The problematic origin of the Pancherot-Cime Bianche-Bettaforca unit (PCB) in the Piemonte zone (Western Alps)

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

doi:10.3301/ijg.2018.21

Cited by 4 publications

(10 citation statements)

References 21 publications

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“…The continental margin-derived Cimes Blanches metasediments are not found exactly at the contact between the Zermatt-Saas Fee and Tsaté ophiolitic nappes but structurally higher inside the Tsaté nappe. Indeed, metabasites devoid of eclogitefacies imprint (even as relict) have been identified between the lowermost bands of the Faisceau Vermiculaire and the Zermatt-Saas Fee eclogites in different localities of the Argand (1908), Bearth (1953bBearth ( , 1964b, Wilson (1978), Crespo (1984) and Sartori (1987); Ba: Barrhorn Series (Siviez-Mischabel autochthonous Mesozoic cover); FV: Faisceau Vermiculaire; SM MB : Siviez-Mischabel monocyclic basement; SR: Série Rousse calcschists; Ts: Tsaté nappe Valtournenche and Ayas valley, where they are referred to as Lower Combin Unit (Ballèvre et al, 1986;Cortiana et al, 1998;Bucher et al, 2004a;Dal Piaz et al, 2015b;Passeri et al, 2018). Metabasites devoid of eclogitic imprint have also been described in an identical tectonic position around Zermatt, in the sectors of the Oberrothorn (Bearth 1973) and Täschalpen (Cartwright and Barnicoat 2002).…”

Section: Geological Settingmentioning

confidence: 99%

“…Caby et al, 1978;Caby 1981;Froitzheim et al, 2006Froitzheim et al, , 2019Pleuger et al, 2007), or from Adriatic margin-derived extensional allochthons (Lower Austroalpine outliers;e.g. Dal Piaz 1999;Dal Piaz et al, 2001, 2015aBeltrando et al, 2014;Passeri et al, 2018).…”

Section: Geological Settingmentioning

confidence: 99%

“…The quartzschists and phyllitic quartzites are mainly composed of quartz, generally in sub-mm grains, white micas and minor chlorite, marking the penetrative cleavage, albite and K-feldspar; accessory minerals include apatite, zircon, calcite, chlorite, tourmaline, epidote, rutile and other oxides (e.g. Girard 1995;Bucher et al, 2004a;Passeri et al, 2018). At Täschalpen, especially on the left (SW) side of the valley, the quartzites have a thickness of a few tens of meters.…”

Section: Quartzitesmentioning

confidence: 99%

“…Identical lithologies at the base of the Pancherot-Cime Bianche-Bettaforca Series in the upper Valtournenche are one hundred meters thick (cf. Passeri et al, 2018). These quartzschists and phyllitic quartzite are classically assigned to Permian (e.g.…”

Section: Quartzitesmentioning

confidence: 99%

“…Staub 1942a;Güller 1947;Iten 1948;Bearth 1953aBearth , 1964aBearth , 1976; (d) early detachment of the sedimentary cover from portions of the subducting continental margin, followed by polyphased folding of variable vergence and later backfolding (e.g. Sartori 1987Sartori , 1990Steck 1989;Sartori et al, 1989;Vannay and Allemann 1990;Escher et al, 1993Steck et al, 2015); (e) shallow depth offscraping, and stacking within an accretionary wedge, of slivers derived either from the continental margins, or oceanic crust and basin (Marthaler and Stampfli 1989;Stampfli and Marthaler 1990); (f ) hyper-extension of the continental margin during Jurassic rifting, leading to the formation of continentderived detachment allochthons inside the oceanic domain, and subsequent occurrence of small continental units pinched within the oceanic units in the Alpine nappe stack (Dal Piaz 1999;Beltrando et al, 2014;Dal Piaz et al, 2015a;Passeri et al, 2018); (g) emplacement of thin continent-derived sheared slices inside the ophiolitic units, along crustal scale shear zones showing a complex history and reactivations with opposite shear senses (e.g. Cartwright and Barnicoat 2002;Reddy et al, 2003;Forster et al, 2004;Groppo et al, 2009;Kirst 2017;Kirst and Leiss 2017); (h) emplacement of continent-derived detached sedimentary slivers at the interface between two distinct ophiolitic units by top-NW directed extraction (Froitzheim et al, 2006(Froitzheim et al, , 2019Pleuger et al, 2007); (i) backshearing and backfolding allowing the emplacement of sheared off continentderived metasedimentary blocks and lenses inside the ophiolites and calcschists complex (Scheiber et al, 2013); (j) diachronous juxtaposition and underplating of slices detached at different depth along the Alpine...…”

Section: Introductionmentioning

confidence: 99%

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Continent-derived metasediments (Cimes Blanches and Frilihorn) within the ophiolites around Zermatt: relations with the Mischabel backfold and Mont Fort nappe (Pennine Alps)

Pantet,

Epard,

Masson

2024

Swiss J Geosci

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The region surrounding Zermatt (SW Switzerland and NW Italy) displays some classic examples of imbrications between continental and oceanic units. In particular, the studied units, called Cimes Blanches and Frilihorn or Faisceau Vermiculaire, consist of a set of thin bands of continent-derived metasediments intercalated at different levels within the ocean-derived units. These bands are locally reduced to only one meter thick but can be traced for several tens to more than one hundred kilometers across the Pennine Alps. The mechanisms leading to such imbrications are a long-standing and still-debated question. Based on detailed mapping and structural analysis of key areas, we present new data on the structure and stratigraphy of the Faisceau Vermiculaire in the area surrounding Zermatt, with particular focus on the Täschalpen sector, where the Faisceau Vermiculaire is locally in contact with basement units. Our observations allow: (i) to confirm the presence of widespread breccias of probable Jurassic age in the Faisceau Vermiculaire; (ii) to interpret the contacts between the Faisceau Vermiculaire and the overlying non-ophiolitic Schistes Lustrés (Série Rousse) as stratigraphic; (iii) to show that the stratigraphy of the Faisceau Vermiculaire and associated Série Rousse contrasts strongly with the cover of the Siviez-Mischabel nappe and that these sequences originate from different paleogeographic domains (Prepiemont basin and Briançonnais platform respectively); (iv) to interpret as stratigraphic the contact of the Faisceau Vermiculaire and the Série Rousse with the basement forming the Alphubel anticline; the local unconformity is interpreted as the result of the activity of synsedimentary Jurassic normal paleofaults; (v) to highlight the trace of a major Jurassic normal fault, that should have marked an abrupt thinning of the paleomargin; it corresponds now to the contact between the Faisceau Vermiculaire (and associated Série Rousse) and the Siviez-Mischabel basement in the hinge of the Mischabel backfold. We propose a new tectonic scheme for the structure of the Faisceau Vermiculaire and adjacent units involving an early northward folding of the Faisceau Vermiculaire with the Série Rousse and the ophiolitic Schistes Lustrés of the Tsaté nappe, followed by major backfolding responsible for the southward emplacement of these units above the HP Zermatt-Saas and Monte Rosa nappes. Our study at regional scale shows that the group formed by the Alphubel basement, the Faisceau Vermiculaire and the Série Rousse share a tectonic position and stratigraphic sequences identical to those of the Mont Fort nappe, which outcrops on the other side of the Dent Blanche klippe. It leads to the proposition that this group constitutes the eastern extension of the Mont Fort nappe.

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Section: Geological Settingmentioning

confidence: 99%

Section: Geological Settingmentioning

confidence: 99%

Section: Quartzitesmentioning

confidence: 99%

Section: Quartzitesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Continent-derived metasediments (Cimes Blanches and Frilihorn) within the ophiolites around Zermatt: relations with the Mischabel backfold and Mont Fort nappe (Pennine Alps)

Pantet,

Epard,

Masson

2024

Swiss J Geosci

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Tectono-metamorphic evolution of UHP Zermatt-Saas serpentinites: a tool for vertical palaeogeographic restoration

Luoni

Rebay

Roda

et al. 2020

International Geology Review

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International Geology Review 42similar to those estimated for D2 in the surrounding serpentinites, which were dated at 65 ± 43 5.6 Ma. These results suggest that portions of ZSZ were subducted at high depth before 70 Ma 44 and widen the time span during which ZSZ recorded PT peak conditions. The comparison of 45 these data with results of a numerical model of an ocean-continent subduction system gives 46 insights on coupling stages of this UHP unit with the surrounding ZSZ rocks during the 47 Alpine convergence and vertical palaeogeography during different time steps.

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Mimicking Alpine thrusts by passive deformation of synsedimentary normal faults: a record of the Jurassic extension of the European margin (Mont Fort nappe, Pennine Alps)

2020

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The Mont Fort nappe, former uppermost subunit of the Grand St-Bernard nappe system, is an independent tectonic unit with specific structural and stratigraphic characteristics (Middle Penninic, NW Italy and SW Switzerland). It consists in a Paleozoic basement, overlain by a thin, discontinuous cover of Triassic-Jurassic metasediments, mainly breccias, called the Evolène Series. The contact of this Series over the Mont Fort basement is debated: stratigraphic or tectonic? We present new observations that support the stratigraphic interpretation and consequently imply that the Evolène Series belongs to the Mont Fort nappe. We moreover show that the Mont Fort nappe was strongly affected by normal faulting during Jurassic. These faults went long unnoticed because Alpine orogenic deformation blurred the record. Alpine strain erased their original obliquity, causing confusion with an Alpine low-angle thrust. These Jurassic faults have been passively deformed during Alpine tectonics, without inversion or any other kind of reactivation. They behaved like passive markers of the Alpine strain. Detailed field observations reveal the link between observed faults and specific breccia accumulations. Areas where the Evolène Series is missing correspond to sectors where the fault scarps were exposed on the bottom of the sea but were too steep to keep the syn- to post-faulting sediments. The Mont Fort nappe thus represents an example of a distal rifted margin. The succession of synsedimentary extensional movements followed by orogenic shortening generated a situation where passively deformed normal faults mimic an orogenic thrust.

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The problematic origin of the Pancherot-Cime Bianche-Bettaforca unit (PCB) in the Piemonte zone (Western Alps)

Cited by 4 publications

References 21 publications

Continent-derived metasediments (Cimes Blanches and Frilihorn) within the ophiolites around Zermatt: relations with the Mischabel backfold and Mont Fort nappe (Pennine Alps)

Continent-derived metasediments (Cimes Blanches and Frilihorn) within the ophiolites around Zermatt: relations with the Mischabel backfold and Mont Fort nappe (Pennine Alps)

Tectono-metamorphic evolution of UHP Zermatt-Saas serpentinites: a tool for vertical palaeogeographic restoration

Mimicking Alpine thrusts by passive deformation of synsedimentary normal faults: a record of the Jurassic extension of the European margin (Mont Fort nappe, Pennine Alps)

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