Abstract

The retrobelts of doubly vergent collisional orogens are classically interpreted as late-stage postcollisional features. Here, we integrate literature data with new structural and thermochronological evidence from the European Alps in order to document the precollisional development of the retrobelt segment exposed in the central southern Alps. During the Late Cretaceous, by inversion of inherited extensional faults of Permian age, the Variscan basement of the central southern Alps was stacked southward onto the Permian–Mesozoic cover sequences of the Adria margin. These thrust systems were first deformed within regional-scale antiforms (the “Orobic anticlines”) and then cut by Eocene magmatic bodies. Our apatite fission-track data show that these units were largely structured and exhumed to shallow crustal levels before the intrusion of the Eocene magmatic rocks. Therefore, thrusting and folding in the Alpine retrobelt took place before the final closure of the Alpine Tethys and subsequent continental collision between Adria and Europe. Final exhumation and uplift in the northern part of the Southalpine retrobelt took place under a dextral transpressional regime largely coeval with the right-lateral strike-slip activity along the Insubric fault. In Neogene times, deformation propagated southward, leading to the formation of a frontal thrust belt that is largely buried beneath the Po Plain.

INTRODUCTION

The doubly vergent structure observed in many collisional orogens worldwide is commonly interpreted as the result of late back thrusting in the retrowedge and building of topographic relief along the suture zone (e.g., Willett et al., 1993; Beaumont et al., 1996; Jamieson and Beaumont, 2013). Such an interpretative framework has long been applied to the doubly vergent orogenic wedge of the European Alps, which formed in response to convergence and collision along the plate boundary between Europe and Africa-Adria (Dewey et al., 1989). In the central Alps, the S-vergent structures of the Southalpine domain are often considered as the Cenozoic retrobelt, developed well after the N-vergent structuring and exhumation of the metamorphic units exposed north of the Insubric and Periadriatic faults (e.g., Dal Piaz et al., 2003; Castellarin et al., 2006; Handy et al., 2010). However, several authors, mainly based on indirect evidence, have suggested that the southern Alps retrowedge began to form during the Cretaceous (Brack, 1981 and 1984; Doglioni and Bosellini, 1987; Laubscher, 1985; Bersezio and Fornaciari, 1994; Bernoulli and Winkler, 1990; Zanchi et al., 1990b; Schönborn, 1992; Zanchetta et al., 2012), i.e., well before the onset of continental collision. In this perspective, a full understanding of the tectonic evolution of the southern Alps may provide valuable insights for the reconstruction of the early stages of the Alpine orogeny and, more generally, new interpretation keys for the analysis of doubly vergent orogenic belts worldwide. In spite of the results obtained through numerical modeling, precollisional doubly verging orogenic wedges are commonly observed in several mountain belts (e.g., Doglioni et al., 2007; Carminati and Doglioni, 2012; Malusà et al., 2015b), among which the Andes are one of the most outstanding examples (e.g., Allmendinger et al., 1990; Ramos, 1999; DeCelles and Horton, 2003).

The aim of this work is to provide new structural and low-temperature thermochronological data and to integrate them with existing geological information from the central southern Alps to demonstrate the formation and polyphase tectonic evolution of the Southalpine domain as an orogenic wedge in a precollisional geodynamic scenario. Our new data also provide constraints on the late Cenozoic syn- and postcollision tectonic and exhumation history of the central southern Alps. The original data are based on extensive field work all across the central southern Alps, from the Variscan basement to the frontal part of the belt, and they provide a full record of the surface structural features of the area.

The proposed reconstruction is then discussed in the context of orogen deformation front migration, and deep-seated structural changes of the Alpine retrobelt. Similarities with other precollisional orogens worldwide are then underlined.

ANATOMY AND TECTONO-STRATIGRAPHIC EVOLUTION OF THE SOUTHERN ALPS

Geological and Structural Setting

The southern Alps are the S-vergent retrobelt of the Alpine orogenic belt, juxtaposed to the N-vergent part of the orogen along the Insubric fault, a major structure active at least since Oligocene times (Schmid et al., 1989; Müller et al., 2001). The southern Alps are divided into two parts by the left-lateral transpressive Giudicarie fault system (Fig. 1). This fault system developed since the Cretaceous along major structures inherited from Permian and Triassic rifting phases (e.g., Castellarin et al., 2006).

Striking differences exist between the eastern and the western sectors of the southern Alps (Fig. 1). The former consists of a very low-grade to nonmetamorphic fold-and-thrust belt, mainly developed since the Oligocene in response to N-S to NW-SE shortening due to Adria indentation beneath the European margin (Castellarin and Cantelli, 2000; Bertelli et al., 2003; Castellarin et al., 2006). Pre-Oligocene deformations were also described by Doglioni (1985) in response to SW-ward propagation of the Dinaric thrusts. In the area between the Giudicarie fault system and Lake Como, the tectonic style changes, with thrusts that deeply involve pre-Alpine basement units (unit 1 in Fig. 1), now exposed in the most-elevated northern sector (Laubscher, 1985; Blom and Passchier, 1997; Schönborn, 1992; Carminati et al., 1997). Despite the common involvement of basement units, Alpine metamorphism was weaker in the southern Alps than in the Austroalpine units exposed north of the Insubric fault, and it only reached lower-greenschist-facies conditions (Crespi et al., 1982; Spalla et al., 1999; Spalla and Gosso, 1999).

The pre-Alpine basement of the central southern Alps was thrust to the south onto the Permian–Mesozoic cover along the Orobic-Porcile-Gallinera thrust system, which extends E-W for more than 80 km (Fig. 1). South of the Orobic-Porcile-Gallinera thrust system, the central southern Alps form several structural belts, each characterized by a peculiar stratigraphy and tectonic arrangement (Figs. 1 and 2). Directly to the south of the basement units, an array of three basement-cored anticlines, with a dextral en échelon arrangement, occurs (the Orobic anticlines of De Sitter and De Sitter-Koomans [1949] and Schönborn [1992]). These regional folds have WSW-ESE–trending axes and mainly consist of Permian volcanic, volcaniclastic, and siliciclastic rocks, also including Upper Carboniferous (Basal Conglomerate) and Lower Triassic units (Servino and Carniola di Bovegno; Fig. 2; Forcella and Jadoul, 2000; Berra and Siletto, 2006).

Moving southward, thrust sheets made of Lower Triassic to Carnian carbonates are back thrust upon the southern limb of the Orobic anticlines along the Valtorta-Valcanale fault system (Figs. 1B and 1C; Laubscher, 1985; Schönborn, 1992). Another S-dipping fault system (the Clusone fault; Zanchi et al., 1990a) forms the tectonic boundary between the Lower to Middle Triassic thrust sheets and another E-W–trending belt consisting of imbricated Upper Triassic successions (Figs. 1B and 1C). The Upper Triassic units are bounded to the south by the Albino thrust (Fig. 1C), which represents the southernmost exposure of the Norian Dolomia Principale. Farther south, the youngest portion of exposed central southern Alps belt (including the Flessura Pedemontana of Desio, 1929) consists of uppermost Triassic, Jurassic, and Cretaceous units, unconformably covered by the Gonfolite clastic wedge accumulated in the Southalpine foredeep and back thrust (Gonfolite back thrust; Fig. 1B) onto the Mesozoic units during the late Miocene (Garzanti and Malusà, 2008; Malusà et al., 2011a).

The frontal part of the belt, ∼50 km wide, is mostly buried below Pliocene to Quaternary clastic sediments of the Po Plain. The belt consists of deformed Cenozoic sediments resting atop of Mesozoic carbonates (Bersezio et al., 2001; Fantoni et al., 2004). The Gonfolite thrust fades out westward, beneath the western Po Plain, in a regional syncline bounded by a N-verging back thrust to the north (Bernoulli et al., 1989; Gelati et al., 1991) and a S-verging thrust to the south (Fantoni et al., 2004). The base of the entire orogenic wedge of the central southern Alps is defined by a major detachment documented by seismic data (Pieri and Groppi, 1981; Schönborn, 1992; Montrasio et al., 1994; Scrocca et al., 2003) that increasingly deepens from the southernmost thrust front toward the Insubric fault (Fig. 1C).

Time Constraints on Alpine Deformation in the Central Southern Alps: A Summary

In the central and eastern parts of the central southern Alps, where basement-cover structures are best preserved and exposed, a straight distinction between Alpine and pre-Alpine deformation has long been established (Milano et al., 1988; Albini et al., 1994; Cadel et al., 1996; Blom and Passchier, 1997). These studies represent the starting point for the classical reconstruction of the deformation history envisaging two main pre-Alpine synmetamorphic phases (D1 and D2; Spalla and Gosso, 1999) followed by two shortening phases that took place during the Alpine orogenesis, involving both basement units and the Upper Paleozoic–Mesozoic cover. Pre-Alpine structures (mainly extensional detachments and normal to strike-slip faults) related to the Permian and Triassic extensional phases (e.g., Jadoul et al., 1992) are preserved along the Orobic-Porcile-Gallinera thrust system and adjacent areas. They were later involved in Alpine compressional deformation, giving rise to complex structures (Blom and Passchier, 1997; Froitzheim et al., 2008).

Two main Alpine deformational events, generally referred to as D3 and D4 in previous works (Carminati and Siletto, 2005), can be recognized in the northern sector of the central southern Alps belt, both in the hanging wall and in the footwall of the Orobic-Porcile-Gallinera thrust system (Figs. 1B and 1C).

Brack (1981 and 1984) first noticed that the western units of the Adamello batholith intruded the eastern termination of the Gallinera thrust and the basement-cored Cedegolo anticline (Figs. 1B and 1C). As radiometric data (Del Moro et al., 1983; Villa, 1983) provided a late Eocene intrusion age, Brack (1984) pointed out that a significant amount of shortening in the northern sector of the central southern Alps occurred before the late Eocene. Since then, pre– and post–late Eocene tectonic stages have been distinguished on the basis of crosscutting relationships between magmatic rocks and deformation structures (Brack, 1984). Other constraints have been derived from crystallization ages obtained through various methods (U-Pb on zircon—D’Adda et al., 2011; K-Ar on amphibole—Zanchi et al., 1990b; Fantoni et al., 1999) from basaltic to andesitic dikes crosscutting Alpine thrust faults and folds, and from 40Ar/39Ar dating of pseudotachylytes (Meier, 2003; Zanchetta et al., 2011).

Nevertheless, the reconstructions of deformation timing in the central southern Alps (Laubscher, 1990; Laubscher et al., 1992; Schönborn, 1992; Castellarin et al., 2006; Schumacher et al., 1997; Fantoni et al., 2004) are still largely based on indirect stratigraphic data and on subsurface seismic data from the buried sector of the belt (Pieri and Groppi, 1981; Cassano et al., 1986).

The first indirect evidence of tectonic activity in the Southalpine domain was indicated by the deposition of a thick succession of clastic deposits (Lombardian Flysch; Fig. 1B) in the Lombardian Basin (the Southalpine retrobelt foredeep; Fig. 1B) during Cenomanian–Campanian times. These deposits were interpreted as a synorogenic clastic infill related to thrust activity in a growing orogenic wedge located north of the basin (Doglioni and Bosellini, 1987; Bernoulli and Winkler, 1990; Bersezio et al., 1993). The modal composition of sandstones is consistent with the erosion of both basement and cover units (Bernoulli and Winkler, 1990; Bersezio et al., 1993). Evidence of pre–middle Eocene deformation also occurs in the buried sector of the central southern Alps, below the Po Plain sediments, where seismic data point to a mild inversion of Middle Triassic and Liassic extensional structures. The age of such an inversion is loosely constrained to the late Barremian–middle Eocene time interval (Fantoni et al., 2004). Late Cretaceous tectonic activity is also indicated, in the easternmost central southern Alps, by the occurrence of an important Turonian unconformity within the Insubric Flysch exposed along the Giudicarie fold-and-thrust belt (Doglioni and Bosellini, 1987; Castellarin et al., 2006).

The 40Ar/39Ar data from pseudotachylytes of the Orobic-Porcile-Gallinera thrust system confirm the occurrence of Late Cretaceous tectonic activity (Meier, 2003; Zanchetta et al., 2011) related to S- to SE-vergent thrusting since 80 Ma. These data suggest that this first deformational phase likely lasted up to the early middle Eocene, and it thus predated the emplacement of the oldest Adamello units (ca. 42 Ma; Callegari and Brack, 2002, and references within) and the oldest andesitic dikes in the central southern Alps (Bergomi et al., 2015).

In the SW sector of the central southern Alps foreland basin, a NW-SE–trending system of extensional faults postdates the hemipelagic Scaglia Formation (middle Eocene; see Fig. 2) and predates the late Oligocene deposition of the Gonfolite clastic wedge (Fantoni et al., 2004), testifying to a short time interval when tectonic shortening was not active.

Since the early Burdigalian, deformation has also affected the Cenozoic clastic units. Folding and N-vergent back thrusting occurred south of Lake Como (Sciunnach and Tremolada, 2004) and in the Varese-Ticino areas (Bernoulli et al., 1989; Malusà et al., 2011a). Folds and thrusts affecting the Gonfolite clastic wedge are in turn unconformably covered by Messinian (“Ghiaie di Sergnano”; Fantoni et al., 2004) and Pliocene (Felber et al., 1994) fluvial conglomerates and marine sediments that mark the end of late Neogene shortening. Deformation was thus not younger than the Tortonian, at least in the external sector of the central southern Alps, although Schönborn (1992) and Fantoni et al. (2004) have proposed the occurrence of Messinian out-of-sequence thrusting along the exposed front of the belt (“Flessura Pedemontana”).

ANALYTICAL APPROACH

Reconstruction of the Alpine deformation history in the central southern Alps greatly relies on the relationships among basement, post-Carboniferous cover successions, and Cenozoic magmatic rocks. Until now, the structural evolution of the central southern Alps was mainly reconstructed on the basis of data separately collected “in” basement or cover units. Our approach addresses this issue “across” faults, by combining structural and kinematic analyses of major faults, existing radiometric data on fault rocks (Zanchetta et al., 2011) and intrusive dikes (D’Adda et al., 2011; Bergomi et al., 2015), and new apatite fission-track (AFT) data.

We selected several key areas (Fig. 1B) characterized by the occurrence of specific features, such as intrusive bodies showing clear crosscutting relationships with major structures, and rocks suitable for AFT studies (see Table 1 for AFT methods and sample description). Study areas include the Variscan basement, the Orobic anticlines, the Lower to Middle Triassic units, and the Upper Triassic units.

The southernmost exposed part of the central southern Alps, south of the Albino thrust (Figs. 1B and 1C), was not investigated due to the lack of Cenozoic magmatic rocks and the unfavorable rock types for AFT dating. Moreover, vitrinite reflectance data on post-Toarcian rocks indicate that organic matter never reached a temperature within the oil window in that area (Bersezio and Bellentani, 1997); therefore, detrital apatites were probably not reset.

Regional Alpine thrusts mapped in previous works (D’Adda and Zanchetta, 2015; Ghiselli et al., 2015) were further investigated to assess their relationships with ductile deformations recorded in the overriding basement units (areas 1, 2, and 6 of Fig. 1B) and Cenozoic intrusives (area 7 of Fig. 1B). The allochthonous Lower Triassic to Carnian carbonate units (areas 3 and 5 in Fig. 1B) and the Upper Triassic units (area 4 in Fig. 1B) were also analyzed.

RESULTS

We illustrate here the relative chronology of Alpine deformation based on our structural and AFT data from the central southern Alps. In describing the deformational events, we adopted the classical distinction between pre-Alpine (D1 and D2) and Alpine phases, and between a “pre-Adamello” (Late Cretaceous–middle Eocene, D3a-D3b) and a “post-Adamello” (late Eocene–Miocene, from D4 onward) stage.

Structural Analysis

First Pre-Adamello Compressional Stage (D3a)

The oldest structures related to the early compressional history in the central southern Alps are the Orobic-Porcile-Gallinera thrust system and associated folds in basement and cover units.

Open to closed chevron folds (Figs. 3C, 3D, 4A, 4B, 5A, and 5B) with no axial planar foliation occur in the basement (areas 1, 2, 6, 8, and 9 in Fig. 1B) in association with S-vergent shear zones characterized by mylonitic to cataclastic fabrics. A N- to NNW-dipping (Figs. 3D and 5A) pervasive slaty cleavage formed parallel to the axial plane of folds in the footwall of the Orobic-Porcile-Gallinera thrust system in the Permian Collio Formation, the Verrucano Lombardo, and the Lower Triassic Servino Formation (Figs. 2, 4C, and 4D).

Structural analysis performed within the basement and Permian to Lower Triassic cover successions (areas 1, 2, 6, 8, and 9 in Fig. 1B) indicates N-S shortening during this early Alpine deformation. Also within unit 3 (Lower to Middle Triassic), kinematic data along thrust zones and folds affecting mainly carbonate cover (Fig. 3A) show a similar southward direction of tectonic transport during this stage.

The Variscan basement (unit 1), in the hanging wall of the Orobic thrust (area 2 in Fig. 1B), is characterized by the occurrence of open to tight folds with N- to NW-dipping fold axes. These folds have been ascribed to Alpine deformation (Carminati and Siletto, 2005). In area 2, however, the folds deviate from the dominant orientation (NE-SW– to E-W–striking fold axes) observed in other areas of the central southern Alps. Field structural analyses suggest that such folds predate the mylonites formed along the Orobic and Porcile thrusts (Figs. 3D and 5C), as mylonitic foliations crosscut and partially transpose these folds. The greenschist-facies mylonitic foliation and later brittle structures within the basement can be geometrically associated with folding and with the development of an axial planar cleavage in the Permian–Triassic cover successions, here exposed in the footwall of the Orobic thrust. We ascribe the mylonites and the S-vergent folds observed in the cover rocks to an early D3 phase (D3a in our reconstruction). At present, no reliable data exist concerning the Alpine versus pre-Alpine (Ghiselli et al., 2015) age of folds with N- to NW-plunging fold axes predating the Orobic thrust.

The transition from plastic (mylonites and folding in cover rocks) to brittle deformation (faulting and formation of pseudotachylytes) along the Orobic-Porcile-Gallinera thrust systems resulted in a strong cataclastic overprint on the existing fabrics, often associated with friction-induced melting of fault rocks. Pseudotachylytes were also found in the eastern portion of the central southern Alps (area 6 in Fig. 1B; Fig. 4) along several splays of the Gallinera thrust system in upper Val Seriana. The eastern portion of the Gallinera thrust, stacking the basement of the hanging wall on Permian and Triassic units in the footwall, is sealed by the intrusion of the Avio (34–32 Ma) and Western Adamello (36–34 Ma) units (Callegari and Brack, 2002, and references therein). Small portions of the hanging wall and the footwall are exposed across the Gallinera Pass, where they both show a pervasive contact metamorphism overprinting previous mylonitic fabrics related to thrust imbrication.

A significant effect of this early stage of compressional deformation was the reactivation of inherited extensional structures of Permian and Triassic age. Structural mapping (Fig. 6) and fault kinematic analyses (Fig. 5A) in area 6 (Fig. 1) allowed us to unravel the incomplete inversion of preexisting normal faults during early Alpine compression. The major branch of the Orobic-Porcile-Gallinera thrust system in area 6 (Vacca thrust; Fig. 6) likely nucleated along an inherited normal fault. The thrust displays an along-strike variation from a true thrust, with an old-over-young structure, to an incompletely inverted normal fault, as testified by the occurrence of young-over-old relationships along a reverse fault. The separation diagram and cross sections of Figure 6 illustrate the structural relationships occurring along the Orobic-Porcile-Gallinera thrust system in this area. At the western end of the map (Fig. 6), the Pizzo del Diavolo and Mount Cà Bianca Formations (Collio Formation Auctorum) overthrust the basement and the Basal Conglomerate (section A-A′ in Fig. 6). Moving to northeast, the thrust is entirely within the cover successions and still displays young-on-old relationships, with sandstones of the Pizzo del Diavolo Formation stacked on ignimbrites of Mount Cà Bianca Formation. Finally, to the north and northeast of locality Simba (Fig. 6), the Orobic-Porcile-Gallinera thrust system returns to its typical setting, with the basement of the central southern Alps thrust on the Permian cover succession.

Second Pre-Adamello Compressional Stage (D3b)

A later deformation stage (D3b) involved deeper structural levels of the central southern Alps belt prior to Adamello batholith intrusion, and it displays a different deformational style. This event was likely related to the southward propagation and wedging of the Orobic anticlines beneath the previously formed thrust sheets.

Mesoscopic structural data poorly describe this stage, as related deformation consists of gentle folding and back thrusting developed at the regional scale. Here, we summarize the key observations to provide a general description of the D3b-related structures.

The effects of the deformation of the previously formed thrust sheet related to the Orobic anticlines are evident in the northernmost part of the Lower to Middle Triassic thrust pile, which has been steeply lifted up and tilted southward along the Valtorta-Valcanale fault in the forelimb of the Orobic anticlines (Fig. 7). Regional tilting of thrust planes is also evident farther south, as all the main thrust surfaces gently dip to the south across the whole study area (Fig. 1C).

Folded thrusts, formed during the D3a phase, occur spectacularly in area 3 (Fig. 7), where older thrust surfaces are folded and dip southward below the Clusone fault, along which Upper Triassic strata have been back thrust onto Middle Triassic strata. Such peculiar structures were likely induced by the southward propagation of deep thrust surfaces that underlie the Orobic anticlines (Zanchi et al., 2012).

Steepening of the older Late Cretaceous reverse fault systems in areas 2 and 6 can be related to the same reactivation of these deep thrust surfaces. Late Paleocene to middle Eocene ages (56.4 ± 1.1–43.4 ± 2.1 Ma) yielded by pseudotachylyte veins along the Orobic and the Porcile thrusts (Zanchetta et al., 2011) point to a reactivation of these faults, also after the Late Cretaceous.

We interpret the Valtorta-Valcanale fault system and the Clusone faults as back thrusts (Figs. 1C, 8, and 9), following the interpretation of Schönborn (1992), who considered the Orobic anticlines as compressional rather than wrench-related structures. Some lines of evidence supporting this hypothesis are: the perfect correspondence to fault-bend fold geometry, with a steep forelimb and a gently dipping back limb; the occurrence of small-scale examples simulating the geometrical features of these basement-cored anticlines; and the occurrence of a well-developed axial planar cleavage parallel to the axial surface of the anticlines in the Verrucano Lombardo and Servino Formation.

A contrasting interpretation of the Valtorta-Valcanale fault as a normal structure was proposed by De Sitter and De Sitter-Koomans (1949) and De Jong (1979) in the past, but no reasonable evidence is available in the field.

The D3b event is constrained to a pre–middle to late Eocene age, because the easternmost Orobic anticline (Cedegolo anticline; Fig. 1B) is sealed by the intrusion of the southwestern part of the Adamello batholith (Brack, 1984), which was emplaced between ca. 42 and 39 Ma (Callegari and Brack, 2002). Furthermore, in areas 3 and 4 (Fig. 1B), near-vertical E-W–trending andesitic dikes intruded the stack of the Triassic imbricates between 42 and 39 Ma (D’Adda et al., 2011).

Oligocene Compressional Stage (D4)

Available structural and stratigraphic evidence indicates a strong résumé of tectonic activity since the Oligocene, i.e., during the late stages or shortly after the intrusion of the Adamello batholith, as also suggested by significant deformations along the Insubric and Giudicarie faults (Martin et al., 1991; John and Blundy, 1993; Stipp et al., 2004; Garzanti and Malusà, 2008).

This tectonic event postdates the thrust-related folds in the northern part of the central southern Alps, the growth of the Orobic anticlines, and the southward to southeastward thrusting along the Orobic-Porcile-Gallinera thrust system.

The northern portion of the central southern Alps also shows evidence of important reactivations along previous fault zones during this stage. Rotated and steepened mylonitic to ultracataclastic shear zones active during the D3a and D3b phases are crosscut by brittle shear planes ascribed to the D4 deformational event (Fig. 3D, area 2), together with a spaced disjunctive cleavage cutting the D3a-b fabrics (Albini et al., 1994; Carminati and Siletto, 2005). Our mesoscale analyses along these fault zones point to mainly dextral oblique and strike-slip motions along the major thrust zones. The Orobic thrust is crosscut by a complex pattern of right- and left-lateral strike-slip faults in area 2 (Figs. 1B and 3D), whereas left-lateral faults dominate in area 1 (Figs. 1B and 5B). Dip-slip reverse motions suggesting NW-SE shortening, and mixed populations of reverse and dextral reverse faults (Fig. 5C) crosscutting older cataclastic and mylonitic shear zones characterize the evolution of the eastern segment of the ENE-WSW Porcile Line. Reverse dip-slip and oblique reverse dextral faults (Fig. 5A) occur in area 6 (Fig. 1B), along major splays of the Gallinera thrust. In this area, crosscutting relationships between older high-angle brittle-ductile shear zones and the younger brittle oblique faults are particularly evident.

Dextral strike-slip faults cut older fabrics along the Gallinera fault system within the Adamello batholith (Gallinera Pass; area 7 in Fig. 1B). This fault zone directly merges into the Gole Strette fault, which cuts across the West Adamello close to its contact with the Avio unit. The Gole Strette fault is a splay of the Gole Larghe fault zone (Di Toro and Pennacchioni, 2005), which also shows the same kinematics and merges to the west into the Insubric fault. Dextral strike-slip fault populations with minor reverse and normal components have been measured (Fig. 5D) west of Gole Strette Pass and along Gallinera Valley, suggesting a linkage between the western segments of the reactivated Gallinera thrust and the Gole Strette fault, which is younger than the Adamello batholith. Pseudotachylytes occurring along the Gole Larghe fault yielded 40Ar/39Ar ages of ca. 30 Ma (Pennacchioni et al., 2006). It is worth noting that some of the andesitic dikes showing Eocene to Oligocene U-Pb zircon ages (Bergomi et al., 2015) have been displaced by similar fault populations, as well as by conjugated dextral E-W and sinistral N-S strike-slip faults. These fault populations are kinematically consistent with observed motions along the Insubric Line during and after the emplacement of the Bergell pluton (32–28 Ma). Similar fault populations are described in the vicinity of the Insubric fault also NE of the Giudicarie fault system, in the eastern Alps (Agliardi et al., 2009; Bargossi et al., 2010).

Apatite Fission-Track Data

Low-temperature thermochronology data may provide useful time constraints to tectonic activity (e.g., Malusà et al., 2006, 2009). However, in the central southern Alps, such data are only available for the Upper Eocene–Lower Oligocene intrusive units (e.g., Reverman et al., 2012) and are virtually lacking in their country rocks, with very few data reported in Bertotti et al. (1999) and in Viola (2000). Available AFT data in the study area (Viola, 2000; Reverman et al., 2012) are chiefly related to the Insubric fault, and these yielded middle–late Miocene ages ascribed to erosional exhumation during transpressional activity along this structure (Viola, 2000; Stipp et al., 2004) and the nearby Giudicarie fault (“Giudicarie phase” in Martin et al., 1998).

In order to complement the existing data set, we collected additional samples for AFT analyses from Cenozoic basaltic to andesitic dikes and stocks showing clear crosscutting relationships with D3a-b and D4 structures. We additionally collected basement and cover rock samples along a N-S transect orthogonal to the Orobic thrust (San Marco pass, area 2, along section A–A′ in Fig. 1B). Details on sample locations are summarized in Table 1. Results of AFT analysis are reported in Table 2.

Following the structural scheme depicted in Figure 1, analyzed AFT samples belong to unit 1 (Variscan basement in the hanging wall of the Orobic-Porcile-Gallinera thrust system), unit 2 (basement-cored Orobic anticlines), and unit 3 (allochthonous Lower Triassic–Carnian units). Samples belonging to unit 4 (from D’Adda et al., 2011) were collected from Eocene magmatic rocks. When interpreted within the conceptual geochronological model for magmatic complexes illustrated in Malusà et al. (2011a), AFT data indicate that dikes were intruded above the partial annealing zone (PAZ of Gleadow and Duddy, 1981) of the AFT system, because AFT ages are indistinguishable (within error) from U-Pb ages yielded by magmatic zircons hosted in the same rocks (D’Adda et al., 2011; Bergomi et al., 2015). Assuming a paleogeothemal gradient of 30 °C/km (cf. Malusà et al., 2006), the inferred intrusion depth was thus shallower than 2–3 km.

Unit 3, consisting of Lower Triassic to Carnian rocks, is bounded to the north by the Valtorta-Valcanale fault system and to the south by the Clusone fault. Samples VZ1 and VZ2, deriving from andesitic dikes (Table 2; Figs. 8 and 9), display younger AFT ages (VZ1: 25.0 ± 1.9 Ma; VZ2: 24.3 ± 1.7 Ma) than U-Pb ages yielded by magmatic zircon in the same rocks (ca. 42 Ma; D’Adda et al., 2011; Bergomi et al., 2015). Therefore, these dikes were intruded within or below the apatite PAZ (Malusà et al., 2011a).

Samples from the Variscan basement in the hanging wall of the Orobic-Porcile-Gallinera thrust system yielded AFT ages of 24.4 ± 4.4 Ma (PU1) and 25.5 ± 5.6 Ma (SS1). One sample collected in the Orobic anticline (CU1) yielded an age of 17.0 ± 1.0 Ma, whereas sample DV1 yielded a loosely constrained age of 41.6 ± 26.3 Ma. Samples from the San Marco Pass transect were collected along an elevation range exceeding 1600 m, from the bottom of Valtellina (320 m, MOR3) to the main ridge just north of the main Orobic thrust surface (1995 m, MOR1). Resulting AFT ages span from 36.5 ± 9.5 Ma to 14.5 ± 0.9 Ma in the hanging wall, and from 32.5 ± 4.8 Ma to 13.1 ± 2.4 Ma in the footwall (Table 2; Fig. 9). These samples show a normal age-elevation relationship, i.e., the AFT age systematically increases with elevation (Table 2; Fig. 9). Samples collected at the same elevation in the hanging wall and in the footwall of the Orobic thrust yielded indistinguishable ages within error.

Time-Temperature Paths

The time-temperature paths derived from modeling of confined track-length distributions are shown in Figure 10A (see Table 2 for raw data). These paths refer to samples from unit 3 (VZ1 and VZ2), from the San Marco Pass transect (MOR3, MOR2, FIOR1, MOR1, SRV1, SE1), and from the basement of unit 1 (CU1b). Results show two well-defined cooling pulses: a Burdigalian pulse recorded by the Lower–Middle Triassic rocks of unit 3 (samples VZ1 and VZ2 in Fig. 10A), and a younger pulse (Tortonian) consistently recorded by samples from different elevations in the northern units (Orobic anticlines and Variscan basement). This later cooling pulse was not recorded by the southern units, because they were already exhumed above the PAZ of the AFT system at that time.

Figure 10B illustrates a possible erosional evolution that is simultaneously consistent with both the thermal histories of samples from the Orobic thrust hanging wall (MOR3, MOR2, FIOR1, MOR1) and with available geological constraints pointing to a Tortonian tectonic phase in the central southern Alps (Fig. 11). The thermal evolution under an eroding topography shown in Figure 10B was modeled with the program TERRA (Ehlers et al., 2005) according to a steady-state two-dimensional (2-D) advection-diffusion equation. The wavelength and amplitude of the topographic surface were derived from the elevation profile along the study transect, as the drainage network in the study area was already established in Oligocene times (Garzanti and Malusà, 2008). According to AFT data, all of the samples in the hanging wall of the Orobic thrust still resided within the apatite PAZ in the middle Miocene (first stage of Fig. 10B), when erosion rates were on the order of 0.1 km/m.y. Relatively fast erosion (1 km/m.y.) during the Tortonian tectonic phase was responsible for heat advection and upward motion of isothermal surfaces (second stage in Fig. 10B), followed by thermal relaxation since 7 Ma, when erosion rates slowed down to 0.2 km/m.y. (third stage in Fig. 10B). Such a thermal reequilibration is consistently recorded by fast cooling in samples from different elevation, as illustrated by the time-temperature paths in Figure 10A.

DISCUSSION

Early Evolution of the Central Southern Alps

Reactivation of inherited structures and pervasive basement folding took place in the central southern Alps during the earliest stages of Alpine compression. According to Laubscher (1985) and Schönborn (1992), this early Alpine compressive event led to the emplacement of the Orobic thrust sheet (thrust system 1 in Schönborn, 1992), including basement and Lower to Middle Triassic units. Both basement and cover rocks were thrust southward, forming an imbricate thrust stack.

This long-lasting compressive phase was Late Cretaceous in age (80–68 Ma), as indicated by 40Ar/39Ar dating of fault-related pseudotachylytes along the Orobic and Porcile thrusts (Meier, 2003; Zanchetta et al., 2011). Most of the Lower to Middle Triassic imbricates between the Valtorta-Valcanale and Clusone faults (Figs. 1B and 1B) were possibly stacked southward during this phase following a break-forward thrusting sequence (D’Adda et al., 2011; Zanchi et al., 2012). Evidence for thrust stacking south of the Clusone fault was recently suggested by the occurrence of Upper Triassic klippen that have been related to the oldest stages of thrust emplacement (area 4; for details, see D’Adda et al., 2011).

Close to the basement-cover boundary along the northern margin of the easternmost of the Orobic anticlines (TC-A in Fig. 8), the early Alpine deformation was strongly influenced by the structural grain inherited from the Permian to Middle Jurassic tectonic evolution of the Adriatic margin (Milano et al., 1988; Bertotti et al., 1993a, 1993b, Albini et al., 1994; Cadel et al., 1996; Blom and Passchier, 1997). ENE-WSW– to N-S–striking extensional faults, which controlled the tectono-stratigraphic evolution of the Collio basins since the Early Permian (Cadel et al., 1996; Cassinis et al., 2008), were inverted during Alpine convergence, thus explaining the structural complexity of the eastern part of the central southern Alps belt. Evidence of complete to partial inversion of Permian extensional faults is illustrated in area 6 (Fig. 1B) by the systematic analysis of the vertical throw along one branch of the Orobic-Porcile-Gallinera thrust system (Fig. 6), which highlights an incomplete progressive inversion of a Permian normal fault that was part of a fault system originally bounding the basins in which the Permian volcaniclastic succession was deposited.

Folding in basement and cover rocks associated with an axial planar cleavage in the sedimentary successions, and S-vergent thrusting along the Orobic-Porcile-Gallinera thrust system also occurred during the D3a phase. The D3 event has been already considered pre- to synthrusting (Carminati and Siletto, 2005), marking the transition from plastic to brittle deformation during Alpine shortening.

The oldest pseudotachylyte ages (80–68 Ma; Zanchetta et al., 2011) in the San Marco Pass area constrain the onset of fault activity along the Orobic and Porcile thrust to the Late Cretaceous (area 2 in Fig. 1B). Microstructural analysis of deformation processes along fault planes attests that pseudotachylytes and cataclasis postdate greenschist-facies mylonites. Zircon fission-track data from the Southalpine basement west to the Giudicarie Line (99–92 Ma; Viola, 2000) suggest a Cenomanian age for basement exhumation above 7–8 km, under the assumption of a 30 °C/km paleogeothermal gradient (Malusà et al., 2009, their Fig. 3). Because pseudotachylytes along the Orobic thrust were formed under brittle conditions, they must postdate basement exhumation above the zircon PAZ.

Indirect evidence of the D3a tectonic stage in the Southalpine foredeep is the deposition of the Cenomanian to Campanian turbiditic successions of the Lombardian Flysch (Doglioni and Bosellini, 1987). Petrographic analyses on these turbidites indicate an orogenic provenance (Bernoulli and Winkler, 1990; Bersezio et al., 1993), including both basement and sedimentary cover sources (Figs. 11 and 12). These features are consistent with a tectonic scenario of a growing orogenic wedge involving Adria-derived basement and cover units since the Late Cretaceous (Zanchetta et al., 2012).

A second deformation phase (D3b) involved deeper structural levels, leading to the southward propagation of the Orobic anticline system. The northward back thrusting along the Valtorta-Valcanale and Clusone faults, together with the tilting of the imbricated Middle Triassic carbonate units (Fig. 9), is ascribed to this second deformation phase. AFT data from the footwall of the Orobic thrust indicate that the northern limb of the Orobic anticline was exhumed above the apatite PAZ in the late Eocene (sample SRV1: 32.5 ± 4.8 Ma). Therefore, the Orobic anticlines were structured at crustal levels deeper than the apatite PAZ before the late Eocene. No differential exhumation is observed between the Orobic anticlines and the basement rocks north of the Orobic thrust (unit 1 in Fig. 1B).

In the eastern sector of the central southern Alps, the Re di Castello and Western Adamello magmatic units intruded the Cedegolo anticline at a depth of 5–6 km (John and Blundy, 1993) during the 40–35 Ma time interval (Callegari and Brack, 2002, and references therein). The regional fold axes of the Orobic anticlines gently plunge to the southwest. This structure results in a deeper structural level exposed at the eastern termination of the Cedegolo anticline, at the contact with the Adamello batholith. The now-eroded upper part of the Cedegolo anticline should therefore have been exhumed to a depth of less than 2–4 km before the intrusion of the Adamello batholith. Exhumation of the Cedegolo anticline may thus be considered broadly coeval with the exhumation of the Orobic anticline farther west. The 40Ar/39Ar dating of pseudotachylytes along the Orobic and Porcile thrusts (Zanchetta et al., 2011) displays, in addition to Late Cretaceous ages, an age cluster at 55–43 Ma (Fig. 11), which may be ascribed to D3b reactivation of the thrust system during the formation of the Orobic anticlines.

The oldest AFT ages found in the central southern Alps are from the central-southern part of the belt, where 42–30 Ma dikes crosscut major thrust surfaces in the thrust stacks consisting of Lower Triassic–Carnian units (areas 3 and 4 in Fig. 1B; Fig. 12). Fission-track ages obtained on magmatic apatites separated from the dikes (D’Adda et al., 2011) are virtually indistinguishable from U-Pb ages on zircon grains from the same rocks (Bergomi et al., 2015), suggesting that intrusion occurred in country rocks already exhumed above the PAZ of the AFT system (2–4 km depth; Malusà et al., 2011a). The frontal part of the central southern Alps north of the Albino thrust (units 4 and 5 in Fig. 1B) has been thus largely exhumed above the apatite PAZ since the Bartonian. Therefore, the presently exposed central southern Alps belt was largely structured and exhumed in pre–late Eocene times (Figs. 9 and 12), and most crustal shortening and thrust stacking were accomplished during the pre-Adamello D3a and D3b phases.

Indirect evidence of the time lapse that occurred between D3a and D3b may be found in the sedimentation history of the Southalpine foredeep. The sedimentary record shows a starved stage from the latest Cretaceous up to the early Eocene (Bersezio et al., 1993; Di Giulio et al., 2001). Both in the Lombardian basin and in the Giudicarie area, the up to 2000-m-thick Cretaceous turbiditic succession was followed by hemipelagic sedimentation (Scaglia and Scaglia Rossa Formations; Fig. 2; Doglioni and Bosellini, 1987). This kind of sedimentation could be related to a relative tectonic quiescence, in the absence of uplift and exhumation in response to crustal shortening (Fig. 11). An extensional tectonic regime is recorded in the western Po Plain during this period (Di Giulio et al., 2001).

Late Evolution, Exhumation, and Unroofing of the Central Southern Alps

The central southern Alps foreland belt buried beneath the Po Plain (the Milano belt Auctorum) was chiefly structured during the D4 compressional phase, when the deformation front migrated 30 km southward (Figs. 9 and 12), and the belt developed with a break-forward sequence in the footwall of the southernmost exposed structures, i.e., south of the Albino thrust (Figs. 1 and 9) and of the pre-Alps foothills (Flessura Pedemontana). Seismic data locate the basal décollement of the Milano belt, also representing the sole thrust of the central southern Alps orogenic wedge, within the Carnian units beneath the Dolomia Principale (Figs. 2 and 9; Schönborn, 1992; Fantoni et al., 2004; Ravaglia et al., 2006). Décollement localization was controlled by the rheological behavior of the sedimentary succession, as also observed farther north (Berra and Siletto, 2006) in unit 3 and unit 4 (Fig. 1). Deformation in the buried frontal belt is documented until the Tortonian, and it involved the Cenozoic clastic wedge (Gelati et al., 1991; Fantoni et al., 2004; Malusà et al., 2011b). The already structured northern sector of the central southern Alps was poorly involved in this deformation phase and was passively transported toward the south, leading to an out-of-sequence reactivation of early structures along the Flessura Pedemontana at the end of the Miocene (Schönborn, 1992; Fantoni et al., 2004).

Postcollision indentation of Adria beneath the orogenic wedge was accommodated on the northern side of the central southern Alps by the Insubric fault (Schmid et al., 1989; Stipp et al., 2004), leading to the exhumation and back thrusting of the central Alps nappe pile (including the Lepontine Dome and the Bergell pluton) within a dextral transpressional regime (Garzanti and Malusà, 2008; Malusà et al., 2011b). The effects of such dextral transpression are also observed in the central southern Alps. Dextral strike-slip motion is documented in fact both along the Orobic-Porcile-Gallinera thrust system and the Tonale segment of the Insubric fault, during the intrusion of the northern Adamello magmatic units (Martin et al., 1991; Stipp et al., 2004). The post-Eocene vertical throw accommodated by the Orobic-Porcile-Gallinera thrust system cannot be resolved by using thermochronological data, and it is possibly negligible, as these structures were chiefly reactivated as strike-slip or transpressional faults.

AFT data show that exhumation of the major units of the central southern Alps was diachronous, and of decreasing amount toward the south (Figs. 8 and 9). Structural units in the south were already exhumed by the Eocene; the Lower Triassic unit 3 records an exhumation pulse of Burdigalian age, whereas the northern units 1 and Unit 2 were exhumed in the Tortonian. Rapid late Neogene cooling is documented not only in the study area, but also in nearby areas such as in the Adamello region and north of the Insubric fault (Martin et al., 1998; Viola, 2000; Reverman et al., 2012).

Because Tortonian exhumation is only observed in the northern sector of the central southern Alps but did not take place to the south, a climatic trigger (Willett et al., 2006) can be ruled out. Data presented in this work favor instead a prominent tectonic control on erosion, as also demonstrated by recent AFT data from the Northern Apennines (Malusà and Balestrieri, 2012). We propose that uplift and erosion of the central southern Alps basement north of the Orobic-Porcile-Gallinera thrust system was possibly linked to the progressive indentation of Adria lithosphere beneath the Cretaceous wedge.

The pre-Tortonian thermal history of the central southern Alps strongly differs from that experienced by other parts of the Alps retrobelt. East of the Giudicarie fault system, in the Dolomites area, the basement in the hanging wall of the Valsugana thrust recorded an exhumation pulse around 10 Ma (Zattin et al., 2003; Zattin et al., 2006), with the entire Dolomites area being exposed to subaerial erosion from the Serravallian onward. The Cretaceous orogeny that shaped the central southern Alps belt was related in the eastern southern Alps to dramatic changes in basin sedimentation and configuration (e.g., Doglioni, 1985), but it was not accompanied by significant shortening or exhumation. Compressional deformation is recorded in the eastern southern Alps only from the middle to late Miocene onward (Zattin et al., 2006).

Central Southern Alps as a Precollisional Orogen

The central southern Alps thick-skinned fold-and-thrust belt has been active since the Late Cretaceous and was structured well before Adria-Europe continental collision (Fig. 12A). With the early subduction of the Alpine Tethys, the broadly S- to SE-directed Cretaceous subduction beneath the Africa-Adria margin promoted a doubly vergent orogenic wedge (Zanchetta et al., 2012; Malusà et al., 2015b).

As a fold-and-thrust belt developed in the Alps retrowedge, the central southern Alps would be expected to grow following propagation of deformation toward the retroforeland (e.g., Davis et al., 1983; DeCelles and Horton, 2003). However, deviations from this classical critical taper model (Dahlen, 1990) frequently occurred, as also observed in other noncollisional and collisional orogens. In the Colombia Andes, for instance, the earliest shortening and exhumation developed in the external sector of the belt, and subsequent reactivation of hinterland structures occurred during foreland deformation advance (Parra et al., 2012). Contractional reactivation of existing crustal anisotropies such as inherited normal faults, and platform and basin boundaries can be regarded as one of the key features for the nonsystematic propagation of deformation. The importance of fault reactivation has been reported both from thin-skinned belts (Canadian Rockies; Price, 2001) and thick-skinned fold-and-thrust belts (North American Cordillera; DeCelles et al., 1995). In the North American Cordillera, the occurrence of hindward reactivation of thrust faults was accompanied by the out-of-sequence growth of large structural culminations with basement in their cores (DeCelles et al., 1995), showing large similarities with the Orobic anticlines.

The precontractional structure of the central southern Alps was characterized by a complex pattern of major faults and abrupt lateral changes of the mechanical stratigraphy inherited from the Permian to Jurassic evolution of the area (Gaetani and Jadoul, 1987; Schönborn, 1992; Bertotti et al., 1993a; Cadel et al., 1996; Blom and Passchier, 1997; Bernoulli, 2007; Froitzheim et al., 2008). Such features promoted later changes in structural style, out-of-sequence deformation, and inhomogeneous shortening along the orogen strike and were reactivated as reverse faults or as orogen-scale transverse zones depending on their orientation with respect to later shortening directions (Laubscher, 1985; Schönborn, 1992; Castellarin et al., 2006; Zanchi et al., 2012).

CONCLUSIONS

We combined original structural and thermochronological analyses from key areas of the central southern Alps with literature data from surrounding areas to unravel the tectonic evolution of the central southern Alps belt. We describe two main early deformation phases (D3a and D3b). The first phase (D3a), of Late Cretaceous age, is ascribed to the southward thrusting of the central southern Alps Variscan basement onto Permian–Mesozoic sedimentary cover sequences. Thrusting was associated with folding and with partial to complete inversion of inherited extensional structures of Permian and Triassic age. The second deformation phase (D3b) led to the growth and southward propagation of the basement-cored Orobic anticlines. Crosscutting relationships with the Adamello pluton and fault-rock ages constrain this deformation phase to the early to middle Eocene.

Our low-temperature thermochronological data combined with available geochronological data on fault rocks and intrusion ages of magmatic rocks demonstrate that the central southern Alps retrobelt was already structured and largely exhumed in pre–late Eocene times, at least to the north of the present-day position of the Albino thrust front. Our new structural analyses suggest that post-Eocene shortening (D4 phase) was mostly confined to the frontal part of the central southern Alps belt, buried beneath the Po Plain. Postcollision deformation in the present-day exposed central southern Alps was mainly accommodated by transpressional to strike-slip reactivation of the Orobic-Porcile-Gallinera thrust system, which was linked to right-lateral strike-slip activity of the Insubric fault further north. The late Cenozoic evolution of the central southern Alps was eventually characterized by the diachronous exhumation of tectonic units that culminated in the Tortonian pulse recorded by the crystalline basement unit in the hanging wall of the Orobic-Porcile-Gallinera thrust system.

We can thus conclude that the central southern Alps, unlike current models of postcollisional retrobelt activation, was initially built through a polyphase tectonic evolution on the upper plate of the Alpine Tethys subduction zone, well before Adria-Europe continental collision. This implies that the European Alps developed as a doubly vergent orogenic wedge since latest Albian–Cenomanian times.

This work was supported by FAR (Fondo per le Agevolazioni alla Ricerca) grants to A. Zanchi, University of Milano–Bicocca. Bianca Heberer, Eugenio Carminati, and two anonymous reviewers provided useful critical comments to a first draft of the paper. We gratefully acknowledge Kurt Stüwe for editorial handling of the manuscript.