Schistes Lustrés in a hyper-extended continental margin setting and reinterpretation of the limit between the Mont Fort and Tsaté nappes (Middle and Upper Penninics, Western Swiss Alps)

Abstract

The Schistes Lustrés form a large and complex unit at the top of the Penninic nappe stack of the Alpine belt. Calcschists, partly of Late Cretaceous age, constitute the dominant lithology. They are closely associated both with blueschist facies Piemont-Ligurian ophiolites and continent-derived Mesozoic metasediments. The question of whether the Schistes Lustrés originated on continental or oceanic crust has been extensively debated among Alpine geologists and is locally still controversial. We present here new structural and stratigraphic observations, as well as Raman graphite thermometry (RSCM) data, for the Schistes Lustrés complex of the Combin zone in the Hérens, Dix and Bagnes valleys. Our observations indicate that the basal part of this Schistes Lustrés complex (defined as the Série Rousse) is systematically devoid of ophiolitic material, and rests in stratigraphic contact on the underlying Triassic - Lower Cretaceous metasediments and Paleozoic basement of the Mont Fort nappe (Prepiemont paleogeographic domain). The unconformity at the base of the Schistes Lustrés complex is interpreted as resulting from the sedimentation of the Série Rousse on a paleorelief formed by remnants of Jurassic normal fault scarps, and not as an Alpine tectonic contact, as previously proposed. The lithostratigraphic comparison with the Breccia nappe in the Prealps, as well as a foraminifer discovery, allows us to better constrain the age of the Série Rousse. It extends from the middle of the Early Cretaceous (Aptian?) to the Late Cretaceous (Campanian to earliest Maastrichtian?). In contrast, the upper contact of the Série Rousse with the ophiolite-bearing Schistes Lustrés clearly corresponds to an Alpine thrust. The thrust zone is underlined by thin and discontinuous slices of highly strained continental-margin derived Mesozoic metasediments (Frilihorn slices). RSCM data show that the recrystallization of the organic matter progressively increases on both sides towards this contact. This contact, internal to the Schistes Lustrés complex, is reinterpreted as the major tectonic contact separating the Middle Penninic Mont Fort nappe from the Upper Penninic Tsaté nappe (defined here as including only the ophiolite-bearing Schistes Lustrés and associated meta(ultra-)basites). This study clearly documents that the Schistes Lustrés consist of sediments either deposited on oceanic crust, showing locally preserved stratigraphic contacts with ophiolitic or serpentinized sub-continental mantle slivers, or sediments still resting stratigraphically on a former hyper-extended continental margin.

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

“…The attribution of the non-ophiolitic russet calcschists from the area of Zermatt to the Série Rousse has been proposed by Sartori (1987). The cartographic continuity Continent-derived metasediments within the ophiolites around Zermatt of the Série Rousse from the Hérens valley, where it rests on top of the Triassic-Lower Cretaceous cover of the Mont Fort nappe and its Paleozoic basement (Pantet et al, 2023), through the upper Turtmann valley, to the area of Zermatt, has then been mapped by Escher (1988). Our observations confirm that the Série Rousse outcropping north of the Dent Blanche klippe (cf.…”

“…Lithologies very similar to these non-ophiolitic russet calcschists are found both in the upper part of the Roisan Series (Ciarapica et al, 2016) and north of the Dent Blanche klippe, where they are referred to as the Série Rousse (Marthaler and Escher in Masson et al, 1980;Marthaler 1984) and have been interpreted as forming the upper part of the autochthonous cover of the Mont Fort nappe (e.g. Escher 1988;Pantet et al, 2023). Relics of planktonic foraminifera have been described in various localities of the Série Rousse (e.g.…”

“…Relics of planktonic foraminifera have been described in various localities of the Série Rousse (e.g. Marthaler 1981Marthaler , 1984Pantet et al, 2023) and indicate a Late Cretaceous age (Cenomanian-Campanian? ).…”

“…Particular attention was paid to the study of basement-cover contacts on the SW slope of the Alphubel and to the associated sediments on the left side of the valley. Data from our previous work on the Mont Fort nappe in the area located to the north and west of the Dent Blanche klippe (Pantet et al, 2020(Pantet et al, , 2023 have also been integrated. Relationships between the Faisceau Vermiculaire and the Mont Fort nappe (incl.…”

Tectonic architecture of the northern Dora-Maira Massif (Western Alps, Italy): field and geochronological data

“…By comparison with the “classical” Mesozoic successions in the External Briançonnais (e.g. Ellenberger, 1958 ) or with the “Pre-Piemonte-type” successions in the Internal Briançonnais (including the Val Maira-Sampeyre Unit and Val Grana Unit along the south-eastern margin of the Dora-Maira Massif; Balestro et al, 2022 ; Michard et al, 2022 ; Pantet et al, 2022 ; Dana et al, 2023 ), dolostones are typically considered Middle to Upper Triassic in age and limestones and calcschists can be considered Jurassic and Cretaceous in age, respectively (Fig. 10 ).…”