1
1 The Maggia nappe: an extruding sheath fold basement nappe in the Lepontine gneiss
2 dome of the Central Alps
3
4 Albrecht Steck, Jean-Luc Epard, Henri Masson
5 Institut des Sciences de la Terre, Université de Lausanne, bâtiment Geopolis, Dorigny, 1015 Lausanne,
6 Switzerland.
7 e-mail: [email protected]
8
9 Abstract
10
11 The Lepontine gneiss dome represents a unique region of the Central Alps where Oligocene-Miocene
12 amphibolite facies grade rocks and fold nappes of the deepest tectonic level of the Alpine orogenic belt are
13 exposed in a tectonic window. The Cenozoic structures of the Maggia nappe reveals a giant tens of kilometre
14 scale tubular fold structure that cross-cut through the surrounding lower Penninic nappes from its root situated in
15 the southern steep-belt of the Alps near Bellinzona. The Mesozoic sedimentary cover of the Maggia nappe is
16 typical for the Helvetic stratigraphic domain. The age of formation of the lower Penninic fold-nappes by ductile
17 detachment of the upper European crust during its underthrusting below the higher Penninic and Austroalpine
18 orogenic lid and Adriatic indenter was estimated between 40 and 30 Ma. Maximal pressures of 8-9 kbars and
19 temperatures of 600-700 °C were attended during and after the nappe emplacement some 30-22 Ma ago. The
20 Maggia and surrounding nappes are crosscut by the isograds of the Barrovian regional metamorphism.
21
22 Keywords Swiss Alps, Alpine tectonics, Penninic nappes, sheath fold structure
23
24 1. Introduction
25
26 The block diagram of the Central Alps of Switzerland and Italy illustrates the tectonic position of
27 Maggia nappe in the Lepontine gneiss dome (Fig. 1, modified after Steck, 2008)). The geology of the Lepontine
28 gneiss dome of the Central Alps of Switzerland and Italy is documented on numerous geological maps and 2
29 stratigraphic studies of the twenties century (e.g. Geologische Karte und Tektonische Karte der Schweiz (2005).
30 The here considered lower Penninic nappes are composed of polycyclic basement gneisses intruded by Permo-
31 Carboniferous granites and overlain by their autochthonous Mesozoic-Cenozoic sedimentary cover of European
32 affinity (Steck et al., 2013). A modern geological study of the Central Alps started with the fieldwork of Schmidt
33 & Preiswerk (1905), Preiswerk et al. (1934) and Niggli et al. (1936) after the discovery of the lower Penninic
34 fold nappe structures during the perforation of the Simplon railway gallery by Schardt (1903). The first
35 description of the Antigorio fold nappe with its amplitude of 9 km by Gerlach (1869) was followed by the
36 confirmation of the fold nappe geometry of the Simplon structures and the higher Saint Bernard and Monte Rosa
37 nappes by Schardt (1903), Schmidt & Preiswerk (1905), Argand (1916), Milnes (1974), Steck (1984), Escher
38 (1988) and Froitzheim (2001). Schmid et al. (2004) and Berger et al. (2005) considered the Maggia nappe as a
39 thrust sheet situated in a high tectonic level of the lower Penninic nappe stack and attributed it to the middle
40 Penninic Briançonnais domain as already proposed by Preiswerk et al. (1934). The high tectonic position on top
41 of the Cimalunga unit was also formerly proposed by Steck et al. (2013). New field work and the integration of
42 the observations of Bächlin et al. (1974) and Spicher and Wenk (1981) conducted us to the conclusion, that the
43 Maggia nappe plunges in the southern steep belt of the Alps as a tubular fold structure below the higher
44 Mergoscia and Cimalunga units (Fig. 2). The interpretation of the Maggia nappe as a sheath fold like tubular
45 fold structure will be discussed in the present study. The study of the Mesozoic Cristallina sedimentary cover of
46 the northern Sambuco lobe of the Maggia nappe similar to that of the Helvetic Gotthard massif conducted us to
47 the conclusion that the Maggia nappe belongs to the Helvetic or European domain. Some units of the Valais
48 suture, as the Mergoscia and Cimalunga Mélanges, possess relicts of an eclogitic late Eocene high-pressure
49 metamorphism. The entire nappe pile was afterwards formed concomitant with and overprinted by an Oligocene-
50 Miocene Barrovian-type orogenic regional metamorphism. The aim of this study is to describe the enigmatic
51 Maggia nappe in the frame of the Alpine orogeny and discuss possible models of its formation.
52
53 2. Geological Setting of the Maggia and surrounding nappes.
54
55 The Lower Penninic nappe stack is composed from base to top of the Leventina, Verampio, Antigorio, Simano
56 (the latter subdivided in Simano and Campo Tencia units, after Preiswerk, 1934), Lebendun, Maggia, Mergoscia,
57 Cimalunga, Adula, Orselina-Bellinzona, Monte Leone and Valais calc-schist nappes that were detached from the
58 upper European crust during its SE-directed underthrusting below the middle and higher Penninic nappes, the 3
59 Dent Blanche-Sesia thrust sheet and Adriatic indenter (Table 1). Milnes (1974; Steck, 1984; Steck et al. 2013)
60 considered the Lower Penninic nappes of the Lepontine gneiss dome as fold-nappes. The fold-nappe geometry of
61 the Maggia nappe is well exposed in its northern Sambuco lobe with its autochthonous Mesozoic sedimentary
62 cover the Cristallina unit on its upper and lower fold limb (Figs. 2 and 4).
63 The deepest level of the Alpine nappe stack is overprinted by an amphibolite facies grade Barrovian-
64 type regional metamorphism (e.g. Niggli & Niggli, 1965; Trommsdorff, 1966; Engi et al., 1995; Rubatto et al.,
65 2009). An older late Eocene eclogite facies metamorphism affected the Mergoscia, Cimalunga and Adula nappes
66 (e.g. Heinrich, 1986; Trommsdorff, 1990; Pfeifer et al. 1991; Becker, 1993; Gebauer, 1996; Brower et al. 2005;
67 Herwartz et al. 2011; Sandmann et al. 2014). The external limit of the Oligocene-Miocene amphibolite facies
68 grade metamorphism is indicated by the albite An0-3-hornblende = plagioclase An >17-hornblende-isograd
69 (Wenk & Keller, 1969; the isograd position is modified in the Monte Rosa region after Steck et al., 2001, 2015).
70 The nappe stack of the Central Lepontine gneiss dome is represented on Table 1 and the Alpine structures of the
71 central part of the Lepontine gneiss dome are illustrated on the map and profiles of Figs. 2 and 3.
72 The Maggia nappe was for a first time represented on a geological map of the Lepontine gneiss dome
73 by Preiswerk,1932 in Niggli et al. (1936) and Preiswerk et al. (1934) as a NW-SE oriented synform
74 perpendicular to the SW-NE trend of the Central Alps. Already Preiswerk (1919), in a structural study of the
75 Campolungo pass region recognised the superposition of the Maggia transverse synform on the deeper Antigorio
76 nappe to the west and the Campo Tencia and Simano (=Verzasca gneiss) nappes to the east. Preiswerk’s model
77 was confirmed by our own observations (Steck, 1998; Steck et al., 2013). Grütter and Kündig considered in the
78 same review paper of Niggli et al. (1936) the Maggia nappe as a tectonic root that separates the deeper Antigorio
79 from the higher Campo Tencia nappe. An idea that was also suggested by other studies (Grujic & Mancktelow,
80 1996, Maxelon & Mancktelow, 2005, Rütti et al., 2005). The latter model is contrary to the Alpine structural
81 data collected by Preiswerk (1919), Wenk (1955), Steck (1998) and correctly documented on Fig 12 of Maxelon
82 & Mancktelow (2005). The superposition of the Antigorio and Campo Tencia nappes by the frontal Maggia
83 nappe proposed by Preiswerk in Niggli et al. (1936) is confirmed by new fieldwork on the Pizzo Massari – Pizzo
84 Prevat geological profile (Fig. 4). The frontal fold hinge of the Campo Tencia Nappe and the sedimentary
85 syncline of the Campolungo pass transect form a down facing fold structure testifying that the Sambuco klippe is
86 situated above the Campo Tencia nappe front. The geometry of the down facing Mogno Mesozoic sedimentary
87 syncline was also confirmed by observations in the Prato-Camblee water gallery (personal communication by
88 Franz Keller, Fig. 5; Steck et al. (2013). 4
89 Keller et al. (1980) separate on their 1:25’000 scale geological map “Campo Tencia” the northern
90 Sambuco klippe from its southern continuation the Maggia synform. The Alpigia gneisses, separating the
91 Sambuco klippe from the southern Maggia synform, were attributed by Keller et al. (1980) to the Antigorio
92 nappe. This attribution is supported by the identical Early Permian U-Pb zircon ages of the tonalites,
93 granodiorites and granites dated between 289 ± 2 Ma and 296 ± 2 Ma by Bergomi et al. (2007) in the Antigorio
94 nappe and between 288 ± 2 and 294 ± 2 Ma in the Alpigia unit by Hirsiger et al. (2015). The tonalites,
95 granodiorites and granites of the Matorello gneiss in the northern Sambuco klippe and the similar Cocco gneiss
96 in the centre of the Maggia synform have a Late Carboniferous age dated at 300-304 ± 8 Ma for the Matorello
97 and at 308 ± 7 Ma for the Cocco gneiss (U-Pb zircon ages, Bussien et al., 2011). The latter data corroborate the
98 Keller et al. (1980) proposal that the Sambuco spoon and the Maggia synform belong to the same Maggia
99 complex and not the Sambuco unit to the Helvetic and the southern Maggia synform to the Briançonnais domain
100 as proposed by Berger et al. (2005). The Early Permian Mezzalama granite of the Monte Rosa nappe and the
101 Randa-granite of the Briançonnais Siviez-Mischabel basement are dated at 270±4 Ma (Bussy in Steck et al.
102 1999; Pawlig & Baumgartner, 2001). Based on the structural study of the northern and middle part of the
103 Maggia synform we first concluded that the Maggia synform continues to the south and superpose there the
104 deeper Mergoscia and Cimalunga nappes (Steck et al. 2013), an idea also supported by Schmid et al. (2004,
105 2017), Berger et al. (2005), Berger & Mercolli (2006) and Bousquet et al. (2012). New field observations on the
106 southern end of the Maggia nappe conducted us to a totally different model of the Maggia structure that we
107 discuss later in this study. The Maggia nappe plunges near Bellinzona as a tubular fold below the Mergoscia
108 nappe in the southern steep belt (Chapter 3. Alpine structures).
109 Preiswerk in Niggli et al. (1936) considered the Maggia nappe as a synformal klippe situated on top of
110 the Lower Penninic nappes and as a lateral equivalent of the Middle Penninic Sankt-Bernhard nappe of the
111 Briançonnais domain. This attribution to the Briançonnais domain was later adopted by Schmid et al. (2004;
112 2017), Berger et al. (2005) and Bousquet et al. (2012). It is in agreement with Preiswerk’s idea based on the high
113 level tectonic position of the Maggia nappe. New stratigraphic studies of the Mesozoic cover sediments suggest
114 an attribution of the Maggia nappe to the Helvetic palaeographic domain (Masson & Steck, 2015). The Sambuco
115 klippe of the northern Maggia nappe retains its autochthonous stratigraphic cover, the Cristallina Mesozoic
116 sediments of a typical Helvetic affinity, similar to the autochthonous cover of the Gotthard massif (Liszkay-
117 Nagy, 1965). The Triassic starts with a thin band of 30-50 cm of quartzose and arkosic sandstone overlain by a
118 band of < 2 m of white dolomite. The latter dolomite is followed by the Naret Formation, a series of 100 m of 5
119 heterogeneous, garnet, staurolite and kyanite bearing sandstones with some thin layers of yellow calcite and
120 dolomite marbles similar to the Rhaetian of the Gotthard cover, the lower and upper garnet schists of Liszkay-
121 Nagy (1965). The Lower and Middle Jurassic is composed of variable types of some garnet, kyanite and
122 staurolite bearing graphitic sandstones and conglomerates, gritty and banded marbles, overlain by pure Upper
123 Jurassic white calcite marble on top. The Pertusio zone, at the limit of the Maggia and Campo Tencia nappes, is
124 composed of complex refolded centimetre-decimetre slices of quartzite and white calcite marble of a probable
125 middle and upper Jurassic age respectively and the up to ten metre white marbles of the Someo zone at the limit
126 between the Maggia and Mergoscia nappes of a probable Upper Jurassic age are interpreted as the reduced
127 Mesozoic cover of the Maggia nappe forming the southern Helvetic continental margin. The fossil-free and
128 strong deformation of these sedimentary slices makes their stratigraphic interpretation questionable and open.
129 Never the less it is herein suggested that the Sambuco-Maggia nappe belongs together with the Antigorio and the
130 Gotthard fold nappe to the Helvetic stratigraphic domain. The Campolungo Triassic sedimentary cover of the
131 Campo Tencia nappe is characterized by its huge over 300m wide zone of banded black and white dolomitic
132 marbles overlying a thin band of basal quartzite. It may be attributed to a transition zone between the Helvetic
133 and Briançonnais domains. A similar large series of Triassic dolomitic marbles characterize the units of the
134 Briançonnais domain (Ellenberger, 1953; Escher, 1988)
135 The Mergoscia and Cimalunga nappes are composed of tectonic mélanges of polycyclic basement
136 gneisses, Meso-Cenozoic continental and marine sediments and oceanic ophiolites, successively overprinted by
137 an approximate Late Eocene eclogite and Oligocene-Early Miocene amphibolite facies regional metamorphism
138 (Engi et al. 1995; Rubatto et al. 2009; Sandmann et al. 2014). The Orselina nappe is a similar tectonic mélange
139 overprinted by the Oligocene-Early Miocene amphibolite facies metamorphism but without eclogite facies relicts
140 (Fig. 2; Steck et al., 2013; Pfeifer et al. 2018). The Mergoscia and Cimalunga tectonic mélanges and the Adula
141 nappe with their Late Eocene eclogite facies metamorphism (Becker, 1993; Brouwer et al. 2005; Herwartz et al.
142 2011; Sandmann et al. 2014), the Bosco and Bombogno-Orselina-Bellinzona mélanges, the Monte Leone and
143 Valais calc-schist nappes mark a major zone of deep subduction and accretion in the Nappe stack of the
144 Lepontine Alps situated below the middle Penninic nappes of the Briançonnais paleogeographic domain. This
145 Late Eocene zone of deep subduction and accretion in the Alpine thrust belt is named Valais suture on Table 1
146 (Engi et al. 1995; 2001; Gebauer, 1996; Cavargna-Sani et al. 2014).
147
148 3. Alpine structures of the Sambuco – Maggia nappe 6
149
150 The decode of the Alpine structure and history of the Maggia nappe was a main subject of the present field work.
151 The Maggia unit forms a high Alpine nappe of the Lepontine gneiss dome. It overlies the deeper Antigorio
152 nappe to the west and Campo Tencia and Verzasca nappes to the east. The measured fold axis and stretching
153 lineations of this study corroborate the data of the structural map of the Ticino Alps by Wenk (1955). The gneiss
154 structures result from multiple ductile folding during and after the nappe emplacement. The unequivocal
155 distinction of the successive formed folds and their axial surface schistosities was established in the Helvetic Aar
156 and Gotthard F2 basement folds and in the frontal parts of the lower Penninic F2-F3 Verampio, Antigorio,
157 Maggia and higher basement fold nappes (Liszkay-Nagy, 1965; Masson et al. 1980; Steck, 1984, 1987, 1990,
158 1998, 2011). Four schistosities S1-S4 and six phases of folding F1-F6 are distinguished in the Maggia and
159 surrounding nappes. Late crenulation folds F7 with a sub-horizontal to low-angle west dipping axial surface
160 were formed during a phase of vertical shortening below the Simplon normal fault (Table 2, Steck, 1987). Note
161 that the first alpine schistosity S1, sub-parallel to the lithologic banding observed by Liszkay-Nagy (1965 and
162 this study) in the Nufenen-pass region is missing in many descriptions (Huber et al. 1980; Huber, 1981; Maxelon
163 & Mancktelow, 2005). This early first schistosity is also clearly observable in the Mesozoic sediments of the
164 autochthonous cover of the Aar and Gotthard massifs F2 basement folds. But first folds F1 are infrequent and
165 rarely demonstrable (Steck, 1984, 1987, 2011).
166 Early schistosites S1-S3 and a SE-plunging stretching lineation L1-L3 were developed in the gneiss
167 fold nappes during their NW-directed ductile shear décollement from the upper European crust during its
168 underthrusting below the middle and higher Penninic nappes, the Dent Blanche-Sesia thrust sheet and Adriatic
169 indenter. A dominant and late axial surface schistosity S4, post nappe folds F4 and an associated SE-plunging
170 stretching lineation L4 were developed after the NW-directed nappe emplacement, creating the southwest-
171 verging Verzasca antiform and Sambuco-Maggia synform during a phase of dextral shear in the zone of Alpine
172 collision between the European plate and the Adriatic indenter (Fig. 2 and 3). The pre-existing NW-directed S1-
173 S3-related sense-of-shear structures were inverted by rotation around the axis and stretching lineation of the
174 younger Verzasca F4-antiform (Allaz et al., 2005; Steck et al., 2013). The unequivocal discernment of the
175 different phases of schistosities and stretching lineations is difficult and in many places impossible. The final
176 planar and linear gneiss structures result from internal rotation and transposition by continuous ductile
177 deformation (Figs. 2 and 3). The post-nappe steepening of the southern Alpine belt with the formation of the 7
178 Vanzone and Cressim F5 backfolds occurred during the so-called Argand’s phase of backfolding (Fig. 1 and
179 Table 2; Argand, 1911; Milnes, 1974).
180 The northern end of the SE-plunging Maggia F4-synform was later overprinted by younger NE-trending
181 Foroglio and Cristallina F6 folds creating a type 2-fold interference pattern after Ramsay (1967). The Neogene
182 uplift of the Lepontine gneiss dome since over 18 Ma (Steck & Hunziker, 1994) accompanied by its erosion
183 were responsible for the separation of the Sambuco lobe from its southern continuation the Maggia synform
184 (Figs. 1 and 2).
185 The fold axes and parallel stretching lineations of the synformal Maggia nappe plunge from north to
186 south, between the Val Lavizzara, Peccia, Bignasco and the Verzasca reservoir, over a distance of 30 km with a
187 regular low angle of 0-10° to the SE. To the south-east of the Verzasca reservoir the fold axes plunge increases
188 rapidly from 10° to 70°, ending in an east closing steep plunging F5 antiform of the Mergoscia nappe (Fig. 2).
189 New detailed field work in the Sementina and upper Gorduno valleys confirm the geological observations of
190 Bächlin et al. (1974): The Ruscada- and Cocco-gneisses of the Maggia nappe dip to the SE with a steep angle of
191 over 75° in a tubular fold structure below the eclogitic Mergoscia mélange nappe (Fig. 2). It is “der
192 schlundartige Wirbel von Bellinzona”, the gorge like whirl of Bellinzona represented by Wenk (1955) on his
193 map of Alpine fold axes. Spicher & Wenk (1981) describe the steep converging structure near Bellinzona as a
194 late vortex-like root zone, in a neutral sense of the term, formed under highly ductile conditions in Oligocene-
195 Early Miocene time, after the nappe emplacement and during a late orogenic phase of folding and re-
196 crystallisation in the deep mobile belt (Read 1955) of the Alps. It is herein considered that the on the geological
197 map established sheath fold like tubular structure originate from the overprint and squeeze of the Maggia F1-F3
198 thrust root during the late F4-F5 folding phases related to dextral shear in the southern steep belt of the Alps
199 (Figs. 2, 3 and 6). The southern root of the Maggia nappe possesses the geometry of a tubular sheath fold like
200 structure surrounded by the Antigorio and Campo Tencia-Verzasca nappes at its base and the eclogitic
201 Mergoscia and Cimalunga mélanges on the top (Fig. 2 and 6). The geometry of this sheath fold like tubular
202 structure results from multiple folding and not from the mechanism of simple shear alone proposed by the
203 Cobbold & Quinquins (1980) model. Sheath folds or sheath fold like structures of centimetre- to metre-
204 dimension are common in other Alpine ductile thrust zones, for instance at the base of the Helvetic Doldenhorn
205 thrust fold (Krayenbühl & Steck, 2009), there characterised by multiple ductile folding. Lacassin and Mattauer
206 (1985) described a kilometre-scale sheath fold in the Mattmark region on top of the NW-directed Monte Rosa
207 nappe thrust. The pictures of fold interference patterns published by these authors suggest also a zone of multiple 8
208 shear folding. Cobbold and Quinquins (1980) discussed and studied experimentally a sheath fold model as a
209 mechanism of simple shear affecting a pre-existing deflection of an irregular surface. The formation of the
210 Maggia sheath fold like tubular structure results not from the process of simple shear alone described in the
211 Cobbold and Quinquins (1980) model, but probably by a more complex process of multiple shear folding in a
212 thrust zone. The intersection of folds of successive generations may be an ideal site for the formation of a sheath
213 fold like structure.
214
215 4. Age and pressure-temperature conditions of the ductile Lower Penninic nappe
216 detachment zone
217
218 The ductile detachment of the Lower Penninic nappes from the upper European crust started at the base of the
219 Alpine Middle and Upper Penninic nappe stack, an overburden of 25 km, after its emplacement dated between
220 52-35 Ma (Markley et al., 1998; Skora et al., 2015) and after or synchronous of the 40-35 Ma of the Paleocene-
221 Eocene planktonic foraminifera in the Sion-Courmayeur zone, Valais flysch (or contourite) nappe near Sion
222 (Bagnoud et al., 1998) and before the formation of the Vanzone and Cressim back folds and the Southern Steep
223 belt of the Central Alps since about 30 Ma (Milnes, 1974; Berger et al., 1996; Steck and Hunziker, 1994; Schmid
224 et al., 1996; Steck et al., 2013). The detachment of the Lower Penninic nappes in the zone of underthrusting of
225 the European crust below the higher Alpine orogenic lid occurred consequently in the Early Oligocene between
226 40 and 30 Ma by ductile shear under increasing greenschist to amphibolite facies metamorphic conditions and
227 this before reaching the thermal peak. Stable assemblages of kyanite-staurolite-garnet-biotite crystallised during
228 the formation of the main S3 schistosity and the NW-directed L3 shear and remained stable during the W-
229 verging Verzasca and Maggia F4 folding. Most of the acicular or prismatic hornblende, staurolite and kyanite
230 synkinematic crystals are oriented parallel to L2-3 stretching lineations, others postkinematic are non-oriented
231 (Allaz et al. 2005). Maximum pressures reaching 8-9 kbars and temperatures of 600-700°C where attended in the
232 Maggia region (Todd & Engi, 1997; Allaz et al., 2005; Brower et al., 2005; Burri et al. 2005; Boston et al. 2017).
233 The isograds of the Barrovian regional metamorphism cut through the nappe limits (Fig. 7, e.g. Niggli & Niggli,
234 1965; Trommsdorff, 1966; Wenk & Keller, 1969; Burri et al. 2005). The northern limit of the over 6 km wide
235 Tertiary migmatite zone cross the southern root of the Maggia nappe to the south of the village of Maggia and
236 occupies the southern steep belt of the Alps from the entrance of the Valle Onsernone and Locarno to the west to
237 the Bergell intrusion to the east (Fig. 7, Burri et al., 2005; Steck et al., 2013). In situ migmatites were dated in 9
238 the Bellinzona region by the age of Cenozoic synkinematic leucosome veins of the high amphibolite facies grade
239 metamorphism of the K-feldspar-silimanite zone at 32-22 Ma with U-Pb SHRIMP zircon ages by Rubatto et al.
240 (2009), allanite U-Pb ages by Gregory et al. (2012) and U-Pb monazite ages by Boston et al. (2017). These
241 Cenozoic migmatites are of the same age as cross cutting crustal derived pegmatites and aplites (Romer et al.,
242 1996; Schärer et al., 1996). The concentration of Tertiary migmatite and discordant aplites and pegmatites in the
243 southern steep belt and the geometry of the Andesine An >40 amphibolite-isograd around the southern Maggia-
244 root suggest a greater terrestrial heat flow in vertical zones of the Lepontine gneiss dome (Fig. 7, Wenk &
245 Keller, 1969). Schärer et al. (1996) suggest that frictional heating in the dextral shear zone of the southern steep
246 belt of the Alps may also be responsible for this phase of post-nappe heating. This signifies that the up heating to
247 thermal peak conditions of the southern steep belt outlast the pre-existing extrusion and emplacement of the
248 Maggia nappe. Thermal peak conditions and emplacement of crustal derived aplites and pegmatites are coeval to
249 a Late Oligocene to Miocene incipient phase of vertical shortening and lateral, mainly SW-NE directed
250 extension of the Alpine nappe stack, dextral shear in the southern steep belt of the Alps and the formation of the
251 Simplon low-angle detachment of the Lepontine dome and Dent Blanche depression pull-apart structure (Fig. 1;
252 Steck, 1990; Steck & Hunziker, 1994; Steck et al., 2013).
253
254 5. Discussion of models for the extrusion structures in fold nappes.
255
256 A fundamental question exists in the interpretation of the Penninic nappes, are they simple thrust sheets as the
257 Sesia-Dent Blanche (Salassic) and Australpine thrust sheets, are they folds deforming crustal sheets or are they
258 nappes formed by a mechanism of ductile folding alone. The question is still discussed. Froitzheim et al. (1996)
259 and Schmid et al. (2004) consider the Maggia nappe as a thrust sheet. In contrast, Wenk (1955), Milnes (1974),
260 Maxelon & Mancktelow (2005), Steck (1984, 2008) and Steck et al. (2013) interpreted the Lower Penninic
261 nappes of the Lepontine gneiss dome as fold-nappes formed by multiple ductile shear folding and decollement of
262 the upper European crust under greenschist to amphibolite facies metamorphic conditions during its south
263 directed underthrusting below the higher Penninic and Salassic orogenic lid and Adriatic indenter. The geometry
264 and penetrative texture of the gneiss fold nappes suggest a mechanism of ductile folding under greenschist to
265 amphibolite facies conditions. The study of Cenozoic deformed granitic basement rocks of the Alpine Aar
266 massif – Leventina nappe transect by Voll (1976) reveal their ductile behaviour related to the quartz
267 synkinematic crystallisation at temperatures over 300°C and feldspar crystallization at amphibolite facies 10
268 conditions. Early brittle thrusting under lower temperature conditions cannot completely be excluded. A model
269 for fold nappe extrusion by ductile squeezing of the deep zone of the Europe-Adria collision is suggested by
270 Merle & Guillier (1989). A more realistic mechanism of ductile detachment of basement gneiss folds by pure
271 and simple shear from the under-thrust European crust was proposed by Epard & Escher (1996), Escher &
272 Beaumont (1997) and Pfiffner et al. (2000). Field data confirm the fold nappe geometry of the Maggia nappe
273 front. The northern Sambuco lobe of the Maggia nappe possess slices of its autochthonous Mesozoic
274 sedimentary cover on its upper and lower fold limb. The sedimentary cover is farther south reduced and strongly
275 deformed to thin quartzite and limestone bands of the Pertusio and Someo cover series (Fig. 2 and 4). The
276 question of the autochthony of these sediments is open. It is herein suggested that these slices of Mesozoic
277 sediments represent a reduced autochthonous cover of the Maggia nappe and the southern border of the
278 European continental margin. The Maggia nappe roots in the southern steep belt of the Alps in direct contact
279 with the eclogitc Mergoscia and Cimalunga mélanges units (Fig. 2 and Table 1; Bächlin et al.1974). It is
280 probable that the Maggia nappe emplacement is related to the deep subduction and later extrusion and accretion
281 of these ultra-high pressure units in the Valais suture (Table 1). The action of strong buoyancy and diapirism of
282 deep subducted and later extruded and accreted upper crustal slabs was suggested in the model of Chemenda et
283 al. (1995). A vertical pull and imbrication of the Maggia fold nappe by the deep subducted high pressure rocks
284 of the Valais suture during their extrusion is probable. Engi et al. (2001) suggest the notion of tectonic
285 accretionary channel (TAC) for such enigmatic zones of deep subduction, extrusion and accretion of the Alps.
286
287 6. Conclusions
288
289 The tectonic window of the Central Alps allows the study of the structural variation through superposed
290 tectonic levels of the orogenic belt. It permits a modern image of the so-called “Stockwerk Tektonik” of
291 Wegmann (1953). NW-directed thrusting and multiple ductile folding of detached European crustal rocks
292 characterise the deepest exposed tectonic level of the Central Alpine nappe stack. The Maggia nappe represents a
293 north-directed pipe-like extruding fold-nappe structure. It has the geometry of a tubular sheath fold like structure
294 that roots in the southern steep belt of the Central Alps (Figs.2 and 6). Its Mesozoic stratigraphic cover assigns
295 the Maggia fold nappe to the Helvetic paleogeographic domain. It is bordered by the higher tectonic units of the
296 Valais suture composed of the high pressure Mergoscia and Cimalunga mélanges, the Adula high pressure
297 nappe, the medium pressure Orselina mélange and the Valais calc-schist nappe situated below the Middle 11
298 Penninic nappes of the Briançonnais domain and the Monte Rosa nappe. The formation of the Lower Penninic
299 nappe pile occurred between some 40 and 30 Ma ago by ductile detachment of the upper European crust starting
300 with high-pressure conditions in the Mergoscia and Cimalunga mélanges followed by increasing greenschist and
301 amphibolite facies conditions in the zone of underthrusting below the higher orogenic lid and Adriatic indenter.
302 The thermal peak of the Tertiary Barrovian regional metamorphism was attended between 30 and 22 Ma after
303 the Alpine nappe emplacement and related to the uplift and E-W extension of the Oligocene-Miocene Lepontine
304 gneiss dome structure.
305
306 Acknowledgments We thank Djordje Grujic and an anonymous reviewer for their constructive suggestions that
307 helped improving this manuscript and Prof. Wolf-Christian Dullo for editorial handling and comments. 12
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528 (Northern Italy and southern Switzerland)., 83/3 Eclogae Geologicae Helveltiae, 629-644.
529
530
531
532
533
534 20
535 Table and figure captions
536
537 Table 1: Tectonic units of the Central Alps of Switzerland and Italy enumerated from north to south and bottom
538 to top. The radiometric ages of the phases of high pressure metamorphism suggest a displacement of the zones of
539 deep subduction and accretion from south to north in the Alpine nappe stack: the Senonian Canavese, Eocene
540 Piemont and Late Eocene - Oligocene Valais sutures (Zingg & Hunziker, 1990; Zingg et al. 1990; Brower et al.
541 2005; Sandmann et al. 2014; Skora et al. 2015).
542
543 Table 2: Comparison of models of the Cenozoic structural evolution of the lower Penninic nappes published by
544 Huber et al. (1980), Huber (1981), Simpson (1982), Grujic & Mancktelow (1996), Maxelon & Mancktelow
545 (2005) and this study.
546
547 Fig. 1: Blockdiagram of the Central Alps, modified after Steck et al. (2008). The Maggia-Sambuco nappe is
548 situated in the centre of the Lepontine gneiss dome. CT = Campo Tencia nappe, GN = Gurnigel flysch, 2DK =
549 Seconda zona diorito-kinzigitica, UH = Ultrahelvetic, ZH = Zone Houillère. Campo Tencia and Simano nappes
550 of Preiswerk (1934) form a composite nappe designated as Simano nappe on the Tectonic map of Switzerland
551 (2005) and on Figs. 2 and 3.
552
553 Fig. 2: Tectonic and structural map of the central Lepontine gneiss dome. A-E indicate the positons of the
554 geological profiles through the Maggia nappe of Fig. 3. Abbreviations: Cr = Cristallina Mesozoic cover, H =
555 Helvetic nappes, ML = Monte Leone nappe, Pe = Pertusio Meszoic cover, P.d.L. = Poncione dei Laghetti Klippe
556 (Maggia nappe), PV = Pizzo del Vallone nappe, So = Someo Mesozoic cover, VALAIS CALC-SCHISTS
557 designates a complex zone, the eastern continuation of the Sion-Courmayeur zone composed of the Bedretto
558 zone of Bianconi & Starsky (2015). Fig. 2 is based on the 1:25 000 Geologischer Atlas der Schweiz 1313
559 Bellinzona (Bächlin et al. 1974), 73 Campo Tencia (Keller et al. 1980), 1262 Ambri-Piota (Bianconi & Strasky,
560 2015), 1291 Bosco/Gurin (Della Torre et al. 2015), 1312 Locarno (Pfeifer et al. 2018), unpublished originals of
561 Eduard Wenk and Paul Graeter of 1292 Maggia and our one new structural data.
562
563 Fig. 3: Geological profiles A-E (on Fig. 2) and representation of Alpine deformational structures on Schmidt net,
564 lower hemisphere. The Maggia nappe forms a F4-synform to the north in the profiles B – E and plunges to the 21
565 west of Bellinzona in a steep structure (profile A). The late steepening of the fold axis is related to back folding
566 and designated F5. Note the frontal down facing fold of the Campo Tencia nappe (profile E).
567
568 Fig. 4: Tectonic map and geological Pizzo Massari-Pizzo Prevat profile through the frontal down facing Campo
569 Tencia nappe F2-fold and its autochthonous Campolungo Triassic sedimentary cover, the Bedretto calc-schist
570 nappe (Bianconi, 1971; Bianconi & Starsky, 2015) and the Sambuco gneisses with its autochthonous Cristallina
571 Mesozoic sedimentary cover in an inverted position at the nappe base and an upper normal position in the
572 syncline to the west of the Pizzo Massari.
573
574 Fig. 5: The cut out of the geological map of Keller et al. (1980) illustrates the uniform about 20° S-plunging axis
575 of the Mogno F2 down facing syncline. The southern and deeper continuation of the Triassic dolomite and calc-
576 schist, observed by Dazio Dal Vesco in the Prato-Camblee water gallery confirms the geometry of the down
577 facing Mogno syncline (oral communication by Franz Keller).
578
579 Fig. 6: Block diagram of the Sambuco – Maggia sheath fold like structure.
580
581 Fig. 7: Map of isograds of the Cenozoic Barrovian amphibolite facies metamorphism of the Central Alps. The
582 limits of the Lower Penninic nappes are crosscut by the isograds of the amphibolite facies metamorphism.
583 Highest temperature where attended at 30-22 Ma after the nappe emplacement. Isograds of metapelites after
584 Niggli & Niggli (1965), siliceous dolomites after Trommsdorff (1966), metabasites after Wenk & Keller (1969),
585 Steck et al. (2001) and migmatites after Burri et al. (2005) are represented. ALPINE NAPPES METAMORPHISM CENOZOIC METAMORPHISM TECTONIC DIVISION MESO-CENOZOIC CRYSTALLINE BASEMENT SEDIMENTARY COVER AGE (Ma)
ZEOLITE FACIES N AAR MASSIF FOLD LOWER HELVETIC NAPPES HELVETIC GOTTHARD NAPPE WILDHORN NAPPE GREENSCHIST FACIES ~32-15 bottom ULTRAHELVETIC LUCOMAGNO-LEVENTINA NAPPE PIORA SYNCLINE
VERAMPIO NAPPE TEGGIOLO SYNCLINE
ANTIGORIO NAPPE TEGGIOLO COVER
SIMANO NAPPE FRASCO COVER
CAMPO TENCIA NAPPE CAMPOLUNGO COVER AMPHIBOLITE FACIES
LEBENDUN NAPPE
OSBARINO-VALGRANDE NAPPE ~38-30 LOWER PENNINIC PIZZO DEL VALLONE NAPPE
European crust MAGGIA (-SAMBUCO) NAPPE CRISTALLINA-PERTUSIO-SOMEO COVER
ADULA NAPPE ECLOGITE→ AMPHIBOLITE FACIES ≥38 MERGOSCIA-CIMALUNGA TECTONIC MELANGES VALAIS SUTURE ORSELINA-BELLINZONA TECTONIC MELANGES AMPHIBOLITE FACIES
MONTE LEONE NAPPE HOLZERSPITZ COVER GREENSCHIST FACIES
VALAIS CALC-SCHIST NAPPE BLUESCHIST FACIES
SIVIEZ-MISCHABEL NAPPE BARRHORN COVER 40-35 MIDDLE PENNINIC (BRIANÇONNAIS) MONTE ROSA NAPPE FURGG COVER PIEMONT SUTURE ~50-38 ECLOGITE→ AMPHIBOLITE FACIES UPPER PENNINIC ANTRONA - ZERMATT-SAAS FEE OPHIOLITIC NAPPES
TERTIARY MAGMATISM BERGELL TONALITE 30 NON METAMORPHIC
SALASSIC SESIA ZONE CANAVESE SUTURE ≥70-65 ECLOGITE→ AMPHIBOLITE FACIES GREENSCHIST FACIES BORDER
top IVREA ZONE CANAVESE COVER SOUTHERN ALPS S STRONA-CENERI ZONE LOMBARDIAN COVER NON METAMORPHIC Steck, Epard & Masson (this study) and Liszkay-Nagy (1965), Steck (1987, 1998), Steck et al. (2013)
Main deformational early deformation in NW-directed W-verging recumbent-folds Southern late backfolds F6 phases Mesozoic sediments main nappe emplacement steep belt vertical shortening S1, F1 S2-S3, F2-F3 S4, F4 F5 F7
first schistosity Maggia and Lower Verzasa antiform Vanzone antiform Chièra synform Main structures and Penninic nappes Maggia synform Cressim antiform Cristallina synform rare early folds Gotthard fold Ziccher fold verticalisation of Foroglio antiform Bosa and Wandfluhhorn antiform Maggia root to the Salmone antiform W of Bellinzona Masera synform Simplon fault
Metamorphic grade greenschist facies amphibolite pfacies retrograde prograde ~ 32-30 Ma Age of deformation temperature culmination at 30-22Ma and metamorphism ~Late Eocene Oligocene 18 Ma - present Late Oligocene-Early Miocene Huber, Ramsay & Simpson (1980), Grujic & Mancktelow (1996), Maxelon & Mancktelow (2005)
Main deformational D1 D2 D3 D4 D5 phases thrusting and main post-nappe folding transverse folds Northern Backfolds vertical shortening isoclinal folding Southern Steep Belt
Main structures Teggiolo zone Antigorio antiform (-nappe) Peccia-Basodino synform Cristallina synform Simplon fault Ledendun synform Campo Tencia synform Chièra synform Campolungo synform Maggia steep zone Southern Steep Belt Wandfluhhorn antiform
Metamorphic grade prograde amphibolite facies amphibolite facies retrograde
Age Late Eocene-Oligocene Oligocene (pre-Bergell) Late Oligocene Neogene Vaduz
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E Campolungo section N F NW 4 SE F2 Pizzo Massari 3 000m F 2760m Pizzo Meda 3000m 2614m 4 F2 2000m 2000 m Cs Do 2000m Qz F F2 D Mogno section 4 antiformal syncline
W F4 E C Campo-Tencia section F4 Pizzo del Piatto di Röd F2 Verzasca antiform CAMPO TENCIA NAPPE Pizzo Mascarpino l’Uomo ALPIGIA Corona di Redorta N Pizzo di BrunescD Mogno 2000m F2 VERZASCA COCCO NA 1000m ANTIGORIO PP Maggia E 0m C B Val di Coglio-Osura section F4 F4 Maggia-Verzasca fold
B SW F4 NE
F 2278m VERZASCA NAPPE 2000m 4 Verzasca 2000m Someo Brione V 1000m Maggia al d’Osura Val Verzasca 1000m COCCO EVENTINA 0m V L 0m M ER A Z G A G SC IA A S Structures of the Maggia transect: Y A N N F TI N O FO R R F4 antiform M M Bellinzona steepbelt F A V 4 E synform R Z A S F C 5 A
A N Schmidt net, lower hemisphere: T IF O R schistosity S1-3 A S M N stretching lineation L1-4 (+fold axis) 2390m Cima dell’Uomo MAGGIA 2000m 2000m foldaxes A A G ROOT I UN G L M 1000m 1000m G E A
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20 axi al plu nge Triassic dolomite and Valais calc-sch ist ou tcrops in Prato-Camblee w ater gallery (alt. 1135m), after Ezi o Dal Vesco,
PRATO - CAMBLEE GALLERY personal commu nication by Franz Keller 45° stau rolite and kya nite bearing garnet micasch ists of Campo Tencia nappe
20 cu t ou t of map sh eet 1272 P. Campo Tencia 0 300m by Keller et al. (198 0) © sw isstopo A D U L CRISTALLINA NCIA A CAMPOLUNGO O TE MP CA E O LOB BUC PE) SAM NAP GGIA (MA 20 PERTUSIO 10 RIO O SIMAN TIG O AN 5 10 VENTINA LE
P. 10 LA GH ETTI K. 10 A V C ADUL M E IM R A Z AL G A U A G SC 12 NGA CI IA A A OS 10 S NTIFORM G SOMEO Y ER NF M M OR ITE A M NAL GG 10 TO 2000 m IA N 15 E APPE 75 LIN ALE 0 m TON
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Steck, Epard & Masson 2019 AROSA N VERRUCANO RN O nz H rie R B E of HELVETIC MESOZOIC L D a e VALAIS CALC-SCHISTS k IE ak BÜNDNERSCHIEFER e N L o f Aar AUSTROALPINE Th RD un N A R TAVETSCH TH
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19 IS BOSCO A z 4 R M a S n> 3 E G C B IO a s Moesa . A Diopside-Calcite in A P g c g g G a Pla M i A A a I R N A NOVATE ZH E MARGNA ANTIGORIO A V R S C IVIEZ ZH R-S L I BERNINA -M N D IS E BELLINZONA TE C D O A LI L N H I A A D MERGOSCIA N A R Migmatite Belt O T T TONALE A O B I S B E S P O L Ticino VEROSS O C LORSELINA Como TONALE LINE
E H A C AT R TSATÉ GR E N JEN A H RT G A O N - A L P 50 km T N B O E R T T IN E N N U G N E A M U I D CA R L O L FURGG L FRILIHORN A
Toce G I STO O R TSATÉ CKHORN P E N E ISOGRADS OF THE OLIGOCENE-MIOCENE GORNERGRAT re C o i MONTE ROSA g g AMPHIBOLITE FACIES GRADE METAMORPHISM CIMES BLANCHES A a E M
R o V g OF THE CENTRAL ALPS ZERMATT- I a SAAS FEE 2DK E L N - I A L N O E R after 23 S T S 23 E V NIGGLI & NIGGLI, 1965; TROMMSDORFF, 1966; TSATÉ A 23 2DK N WENK & KELLER, 1969; STECK et al. 2001 A
9 C A LINE AOST SESIA and BURRI et al. 2005