Geophysical‐geological transect and tectonic evolution of the Swiss‐Italian Alps
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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.Keywords:
Continental Margin
Orogeny
Alpine orogeny
Adakite
Incompatible element
Planetary differentiation
Underplating
Primitive mantle
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Orogeny
Dalradian
Phyllite
Devonian
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Orogeny
Alpine orogeny
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Pegmatite
Orogeny
Alpine orogeny
Muscovite
Dalradian
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The stretching and thinning of the continental crust, which occurs during the formation of passive continental margins, may cause important changes in the velocity structure of such crust. Further, crust attenuated to a few kilometres' thickness, can be found underlying 'oceanic' water depths. This paper poses the question of whether thinned continental crust can be distinguished seismically from normal oceanic crust of about the same thickness. A single seismic refraction line shot over thinned continental crust as part of the North Biscay margin transect in 1979 was studied in detail. Tau-p inversion suggested that there are differences between oceanic and continental crust in the lower crustal structure. This was confirmed when synthetic seismograms were calculated. The thinned continental crust (α ≥ 7.0) exhibits a two-gradient structure in the non-sedimentary crust with velocities between 5.9 and 7.4 km s-1; an upper 0.8 s-1 layer overlies a 0.4 s-1 layer. No layer comparable to oceanic layer 3 was detected. The uppermost mantle also contains a low-velocity zone.
Continental Margin
Convergent boundary
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We investigate how subduction may be triggered by continental crust extension at a continental margin. The large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement. Subduction requires the oceanic plate to get submerged in mantle, so that negative buoyancy forces may take over and drive further descent. This is promoted by two mechanisms. Loading by continental crust bends the oceanic plate downwards. Extension in the continental domain induces crustal thinning, which acts to raise mantle above the oceanic plate. In this model, the width of the continental region undergoing extension is an important control parameter. The main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading. Three governing dimensionless parameters are identified. One involves the poorly constrained oceanic plate buoyancy. We find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant, enabling metamorphic reactions and density increase in the oceanic crust. Another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate. Initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong. The third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness, such that a weak plate and a thick crust do not favour subduction. Thus, the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere. It is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides. Evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed.
Convergent boundary
Continental Margin
Eclogitization
Adakite
Passive margin
Underplating
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Flysch
Orogeny
Trough (economics)
Alpine orogeny
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In the pre-Alpine basement of the Aspromonte Nappe (southern Calabria and northeastern Sicily), a Barrovian greenschist to amphibolite facies tectono-metamorphic overprint has been recognized since the end of the last century. It was tentatively ascribed to the Alpine cycle from field and petrological evidence. The results of a systematic isotopic study on Southern Calabria rocks, supporting that interpretation, are reported in the present paper. Rb-Sr isotopic data from biotites and white micas show ages ranging from 314 to 22 Ma, with a maximum frequency around 30-25 Ma. The oldest values (314-308 Ma) are interpreted as Variscan ages, whereas the youngest values (32-22 Ma) support, for the first time isotopically, the inferred occurrence of an Alpine metamorphism in the Aspromonte Nappe. The intermediate values (273-48 Ma) are considered as mixed ages. The thermal peak of the Alpine metamorphic overprint indicates the actual age of deformation in the Aspromonte Nappe, related to a major tectonic phase responsible for the nappe stacking. The age of the metamorphic overprint is in agreement with the available stratigraphic data and further strengthens the distinction in the Calabria-Peloritani Arc of two terranes, characterized by significant differences in the Alpine tectono-sedimentary and tectono-metamorphic evolution. The occurrence of a late Oligocene-early Miocene metamorphism also in the Tellian Maghrebids and in the Sardinia Channel further supports the interpretation which considers these portions of the circum-Mediterranean mountain belt and the northern and southern sectors of the Calabria-Peloritani Arc as fragments of a former continuous orogenic belt, dismembered by the opening of the Algero-Provencal and Tyrrhenian basins.
Alpine orogeny
Greenschist
Basement
CYCLADES
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