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 Email alerting click here to receive free email alerts when new articles cite this service article Permission click here to seek permission to re-use all or part of this article request Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by on 30 May 2007 © 1989 Geological Society of London The role of the Periadriatic Line in the tectonic evolution of the Alps S. M. Schmid, H. R. Aebli, F. Heller & A. Zingg SUMMARY: The Periadriatic Line and related lineaments formed as a result of post- collisional 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. ~=WIE~l~ i CA ~" " /, ' f f p::ina::iaticLine and related ~(T 0~a ~b ~ Helvetics and Jura ORIN (European continental margin) ~~ Pennine units (distal European margin incl. oceanic fragments) Austroalpine nappesand Southern Alps (Southern continental margin) ~ =WIEN . .. O9 BEN ~~~ ~ ~ Tertiar,:amphibolite facies . fac=es (Eastern AIps) ~'~ ~... _ r --m ::'t:::::::::h i bolite facies 0 km ...-35 krn / j ~~ J km ;..5o.m ~50 km "'.......... .30krn ...... ~ ~ isostatic anomaly /'?~.,.o'~: ::: } +100 (~ k~ r/~--(MLiller etal. 1980) \ ,,Moho, | ~.~-",,Europeanplate" \ contour t " (Gieseet al. 1982) (c) k map / ....... ,,Adria microplate" L (Giese et al. 1982) The Periadriatic Line 155 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.