Tectonics of the Lepontine Alps: ductile thrusting and folding in the deepest tectonic levels of the Central Alps

Abstract

The Lepontine dome represents a unique region in the arc of the Central and Western Alps, where complex fold structures of upper amphibolite facies grade of the deepest stage of the orogenic belt are exposed in a tectonic half-window. The NW-verging Mont Blanc, Aar und Gotthard basement folds and the Lower Penninic gneiss nappes of the Central Alps were formed by ductile detachment of the upper European crust during its Late Eocene-Early Oligocene SE-directed underthrust below the upper Penninic and Austroalpine thrusts and the Adriatic plate. Four underthrust zones are distinguished in the NW-verging stack of Alpine fold nappes and thrusts: the Canavese, Piemont, Valais and Adula zones. Up to three schistosities S1-S3, folds F1-F3 and a stretching lineation XI with top-to-NW shear indicators were developed in the F1-F3 fold nappes. Spectacular F4 transverse folds, the SW-verging Verzasca, Maggia, Ziccher, Alpe Bosa and Wandfluhhorn anticlines and synclines overprint the Alpine nappe stack. Their formation under amphibolite facies grade was related to late ductile folding of the southern nappe roots during dextral displacement of the Adriatic indenter. The transverse folding F4 was followed since 30 Ma by the pull-apart exhumation and erosion of the Lepontine dome. This occurred coevally with the formation of the dextral ductile Simplon shear zone, the S-verging backfolding F5 and the formation of the southern steep belt. Exhumation continued after 18 Ma with movement on the brittle Rhone-Simplon detachment, accompanied by the N-, NW-and W-directed Helvetic and Dauphiné thrusts. The dextral shear is dated by the 29-25 Ma crustal-derived aplite and pegmatite intrusions in the southern steep belt. The cooling by uplift and erosion of the Tertiary migmatites of the Bellinzona region occurred between 22 and 18 Ma followed by the exhumation of the Toce dome on the brittle Rhone-Simplon fault since 18 Ma.

The Grand St Bernard‐Briançonnais Nappe System and the Paleozoic Inheritance of the Western Alps Unraveled by Zircon U‐Pb Dating

“…Evidence of pre‐Variscan magmatism possibly resembling what observed in the Ruitor unit is instead reported in the Lower Penninic units of the Lepontine dome (Monte Leone nappe; Bergomi et al, ). In the Ossola Valley (NW Italy), the Lepontine nappe stack includes (from the bottom to the top) the Verampio, Antigorio, Pioda di Crana, and Monte Leone nappes (Berger et al, ; Bigi et al, ; Escher et al, ; Grujic & Mancktelow, ; Maxelon & Mancktelow, ; Steck et al, ), mainly consisting of Upper Carboniferous‐Lower Permian granitoids (305–290 Ma; Bergomi et al, ) with minor metapelites, marbles, and amphibolite lenses. The Monte Leone nappe includes fine‐grained banded orthogneisses and minor coarse‐grained augengneiss interlayered with paragneisses, hornblende gneisses, and amphibolites and shows a penetrative amphibolite‐facies metamorphic overprint of Alpine age that is common to other nappes of the Lepontine dome (Maxelon & Mancktelow, , and references therein).…”

“…This article is mainly focused on the Paleozoic evolution of the Penninic basement units of the Grand St Bernard‐Briançonnais nappe system (Western Alps) and provides new sensitive high‐resolution ion microprobe (SHRIMP) U‐Pb zircon and geochemical whole rock analyses on gneissic bodies derived from acidic intrusive and subvolcanic protoliths sampled in the Ruitor, Leverogne, Grand Nomenon, and Houillère units exposed in the Aosta Valley (NW Italy). Results are compared with unpublished data from similar granitoids exposed in the Central Alps, in the Lower‐Penninic gneissic nappe of Monte Leone (Lepontine dome; Steck et al, , and reference therein) and in the Lower Austroalpine Gneiss del Monte Canale unit (Bernina nappe s.l. ; Spillmann & Büchi, ).…”

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

“…Because the age of the Vanzone fold is ca. 25-20 Ma (Steck and Hunziker 1994;Keller et al 2005;Steck et al 2013), the low-angle brittle faults are assumed to be slightly older (ca. 30 Ma, early Oligocene).…”

Shear zone and nappe formation by thermal softening, related stress and temperature evolution, and application to the Alps

“…Many nappes in the W‐Alps can be attributed to kilometre‐scale shear zones, because (i) their length is usually much larger than their thickness; (ii) they often exhibit a penetrative shear strain usually increasing towards the base (e.g. Epard & Escher, ; Steck, ; Steck et al ., ); (iii) shear sense indicators usually denote a consistent top‐to‐the‐foreland shearing (e.g. Steck, ; Pleuger & Podladchikov, ; and references therein); and (iv) many nappes usually still preserve more or less complete remnants of their original Mesozoic–Tertiary sedimentary cover (especially in the northern Lepontine dome; e.g.…”