Insubric Line and Root Zone of the Penninic Nappes”, Annual Convention 2017, Monday June 19

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Excursion Guide for Excursion “Insubric Line and Root Zone of the Penninic nappes”, Annual Convention 2017, Monday June 19

S.M. Schmid, Institut für Geophysik, ETH Zürich with contributions by Diego Pozzorini, Dr. Baumer SA Viale Monte Verità 54, 6612 Ascona

Morning program:

Walk from the Monte Verita hotel, ending at Grotto Zelinda at lunchtime page 1

Afternoon program: bus tour leading to the Verzasca dam and the Lavertezzo bridge page 13

Topographic map showing the area visited in the morning 2 A. Introduction into the regional geology

The area visited by this excursion exposes, from S to N: (1) the northwestern rim of the Ivrea Zone (lower crustal basement of the Southern Alps) at its eastern termination, (2) the Insubric mylonite belt, consisting of protoliths derived both from the Southern Alps (Ivrea Zone) and the Central Alps (Sesia nappe), south of (3) various units of the Southern Steep Belt (root zone) of the Central Alpine nappe stack visited in the afternoon (Figs 1 & 2).

1. The Ivrea Zone

The Ivrea Zone consists of a part of the basement (lower crust) of the Southern Alps (Figs. 3-5). The Southern Alps belong to the Apulian microplate and lack Alpine-age penetrative tectonic and metamorphic overprint. The Ivrea zone is paleogeographically located at the distal passive continental margin of the Apulian microplate, near the transition into the Piemont-Liguria ocean (now part of the Central Alpine nappe system) located to the W (Fig. 3). Uplift related to substantial crustal thinning lead to cooling below 300°C during the Early Mesozoic. Hence, Late Oligocene to Miocene Alpine deformations ("Insubric phase", contemporaneous with faulting across the Insubric mylonite belt), affected these lower crustal lithologies at moderate temperatures (i.e. below 300°C).

Given the fact that a large portion of the Ivrea Zone is made up of quartz-free lithologies, deformed at temperatures below 300°C during Alpine orogeny (mafics, ultramafics), it is not surprising that the Ivrea zone (1) acts as an essentially rigid indenter (see profile Fig. 2), controlling the shape of the internal part of the arc of the Western Alps, and, (2) that there is a very abrupt transition from pre-Alpine fabrics preserved in the Ivrea mafics into Alpine-age quartz- bearing greenschist mylonites of the Insubric mylonite belt ("Fabric boundary" in Fig. 10). Note that the strictly E-W trending Insubric line west of Locarno starts to swing into a NE-SW-strike exactly in our area, where the Ivrea zone (and its associated positive gravity anomaly) terminates towards the NE (Figs. 1 & 6). The Insubric line accommodates the backthrusting of the Penninic units over the Southern Alps as well as coeval dextrally transpressive movements between Central Alps and Southern Alps (Figs. 6 & 7). The amount of the strike slip component has to decrease as the line swings into a NE-SW strike, predetermined by the Ivrea-indenter (Fig. 7). Alpine penetrative strain is very modest within the Ivrea Zone, and hence there is an enormous strength contrast between the Ivrea Zone (low metamorphic grade during the "Insubric phase", largely composed of quartz-free lithologies) and the Central Alps (Alpine greenschist to amphibolite facies metamorphism, weak quartz-bearing lithologies).

Nevertheless, the Ivrea Zone exhibits some Alpine-age strain features in our area: we observe that the Ivrea Zone is affected by an east-plunging antiform of Alpine age (Fig. 8 & 10), formed by foliation-parallel flexural slip (numerous slickensides), but preserving the pre-Alpine fabric (Fig. 3 10). According to Schmid et al. (1987) and Zingg et al. (1990) tilting of the Ivrea crustal section was caused by the rotation of the southern limb of this antiform. This asymmetric fold brought upper crustal levels of the Southern Alps (Canavese Mesozoic) into direct contact with the lower crustal Ivrea rocks to the south and the the Central Alps to the north. Mylonitized relics of these upper crustal levels are merely preserved within the Insubric mylonite belt decribed below ("Canavese", see Figs. 9 & 10).

2. The Insubric line

The Insubric line is sometimes subdivided into the SW-NE trending Canavese line southwest of the area visited and the EW trending Tonale line further to the east (Fig. 1). The Insubric line is part of the Periadriatic line, a late Alpine system of faults delimiting the northern edge of the Adriatic block (part of the Apulian microplate), extending from the Piemont in Italy all the way to Slovenia. The eastern part of the Canavese line and the western part of the Tonale line are associated with very substantial vertical displacements which exhumed the Lepontine dome, characterized by high grade Alpine metamorphism.

More than one feature characterize the Insubric line (IL):

(1) The Insubric line marks the N boundary of the Southern Alps, characterized by relatively modest shortening (some 50-60km during the Tertiary) by S-vergent foreland thrusting (Fig. 2). (2) The IL forms the southern limit of the Central Alps, deformed into an impressive pile of nappes (about 500km shortening during the Tertiary) and associated with a substantial metamorphic overprint of Alpine age (Fig. 12). (3) The IL is genetically linked to substantial backfolding and backthrusting of the Southern part of the Central Alps. It parallels the root zone of the Alpine nappe stack (Figs. 1, 7 & 10). (4) The IL is the site of important dextral strike slip movements (in the order of 60-100 km along the Tonale line). (5) Geophysically, the Insubric line may be traced into a depth of 30km (Fig. 2).

Sense of shear criteria within the 1 km wide Insubric mylonite belt indicate a complex kinematic history. The southern part of the mylonite belt indicates dextral shear with subhorizontally oriented stretching lineations in the Ascona-Locarno area, exhibiting a modest pitch to the NE further to the southwest (Fig. 6). In the northern part of this mylonite belt the lineations plunge approximately down-dip, senses of shear indicating backthrusting (Fig. 6). Hence, the Insubric line may viewed as a dextral strike slip zone and a steeply N-dipping (45°-70°) backthrust (or retroshear) at the same time.

In the Ascona-Arcegno area overprinting relationships do indicate that some of the mylonites formed during backthrusting have been re-mylonitized during strike slip shearing. However, these 4 local overprinting relationships have to be viewed as part of a continuous deformation process related to dextral transpression (see arrows in Fig. 7). During W- to NW-directed movement of the Adriatic block, which acted as an indenter with the Ivrea geophysical body at its western edge, individual parts of the Central Alps are first backthrusted in the area of the Canavese line and subsequently displaced to the east by later strike slip movements along the Tonale line (Fig. 7). The cooling ages within the Lepontine metamorphic dome, related to exhumation by backthrusting, systematically decrease in age going from W to E (Schmid et al. 1989). Hence the eastern parts of this dome were uplifted first. This results in the formation of a complicated internal structure within the Lepontine dome, formed during the Insubric phase, as discussed by Merle et al. (1989).

Towards the end of these transpressive movements and after cooling below 300°C, the Tonale line (but not the Canavese line) was overprinted by cataclastic dextral movements under brittle conditions. Westwards, and N of the Canavese line, these brittle dextral movements continue into the Centovalli line, associated with cataclastic Riedel faults affecting the part of the area immediately north of Arcegno (Fig. 10).

Timing and amount of backthrusting are well constrained by cooling ages (Hurford 1986) and the offset in metamorphic grade between Central and Southern Alps, respectively. Note, however that this timing is not valid further to the east where earlier cooling was inferred from cooling ages (Giger & Hurford 1989). Accounting for the fact that (1) cooling started at temperatures in excess of 500°C (local anatexis in granitoids) and that (2) rapid exhumation is likely to substantially predate rapid cooling, the onset of backthrusting predates 23 my. In the Bergell area backthrusting sets in immediately after the emplacement of the Bergell pluton (33-28 my), i.e. at around 28 my (Berger et al. 1996). The amount of vertical displacement of the Central Alps in respect to the Southern Alps is in the order of 20 km south of the center of the Lepontine area (i.e. in the area visited by our excursion), as estimated from the difference in Alpine metamorphic overprint across the IL.

From S to N one can map the following groups of mylonites, based on the nature of the protoliths these mylonites were derived from:

The Ivrea-derived mylonites predominantly consist of former Ivrea paragneisses (the kinzigites), former amphibolite grade rocks, transformed into brownish mica- and chlorite-rich greenschist mylonites (qtz-ab-czo-wmica-chl-sphene). Complete retro-gradation of the pre-Alpine plag-grt-bi- sill-kf assembledge induces reaction-enhanced softening and an associated inversion of the relative strength of quartz in respect to the other mineral phases. Quartz, deforming by dislocation creep, initially represents the weakest mineral phase. With progressive deformation quartz is boudinaged within the fine-grained matrix of greenschist facies reaction products, which presumably deforms by some grain size sensitive mechanism. This phenomen is widespread in mylonites and has been 5 extensively discussed by Stünitz and Fitzgerald (1993). It indicates that the strength of quartz overestimates the strength of quartz-bearing rocks. Thin amphibolite and pegmatite layers, interbedded with former kinzigite, are relatively more flow resistant. The northern edge of a massive body of Ivrea mafics totally escaped mylonitization in our area (Fig. 10). There, modest amounts of Alpine strain are produced by brittle faulting with associated epidote-bearing slicko- fibres. Hence, the transition between pre-Alpine fabrics preserved in this large mafic body and the mylonites is extremely abrupt ("fabric boundary" in Fig. 10). The thin layers of amphibolite contained within the kinzigites north of the fabric boundary are mylonititized, presumably due to sufficient water access, allowing a breakdown of the original mineral composition of the amphibolites into ab-act-chl-czo-sphene. This again implies reaction-enhanced work softening.

The Canavese mylonites consist of mylonitized granitic rocks and former paragneisses (now chlorite-sericite phyllonites) representing upper crustal levels of the Southern Alps (equivalent to the Strona-Ceneri Zone) and Mesozoic cover of the Southern Alps (predominantly siliceous limestones of Lower Jurassic age). The Mesozoic Canavese sediments underwent prograde Alpine lower grenschist facies metamorphism, hence they contrast in this respect with the rest of the Insubric mylonites which formed under retrograde conditions in respect to pre-Alpine (Ivrea zone) or Alpine (Sesia unit) metamorphism.

The Sesia-derived mylonites formed in the highest tectonic unit of the Central Alps preserved in the area, the Sesia unit. Only the southernmost rim of the Sesia nappe was affected by mylonitization under greenschist facies conditions. The lithologies found in these mylonites are granitic (former orthogneisses) or phyllonitic (former biotite gneiss or schist). The transition into the rest of the Sesia nappe is gradational. In Fig. 10 the northern margin of the Insubric mylonites was mapped where no substantial retrograde overprint is observable any more. The gneissic units to the N, which preserved their Alpine amphibolite facies assemblages, also suffered very large strains. Hence, they may also be considered as mylonites if that term is used to merely imply very high amounts of strain (Schmid and Handy 1990). However, we reserve the term "Insubric mylonite belt" to greenschist facies mylonites. This is also because it is not always easy to distinguish fabrics formed during the Insubric phase from earlier fabrics found within the gneissic amphibolite grade region to the north.

3. The Southern Steep Belt north of the Insubric mylonite belt

Foliation remains steeply N-dipping for another 5-7km north of the Insubric line (Fig. 13. From S to N, and within the area covered by Fig. 10, one crosses the root zone of the Sesia nappe (Austroalpine), the Zermatt-Saas Zone (ophiolite-bearing unit marking the S-Penninic suture zone between the Apulian microplate and Europe) and the root of the M. Rosa nappe. 6

Fig. 1 Tectonic map of the Alps and profile trace of Fig. 2 Ticino (Schmid et al. 2017)

Fig. 2 Profile “Ticino” (Schmid et al. 2017) 7

Fig. 3: Scenario during passive margin formation in Mid-Jurassic times

Fig. 4: Subduction and exhumation of the Sesia Zone below the Ivrea mantle wedge (Late Cretaceous)

Fig. 5: present-day setting Figs. 3-5 from Schmid et al. (1987) 8

Fig. 6: Orientation of foliations and lineations in the Insubric mylonites (Schmid et al. 1987)

Fig. 7: Sketch illustrating backthrusting followed by strike-slip kinematics along the Insubric line (Schmid et al. 1989) 9

Fig. 8: Detailed map of foliations in the Ascona-Arcegno area (Pozzorini 1989). The foliations are pre-Alpine but folded during Alpine orogeny. 10 N 1 km L e" +1989+ + Rilevamento + Geologico + Ascona +–+Monte+ Verità + Pozzorini Diego+

Fig. 9: Detailed map by D. Pozzorini (inpublished diploma thesis ETH Zürich 1989) 11 1 2 6$ 3$ 7$ 4$ 5$ 8$

Fig. 10 Geological map with indication of the excursion stops

B. Excursion stops in the Arcegno area (for stops, see Fig. 10)

Stop 1: Inspection of the hinge of the Alpine-age antiform of the Ivrea Zone near the Hotel Monte Verita. The mechanism of folding is flexural slip (“Biegegleitfalte”), as is the case in the Jura Mountains.

Walk from Stop 1 to Stop 2: Inspection of the two predominant lithologies making up the Ivrea Zone in the area: (1) amphibolites mostly derived from mafic intrusions, occasionally with retrogressed garnets und (2) paragneisses, which are known as “Kinzigites” in the local literature, bearing sillimanite and garnet.

Stop 3: One group will now climb the Balladrüm hill to see nice exposures of pre-Alpine folds on top of this steep hill. The other group will take a more leisurely towards west with only a short ascent.

Stop 4: The bench along the road not only offers a splendid view over Lago Maggiore but also spectacular outcrops of the amphibolites of the Ivrea Zone with pre-Alpine structures.

Stop 5: This stop illustrates the “fabric boundary”, i.e. the southern rim of the ductilely deformed Insubric mylonites. This locality contains abundant pseudotachylites. These fault rocks contain melt pockets and even small dykes that intrude opening fractures. These melts are the result of very high friction that is generated very quickly during seismic rupture (“paleo-earthquakes”). This is indicative of the high strength of the Ivrea mafics under low temperature conditions (below some 300°C).

Stop 6: Here we study Insubric mylonites produced by the intense deformation of granitic basement (former granitoids of the Sesia Zone). Note the extreme lamination of the mylonites and stretching lineations visible on foliation planes.

Stop 7: Inspection of mylonitized Canavese sediments, in this case derived from the silicious Lombardic Kieselkalk (Montrasio limestone) that we visited on day 1 in the Breggia gorge.

Stop 8: Opposite Grotto Zelinda we inspect a ca. 60 cm thick late Alpine andesitic dyke as a part of the magmatic suite of the Bergell intrusion which formed at around 30-32 Ma ago (Berger et al. 1996) and was partly also affected by the deformation related to back-thrusting along the Insubric Line. This dyke intrudes finegrained gneisses of the Sesia Zone. Such dykes can be observed all the way to the Valle d’Ossola further to the SW, and always near to or within the Insubric mylonites.

C. The Southern Steep Belt

“Southern Steep Belt” denotes what was formerly referred to as “root zone” and represents a belt consisting of mostly steeply dipping packets of gneisses that can be followed further north into the flat- lying part of the Lepontine dome (Figs. 11 & 13), consisting of a stack of nappes. However, due to intense strain and the frequent lack of clear lithological criteria it is often a matter of interpretation as to which part of this steep belt represents the “root” of which nappe. Moreover, very tight folding, such as depicted in the profile of Fig. 2 (also see Figs. 14, 16 & 17), and to be visited at the Verzasca dam, complicate such compilations. Additional complications arise from the complicated 3-dimensional structure of the Maggia transverse structure (Fig. 13). The Maggia nappe forms an almost orogen- perpendicular synclinorium, bending into a strike that is sub-parallel to the Insubric Line in the lower Verzasca valley around the area of the Verzasca dam (Fig. 12), pointing to non-cylindrical geometries formed during post-nappe folding younger than about 30 Ma.

From S to north we distinguish the following tectonic units (see Figs. 11 & 12): (1) Sesia Zone; (2) root of the Monte Rosa nappe (Briançonnais) divided from the Sesia Zone by a thin band of lithologies attributed to the Piemont-Liguria Ocean; (3) gneiss complex attributed to the Valaisan paleogeographical domain by Schmid et al. (2004), named Bellinzona-Dascio Zone, with an antiform exposing rocks attributed to the Antigorio nappe (Onsermone Zone) in the middle (eastern part of Figs. 12 & 13); (4) Maggia nappe, attributed to the Briançonnais by Schmid et al. (2004), occupying the core of a syncline, and with a band of high-pressure rocks at its base (the Cima Lunga Unit, a western euqivalent of the high-pressure Adula nappe); (5) Simano nappe (lateral equivalent of the Antigorio nappe) forming a rather flat-lying domal part of the Lepontine gneiss area (Leventina dome) and thrust onto the Leventina gneisses outside the eastern margin of Figs. 11 and 13, considered to represent the basement of the Helvetic nappes and are part of the distal part of the European continent.

Stop 9: Outcrops of tightly folded thin-banded gneisses and marbles (Bellinzona Dascio Zone) at the Verzasca dam (see Figs. 16 & 17).

Stop 10: Rather flat-lying banded gneisses, partly migmatitic, exposed within the Simano nappe (mostly meta-sediments plus pre-Alpine granitoid intrusions (see Fig 18).

15 Fig. 11: detail from the 1:100’000 map Geologische Spezialkarte No.127 (Berger, A. & Mercolli, I. 2006)

Fig. 12: Metamorphic map of the Central Alps (Oberhänsli et al. 2004)

16 Fig. 13: Unpublished foliation and lineation map (mostly after Wenk) superimposed onto the map of Fig. 11

Fig. 14: Profiles across the Southern Steep Belt; profile traces are approximate. From: Geology of Switzerland Field Guidebook

17 Fig. 15: profile traces of the composite profile depicted in Fig. 13, superimposed onto the map of Fig. 11

Fig. 16: detailed map of the chevron folds in the Southern Steep Belt at the Verzasca dam.

18 Fig. 17: chevron fold at Verzasca dam. Such as visible at low water level.

Fig. 18: Structures at Lavertezzo Lavertezzo outcrop (Sharma 1969) 19 References

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