western to the Central Alps
Romain BousquetMartin EngiG. GossoRoland OberhänsliA. BergerMaria Iole SpallaMichele ZucaliBruno Goffé
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The Zermatt-Saas ophiolite of the Swiss Alps represents a complete sequence of Mesozoic Tethys oceanic lithosphere. The ophiolite was subducted during early phases of the Alpine orogeny and the mafic rocks were transformed to eclogites and blueschists. Metabasalts locally preserve pillow structures in which glaucophanite forms rims on eclogitic pillow cores. Omphacite-garnet-glaucophane-epidote-ferroan dolomite-Mg-chloritoid-talc-paragonite-chlorite. rutile form characteristic coeval blueschist- and eclogite-facies assemblages.
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A complete Alpine cross section integrates numerous seismic reflection and refraction profiles, across and along strike, with published and new field data. The deepest parts of the profile are constrained by geophysical data only, while structural features at intermediate levels are largely depicted according to the results of three‐dimensional models making use of seismic and field geological data. The geometry of the highest structural levels is constrained by classical along‐strike projections of field data parallel to the pronounced easterly axial dip of all tectonic units. Because the transect is placed close to the western erosional margin of the Austroalpine nappes of the Eastern Alps, it contains all the major tectonic units of the Alps. A model for the tectonic evolution along the transect is proposed in the form of scaled and area‐balanced profile sketches. Shortening within the Austroalpine nappes is testimony of a separate Cretaceous‐age orogenic event. West directed thrusting in these units is related to westward propagation of a thrust wedge resulting from continental collision along the Meliata‐Hallstatt Ocean further to the east. Considerable amounts of oceanic and continental crustal material were subducted during Tertiary orogeny, which involved some 500 km of N‐S convergence between Europe and Apulia. Consequently, only a very small percentage of this crustal material is preserved within the nappes depicted in the transect. Postcollisional shortening is characterized by the simultaneous activity of gently dipping north directed detachments and steeply inclined south directed detachments, both detachments nucleating at the interface between lower and upper crust. Large scale wedging of the Adriatic (or Apulian) lower crust into a gap opening between the subduced European lower crust and the pile of thin upper crustal flakes (Alpine nappes) indicates a relatively strong lower crust and detachment between upper and lower crust.
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Previous work suggested a double collision suture, including ophiolites from two oceanic basins (Valais and Piemont‐Liguria), on the southern side of Monte Rosa (Penninic Alps, northern Italy). This area was studied using field mapping, microstructural analysis, and neutron texture goniometry. After its formation and eclogite‐facies metamorphism of continental and oceanic units, the suture was deformed by four successive folding and shearing events under greenschist‐facies conditions, all of them taking place between 40 and 28 Ma. After fold retrodeformation, the following tectonostratigraphy results, from base to top: Monte Rosa gneiss (European margin), Balma serpentinite/eclogite unit (Cretaceous crust of Valais ocean), Stolemberg gneiss (Iberia‐Briançonnais continent), Zermatt‐Saas and Tsaté ophiolites (Jurassic crust of Piemont‐Ligurian ocean), Sesia nappe (continental fragment off the Adria margin). The preservation of this lithological sequence suggests that deep‐seated deformation during multiple continent collision produces heterogeneous strain and extreme thinning of nappes but their original stacking order can still be reconstructed using kinematic analysis and overprinting criteria. This is due to the ductile nature of the collisional deformation which retains the continuity of tectonic contacts.
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