Alps the Role of the Periadriatic Line in the Tectonic Evolution of The

Geological Society, London, Special Publications

The role of the Periadriatic Line in the tectonic evolution of the Alps

S. M. Schmid, H. R. Aebli, F. Heller and A. Zingg

Geological Society, London, Special Publications 1989; v. 45; p. 153-171 doi:10.1144/GSL.SP.1989.045.01.08

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S. M. Schmid, H. R. Aebli, F. Heller & A. Zingg

SUMMARY: The Periadriatic Line and related lineaments formed as a result of postcollisional deformations which severely modified the Alpine chain. This post-late Oligocene deformation is the result of dextral transpression between the Adriatic sub-plate and the European foreland. Indentation of the western edge of the southern Alps caused uplift, related to backthrusting and associated deformations of the Lepontine region combined with E-directed escape of the central Alps. In the eastern Alps the response to dextral transpression is mainly by lateral escape along conjugate strike slip zones with minor or no vertical movements. Older deformations along this essentially late Alpine lineament can still be inferred locally and include: extension and transfer faulting in the late Palaeozoic to early Mesozoic, Cretaceous deformations, and Tertiary phases of compression (Eocene) and possibly extension (Oligocene). The geometry of crustal thinning associated with the formation of the passive continental margin of the southern Alps (associated with initial uplift of the Ivrea zone) has a profound influence on strain localization and the kinematics of movements along and north of the present day Periadriatic Line.

The polymorphism of the Karawanken (Bauer 1984, Bauer & Schermann Periadriatic Line 1984), interpreted in terms of a flower structure by Laubscher (1983b). Thus, it seems that the notion of a major subdivision into southern The Periadriatic Line as the northern boundary Alpine and Austro-Alpine domains, each of the southern Alps characterized by their own tectonic history and Traditionally, the Periadriatic Line (Fig. la) is later juxtaposed along a first order tectonic considered to mark a first order tectonic bound- boundary is misleading. Over most of its dis- ary between the southern Alps with only minor tance the Periadriatic Line transsects the crust S-vergent thrusting and folding and the main of the southern continental margin (Fig. la) ~body of the Alps with its W- to N-verging nappe and it is merely a matter of definition as to structure. The change in tectonic style is very where exactly (N or S of the Drauzug) one impressive across the western sector of the chooses to draw this 'first order' tectonic Periadriatic Line (Canavese and Tonale Lines) subdivision. where this lineament separates the penetratively deformed area of the Lepontine region from The Periadriatic Line as the southern limit of the southern Alps (including the Ivrea zone) Alpine metamorphism which lack an Alpine penetrative deformation. However, this contrast in tectonic style mainly Ahrendt (1980) puts emphasis on another aspect reflects the metamorphic conditions during of the Periadriatic Line, namely on that of a deformation. South-vergent backfolding also lineament which marks the southern limit of affected the central Alps (Argand 1916) and the Tertiary metamorphism. Such a definition is 'minor' S-vergent thrusting in the southern Alps very useful in regions with a strong metamorphic has recently been found to be rather impress- overprint (northern part of the Canavese Line ive (Laubscher 1985), causing about 40 km and the western portion of the Tonale Line, shortening. Fig. lb). However, as pointed out by Ahrendt Further to the E the boundary between (1980), it is rather the DAV Line which marks Austro-Alpine nappes and southern Alps was the southern boundary of Alpine biotite ages in rather artificially drawn along the Pusteria and the region S of the Tauern window than the Gailtal Lines and S of the Drauzug (Fig. la) Pusteria Line further to the S. Problems with rather than N of the Drauzug and along the this definition arise at the southwestern end of Defereggen-Anteselva-Vals (DAV) Line the Periadriatic Line as well. There, Zingg et al. (see BOgel 1975 for an extensive discussion of (1976) report low grade metamorphism and the literature). Finally, the Periadriatic Line Cretaceous ages from the cover of the southern runs into the central zone of the bivergent Alps (Canavese zone s. str., Fig. 2), S of the

From: COWARD,M. P., DIETRICH, D, & PARK, R. G. (eds), 1989, Alpine Tectonics, I53 Geological Society Special Publication No. 45, pp. 153-171. [ 54 S.M. Schmid et al.

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Periadriatic Line. Two additional problems arise The Periadriatic Line as a strike-slip zone with this definition: (1) The impressive jump in radiometric ages ceases to exist east of the Laubscher (1971a, b) placed emphasis on yet DAV Line; (2) the same lineament marks the another aspect of the Periadriatic Line by southern and eastern boundary of Tertiary postulating some 300 km dextral strike slip at or (Lepontine dome, Tauern window) as well as in the vicinity of this line. This postulate results Cretaceous cooling ages (Sesia zone, eastern from large scale kinematic considerations con- Alps). One feature emerging from inspection of cerning the polarity of thrusting in the Alps, Fig. l(b) is highly conspicuous: Both the Dinarides and Appennines. As the Alpine belt Lepontine dome and the Tauern window with swings into the N-S oriented strike of the their amphibolite grade Tertiary metamorphism western Alps the westward motion of the are situated at the interference points of SW- Adriatic sub-plate was held responsible for EW NE and E-W-trending parts of the Periadriatic directed shortening in the western Alps includ- lineament. This geometry suggests a causal ing obduction of the Ivrea geophysical body relationship between updoming and movements (Laubscher 1984). along the Periadriatic Line. The Periadriatic Line in the light of geophysical The Periadriatic Line as the site of a root zone data A third aspect of the Periadriatic Line is related Figure l(c) suggests that the western sector of to the confusing notion of a 'root zone'. The the Periadriatic Line marks the western edge of Periadriatic Line marks the southern boundary the Ivrea gravity anomaly. Kissling (1980, of this so-called root zone, which is better 1984) showed that the northwestern boundary referred to as the 'southern steep belt' (Milnes of the Ivrea geophysical body is identical with 1974), characterized by steeply N- and NW- the projection of the Periadriatic Line down to dipping foliation planes. The term 'root' is mis- a depth of at least 10 kin. Due to the NW-dip of leading because it comprises both the vision of the Canavese Line (as shallow as 400-60 ~ across an origin for far-travelled nappes and at the the Valle d'Ossola) the gravity anomaly extends same time the subvertical orientation of the below the Sesia zone. The rather sharp bend nappe contacts. This old-fashioned concept of a into the E-W strike of the Tonale Line near highly squeezed root which runs in a subvertical Locarno coincides with the northeastern termin- orientation through the entire crust (as ation of this gravity anomaly. This suggests a suggested in most classical cross sections causal relationship between the geometry of the through the Alps) led several authors to consider Ivrea gravity anomaly and the position of the the Periadriatic Line as a suture zone (e.g. Periadriatic Line. Dewey & Bird 1970). Mesozoic sediments The same bend from a NNE-SSW into an along the Periadriatic Line (e.g. the Canavese) E-W strike is noticable in the contour map for have classically been considered to represent the Moho under the western and central Alps. the root of the Austro-Alpine nappes (Argand Figure l(c) incorporates older published data 1910), although they simply represent the cover (Miiller et al. 1980, Giese et al. 1982), and of the southern Alps (Ahrendt 1980). In fact, according to a more recent re-evaluation of the southern steep belt is genetically linked to seismic data (Ansorge, pers. comm.) this bend late Tertiary post-nappe emplacement folding appears even more discrete. Thus, both the and backthrusting (Milnes 1974). Canavese and Tonale Lines are situated S and

FIG. 1. The Periadriatic Line in the context of Alpine geology. (a) The segments of the Periadriatic Line and related late Alpine faults. The segments of the Periadriatic Line are from W to E: CR-Cremosina; CA-Canavese; CE-Centovalli; TO-Tonale; GI-Giudicarie; PU-Pustertal; DAV-Defereggen-Anteselva-Vals; GA-Gailtal. Related faults are from W to E: SI- Simplon; EN- Engadine; PE- Pej o; B R- Brenner; MO- MOlltal; LA - Lavanttal; B A- B alaton; RA-Raba (Balaton and Raba lines after Kazmer & Kovacs 1985). D-Drauzug; K-Karawanken. (b) Extremely simplified metamorphic map of the Alps (after Niggli & Zwart 1973, Frank 1987). O: thermal dome in the southern Otztal nappe. (c) Contour maps of the crust-mantle boundary (after MOiler et al. 1980 and Giese et al. 1982) and isostatic anomalies of the Ivrea geophysical body (after Veccia 1968). I56 S. M. Schmid et al.

SE of the Moho trough. This close relationship being from the SE towards the NW: Ivrea- is completely lost further to the E. For a short derived mylonites (mainly former paragneisses), distance the axis of the Moho depression Canavese sediments (Permo-Mesozoic cover changes into the southern Alps. This suggests of the southern Alps), and mylonites derived that the character of the Periadriatic Line from the Austro-Alpine Sesia zone (which changes rather drastically towards the E. here lacks evidence for a high pressure Eoalpine event). East of Locarno mylonitization affects Terminology the Tonale zone (a characteristic association of lithologies which are part of the pre-Triassic Although it became customary to use the terms basement of the Austro-Alpine nappes, 'Periadriatic' and 'Insubric' synonymously we Cornelius & Cornelius-Furlani 1931 Lardelli use the term 'Periadriatic' to denote the entire 1981) and parts of the tail of the Oligocene system of fault zones as depicted in Fig. l(a). Bergell intrusion (Jorio tonalite, Fumasoli 1974, This figure also indicates the names used for Heitzmann 1987b, Vogler & Voll 1981). In various segments of the Periadriatic fault sys- this region cover and basement of the southern tem. It is one of the aims of this contribution to Alps are only moderately overprinted by mylo- show that portions of the Periadriatic Line nitization but affected by cataclastic movements accommodated different displacements at dif- (Fumasoli 1974). ferent times. The northeastern portion of the Manifestations of brittle faulting and cata- Canavese Line and the western sector of the clasis are rarely found within the mylonites of Tonale Line play a rather well defined role the Canavese Line W of Locarno. The E-W during the uplift of the Lepontine region associ- oriented Centovalli Line, however, represents a ated with backthrusting (Schmid et al. 1987), rather broad zone of cataclasis and fault breccias and following Spitz (1919) we reserve the term affecting the gneisses of the southern steep belt 'Insubric' to this portion ofthe Periadriatic Line which lack signs of earlier mylonitization. Thus, which was active during the late Oligocene- it appears that the site of earlier ductile move- Miocene postcollisional 'Insubric phase' in the ments is spacially separated from the site of sense of Argand (1916). later brittle movements. This separation no longer exists E of Locarno. A system of dextral Riedel shears affects the southern steep belt of the central Alps (Fig. 2) including the mylo- Dextral transpression during the nites along the Tonale Line (Fumasoli 1974, Heitzmann 1987a,b). A sharp brittle fault Insubric phase and associated uplift (Tonale Fault) forms the southern boundary of the Lepontine region of (i) the Riedel shears, and, (ii) the mylonites. This distinction between Tonale mylonite The Insubric myionite belt south of the belt (N-dipping) and Tonale Fault (subvertical Lepontine region brittle fault) will turn out to be useful for later discussions of the kinematics of movements and A greenschist-facies mylonite belt of approxi- the eastwards extension of the Periadriatic Line. mately 1 km thickness defines the NE portion of The kinematics of movement along the the Canavese Line which curves from a NNE- Canavese Line have recently been analysed SSW into an E-W strike over a short distance (Schmid et al. 1987): mylonitized Canavese (Fig. 2). The mylonites dip with values of 40 ~ sediments with variably oriented stretching 60~ to the NW across the Val d'Ossola. The dip lineations separate an earlier formed, more angle increases towards the E as the mylonite northerly located mylonite belt which indicates belt swings towards an E-W strike (values backthrusting of the Sesia zone to the SE from a around 70~ are typical for the region around later, more southerly located mylonite belt Locarno). The same mylonite belt can be formed during dextral shearing. The change in followed further to the E along the Tonale Line orientation of the stretching lineations from (Wiedenbeck 1986, Heitzman 1987a,b). There, down-dip plunging to subhorizontal orienta- the mylonitic foliation dips at 600-70 ~ to the tions is rather abrupt. However, overprinting N or NNW. However, a late brittle fault (the relationships suggest that at least parts of the Tonale Fault, which will be discussed below) is southern mylonite belt were previously affected in a subvertical orientation and runs exactly by backthrusting as well. E-W. The change in the orientation of stretching West of Locarno the mylonite belt comprises lineations is much more gradual E of Locarno. three types of protoliths (Schmid et al. 1987), In the area of Passo S. Jorio (Fig. 2) the stretch- The Periadriatic Line t57 11 !r 'I 'II .=. C !~ ! i ~ a~ iiiil E OI 0

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ing lineations are in a down-dip orientation within more internal parts of the southern steep (backthrusting) within the strongly deformed belt (Paina marble zone, Heitzmann 1987). tonalites of the Bergell intrusion. Towards the S and within the mylonitized Tonale zone they Formation of the southern steep belt and gradually change from the same down-dip orientation to a pitch to the W (dextral com- verticalization of the northwestern rim of the bined with backthrusting movement) and finally Ivrea zone into a subhorizontal orientation (dextral strike- The contemporaneity of backfolding within the slip, Vogler & Voll 1981, Heitzmann 1987a,b). Monte Rosa and Sesia units with backthrusting This continuous change in the orientation of within the Insubric mylonite belt was postulated the lineation goes hand in hand with decreasing in a section along the Val d'Ossola by Schmid et temperatures (from N to S) during the mylonit- al. (1987). The rapidly and continuously de- ization event. Still further to the E and S of the creasing amount of retrograde overprint main body of the Bergell intrusion (near Mello, towards the NW and away from the Insubric Fig. 2) only subhorizontal stretching lineations Line reflects a horizontal temperature gradient indicating dextral shear are observed within the during backfolding and backthrusting. This mylonitized Tonale zone (Wiedenbeck 1986). gradient results from the rapid juxtaposition of Stretching lineations formed under conditions a hot hangingwall (central Alps) over a rela- of simple shearing with a constant direction of tively cooler footwaU (southern Alps). shearing asymptotically approach the displace- Verticalization is not restricted to the ment vector. The above mentioned changes in southern steep belt. Recent work in the Ivrea orientation of the stretching lineations along zone and along its contact to the Strona-Ceneri and across strike suggest that the displacement zone by Brodie & Rutter (1987) and by Handy vector changed in time and space. This leads to (1986, 1987) indicates an Alpine age for considerable difficulties in inferring the exact late folding and tilting within the Ivrea zone movement path because, strictly, the lineations around a NE-SW oriented axis. However, the merely record the finite stretch accumulated sedimentary cover of the southern Alps, further over some unknown duration in time at a given to the S and E, was not affected by this verti- locality. Although Schmid et al. (1987) recog- calization. Thus, a hinge zone somewhere within nized two mylonite zones with distinct orien- the Strona-Ceneri zone or at the contact to the tations of stretching lineation and sense of shear sedimentary cover has to be postulated. we interpret this configuration to indicate one Contemporaneity of backfolding and back- continuous transpressive dextral strike-slip thrusting N of the Insubric Line was also inferred motion rather than two strictly separate events. for the region E of Locarno (Heitzmann Within the mylonite belt a volume of rock will 1987b). It is very tempting to also relate uplift of first be affected by backthrusting to the SE and the Orobic basement S of the Tonale Line (De perpendicular to the NE-SW strike of the Sitter & De Sitter-Koomans 1949) to the same Canavese Line and it will then be either com- event. According to Bertotti (1987) this uplift pletely overprinted by dextral strike-slip move- can be interpreted to have tilted the northward ments (horizontal lineations in the southern continuation of Mesozoic normal faults, such as mylonite belt), or it will be passively carried to the Lugano Line, situated between the M. Nudo the E due to later strain concentration within and Generoso basins. The continuation of the the southern mylonite belt (preserved steep Lugano Line (the M. Grona Line) appears as a lineations in the northern mylonite belt). The sinistral strike-slip zone in present day map very gradual change in the orientation of the view (see Figs 2 and 7 for localities). The age of lineations within the Insubric mylonites as thrusting in the Grigna mountains (Laubscher observed E of Locarno supports such a view. 1985) is still uncertain since Brack (1981) reports Additional arguments for a continuous move- a pre-Adamello (Cretaceous?) phase of defor- ment during the Insubric phase will be given mation further to the E. later when synchronous deformations outside Both the southern steep belt and the Ivrea the mylonite belt will be discussed. zone were also affected by dextral shearing but Only during the final stages of deformation a clear sequence of backthrusting followed by did dextral strike-slip outlast transpression. The dextral shearing cannot be established yet. Some brittle deformation within the Centovalli fault of this dextral shearing within the southern zone finds its eastward continuation in the steep belt occurred under amphibolite facies Tonale Fault with its associated Riedel shears. conditions (Steck 1984, Merle et al. this volume) This same event also caused strike slip defor- whereas E of Locarno dextral shearing occurred mation under greenschist facies conditions under greenschist facies conditions (Heitzmann The Periadriatic Line I59

1987). A large portion of the very complex two periods of uplift can be distinguished. deformation N of the southern steep belt can Within the southern steep belt rapid cooling also be attributed to the Insubric phase (Merle (65~ corresponding to an uplift rate of et al. this volume). about 2.2 mm/a) from 23 to 19 Ma was followed by significantly slower cooling (14~ corresponding to an uplift rate of around 0.4 mm/a) The cooling history of the Lepontine region compatible with present day uplift rates. The same pattern is recorded for the fiat-lying Radiometric data from the Lepontine area are central and northern parts of the Lepontine so numerous by now that the application of the except that the onset of rapid cooling becomes cooling age concept allows for a rather precise younger. Since the southern limit of this area of evaluation of temperature vs time curves as rapid cooling coincides with the Insubric Line, a depicted in Fig. 3. With K/Ar and Rb/Sr dating correlation of the period of rapid cooling with on biotite and white micas combined with fission uplift due to backthrusting along the steeply track data on apatite and zircon, Hurford (1986) inclined Insubric Line is obvious. Because the was able to constrain the cooling curves for 19 onset of rapid cooling is expected to postdate samples along a N-S profile through the the beginning of rapid uplift (Werner 1980), Lepontine area N of Locarno to such detail that backthrusting along the Insubric Line may have

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9 ' ' " I ' '" ' ' [ ' ' ' ' I ' ; ' ' I ' , I"~ ", '~ ~ ' ' ' I ' ' ' ~ [ ' ' ' i 10 20 30 m.a. Fro. 3. Cooling history derived from published radiometric age determinations. Tertiary amphibolite facies terrains: A and B Oligocene intrusions: C and D Bergell boulders from the Com, formation: E. Cretaceous high pressure terrains of the Sesia zone: F. A: Verampio (KAW 201), reference (2), (3), (4). B: Maggia (KAW 1887), ref. (7). C: Novate (KAW 132), ref. (2), (4). Codera (KAW 553), ref. (3), (4). D: Bergell (Z 6, BM 2, BM 3), ref. (1), (2), (5). E: Com, fm. (KAW 1003, 1004, 1005), ref. (4), (5). F: Mucrone (KAW 987,988,989), ref. (6). References: (1) Armstrong et al. (1966), (2) J~iger et al. (1967), (3) Purdy & Jager (1976), (4) Wagner et al. (1977), (5) Wagner et al. (1979), (6) Hurford & Hunziker (1985), (7) Hurford (1986). 160 S. M. Schmid et al. started somewhat earlier than 23 Ma ago. Based Based on hornblende geobarometry, Reusser on an estimated amount of 9 km rapid vertical (1987) determined a pressure difference of uplift during 4 Ma (Hurford 1986) a minimum 2.5 kbar between eastern and western Bergell shear strain rate of 8 x 10 -14 sec -~ may be at the time of intrusion. This would correspond estimated during mylonitization. Later cooling to a later differential uplift of the western parts and uplift affects both central Alps (14~ of this intrusion in the order of 9 km. and Ivrea zone (7~ Hurford 1986) and may partly be attributed to isostatic uplift. The N-S age trend for the onset of rapid cooling (19 Ma vs 23 Ma for the southern steep Earlier deformations along the belt and the northern and central Lepontine Canavese Line areas respectively) supports the idea of a substantial horizontal temperature gradient during Two mylonite belts along the southwestern backthrusting. This allows mylonitization under portion of the Canavese Line greenschist-facies conditions and along the Insubric Line to be contemporaneous with more SW of the Valle Sesia the character of the distributed deformation under amphibolite- Canavese Line and the adjacent zones changes facies conditions further to the N. significantly. The region considered is situated The age differences along an E-W traverse at the transition between the Upper Cretaceous N of the Insubric Line are much more pro- high P-low T metamorphism and the Tertiary nounced. Cooling ages for most radiometric greenschist-facies metamorphism in the Sesia systems systematically decrease from the Bergell zone. The Ivrea zone consists of Palaeozoic area in the E to the Verampio-Simplon area in granulite-facies rocks with a border of the W (Fig. 3). From these data, combined with (Permian?) metatonalites and metagranites the pattern of mineral isograds which excludes towards the Canavese Line (see Figs 4 and 5). simple tilting around a N-S axis it is clear that Both Ivrea and Sesia zone show Oligocene the Lepontine region cannot have been uplifted magmatism. The Sesia zone was intruded by the as a rigid block without contemporaneous Biella syenite. Along the Canavese Line rem- internal ductile deformation (Merle et al. this nants of related andesites and tuffites are found volume). in a steeply dipping position. Boulders of Upper This cooling pattern can be interpreted in Cretaceous high-pressure rocks, found within the light of the earlier discussed concept of the tuffite, indicate that this volcano- continuous dextral transpression with dominant sedimentary series represents the cover of the backthrusting NW of the Canavese Line fol- Sesia zone (Bianchi & Dal Piaz 1963). Its lowed by dominant strike slip along and N of Oligocene age has been confirmed by K-Ar the Tona|e Line. Thereby the Bergell region age dating (Scheuring et al. 1974). This cover is would have suffered its very rapid uplift at an now at the same topographic level as the intrus- early stage while positioned somewhere around ives. In the Ivrea zone an Oligocene tonalite Locarno whereas the Verampio area was up- was found near Miagliano (Carraro & Ferrara lifted while the Bergell area escaped sideways 1968) and mafic dykes of inferred Tertiary age by dextral strike-slip. In fact, the displacement are observed in the granitoid rim of the Ivrea of the roof of the Bergell intrusion in relation to zone in the Valle Sessera region. the 'Gonfolite lombarda' containing Bergell In the region between the Valle Sessera and boulders (Gunzenhauser 1985) suggests dextral the Valle Sesia (Figs 2 and 4) the Canavese Line strike slip by some 60 km (Fumasoli 1974, swings into a SSW-NNE strike. At the same Heitzmann 1987b) and is compatible with such time the inclination increases to about 70 ~ to the an idea. Fission track data on these boulders WNW. Further to the SSW, a steep ESE dip is (Fig. 3) suggest very rapid cooling and erosion observed, possibly as a consequence of inter- (Wagner et al. 1977). The deposition of the ference with the younger Cremosina Line (Fig. Bergell boulders (latest Oligocene) predates 1). Two mylonite belts can be distinguished. most of the subsequent strike-slip movement. The earlier mylonitization affects the tonalitic Later uplift of the area W of the Bergell intrusion rim of the Ivrea tectonic unit. This earlier mylo- was responsible for a considerable tilt of this nite belt (belt 1) also contains tectonically intrusion around a N-S oriented axis resulting emplaced slivers of Canavese sediments. Based in a well documented contact aureole E of the on sense of shear criteria this mylonite belt 1 Bergell batholith, while no such aureole can be indicates a relative uplift of the Ivrea zone observed in the western parts of the same combined with a dextral component of shear intrusion (Trommsdorff & Nievergelt 1983). (Fig. 4). Mafic dykes of inferred Oligocene age The Periadriatic Line I6I

FI6.4. Sketch map of the southwestern part of the Canavese line intrude the western margin of the Ivrea zone by the cooling path deduced by Hurford & and mylonite belt 1 and they are clearly seen to Hunziker (1985). postdate mylonitization. Granitic rocks (Sesia zone) and the above mentioned Oligocene andesite bearing forma- Palaeomagnetic evidence for a large scale tion are affected by later mylonitization and/or rotation of Ivrea and Sesia units near the cataclasis within mylonite belt 2. It is only this Canavese Line post-Oligocene mylonite belt 2 which represents The remanent magnetism of 26 specimens taken the continuation of post-late Oligocene back- from 4 mafic dykes intruding the mylonitized thrusting movements along the Insubric Line, rim of the Ivrea zone (belt 1) was measured. as discussed in the previous chapter. Sense of The results of these investigations are plotted in shear criteria within mylonite belt 2 indicate a Fig. 6 together with other data taken from the relative uplift of the Sesia zone in respect to the literature. A 60 ~ rotation of the palaeopoles Ivrea zone, combined with a minor sinistral around a horizontal axis with a strike parallel to strike-slip component. South of the Sesia valley the Canavese Line (N20E) in an anti-clockwise the tectonites of the post-late Oligocene mylo- sense viewed to the north brings these poles nite belt 2 pass through the transition from into a position near to that of stable Europe in crystal plasticity into cataclasis. This indicates the Oligocene. A similar rotation has to be that shallower levels of the post-Oligocene applied to the data obtained from the Oligocene Periadriatic Line are exposed in this region and intrusion of Traversella, the mafic dykes found that the amount of post-Oligocene backthrust- in the internal part of the Sesia zone and the ing and uplift is minor. This view is supported andesites of the Sesia cover, whereas the mafic 16Z S. M. Schmid et al.

FiG. 5. A possible kinematic reconstruction (a-e) for the profile across the Canavese Line in the Valle Sessera (f). For further explanations see text. The Periadriatic Line I63 dykes of more external parts of the Sesia zone N axis of rotation require a smaller amount of rotation (Lauza, 1977). After this backrotation the palaeopoles cluster with other measurements from the Piemonte and the central Alps. These findings very strongly support the idea of post-Oligocene verticalization of the Ivrea zone during the Insubric backfolding event, so far based on rather circumstantial evidence (Handy 1987, Schmid et al. 1987). Since the mafic dykes intrude mylonite belt 1, a backrotation has to be applied to this mylonite belt as well in order to restore the pre-Oligocene situation depicted in Fig. 5.

A kinematic model for the southwestern part of the Canavese Line Figure 5 (f) shows the present-day configuration. Although the kinematic evolution is not completely constrained, the following episodes can unrotated rotated by 60 ~ be reconstructed combining palaeomagnetic and 9 rnafic dykes in mylonite belt (1) 0 structural data: pre-Oligocene dextral strike-slip 9 andesites of the Sesia cover (2)+ (3) ,L~ combined with thrusting of Ivrea rocks in a Traversella intrusion (4) 0 9!- dykes of the internal Sesia zone (5) northwesterly direction lead to the formation of "k Bergell intrusion (6)*(7)*(8) mylonite belt 1 (Fig. 5a). This mylonite belt -3(- Lepontine region (8) contains slivers of Canavese sediments and 9 Piemonte (9) mainly affects the (Permian?) tonalitic rim of 0 stable Europe, Oligocene the Ivrea zone. Both Canavese sediments and FIG. 6. Summary of a palaeomagnetic data suggesting tonalites are interpreted to represent shallow a 60~ rotation around a horizontal pole positioned crustal levels of the southern Alps, strongly N20~ (lower hemisphere). References: (1) new tectonized and overridden by the rest of the data, (2) Lanza (1979), (3) Heller & Schmid (1974), Ivrea zone during thrusting and dextral strike (4) Lanza (1984), (5) Lanza (1977), (6) Heller (1971), slip (Figs 4, 5a). Both subduction and ex- (7) Heller (1973), (8) Heller (1980), (9) Van den berg humation of the high pressure Sesia rocks to the (1979). W of mylonite belt 1 occurred in Cretaceous times (Hurford & Hunziker, 1985, report mica ages suggesting 300~ at 60 Ma ago). Mylonite Oligocene andesites between the erosional sur- belt 1 cannot be held responsible for subduction face of the Sesia zone and mylonite belt 1 a (lack of high pressure overprint) nor ex- second post-Oligocene thrusting event is re- humation (opposite sense of shear in Fig. 5a) of quired (Fig. 5c). Subsequently, the Insubric the Sesia high pressure rocks. Thus, movements phase produces large scale rotation of both the along mylonite belt 1 are interpreted to postdate internal Sesia zone and the western rim of the exhumation. Tentatively we relate these move- Ivrea zone (Fig. 5d, e). The component of ments to the Mesoalpine collision event of the backthrusting must have been minor. This western Alps, but the regional significance of explains the above mentioned transition into these movements remains enigmatic. cataclastic deformation along mylonite belt 2. Oligocene magmatism is very widespread all Although the local geology is not yet fully along the Periadriatic Line and Laubscher understood the following conclusions can be (1983b) relates these intrusions to a scenario of drawn from the observations in the Sessera extension affecting a large region all around the area: Alps. Although direct evidence for such an extensional event has not yet been found along (1) Pre-Oligocene deformations are docu- this lineament, this scenario could well explain mented in mylonite belt 1. rapid erosion and redeposition (Fig. 5b) of huge (2) There is a lateral transition from crystal Sesia boulders well exposed NW of Biella near plasticity into cataclastic deformation along Sordevolo (Bianchi & Dal Piaz 1963). belt 2 in post-Oligocene times, i.e. during In order to explain the sandwiching of the Insubric phase. 164 S. M. Schmid et al.

(3) Palaeomagnetic data indicate post- with the Tonale Line to be predetermined by a Oligocene rotation and verticalization of former transfer fault. No deep crustal or upper the Ivrea zone and the internal part of the mantle rocks are preserved along the continu- Sesia zone. ation of this passive continental margin into In the next section a possible model will be the Lower Austro-Alpine units of eastern discussed which helps to explain this rapid Switzerland. The exceptionally good preser- vation of a passive continental margin in the change in amount and direction of shearing along the Insubric Line. southern Alps, including lower crust and upper mantle is attributed to another anomaly, namely the subduction and exhumation of continental crust in the form of the Sesia zone. The quartz- A model of indentation and bearing Sesia zone acted as a buffer inhibiting the deformation and subduction of the deeper sideways escape of the Lepontine parts of this continental margin at its western region during the Insubric phase edge during collision. Dextral transpression during the Insubric In an attempt to understand the complexity of phase led to highly variable amounts of uplift late to post-Oligocene movements along the related to a very complex displacement path Insubric Line and the associated deformation along the curved Insubric Line (Fig. 7) and in within central and southern Alps the role of the front of a rigid indenter in the form of the Ivrea Ivrea zone (and its extension to depth, the Ivrea zone which is bounded by a transfer fault in the geophysical anomaly) as a rigid buttress with north. The lateral escape of the central Alps restricted internal deformation has to be empha- occurred almost entirely in an eastern direction sized. Schmid et al. (1987) interpret the vertical because, (i) the southwestern Canavese Line uplift of this slice of deep continental crust and (mylonite belt 2) acommodates only minor upper mantle to result essentially from crustal amounts of backthrusting and sinistral strike- thinning culminating in the early Jurassic (poss- slip, (ii) a substantial component of dextral ibly starting in the late Palaeozoic, Handy 1986 strike-slip is recorded along the Canavese Line 1987, Brodie & Rutter 1987) combined with northeast of Valle Sesia, i.e. in a region where Cretaceous subduction. Radiometric data this line strikes NE-SW and perpendicular to from the Ivrea zone (McDowell & Schmid 1968, the inferred direction of indentation (Fig. 7), Hunziker 1974) indicate temperatures at around and (iii) further to the E and S of the Bergell 300~ 180 Ma ago. Hence mantle material intrusion the component of backthrusting rap- whose rheology is dominated by olivine, and in idly decreases in favour of dextral strike-slip addition, deeper crustal levels dominated by which clearly outlasts backthrusting (Heitzmann feldspar rheology, were present in shallower 1987b). crustal levels and before the onset of orogenesis. The complex cooling history of the Lepontine This had severe consequences for later com- region with the earlier mentioned migration of pressional movements in that the rheology of late Tertiary cooling and uplift from E to W the Ivrea rocks must have been dominated by (Fig. 3) can now be readily explained by this the flow properties of olivine and feldspar. Both model which suggests a point of indentation these minerals are significantly harder to deform somewhere at the northwestern edge of the at shallow depth than quartz, the mineral which Ivrea body. Parts of the central Alps which will determine the rheology of upper crustal were previously uplifted and backthrusted at levels under normal circumstances (see Ranalli this northwestern edge of the Ivrea zone would & Murphy 1987 for a compilation of depth vs successively move to the E. Extension across strength curves). the Simplon Line (Mancktelow 1985) postdates The mylonites of the Insubric Line closely most of the backthrusting and backfolding. follow the pre-existing northwestern margin of These movements along the Simplon Line indi- the Ivrea zone. Their localization is the product cate a substantial tangential stretch in front of of a pre-existing configuration created by the indenter during the closing stages of dextral Mesozoic rifting (Fig. 7), i.e. a NNE-SSW transpression. For a more detailed discussion of oriented axis of spreading offset by E-W run- the internal deformation within the Lepontine ning transfer faults (Weissert & Bernoulli 1985) dome the reader is referred to Merle et al. (this modified by Cretaceous subduction and under- volume). plating of the Sesia zone (Schmid et al. 1987). Viewed in cross section (Fig. 8) the modifi- We interpret the relatively sharp bend of the cation of the Insubric movements to the pre- Canavese Line into the E-W strike as it merges viously existing Alpine edifice are impressive. The Periadriatic Line ~65

FIG. 7. Three-dimensional sketch illustrating the model of asymmetric escape of the Lepontine region as a result of indentation of the passive continental margin in the Southern Alps (Ivrea geophysical body in particular). Arrows indicate the displacemant path of the central Alps in relation to the southern Alps as inferred from the analysis if stretching lineations (Insubric mylonites) and slickensides (brittle overprint of the Insubric line by the Tonale fault).

Backthrusting and backfolding in the south are and below the central Alps is needed (as largely contemporaneous with the age of major depicted in the model of Laubscher 1984) or not nappe transport and folding in the Helvetic cannot be decided before a serious attempt in nappes as given by Triimpy (1969) and Pfiffner volume balancing is made under consideration (1986). Pfiffner (1985) pointed out that some of the amount of syntectonic erosion. 15-30 km displacement are still required as far Whereas Laubscher envisaged a similar ge- back as point X in Fig. 8(b) along the Garvera ometry during the Eocene collision (fig. 16b in thrust, which accomodates part of the 50 km Laubscher 1983a) we prefer a model of S-sloping displacement along the Glarus thrust. Two subduction during the Eo- and Mesoalpine solutions are possible for the continuation of stages. We propose that the Insubric backthrust this thrust: it either runs down into the Moho as merely displaces the suture between Austro- a ductile shear zone or it has to be kinematically Alpine and Pennine units and that the same connected with backthrusting along the Insubric suture extends further to the S and under the Line. We prefer the second option which would southern Alps. This reasoning can be supported imply that the central Alps form an asymmetric by the following arguments: wedge or orogenic lid (Laubscher 1983a) with (1) The density inversion at the base of the complete decoupling between lower and upper Ivrea geophysical body (Fig. 8a) is readily crust. The asymmetry of the shape of this explained by Eoalpine subduction of an wedge, i.e. low angle thrusts in the Helvetics originally much larger volume of continental and steeply inclined backthrusting along the crust as compared to the present-day Sesia Insubric Line is also reflected in a similar but zone (Schmid et al. 1987). less pronounced asymmetry of the northern and (2) The palaeomagnetic results presented southern slope of the Moho (Figs lc, 8b). It earlier give firm support for late or post- thus appears that the present day topography of Oligocene verticalization of the Ivrea zone. the Moho is closely related to and caused by On the other hand most of the cover of the post-collisional shortening during the Insubric southern Alps further to the S and E re- phase. The question as to whether post- mained in a subhorizontal attitude. This collisional subduction during the Insubric phase I66 S. M. Schmid et al.

Fro. 8. Two scaled profile sketches through the Alps (profile trace indicated in Fig. la). (a) Profile through the Simplon area (after Kissling 1980, Schmid et al. 1987, Escher et al. 1988). E-external massifs; B-Bernhard nappe; R-M Rosa nappe; Se-Sesia- Dent Blanche nappe; lv-Ivrea zone; SC-Strona Ceneri zone; Si-Simplon Line; IL-Insubric Line; P-Pogallo Line. (b) Profile through eastern Switzerland (after Triimpy 1985 and Pfiffner 1985). A-Aar Massif; Go-Gotthard 'Massif'; T-Tambo nappe; AN- austro - Alpine nappes; Gr - Grigna mountains; TF- Tonale fault (brittle); IL- Insubric Line (mylonites); X-see text.

situation calls for a hinge zone somewhere (3) Miiller et al. (1980) provide evidence for a SE of the Periadriatic Line (Fig. 7). Thus, it deep seated lithospheric root S of the Peri- is tempting to bring the suture back into a adriatic Line, and Giese et al. (1982) inter- moderately S-dipping orientation as shown pret their seismic section through a region in Fig. 8(b). further to the E in terms of crustal doubling. The Periadriatic Line I67

(4) In the absence of substantial vertical dis- Oetztal nappe (Schmid & Haas 1987), contem- placements across the Periadriatic linea- poraneous with Cretaceous amphibolite grade ment further to the E (Gailtal Line) a major metamorphism (Th6ni 1981, 1986). This ther- subdivision into Austro-Alpine nappes and mal dome within the southern Oetztal nappe southern Alps becomes rather dubious, at (Fig. lb) must have rapidly cooled during Cre- least in profile view (we will discuss possible taceous times (Th6ni 1986). Pre-Alpine radio- strike-slip components later). metric ages (Del Moro et al. 1982) are found in a region S of the Oetztal nappe but still N of the traditional location of the northern branch of the Guidicarie Line. All this suggests a scenario of backthrusting and westward escape of the Oetztal nappe in Late Cretaceous times, The Periadriatic Line and related associated with movements along or in the vici- lineaments in the eastern Alps nity of the Periadriatic Line (i.e. the northern Giudicarie Line). The following discussion is largely based on Renewed compression during the Insubric reviewing the literature. Considerable uncer- phase produced tangential stretching along the tainties about the kinematics of movements still Brenner fault (Behrmann 1987, Selverstone exist because of the paucity of modern micro- 1988) combined with updoming of the Tauern structural work eludicating the sense of shear. window (Fig. la). The localization of tangential The increasing importance of Cretaceous defor- stretching and updoming in front of an indenter mations and metamorphism in the eastern Alps (southern Alps E of the Giudicarie Line) is very is another complicating factor. similar to what is observed at the western The exact nature of interference between edge of the Lepontine dome. There, tangential Giudicarie and Tonale Lines is still unknown. A stretching is recorded along the Simplon Line simple offset of the Tonale and Pustertal Lines (Mancktelow 1985, Merle et al. this volume) by the Giudicarie Line can be ruled out because and in front of the indenting lvrea zone (Fig. of the non-existence of an extension of the la). Giudicarie Line into the central Alps. It is Based on a modern microstructural and textu- possible that the eastern parts of the Tonale ral analysis of the greenschist-facies mylonites Line only take up the cataclastic components along the DAV line, Kleinschrodt (1987a, b) observed along the Tonale Fault and that the demonstrated sinistral strike-slip movements mylonite belt, which takes up most of the 60 km combined with a minor component of uplift dextral shear continues along the Pejo Line north of the DAV line, responsible for an age where Andreatta (1948) described greenschist- jump in terms of biotite ages (Borsi et al. 1978). facies mylonites. These biotite cooling ages and the deformation The Giudicarie Line runs immediately to the of dykes associated with Oligocene intrusives in W and parallel to a palaeogeographic boundary that region date these movements to be con- between the Trento swell and the Lombardian temporaneous with the Insubric phase. Ad- basin (Castellarin 1972). This boundary per- ditionally, St6ckhert (1984) provides evidence sisted from the early Jurassic into the Eocene that the same lineament was also active during and forms the eastern edge of deposition of the Cretaceous. Dextral movements along the upper Cretaceous fiysch deposits (Bernoulli et Pusteria and Gailtal Lines are indicated by a al. 1981). Furthermore, Brack (1981) showed series of large scale Riedel shears: the M611tal that N-S compression within the southern Alps and Lavanttal Lines (Fig. la). west of the Giudicarie Line predates the Eocene Thus, we are faced with evidence for both Adamello intrusion. Further to the E it appears dextral and sinistral strike-slip movements form- that this pre-Eocene (Cretaceous?) defor- ing at a very acute angle during post-Oligocene mation is transformed into sinistral trans- times. Palaeogeographic evidence can sub- pression along the Giudicarie Line. All this stantiate such strike-slip movements. Bechst~idt suggests that this line may have acted as (1978) calls for a very substantial amount of a transpressive sinistral strike-slip zone in strike-slip between the northern calcareous Alps Cretaceous times, although it certainly was also and the Drauzug based on palaeogeographic active after the Oligocene (Castellarin & Sartori arguments. Kleinschrodt (1987a) proposed the 1979). DAV Line as a possible site for such sinistral Cretaceous movements along or in the vicinity movements. On the other hand there are con- of the northern segment of the Giudicarie Line siderable facies differences across the Gailtal are indicated by east directed thrusting of the Line between Drauzug and the southern Alps I68 S. M. Schmid et al.

(Prey, in Oberhauser 1980 and Bechst/idt 1978) km total strike-slip predates the post-late calling for compression and/or dextral strike- Oligocene movements along the Periadriatic slip. Line. In a recent attempt to reconstruct the palaeo- In summary, it appears that the response of geography of a very large region including the the eastern Alps to post-Oligocene dextral trans- Transdanubian central ranges (Bakony, pression is of a fundamentally different nature Hungary) Kazmer & Kovacs (1985) postulate as compared to the western Alps: lateral escape lateral escape of Drauzug and Bakony unit to between conjugate strike-slip zones rather than the E between the DAV and Raba Lines oper- substantial uplift associated with foreland ating sinistrally and the Gailtal and Balaton thrusting (Helvetic nappes) and backthrusting Lines operating dextrally (Fig. la). Although (Insubric Line). As shown in Fig. lc the Moho these authors date this lateral escape of some trough no longer follows the direction of the 450 km to have taken place from middle Eocene Periadriatic Line E of the Tonale Line and its to late Oligocene times, the closing stages of topography seems to be unrelated to and un- this escape may still coincide in time with the affected by Giudicarie, Pusteria and Gailtal Insubric phase of the western Alps. The fact Lines. that the M611tal and Lavanttal Lines dextrally transect a large area to the N of the Gailtal ACKNOWLEDGEMENTS: This contribution would not Line and the younger age for transpression have been possible without a lively exchange of in the Karawanks (late Miocene, Laubscher ideas with many of our colleagues, particularly G. 1983b) may indicate that dextral transpression Bertotti, P. Cobbold, M. Handy, P. Heitzmann, outlasted this continental escape. J. Hunziker, R. Kleinschrodt, H. Laubscher, N. Mancktelow, O. Merle, A. G. Milnes, L. Ratsch- We regard the total amount of dextral strike- bacher, and V. Trommsdorff. U. Gerber and A. Uhr slip movement N of the Adriatic sub-plate (300 kindly helped to prepare the figures. H. Stfinitz, D. km) to be rather well constrained by the argu- Dietrich, M. Johnson and an anonymous reviewer ments of Laubscher (1971a). However, recent substantially helped to improve the manuscript. We structural work in the Austro-Alpine nappes also acknowledge support by the Schweizerische (Ratschbacher 1986, Schmid & Haas 1987) calls Nationalfonds (project No. 2. 851-0.80 as well as for a regime of dextral transpression already project No. 4. 903-0.85.20 which is part of the during Cretaceous orogeny of the eastern Alps. National Research Programme NFP 20). It thus appears that a large portion of the 300

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S. M. SCHMID & H. R. AEBLI, Geoiogisches Institut, ETH-Zentrum, 8092 Ziirich, Switzerland. F. HELLER, Institut f/Jr geophysik, ETH-H6nggerberg, 8093 Ziirich, Switzerland. A. ZINGG, Geologisch-Pal~iontologisches Institut, Bernoullistrasse 32, 4056 Basel, Switzerland.