Facies and Late Triassic fossils in the Roisan zone, Austroalpine Dent Blanche and Mt Mary-Cervino nappe system, NW Alps

Ciarapica, Gloria; Passeri, Leonsevero; Bonetto, Franco; Piaz, G. V. Dal

doi:10.1007/s00015-016-0207-6

Cited by 8 publications

(10 citation statements)

References 11 publications

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“…These meta-sediments contain a variety of dolomite marbles, classically attributed to the Triassic on the basis of lithology and poorly-preserved fossils (diploporacean algae according to Ballèvre et al 1986). Recently, Ciarapica et al (2010) identified diploporacean algae and a rich association of foraminifera preserved in dolomite marble that was attributed to the Late Triassic (Norian). Carbonate breccias, including dolomite clasts, calcschists and quartz-micaschists are also observed.…”

Section: Geological Settingmentioning

confidence: 99%

“…The scale of pervasive transposition that altered the original stratigraphy appears to be strongly dependent on the viscosity contrast and the relative thickness between layers of contrasting rheology. In the Roisan Zone, owing to the strong deformation that affected the cover sequence, the occurrence of Late Triassic microfossils and depositional structures is restricted to a few pods of undeformed dolomite marble (Ciarapica et al 2010). Little information on the stratigraphy or overall distribution of sedimentary horizons is thus preserved.…”

Section: Tectonic Units and Rock Types Constituting The Rcszmentioning

confidence: 99%

“…In the DBTS, the sedimentary sequences now forming the Mont Dolin Series (Hagen 1948;Weidmann and Zaninetti 1974) and the Roisan Zone (Diehl et al 1952;Ciarapica et al 2010) are remnants of the Mesozoic sedimentary cover that was deposited along the Adriatic passive margin. The Mont Dolin Series (Fig.…”

Section: Passive Margin Evolution In the Cover (Triassic-jurassic)mentioning

confidence: 99%

“…In the Roisan Zone, pervasive transposition of the primary sedimentary contacts precludes a reliable reconstruction of the original lithostratigraphy in most cases. Sedimentary facies and fossil contents can be compared to the Mont Dolin Series (Ciarapica et al 2010). The two Mesozoic sequences differ in sedimentary facies because (1) quartzites of presumed Early Triassic age are not found in the Roisan Zone, and (2) some breccias do occur in the Triassic dolomites of the Roisan Zone (Ciarapica et al 2010), while no equivalents of the monogenic and polygenic breccias have been found.…”

Section: Passive Margin Evolution In the Cover (Triassic-jurassic)mentioning

confidence: 99%

“…Sedimentary facies and fossil contents can be compared to the Mont Dolin Series (Ciarapica et al 2010). The two Mesozoic sequences differ in sedimentary facies because (1) quartzites of presumed Early Triassic age are not found in the Roisan Zone, and (2) some breccias do occur in the Triassic dolomites of the Roisan Zone (Ciarapica et al 2010), while no equivalents of the monogenic and polygenic breccias have been found. This implies that the Mesozoic sequence was detached from its Permian and Early Triassic cover, and that Jurassic sediments are lacking or alternatively, the sediments of the Roisan Zone were deposited in a basinal setting, away from the active normal faults that segmented the Adriatic palaeomargin.…”

Section: Passive Margin Evolution In the Cover (Triassic-jurassic)mentioning

confidence: 99%

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Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

et al. 2014

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The Dent Blanche Tectonic System (DBTS) is a composite thrust sheet derived from the previously thinned passive Adriatic continental margin. A kilometric high-strain zone, the Roisan-Cignana Shear Zone (RCSZ) defines the major tectonic boundary within the DBTS and separates it into two subunits, the Dent Blanche s.s. nappe to the northwest and the Mont Mary nappe to the southeast. Within this shear zone, tectonic slices of Mesozoic and pre-Alpine meta-sediments became amalgamated with continental basement rocks of the Adriatic margin. The occurrence of high pressure assemblages along the contact between these tectonic slices indicates that the amalgamation occurred prior to or during the subduction process, at an early stage of the Alpine orogenic cycle. Detailed mapping, petrographic and structural analysis show that the Roisan-Cignana Shear Zone results from several superimposed Alpine structural and metamorphic stages. Subduction of the continental fragments is recorded by blueschist-facies deformation, whereas the Alpine collision is reflected by a greenschist facies overprint associated with the development of large-scale open folds. The postnappe evolution comprises the development of low-angle brittle faults, followed by large-scale folding (Vanzone phase) and finally brittle extensional faults. The RCSZ shows that fragments of continental crust had been torn off the passive continental margin prior to continental collision, thus recording the entire history of the orogenic cycle. The role of preceding Permo-Triassic lithospheric thinning, Jurassic rifting, and ablative subduction processes in controlling the removal of crustal fragments from the reactivated passive continental margin is discussed. Results of this study constrain the temporal sequence of the tectono-metamorphic processes involved in the assembly of the DBTS, but they also show limits on the interpretation. In particular it remains difficult to judge to what extent precollisional rifting at the Adriatic continental margin preconditioned the efficiency of convergent processes, i.e. accretion, subduction, and orogenic exhumation.

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Section: Geological Settingmentioning

confidence: 99%

Section: Tectonic Units and Rock Types Constituting The Rcszmentioning

confidence: 99%

Section: Passive Margin Evolution In the Cover (Triassic-jurassic)mentioning

confidence: 99%

Section: Passive Margin Evolution In the Cover (Triassic-jurassic)mentioning

confidence: 99%

Section: Passive Margin Evolution In the Cover (Triassic-jurassic)mentioning

confidence: 99%

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Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

et al. 2014

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The tectonometamorphic evolution of the Sesia–Dent Blanche nappes (internal Western Alps): review and synthesis

et al. 2014

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International audienceThis study reviews and synthesizes the presentknowledge on the Sesia–Dent Blanche nappes, the highesttectonic elements in the Western Alps (Switzerland andItaly), which comprise pieces of pre-Alpine basement andMesozoic cover. All of the available data are integrated in acrustal-scale kinematic model with the aim to reconstructthe Alpine tectono-metamorphic evolution of the Sesia–Dent Blanche nappes. Although major uncertainties remainin the pre-Alpine geometry, the basement and coversequences of the Sesia–Dent Blanche nappes are seen aspart of a thinned continental crust derived from the Adriaticmargin. The earliest stages of the Alpine evolution areinterpreted as recording late Cretaceous subduction of theAdria-derived Sesia–Dent Blanche nappes below theSouth-Alpine domain. During this subduction, severalsheets of crustal material were stacked and separated byshear zones that rework remnants of their Mesozoic cover.The recently described Roisan-Cignana Shear Zone of theDent Blanche Tectonic System represents such a shearzone, indicating that the Sesia–Dent Blanche nappes representa stack of several individual nappes. During thesubsequent subduction of the Piemonte–Liguria Oceanlarge-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschistfacies conditions, which indicates partial exhumation of theDent Blanche Tectonic System. The entrance of the Brianc¸onnais micro-continent within the subduction zone ledto a drastic change in the deformation pattern of the Alpinebelt, with rapid exhumation of the eclogite-facies ophiolitebearingunits and thrust propagation towards the foreland.Slab breakoff probably was responsible for allowing partialmelting in the mantle and Oligocene intrusions into themost internal parts of the Sesia–Dent Blanche nappes.Finally, indentation of the Adriatic plate into the orogenicwedge resulted in the formation of the Vanzone back-fold,which marks the end of the pervasive ductile deformationwithin the Sesia–Dent Blanche nappes during the earliestMiocene

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Internal geometry of the central Sesia Zone (Aosta Valley, Italy): HP tectonic assembly of continental slices

Giuntoli

Engi

2016

Swiss J Geosci

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Detailed field mapping reveals that the Sesia Zone is subdivided into two complexes with the Barmet Shear Zone (BSZ) outlining the tectonic contact between them. This greenschist-facies contact reflects a metamor-phic gap between the Internal Complex (eclogite facies, eclogitic micaschists dominant) and the External Complex (epidote blueschist facies). The BSZ comprises a wedge shape area in which fragments and slices of orthogneiss and paragneiss are wrapped by siliceous dolomite marbles displaying a mylonitic foliation. Conspicuous cornieules and high pressure breccias occur along this contact. We propose that the eclogite facies Internal Complex is sub-divided into three basement units, called sheets, delimited by discontinuous metasedimentary trails of probable Mesozoic age. Thin monocyclic bands thus separate kilo-metre scale polycyclic sheets. The External Complex comprises three epidote blueschist facies sheets of com-parable size, which are separated by lenses retaining a pre-Alpine high temperature imprint. These weakly overprinted fragments (parts of the classically termed 2DK zone) are aligned along greenschist facies shear zones that separate the gneissic sheets. The BSZ, with a wedge rich in meta-sediments, chiefly siliceous dolomite marbles, is a key element in which fragmentation and reworking of materials from the internal and external complexes are evident. A carbonate breccia occurs in this shear zone, with clasts displaying a HP foliation randomly oriented in a ductile carbonate matrix. Siliceous dolomite marbles appear to have acted as lubricants to accommodate deformation related to the juxtaposition of the two basement complexes during exhumation. We propose a model of the Sesia Zone, with the BSZ as the thrust responsible for the juxtaposition of eclogite facies rocks of the Internal Complex on top of epidote blueschist facies rocks of the External Complex. The two complexes were already assembled when this shear zone became active. The entire stack was finally rotated (40 60°) during the Vanzone Phase.

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Facies and Late Triassic fossils in the Roisan zone, Austroalpine Dent Blanche and Mt Mary-Cervino nappe system, NW Alps

Cited by 8 publications

References 11 publications

Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

The tectonometamorphic evolution of the Sesia–Dent Blanche nappes (internal Western Alps): review and synthesis

Internal geometry of the central Sesia Zone (Aosta Valley, Italy): HP tectonic assembly of continental slices

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