The Alpine evolution of the Southern Alps around the Giudicarie faults: A Late Cretaceous to Early Eocene transfer zone
Introduction
The Southern Alps are a S-pointing Africa-vergent backthrust belt of the Alps (Fig. 1) which is in contact, along the Periadriatic (or Insubric) lineament, with the N-pointing Alpine Europe-vergent nappe stack. The pre-Permian evolution of the Southern Alps is well documented inside the metamorphic basement and in the non-metamorphic Palaeozoic succession of the Carnic Alps (Vai and Cocozza, 1986). Late and post-Hercynian, Lower Permian extensive magmatic bodies together with those of Middle Triassic age, exposed in the Dolomites, strongly controlled the further Mesozoic and Tertiary structural evolution of the eastern sector of the Southern Alps. In fact, these magmatic events consolidated the upper crust strengthening this sector, which is the less deformed and shortened in the whole Southern Alps (Castellarin and Vai, 1986).
The Southern Alps are considered to be a significantly preserved portion of the southern continental margin of the oceanic Mesozoic Tethys as documented by the synsedimentary extensional tectonics of the Norian–Liassic continental rift affecting most of the Southern Alpine domain (Bosellini, 1973, Bernoulli et al., 1979, Winterer and Bosellini, 1981, Castellarin and Picotti, 1990, Bertotti et al., 1993). At the end of the Early Cretaceous the Eurasian and African plates inverted their motion and the continental margin convergence controlled the subsequent evolution of the Alps up to their present setting (e. g., Coward and Dietrich, 1989, Roure et al., 1990, Dal Piaz, 1999, Dal Piaz et al., 2003). The convergence evolution of the Alps includes the Late Cretaceous pre-collisional, the Eocene collisional and the Late Oligocene–Neogene post-collisional (neo-Alpine) compressional events (Trümpy, 1973).
One of the most fascinating problems of the Southern Alps is the meaning of the strong structural inflection of the Alps along the N-Giudicarie fault, where the Europe-verging nappe stack of the Alps proper and the Africa-verging thrust belt of the Southern Alps are facing each other and appear sinistrally displaced more than 50 km. A further still open question is the southward continuation of the N-Giudicarie fault into the S-Giudicarie fault. Several generations of geologists debated these problems proposing different interpretations.
Since the first half of the last century, the Giudicarie lineament and the associated NNE trending structural belt were considered as a wide oblique shear zone related to N–S Tertiary compressions, separating the western Dolomites of the Trento plateau from the Lombardy sector. In order to produce this tectonic separation, relevant sinistral strike-slip motion was required along the N-Giudicarie lineament (Trevisan, 1939). These interpretations were largely accepted and the subsequent studies viewed the N-Giudicarie fault as displacing the Periadriatic lineament during the Middle–Late Miocene times; the lineament was considered, in turn, as a formerly continuous E−W trending structure, experiencing a large dextral strike-slip displacement (not less than 50 up to 300 km) during Late Paleogene to Early Miocene (Laubscher, 1971a, Laubscher, 1971b, Ratschbacher et al., 1991a, Ratschbacher et al., 1991b, Werling, 1992, Schönborn, 1992, Schönborn, 1999, Schmid et al., 1996, Schumacher et al., 1997, Frisch et al., 1998). Alternatively, the N-Giudicarie zone was interpreted as a Tertiary arc originated by pushing the Dolomite block against the Austroalpine units of the Eastern Alps (“Giunzione Atesina”, by Dal Piaz, 1942). The Giudicarie zone was also described as a structural belt, affected by strong Neogene compressional inversions of an older tectonic style inherited by the Early Permian, Triassic to Lower Liassic extensional tectonics and continental rifting (Castellarin and Vai, 1982, Castellarin et al., 1992, Castellarin et al., 1993, Castellarin and Picotti, 1990, Bertotti et al., 1993, Picotti et al., 1995, Picotti et al., 1997). Recent investigations suggest significant reduction in the sinistral strike-slip displacement of the N-Giudicarie fault (Picotti et al., 1995, Prosser, 1998, Prosser, 2000, Viola et al., 2001) stressing a severe, up to some 20−30 km, E−W shortening across this lineament (Prosser, 2000, Viola et al., 2001). The structural system of the N- and S-Giudicarie faults strongly interacted with the Periadriatic lineament and, very likely, also with other E trending faults (e. g., Defereggen–Anterselva–Valles fault, DAV system) (Mancktelow et al., 2001). Also the amplitude of the Periadriatic dextral displacement is thought to be strongly reduced (to less than 40 km) (Prosser, 1998, Prosser, 2000, Müller et al., 2001, Viola et al., 2001). Furthermore, the Early–Middle Miocene compressions are considered to be the driving forces originating both the North-Alpine orogen-parallel extension (Behrmann, 1988, Selverstone, 1988, Neubauer and Genser, 1990, Mancktelow, 1992, Müller et al., 2001, Mancktelow et al., 2001) and the “crustal escape” of the Eastern Alps toward the E (Ratschbacher et al., 1991a, Ratschbacher et al., 1991b).
The aim of this paper is to clear up the kinematics of the early most internal compression of the Southern Alps as suggested by the distribution of the Late Cretaceous Flysch deposits, taking advantage from some recent results on the younger deformation history provided by the TRANSALP project.
A brief review of the main phases of deformation known from the Southern Alps is useful. Five main structural systems and related deformational phases were recognized (Castellarin et al., 1992, Castellarin et al., 1998):
- (1)
Pre-Adamello phases
Late Cretaceous–Early Eocene ENE trending thrusts, related to the eo-Alpine pre-collisional convergence.
- (2)
Dinaric phases
Eocene NW trending thrusts system related to the Alpine collision.
- (3)
Insubric–Helvetic or “Gonfolite” phase
Early post-collisional Oligocene–Miocene (mostly Chattian to Burdigalian) WNW trending thrusts, mostly affecting the subsurface (Po Plain) and the foothills of Lombardy.
- (4)
Valsugana phases
Serravallian to Tortonian ENE trending thrusts related to the neo-Alpine post-collisional deformation.
- (5)
Adriatic phases
Late post-collisional Messinian to Pliocene NE trending thrusts related to the neo-Alpine deformation.
Section snippets
Structural scheme of the Southern Alps
The keys tectonic elements in order to outline the problems concerning role and interpretation of the Giudicarie zone are summarized here (for further information see also: Castellarin et al., 2005-this volume, with references).
Upper Cretaceous to Early Eocene deposits of the Southern Alps
Thick, mostly silicoclastic Upper Cretaceous to Early Eocene Flysch tectofacies deposits are still widely preserved, especially in Lombardy and in the Giudicarie zone. They are arranged according to an arcuate belt along the southern and eastern edge of the pre-Adamello thrust belt (Fig. 7).
Late Cretaceous–Early Eocene
The distribution of the Upper Cretaceous (Turonian–Maastrichtian to Lower Eocene) Lombardian and Giudicarie Flysch is arranged according to an arcuate belt along the southern and eastern borders of the pre-Adamello thrust belt and along the footwall southern edge of the N-Giudicarie thrust (C.N.R., 1990).
Tectofacies meaning of the Late Cretaceous–Early Eocene Flysch deposits, as in the more recent Alpine and Apennine occurrences (e.g., Roure et al., 1990, Vai and Martini, 2001), has to be
Concluding remarks
- (1)
The Upper Cretaceous to Lower Eocene pre-Adamello belt is present only in the internal Lombardian zone, located W of the Adamello massif. This belt is unknown in the Dolomites and surrounding zones located to the E of the Giudicarie lineament (Fig. 1, Fig. 2, Fig. 3, Fig. 4). Consequently the structural equivalents, homologous of the pre-Adamello belt, have to be located within the Austroalpine domain of the Eastern Alps, N of the Pustertal–Gailtal fault (eastern sector of the Periadriatic
Acknowledgments
Gian Andrea Pini, Rinaldo Nicolich and Franz Neubauer are acknowledged for their useful suggestions and for the constructive review of the manuscript. This research was supported by the Italian Departement of Education (M.I.U.R., ex 60% funds of the years 2002−2004).
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2021, Cretaceous ResearchCitation Excerpt :The Southern Alps emerged during the deformation of the passive continental margin of the Adriatic Plate, uplifted as the South Alpine-Apennine Block (Bosellini, 1998; Cecca, 1998; Bosellini et al., 2003; Dercourt et al., 1993; Fourcade et al., 1993; Stampfli and Mosar, 1999; Scotese, 2001; Stampfli et al., 2002). The Dolomites (Permian to Cretaceous) are an internal part of the Southern Alps; they are a Northern Italian chain that emerged during the deformation of the passive continental margin of the Adriatic Plate (Jud, 1994; Bosselini et al., 2003; Castellarin et al., 2006). The Trento Plateau (= Piattaforma Atesina), which extends S–N from the south around Trento up to the Ra Stua region, was framed by two basins: the Lombardian Basin (= Bacino Italiano) to the west and the Belluno Basin (= Fossa di Belluno) to the east (Bosellini et al., 1981; Geyer, 1993; Figs. 1, 2).