Abstract
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.
Highlights
The tabular quartzite of the Evolène Series is followed by a carbonate sequence, mainly dolomite, whose thickness never exceeds a few tens of meters, again much thinner than in the Briançonnais s.str
5.3 The case of a basal gap As mentioned above, the superposition of the Mesozoic Evolène Series over the Mont Fort Paleozoic basement is frequently discordant, with an angular obliquity of a few degrees, often only noticeable at the map scale (Figs. 6, 7, 8, 9 and 10). This obliquity causes a gap of variable importance along the cover/basement contact, such that any layer of the Evolène Series, Triassic or Jurassic, may rest upon the Mont Fort Paleozoic. (See figure on page.) Fig. 6 The Mont Fort nappe and surrounding tectonic units in SW Switzerland
Contrary to examples frequently described in other regions, the Jurassic faults of the Mont Fort nappe were not reactivated during Alpine tectonics
Summary
The Pennine Alps (SW Switzerland and NW Italy) show a complex tectonic architecture resulting from the Cenozoic collision of the European and Adriatic plates. The Mont Fort nappe which is the subject of the present work is of particular interest in this regard This nappe (first defined by Escher 1988) belongs to the Middle Penninic realm and occupies a central position in the nappe stack of the Pennine Alps (Fig. 2). This study leads to a new appraisal of the importance of extensional faulting of Jurassic age on the European margin of the embryonic Alpine Tethys. The recognition of such paleofaults mistaken for Alpine thrusts by some authors has direct consequences regarding its relationships with Upper Penninic units
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